Zeiss Tessar: How does my vintage lens blur images?

Computer Vision, Optics, Photography, Projects

Modern camera lenses are typically super sharp but can sometimes lack character. Vintage lenses with fewer corrective elements are typically softer and have decreasing contrast and resolution in the corners. But what’s going on in my lens?

In this post, I’ll use computer vision techniques to analyse the performance of this vintage lens and then see what effect this has on photographs.

Lens adapted onto modern mirrorless camera

Background

The lens was a budget product from one of the premier optical manufacturers in the world, Zeiss. Zeiss has long been synonymous with excellent optics, developing many key technologies in its history. Currently, Zeiss produces extremely high-performance optics for a range of applications, including photography and photolithography – the silicon chips in your laptop and iPhone were likely manufactured using Zeiss-made photolithography systems. After the partition of Germany, Zeiss – a high-tech company critical to modern economies and militaries – was also partitioned. Much of the stock, equipment, intellectual property, and staff were taken by the Soviet Union to kick-start its optics industry and the remaining Zeiss company was split into Zeiss Oberkochen in West Germany that largely sold to the West, and Carl Zeiss Jena in East Germany that largely sold to the Soviet Union.

This lens was manufactured in 1967 by Carl Zeiss Jena and is a Zeiss Tessar design. The original Tessar was patented in 1902 which I have previously discussed, but this one was redesigned for smaller formats using the latest glasses and design techniques. It was sometimes called the eagle eye because of its centre sharpness and contrast. This is a 4 element/3 group design that performed well, especially when coating technology was in its infancy when it was important to minimise the number of glass-air interfaces to reduce flaring. This example is single-coated, has a minimum focus distance of 0.35m, uses unit focusing (the whole optical unit moves back and forth to focus), and was designed for use with 135 (35mm) film probably for colour and black and white – which were both available when this lens was manufactured. Films available in 1967 were very different to the ones available now and in the ’90s. They were slower and had more grain, Kodachrome II (I believe the most common colour film available in 1967) was available in ASA 25 and Kodachrome-X had just been released in 1962 with a speed of ASA 64. ASA speed is basically the same as ISO speed – in fact, ASA and ISO are just standard agencies (American Standards Agency and ISO kind of means International Standards Organisation, although it now isn’t an acronym and is just ISO, the Greek prefix for same) which specify how the film speed should be measured, there are ISO standards for bolts, paper sizes, and almost anything else you can think of (including how to make a cup of tea).

The lens has a five-bladed aperture, aperture clicks from f/2.8-f/22 with half stops. It has a depth of field scale, infrared focus mark, and an automatic aperture system – meaning that, on a compatible SLR, the aperture remains open for focusing and then stops down the specified setting when the shutter is depressed. It uses the M42 mount which is easily adapted to other systems due to its long back focal distance. The lens body is made of aluminium and has an attractive brushed/striped design that offers excellent grip. This particular lens had a stuck focusing helicoid, so I removed the old grease using lighter fluid, lint-free paper, and cotton buds and re-greased it with modern molybdenum grease (Liqui Moly LM47).

I wrote some software (this actually took ages) and printed off some test charts to work out exactly what my vintage Zeiss Tessar 50mm 2.8 lens is doing to my images.

Analysing the Blurring

I photographed the test charts at a set distance (0.6m) wide open (f/2.8) and stopped down (f/8). The camera was set on a stable tripod and carefully aligned so that it was perpendicular to the floor where the test chart was placed a 10-second shutter delay was used to reduce camera shake and the camera was set at its base sensitivity to minimise noise.

The software that I wrote found each of the test targets in the stopped-down image and found the corresponding test targets in the wide-open image, this is shown in Figure 0. I then assumed that the stopped-down image was perfect and used it as a reference to compare the wide-open image.

Figure 0. Computer vision algorithm automatically finds the targets in the test chart.

Using some fancy maths (called a Fourier transform) you can work out how to transform between the sharp stopped-down image and the blurred wide-open image. I did this for each target on my test chart because the amount and nature of the blurring are different across the frame.

