Lens Tests (Third Round)
I decided I need to hurry up and choose a lens, so I can get started on the shutter. I have set myself three very simple requirements:
- The image quality must be absolutely as good as possible.
- The working distance must be as long as tolerable.
- The magnification must be 1 or greater.
The Kodak 14″ Commercial Ektar clearly outranks them all as far as item 2 is concerned. To get 1:1 magnification, the working distance is approximately twice the focal length (this is true for all “simple” lenses, that is, those that are not designed to have a camera-specific back-focus distance), or 28 inches (711 mm), which is confirmed by the previous round of lens tests. This means that the distance between the lens and film is also 711 mm, which would make for a terribly unwieldy camera unless fold mirrors were used. The image quality requirement just kills this lens; there are simply too many chromatic problems at all apertures. The only way to get rid of these problems is to filter the color of light allowed through the lens to a relatively narrow bandwidth, and I’m not ready to do that considering that flash illumination with extremely short pulses will be challenging enough as it is.
Items 2 and 3 together essentially eliminate the 105mm Nikon macro lens as an option. At the higher magnifications the working distance is too short—no sense in making a portable system if the insects will all be scared of it. The image quality is better than for the Kodak, but it is not stunning.
The Hasselblad 120mm f/4 macro lens is a good option; although the working distances are on the short side, the image quality is excellent.
For this round of lens tests, I added a diffuser to the resolution target, so I wouldn’t get any weird diffraction effects and so that I could get much more even illumination. The diffuser is simply a small piece of white plastic that is smooth on one side and rough on the other (it is meant to diffuse fluorescent bulbs in light fixtures). I put the smooth side to the target, and, to my surprise, I could see the roughness of the backside in the images where the depth of field was long enough. I really should stop being lazy and simply make a photographic contact print of the target and mount it to something rigid.
This time I also wanted to get an idea for the depth of field at different apertures, so I imaged the target tilted at about 45 degrees relative to the focal plane. The results for the Kodak lens I won’t even show—they are very disappointing. I tested also the Hasselblad macro, and then I figured I may as well test my 135mm enlarging lens—a Schneider Componon-S 135mm f/5.6. Only after the test did I realize what spectacular lenses these are—and although the newer ones are not dirt cheap, they are not terribly expensive in the used market, especially with so many photo labs going out of business or going all digital. The results with this lens are absolutely stunning. Interestingly, it seems to work fantastically at a 1:1 ratio, even though they are claimed to be made for ratios ranging from 2:1 to 20:1.
Two notes before I discuss the results:
First, if you’re wondering why the numbers are always backwards on the target, it’s because the chrome layer from which the actual target bars are cut I always place facing the lens, so that the glass substrate does not interfere. The target is printed such that it should be read from the glass side, so it appears backwards when viewed from the chrome side. I believe this is done so that contact prints of the target (which should be done with the chrome layer directly on the emulsion) will appear in the correct handedness.
Secondly, for this round of tests I used Nikon’s Camera Control Pro software. At first I was confused about it coming with a special cable, since it only seemed to be sold in boxed editions. Then I realized there is a free trial, and indeed, the only requirement is the USB cable that comes with the camera and to set the camera’s USB interface to “PTP” (point-to-point, I suspect) mode rather than mass-storage device mode. The software is fantastic for this kind of thing, especially in changing camera settings (like non-CPU lens data) since it’s all done from the computer rather than navigating the maze of camera menus. With the ease of use I set the lens data and then used the camera in aperture-priority exposure, which worked well except for the fact that it underexposed in the smaller apertures. I adjusted the images so that they look similar, though towards the end of the test I used the exposure compensation to account for that. There are two downsides to the software: one is that it’s really easy to change the settings—which means one should remember to change everything back before disconnecting or the camera (in my case) will be left in a horrific state for a quick snapshot. Secondly, when the option to automatically display the last image is selected, the software remembers the zoom level but does not remember the relative image position, so if one is zoomed in close to examine the center of the image, the next image loaded resets the window to the top left. This is contrary to one of the most surprising feature of the D200 camera—zoom level and position in play mode is maintained as one browses through the images in the CF card. Still, the software is fantastic; the free trial is here and it also contains the link to buy the registration key directly from Nikon. Note that the trial is version 1.0, but the current version (for Windows anyway) is 1.3, and according to Nikon you have to have 1.0 installed to install 1.3 (so basically the download size is doubled). When I installed 1.0 it immediately asked if I wanted to update to 1.3.
So on to the results!
The Schneider lens was tested at nominal magnifications of 1, 2, and 3, and images were taken at each standard f-stop from f/5.6 to f/45. Nominal means I didn’t measure it, but I tried to put the lens at the right distance from the camera to obtain those magnifications. (In the M=3 case, there is a big difference in magnification between the two lens orientations.) For each setup, the lens was mounted “forwards” (meaning the side that normally faces the photo paper is facing the object) and backwards. The images are linked in a way that each setup will open a new window. So if you click on all apertures for a particular case, then you can use your browser’s forward and back buttons to navigate through the sequence.
