I collimated a couple of Olympus Trip 35's recently and figured out some refinements to the SLR collimation method. They work for me in any case and may be helpful to others.
For a camera such as the Trip 35, there is no B setting to open the shutter for a long period to inspect the ground glass image for infinity focus through the SLR viewfinder.
To get around this I set my Nikon DSLR to Bulb. With the SLR pointing into the Trip 35 lens (as per the regular collimation method) and both set to infinity focus, I set the Trip 35 to manual f2.8, opened the DSLR shutter and then released the Trip 35 shutter.
This results in a digital image of the ground glass, taken through the Trip 35 lens. I made sure there was a good back light for the marked ground glass mounted in the Trip 35 film chamber.
With the DSLR it's then easy to zoom and magnify the image, to see with incredible detail how in focus the markings are on the ground glass. I was able to make minute front cell adjustments and repeat until I could achieve very precise focus, as well as compare two settings.
The photos below are not originals, just phone pics of the rear screen of my DSLR. But you get the idea.
I have seen people using a similar approach to test shutter speed accuracy, by taking one photo with the good SLR shutter (camera to be tested on Bulb) and one with the shutter to be tested (DSLR set to Bulb) and comparing the average light on the resulting images.
Anyway, feedback welcome. Maybe I made a mistake doing this so please point out any errors.
Seems basically sound, Hugh. I tend to use a ground glass and high powered loupe out of habit more than anything else, but I have used an SLR as a substitute autocollimator with mostly success (and one failure which proved a useful learning exercise when resolved). I have never used a digital SLR for this—eventually, I decided that of all the various cameras I have on hand, the large finder and excellent calibration of my 500C/M made it the best candidate for inspecting film planes. Anyway—I suppose the only thing I worry about when someone uses a DSLR and zoom lens, for example—is the accuracy of the DSLR lens/body infinity calibration.
It might—possibly—have been the late Ed Romney who first suggested using a SLR to inspect a camera's film plane focus at high magnification. If not; he certainly popularised it in his camera repair book. (He described it in his main repair book as "back sighting", from memory). It's a very clever technique for those of us who do not have an actual autocollimator. Of course, the principle is: when two lenses are focused directly into each other, to the same distance, the light rays will agree and a target will appear in focus. So far, so good. But here's the thing: by using this method, what you are really doing is matching the calibration of one camera, to another. Or, to put it another way—the accuracy of adjustments to your patient camera/lens focus, can never be any better than the quality of calibration of your examining camera/lens. Ie. everything hinges on the standard of the auto-collimating camera adjustment. The takeaway, is that, when you set whichever camera lens you are looking through to its infinity stop—it had better be spot on. Otherwise, all you'll be doing is adjusting your other camera to equal the same "error factor".
I don't pretend to know a huge amount about zoom lenses, frankly (I've never used very many, or very often). However I understand that in some cases they may be able to focus past infinity before hitting their stop for various reasons. (To be fair, some longer manual focus primes could, also, to accomodate Eg effects of temperature on material dimensions etc). Nevertheless—whichever lens one uses—zoom, or prime—really does need to be both: well calibrated to the lens register, or "back focus" of the body system employed, and; utterly trustworthy, in its focus at the infinity stop. Otherwise, the whole exercise will be compromised (to a greater or lesser extent, depending on any error).
Autocollimators are invariably fitted with a lens of long focal length. Those made specifically for calibrating photographic equipment, such as the various Gokosha models, for example: tend to have lenses of varying focal length, according to the camera format they're optimised to check. The reason for this is so that an appropriate amount of target magnification may be used to check cameras usually fitted with standard lenses for the format. Ie. Their instruments for medium format camera repair tend to have a lens of longer focal length, than those marketed primarily for 35mm camera inspection—because, of course, a medium format camera will have a longer standard lens. (Our own RFF member Hans Kerensky could speak with more insight on this point, owning—or at least having, at one time, owned, simultaneously—a couple of different Gokoshas, of different types).
I suppose I have veered off track somewhat, above—but this is the internet—veracity of peoples' opinions/accuracy of info conveyed can be, well. . . variable. Using a lens of appropriate focal length with which to examine a film plane of another camera
matters. It affects the degree of magnification, and, hence, the accuracy, of one's adjustments. The preceding paragraph, whilst not, directly, relevant (hopefully) helps to demonstrate that using a decently long lens of at least 2 x the focal length of that you are examining—is very desirable. If it didn't matter, instrument manufacturers wouldn't bother specifying different focal lengths, for different format cameras. OK?
Having said all of that—assuming your DSLR and its lens are well matched and the lens is, ideally, at least 80-odd millimetres focal length (based, of course, on the Trip's Zuiko) what you've described, Hugh, is a clever way of sidestepping the absence of a Bulb setting, and ought to be fine.
Cheers,
Brett