Fig 1. Reference test chart taken at f/8 with centre and edge targets marked. The edge targets are less sharp than the centre even in the reference image.

Fig 2. Wide open test chart taken at f/2.8 with centre and edge targets marked

The above images are the photographs of the test charts photographed at the sharp aperture (f/8) and the blurred aperture (f/2.8). This method of using the sharpest aperture of the lens as the reference was used because it allows for perfect alignment of the images – both images have the same geometric distortion. A limitation of this method can be seen in Figure 1 in which the corners of the image are not perfectly sharp even at f/8. I was inspired to use this method after reading the excellent paper High-Quality Computational Imaging Through Simple Lenses by Heide et al, 2013, although the actual method that I used was different.

Fig 3. Comparison of reference and wide open targets. The central targets are slightly blurred and the edge targets are very blurred.

Enlarged examples of the test target pairs are shown in Figure 3. I then worked out exactly what blurring function you need to apply to the sharp image to produce the blurred image for each pair of targets. The result of this is shown in Figure 4, which is an array of point spread functions (PSFs). These PSFs show how each point in the image is blurred by the lens – the PSFs in Figure 4 are zoomed in 7x. Figure 4 also includes a second array of targets that is offset from the one shown in Figures 1 and 2, that’s why the PSFs are arranged in two grid patterns. The results from the two test charts agree.

Figure 4. Point spread functions at different locations over the frame. Zoomed in 7x

Figure 5. PSF from the centre of the frame. The PSF is compact with a halo around it that is mostly symmetrical. Left is the original and the right panel has increased contrast.

Figure 6. PSF from the centre of the frame. The PSF has a core and a surrounding extended diagonal line. Left is the original and the right panel has increased contrast.

Figures 5 and 6 show enlarged PSFs from Figure 4. The nature of the PSF changes over the image. The PSFs in the centre are more compact and have a distinct halo around the central region, an ideal PSF would have a central point and no surrounding detail. The halo around the points is spherical aberration, this is an optical aberration caused when rays parallel to the optical axis of the lens are focused at different distances from the lens depending on the distance from the optical axis. This causes a glowing effect in the image, since a sharp image is formed (the sharp central core of the PSF) and a blurred image of the same object is also formed but superimposed on the sharp image. This would not greatly reduce resolution but would reduce contrast. Spherical aberration should be constant over the image frame, but varies a lot with the aperture size used in the lens, stopping the lens down reduces spherical aberration quickly.

The PSF in Figure 6 mostly shows sagittal astigmatism. Astigmatism is when rays in two perpendicular planes have different focal points. Two kinds of astigmatism that occur in systems like this are tangential astigmatism and sagittal astigmatism. Tangential astigmatism points towards the optical centre of the frame and sagittal astigmatism points perpendicular to the centre of the frame. Sagittal astigmatism can cause a swirly effect in images as it blurs points into circular arcs around the optical centre. A lens with sagittal astigmatism can be refocused to give tangential astigmatism so the orientation of the astigmatism will flip around. This is because the best sagittal focus and the best tangential focus occur in different planes. Astigmatism doesn’t occur in the centre of the frame and increases rapidly towards the edge of the frame and with increasing aperture size. This aberration is sometimes called coma which is a similar but distinct aberration that looks a bit like a comet with a sharp point pointing towards the centre of the image and a fatter tail pointing away.

Above are maps of the Strehl ratio of the PSF. Strehl ratio is a simple measure of PSF quality, where higher values (maximum 1) indicate that the PSF is most similar to an ideal PSF e.g., a single point in this case. The bright greens are the regions of highest sharpness and the dark blues are the regions of lower sharpness. There is likely some noise in this estimate – I don’t completely trust it. However, from this analysis, it seems that the lens is somewhat decentred, as the sharpest region of the lens is not in the direct centre. This could be due to manufacturing defects such as the lens elements not being correctly seated, or due to damage that occurred during the previous 57 years, or due to an alignment issue with the lens adapter or the target was photographed.