Schneider 135mm f/5.6, M=1
The first thing to notice is that in all cases, sharpness at f/32 and f/45 suffers considerably due to diffraction blur. Moreover, these are typically the worst images because the longer exposures make them more prone to light leaks in my bellows-less setup.
The next thing to notice is the axial chromatic aberration at the larger apertures. The f/5.6 image is of course the one that makes this most obvious. You should notice that the blurred target portions on the left of the image (closer to the lens) have a red glow, whereas the right side has a blueish or greenish glow. At f/16, it is still there, but minutely so. At f/22 it is indiscernible, at least in the presence of JPEG compression artifacts. The magnitude of this aberration seems to be very close in both orientations of the lens, suggesting that the design is nearly symmetrical across the aperture plane. A quick look at the specifications confirms this. (Note that in the extensive Schneider specifications, the magnification is referred to as the Greek character beta.) Don’t be alarmed by the absurd list price; the lenses are much cheaper on the used market (~$150).
In this case the chromatic performance at the larger apertures is similar to the Hasselblad macro lens’s. However it seems to disappear by f/16 rather than f/22 as in the M=1 case. Interestingly, the lens seems to perform better facing the object than the other way around. This is especially true at the full-open aperture.
Note that magnification as I write about it here is optical magnification, that is, the size of the image divided by the size of the object. In the brochure for the enlarger lenses, Schneider says they are designed to be used at 2x to 20x magnification, which implies M=0.5 to M=0.05 according to my notation (replace M with for their notation). This confused me at first; in fact, I had to go back and edit the post after I realized it.
So, not to confuse anyone, but as far as the lens is concerned, when the it is facing the object the magnification is 2 whereas when it’s backwards it’s 0.5—in other words, the lens backward at an optical magnification of 2 should perform better than the lens forward. The opposite seems true at the larger apertures, and at the smaller ones they are indistinguishable.
This case (that of the lens backwards) has published MTF curves by Schneider. With our M=3 and the lens facing backwards we have the equivalent situation to Schneider’s (I’m not sure what the minus sign implies). I believe that in the plots, the x-axis is the relative (radial) height of the ray on the image. Note, for example, that decreasing the aperture from f/8 to f/11 does not increase the resolution at the center but it does make a difference at the edges.
In my tests there is a clear difference in the two lens orientations, with the backwards lens performing better. The higher the optical magnification with the lens facing forward, the farther we are from its design space. The behavior seems to be similar to the previous case; that is, by f/16 any chromatic problems are gone. These are the worst images by far simply because I do not have bellows long enough (and not enough cardboard boxes lying around) to block out stray light. I chose not to pursue better images because I was tired and because this particular lens still has a shorter working distance than I want.
Hasselblad Makro-Planar CF 120mm f/4, M<1
The magnification in this case is less than 1; in fact, it is less than in the previous test where the lens was tested at M~0.8. Since the target is tilted, I won’t try to guess what it is, but it can be compared directly to the images above in terms of scale.
You should notice immediately that even at f/5.6 the chromatic aberration is much worse than in the case for the Schneider lens at M=1. This is precisely why the performance of the Schneider surprised me so much, since this Zeiss lens is no joke. The chromatic aberration is apparent all the way up to (but excluding) f/22, where diffraction blur is already affecting the image. It is impossible to compare sharpness between cases at different magnifications, but I think it’s safe to say both these lenses are very sharp.
Chromatic aberrations are obviously a problem in color images, even if the aberration is only present in the out-of-focus regions. In black-and-white pictures, all it really does is increase blur, so if it is not present at the focal plane, then there’s no reason to discard a lens completely because of it. Although I can certainly see myself doing plenty of black and white with this camera (seeing as how I can print nice prints in my garage/darkroom), I’m not going to allow a loss in performance and justify it with that. Thus so far the first choice lens has become the Schneider 135mm enlarger lens, both because of it’s superb performance and its reasonable working distance at M=1. Still, I believe the camera could benefit from a longer focal length (thus giving a longer working distance). The surprising performance of this lens has also enticed me to test an old Goerz 12-inch f/9 apochromatic “red dot” lens, since an apochromatic lens is better corrected for chromatic aberrations than simple achromats (which is everything that doesn’t say “apo” in front of it).
But this has also taken me back to eBay in search of longer focal length Schneiders and any long-focal length apochromatic enlarger lenses I can find.
As a side note, I guess the performance of an enlarger lens being so great should not have surprised me. Schneider’s philosophy, at least, is that the enlarger lens should out-perform the image-recording lens.
In summary, I will test my 12 inch Goerz next, since that will have a nice working distance of around 20 inches at M=1 (and it is a camera lens and thus meant to have a shutter mounted between the front and back cells). Based on that, I may have to spend some time on eBay before settling on a lens and start working on the other parts.
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- 29.07.07 / 5pm