An interesting feature of this lens is the apparent lack of axial chromatic aberration. The PSFs are very similar in each colour channel and the Strehl maps are also very similar, these tests are not very demanding and are not able to test at all for transverse chromatic aberration. For a lens likely designed with black and white photography in mind this is a pleasant supprise.

Example Images

Below are sample images taken with the lens at various apertures, mostly f2.8 (wide-open) and f/8. The second gallery includes zoomed-in regions to show the character of the lens in more detail. Simple colour and exposure correction was applied in Adobe Lightroom, no texture, clarity, or dehazing was added. Images were resized to 1600px on the long edge with sharpening for screen added at export.

A selected sample of images with zoomed-in regions. The image with daisies shows much greater sharpness in the centre of the image compared with the edge and some swirling effect caused by the astigmatism in the lens. The pair of images of the graffiti rabbit (f/2.8 and f/8) show the increase in contrast and sharpness as the lens is stopped down. All areas of the image show an improvement in sharpness and contrast, but the edges improve more. This is also shown in the pair of leaf images, however, the increase in depth of field in these images makes it harder to determine sharpness changes in the plane of focus. The train track images show the same trend with a distinct increase in contrast in the centre of the image (the clocktower).

The bokeh image is of running water in a canal and was taken with the lens set to close focus at f/2.8. The bokeh is bubbly with distinct outer highlights which indicates over-corrected spherical aberration, sometimes this is considered attractive such as in the Meyer Optik Goerlitz Trioplan lens, however, the effect is less strong in this Tessar. This also leads to the distracting out-of-focus elements in the last image of the dog (my beagle, Saturn) where a line out of the plane of focus is blurred into a sharply defined feature.

None of these images show chromatic aberration, this would likely be apparent on the image of the tram powerlines.

f/2.8 red lines showing the direction of the blurring causing the slight swirl effect.

Conclusion

Despite the lens being 57 years old it is still capable of producing sharp images. The lens is sharper in the centre of the frame than at the edge and the edge has a small amount of swirl effect. The lens doesn’t offer much ‘character’, which is likely expected as it was a standard lens for decades and most people want low-cost, small, sharp-enough lenses that don’t detract from the subjects being photographed. There some pleasant aspects of the lens, such as the slightly soft look that may be pleasing for some portrait work, and the bubbly bokeh may be desirable for some creative effects.

To get the most character from this lens, have a bit of separation between the subject and the background and foreground elements. Strong highlights may have a distracting or exciting bubbly effect, so stay cognisant of this. The lens has a great minimum focusing distance and so can produce quite a lot of out-of-focus blurring, it’s also a bit softer up close, so use this when you want to knock out very sharp details, such as skin texture in portraits. The lens has little chromatic aberration so don’t worry too much about that. The vibes of the lens tie the image together nicely, although they make it a little flat before contrast is added back in editing.

Aruora Borealis in England

Photography

Last night people in Northern Europe were able to experience a rare sight, the northern lights.

Here are some photos of the night sky, the earlier ones contain only the moon, and the later images contain the aurora.

Macro photos April 2024

Photography

These images were shot in my garden at low magnification using my new Laowa 90mm macro lens. It’s a great lens, apochromatically corrected, super sharp, and with up to 2x reproduction ratio. Here are a few images I captured using it.

These images were single exposures taken with a diffused flash. The diffused flash yields rich warm colours when the white balance is set correctly. It also avoids obvious specular highlights and increases sharpness. It isn’t always the best choice for macro photography, but I do enjoy the depth caused by the light fall off.

How to get the most out of budget film scanners

Photography

A few simple tips to get the best image quality from budget film scanners. Scanning film can be challenging, high quality dedicated scanners are expensive, and even entry level flatbed for film are pretty pricey. My preferred method is to scan film with a macro lens – perhaps I’ll talk about this method at some point in the future – but today I was going to mention some tips for using the basic 35mm film scanner units which are available on ebay/amazon for between £30-£50. These units all seem to be the same or at least very similar: they are powered by USB, scan to SD cards, and claim a resolution of 5Mpx. Maybe a new generation will come out with a higher resolution sensor, but there are plenty of issues with these scanners and the sensor is not the worst of them.

Typical ebay listing for these sorts of units.

A good 5Mpx scan could be sufficient for digitising photos, especially if you don’t currently shoot film and want to digitise family photos from a few decades ago, as it’s likely that the photos where shot on basic point-and-shoot cameras. If you want to squeeze every drop of image quality from your Leica M7 you probably know that a £30 scanner isn’t for you. To be honest, I don’t think these scanner are for anyone, but if you happen to have one you might be able to eke out a little more image quality with this method.

Dust is the enemy of film scanning. Your film should be as clean as you can make it before scanning. Compressed air, or a hand pumped air puffer, can help to clean up your film and so can water (ideally deionised), however you should make sure not to scan your film wet. If you process your own film you’ll know that you can wash it with tap water and even dish soap, however tap water can leave streaks in your film from the minerals in the water (but if you just dropped your wedding photos on the carpet and they are covered in fluff this is probably a reasonable proposition – and you can re-wash your film if needed).

However, with this scanner you don’t need to clean your film at all. This film scanner was sitting in my dad’s computer cupboard for several years before I tried it, and all that time is has been filling up with uncleanable dust. Not only does the design of the scanner make it very likely to collect dust, it also makes sure that all of the dust is visible in your scans. The scanner has a slot from left to right which accepts the film tray, this slot doesn’t have a dust cover, so is constantly collecting dust. Inside the film scanner is a sensor and lens (just like in a digital camera) and a light source and diffuser. When the film is inserted the light source shines through the film. However, as the diffuser for the light source (a sheet of milky plastic) is very close to the film, dust on this sheet will appear in your scan.

If you scan photos with a dSLR you want the light source to be far from your negative so that any dust on the light source is out of focus.

Scan from the film scanner without any film. All of this dust is superimposed onto each frame which you scan. Also, you can see that the illumination isn’t even.

Scan with dust removal and some simple colour work

The images above were ones which I had processed a few years ago. I thought that I should redo the process with a few intermediate steps to prove to myself that I had remembered all of the steps.

First you need to scan each frame multiple times. I did this two ways, the first was better and the second was faster. The first way is to scan the frame, then wiggle the tray around a little and scan again (you should repeat several times until you have at least 5 scans of each frame). The second method is to scan the whole strip 5 times and just hope that you moved the film holder a little each time.

At the moment you’ll have several images where the dirt is in the same place but the picture moves, we can swap that up so that the picture is in the same place and the dirt moves.

Unregistered images from the scanner. The image moves, but most of the dust stays still.

I found all of the images from the same frame. As you can see the scan area is significantly smaller than the 35mm film frame – another issue with theses scanners. To complicate issues, there are three different kinds of artifact which we want to remove, marks which are fixed in the scanner frame, marks which move a little in the scanner frame, and marks which are on the film. The distinction of marks which move a little in the scanner frame is important as if these didn’t exist we might be able to perform background subtractions, or use the image with no film as a mask to show where the dirt is.

Registered images, for some reason the automatic registration failed.

The registered images show just how much fluff and rubbish is inside the scanner. For a more accurate alignment it is helpful to increase the resolution size (by interpolation), I doubled the number of pixels in each direction, and downsampled the image at the end of this whole process.

Next is where the magic happens. How do we keep the bits which are the same, but remove the bits which move? The median filter! Just as a quick recap, you probably know the median as a centre point estimate, you might use it like the mean if you have outliers in your data. Well, that’s exactly what we have here. Hopefully, if we stack each image on top of one another and look at each pixel we’ll have 5 pixels which are very close in value, and one which is bright white (the dirt is opaque and the negative has the intensity reversed). Now, if we sort the pixels by intensity and the select the one in the centre there is a very good chance our selected value will be close to the true intensity value on the film without the artifact from the dirt.

This process isn’t perfect of course, one issue is that if each image is very dirty then there might be many regions where all of the stacked up pixels happen to be in dirty regions. However, the more images we include the better we can remove the dirt. Also, we will average out noise and JPEG artifacts. For removing noise and JPEG artifacts taking the mean would be slightly more effective, however, for dirty regions which are close to 100% white caused by the dirt we would be left with 6 copies which all have 16.7% brightness all over the image – which might be worse than just one copy of them. Especially if you had to clone-stamp them all out.

The result of the median filter. I did call it a magic trick, but it’s less Wingardium Leviosa, and more ‘Is this your card?’.

The median filter cleaned up the image a lot, but there is still a lot of mess in the frame. Some of this is dirt on the film, or small scratches.

We can easily clone-stamp out the remaining fluff and brighten up the image.

After touching up the image with the clone-stamp and adjusting the brightness we could say that we’ve got the most out of the scanner (and the frame). Zooming in you can the significant noise in the sky, and the halos around the birds caused by the sharpening algorithm in the scanner.

This film cost £1 (it’s Agfa Vista 200) and if you can’t see the grain in £1 colour film then there is probably something pretty bad going on in the scanning.

So, what are your scanning tips? Do you have one of theses scanners? Do you think you’ll use this technique?

The Humble Tessar

Optics, Photography, Projects

In April 1902 Paul Rudolph, of the Carl Zeiss firm, applied for a patent for a camera or projection lens:

‘[Constructed by]… arranging four single lenses in two groups separated by the diaphragm, the two components of one of the groups inclosing an air-space between their two surfaces, facing one another, while the two components of the other group are joined in a cemented surface, and the pair of facing surfaces having a negative power…’

The full specification of the lens is included in the patent, but the wording is extremely broad. The Tessar is considered to be any lens of four elements, with a doublet and a pair of singlets. However, Rudolf Kingslake in The fundamentals of lens design gives more insight, describing them as like Protars but with an air-spaced front group, the Protar being another design from Rudolph and produced by Zeiss from 1890.

If we want to swap out those refractive index values for Abbe values we can simply fit Cauchy’s equation and use that to estimate the C-line index, giving the table below.

Simply fit the three wavelength/index pairs to Cauchy’s equation to calculate the missing values. Only the first two terms were needed.

	nd (589.3nm)	Vd	∆P(g,f)
Lens 1	1.61132	58.4128	0.020117
Lens 2	1.60457	43.2243	0.012310
Lens 3	1.52110	51.5109	0.013207
Lens 4	1.61132	56.3675	0.021767

Refractive index Abbe number and anomalous partial dispersion for the glasses listed in the Tessar Patent

The original patent lists most of the values needed to model and construct the lens, however, you might struggle to find those particular glasses. A more modern formulation, from Kingslake, would find the same basic lens updated with modern (for 1978 in the case of my copy of lens design fundamentals) glasses, SK-3, LF-1, and KF-3, of note is the significant decrease in index of the second lens, but still flowing the same idea of a dense crown for the outer elements and medium flint for the second, and a light flint for the third.

The system focal length seems to be nominally 1000mm and back focal distance is 907mm. For the purposes of this exercise the presented designs will be scaled to exactly 1000mm focal length.

The lens produces an image circle of approximately 500mm. Requiring good performance over this size field is a challenge, but would be necessary if the camera was intended to be used without an enlarger. However, if only contact printing was intended then the definition requirement would be significantly lower. If the camera was to be used with wetplates then the chromatic aberration requirements become quite challenging, as the camera needs to be corrected for longitudinal chromatic aberration in the visible light (where it is focused) and the near-UV where most of the exposure comes from.

My initial hope/plan was to simply re-optimise this lens with modern computer optimisation and to get a slightly better performing lens at f/4 over the f/5.5 lens. This did not happen. It seems that Zeiss’s reputation was well earned. However, what I did do was significantly alter the lenses character and learn a lot on the way. For one, I didn’t really understand the importance of vignetting for aberration control, as you can clip off some of the problematic rays near the edge of the field of view, with only a small loss in edge illumination.

We can assess the performance of a photographic lens in a number of ways. From a design point of view one of the most obvious is spot size. This is the size that a single point of light would make at the focus of the lens. Different object distances can be considered, but for this I only looked at objects at infinity. Lenses tend to have better definition in the centre than at the edge, so it is important to examine the spot size at different field angles. Also, since lenses have dispersion it is important to also examine the effect of wavelength on the system. I used three main methods to judge image quality, the polychromatic spot size, chromatic focal shift, and image simulation. The image simulation also gives an idea of the performance of the whole system, including sharpness, chromatic aberration, and vignetting.

Layout of the Patent version of the Tessar, at f/5.5.

Raytrace of the Patent Tessar, at f/5.5.

Chromatic focal shift in the Patent Tessar. The maximum focal shift is 916µm

Spot diagram for the patent Tessar at 0˚, 7.5˚, 15˚, 20,˚ and 30˚ at f/5.5. The colours show the wavelength.

There are some things we do know about the lens such as properties of the glass and the radii of the curvatures. But there is also other information which we don’t know, such as the semi-diameters of the lenses, or the manufacturing tolerances of the system. If we guess at the first and ignore the second we can model the system as shown in the figures. The rear lens group is slightly smaller than the front group, vignetting some of the rays – this is set by the edge thickness of the rear lens.

To characterise this lens we might say that it is well optimised over the whole field, with the spot size increasing by more than a factor of 3 from the centre to the edge. The chromatic shift isn’t significant and at f/5.5 there isn’t any obvious lateral chromatic aberration.

I re-optimised the lens several times, tweaking the weightings of various factors. I decided that distortion wasn’t an issue, and that over-all central performance was more important the edge or the very centre. I also kept the same glass as the original. The prescription which I arrived at is

	Radii/mm		Thickness/mm
r1	213.366	L1	40.881
r2	-3276.842	gap	19.710
r3	-648.011	L2	11.081
r4	197.148	to stop	39.115
r5	-777.429	from stop	19.649
r6	221.080	L3	8.382
r7	-340.573	L4	46.140
		Backfocus	887.795

Re-optimised prescription for lens

As can be seen from the table and the layout diagram the first lens of the re-optimised lens is almost unchanged. The second lens is slightly strengthened on both surfaces. The rear doublet is thickened and has more power. This might have been avoided in 1902 due to the cost of the ‘new achromat’ glass. Overall, the lens is not much changed, at least by examination. I expect that the 1902 patent lens would be less expensive to make due to the weaker surfaces and thinner lenses. However, in the re-optimisetion I did squeeze an extra stop of speed out of the system.

Raytrace of the re-optimised Tessar at f/4

Focal shift in the re-optimised Tessar, maximum focal shift is 766µm

*Spot diagram for the re-optimised Tessar at 0˚, 7.5˚, 15˚, 20,˚ and 30˚ at f/4. The colours show the wavelength.*

The re-optimised Tessar is a slightly better achromat with a smaller maximum chromatic focal shift of 766µm instead of the 916µm of the original Tessar. This is probably not significant. I don’t know exactly how the original lens was achromatised, however, my choice was to achromatise 0.54µm and 0.4861µm. These value were chosen as they are close to the peak sensitivity of the eye and of the collodion process, hopefully, a photographer could focus in the visible light and expose with minimal focus shift in the blue/near UV.

In the spot diagrams of the re-optimised lens you can see an obvious design choice, the centre spot has been allowed to increase in size slightly, and the very edge spot has increased significantly, all of the other regions show significant spot size decreases. This is due to a difference how I would personally like to compose images, with a less strong centre bias than 1902-Zeiss expected.

The average spot size for the re-optimised lens is significantly larger than for the patented example although almost all of that is in the very edge, but we can’t judge it too harshly as the re-optimised version is almost a stop faster at f/4 rather than f/5.5. If we stop it down to f/5.5 we get a slightly different result.

Raytrace for the re-optimised Tessar, at f/5.5.

*Spot diagram for the re-optimised Tessar at 0˚, 7.5˚, 15˚, 20,˚ and 30˚ at f/5.5. The colours show the wavelength.*

The spots have decreased significantly over the field when stopped down, as would be expected. The central spot size is now almost the same as in the patent design, and the 15˚ spot size is now smaller than the 7.5˚ spot size in the patent design – this significantly increases the region of good definition of the system.

Perhaps a more meaningful way of comparing the lenses is by simulating an image made from them.

Comparison of the image quality between the original patented Tessar and an re-optimised version of the lens.

Examining the simulated image (which doesn’t take into account depth) we can see some of the character of each lens. Like with any other artistic tool, the final judgement is based on the desired use.

The actual imaging properties of a real Zeiss Tessar lens (a 1960s Carl Zeiss Jena Zebra Tessar 50mm f/2.8) are analysed in this post.

Computer Controlled Macro focusing rail

Macro Photography, Photography, Projects

Stack of 128 frames of a wasp using a 3.7x 0.11NA objective on a Pentax K-1

Focus is often used creatively in photography to separate the subject from the background of an image. In microscopy ‘optical sectioning’ to resolve details out of the plane of the image. In macrophotography, however, we often want to capture images which are pin-sharp front to back. Doing this is quite hard.

The depth of focus is very narrow at high magnification. In fact, in the wasp portrait the depth of focus was only 58 microns, or only 0.058mm. You can see what a single image looks like bellow. Only few hairs are in focus. In total for this photo I took 128 images in 20 micron steps.

It’s pretty hard to move in 20 micron steps by hand, so for a little while I’ve been putting together a focusing system.

Version 1 was just a z-axis rail for a CNC machine. A rail with a carriage supported on it, with a screw thread and a like-threaded insert on the carriage, and a stepper motor. This was controlled by an arduino and stepper driver. The camera was set up with an interval timer, and the arduino code had periods in it for the camera to take a photo.

This setup had several disadvantages, securing the camera to the camera was difficult. an M5 to 1/4″ bolt was used, but this didn’t allow for the camera to be securely fastened. Also, the minimum step size was ~50µm which wasn’t fine enough. Lastly, the camera needed to stay in sync with the arduino, which was achieved by starting the arduino code a few seconds before the camera, not ideal.

Version 2 has a number of improvements. By cannibalising a shutter release cable I’ve been able to control the camera from the arduino, by just bringing a pin high. I also drilled out a tripod base plate to give sufficient clearance for the camera plate to slide into it, while everything is bolted together. Lastely, I swapped out the threaded rod for a M8 fine pitch rod. This rod has a pitch of 1mm, and only 1 thread cut into it, instead of the ~2.5mm pitch and 4 threads cut into the rod I was previously using. This improves the stepping precision by a factor of 10. A single step on the new system is only 5µm, which is only about 10 wavelength of light.

The thread was cut into a small block of wood which was pre-drilled with an 8mm hole. The wood offers quite a lot of resistance, but also doesn’t produce any backlash.

1:1 zoom of the eye of the wasp. 1px is 1µm in size. The facets of the compound eye are most interesting as they transition from the hexagonal packing into a square packing.

Mounting unusual lenses on digital cameras

Macro Photography, Photography, Photography Equipment, Projects

*lens recovered from Polaroid Sprintscan 35*

Pleasing effects can often be found when using unusual lenses on modern digital cameras. Sometimes they give a highly aberrated image which is useful in creative situations, sometimes the history of the lens enhances the image in a meaningful way, and sometimes the unusual lens offers a quality which is not possible with typical lenses from first party manufactures.

The last instance is often the case with macrophotography. Many people use objectives to photograph small insects as they offer much higher magnification ratios and resolution than macro lenses. Occasionally, lenses used for obscure purposes in industry can find use in various areas of photography. Surplus aerial photography lenses, such as the Kodak Aero-Ektar are highly sought after for their high resolution and high speeds over large flat film planes. Occasional lenses with have excellent performance are found for macrophotography.

It is one of these which I have acquired recently from an old film scanner. Film scanners image a flat sheet of film onto a flat sensor at high resolution. Unlike lenses for general photography they are optimised for magnification ratios of around 1. Most photography lenses are optimised for a magnification of almost zero.

Mounting a lens can be easy. Adapters often exist allowing a camera lens to be mounted on other cameras. However, when we look at older lenses, or lenses which were designed for industrial use the adaptation is more difficult.

In order to mount the lens securely, I designed a small device to securely hold the lens. The bore has 0.5mm of clearance for the lens barrel, and a flange at the base so that the lens can be reliably positioned the same distance from the sensor. This was 3D printed by a friend of mine. There is also a place for a grub screw to be installed so as to secure the lens in place. The base is so sized so that it may be bonded to a body cap. The injection moulding used on the body cap left the inside surface shiny, this was sanded to reduce reflections. The body cap can the be fitted to a set of bellows or extension tubes.

*3D printed lens hold affixed to body cap and with a screw to hold the lens in place*

Lens mounted to an old set of bellows. These bellows are backwards, so that the remaining rails are on the camera end.

I was surprised to realise that the top of the body cap was quite convex. This cased two problems. Firstly, the contact area which the lens holder made was rather small, and secondly, it would slide about as the glue dried. To compensate for the first issue I used quite a lot of adhesive. To compensate for the first I used a quick curing epoxy resin. This turned out not to be so quick curing, and I spent about 30 minutes poking the parts into alignment.

I intend to test the lens both ways around and at different magnifications. I don’t know exactly what magnification it will perform best at, presumably at it’s design magnification. However, it may surprise us. The lens is not a symmetric design, the front (dot end) has a convex element, the rear surface is plano.

I took the lens out for a short while and tried to photograph some insects. Unfortunately, I didn’t bring my flash light guide, so most of the picture turned out greatly under-exposed. The lens is not exceedingly sharp, at least, not at the magnifications which I tested it at. However, this is not a well designed resolution test (that will come later).

Full image (resized) and 100% zoom on a 36MP full frame sensor

As can be seen from the above frame, the lens is not sharp to the pixel. However, it shows nice contrast and has very little longitudinal chromatic aberration.

Not many insects would stay still for me. This guy did, but he was really small.

The insect above was very small. I’d be interested to know what species it is. Part of the issue with this photo is the due to a heavy exposure pull due to a lack of flash power. The Pentax K-1 isn’t known for its dynamic range, but this is pulled 3.5 stops, and I don’t think it is so bad. I tried a few different magnifications, but I didn’t keep track of it. The working distance is quite short, but this is also a lot higher magnification than my macro lens.

The resolution is probably what I should expect. The scanner that this lens came from was a 2,700 dpi scanner and the resolution of my sensor is 5,200 dpi, so it isn’t surprising that the sensor out resolves the lens. However, image space resolution isn’t the only important property.

Photographs from NNT Adam and Eve

Photography

On Saturday I did the production shots for Adam and Eve, a production put on by Tara Anegada. The show was excellent from the acting to the set and lighting. Some more information about the show is available on https://history.newtheatre.org.uk/years/18_19/eve_and_adam/

Photographs from Birdworld

Photography

Over Christmas two photography related events happened. I acquired a 70-200, and I also went to Birdworld. These are some of the results. I really liked the lens (an older tamron) it’s quick to focus and brilliantly sharp at f4. The screw af is quite loud though and swapping between manual and autofocus is slow on pentax cameras.

My keep rate was about 7% for this outing, which is pretty good for me. Hopefully, as I become better acquainted with the lens that’ll go up.

The focus on the lens is a little slower than my SDM lens. But it is still snappy. I found that, to nail focus on the exact part of the eye I wanted, I needed to manual focus when the scene had foreground elements such as mesh or fence – which is completely normal.

Photographs from NNT Heather

Photography

On Sunday I photographed the dress rehearsal of Heather, by the Nottingham New Theatre. The play was written by Thomas Eccleshare, and directed by Tara Phillips. The entirely student-run, theatre put together an excellent performance. From the acting to the technical effects.

Below are some of the photos.

More of the photos can be found https://history.newtheatre.org.uk/years/18_19/heather/