Correctly Calibrating Film Plane & Viewfinder Focus
Correctly Calibrating Film Plane & Viewfinder Focus
Hi Brett, thanks for the reply. Although I took the shutter out I didn't manage to take one of the covers off, it was a bit of a mystery how it all went together. There might be something under there. The focus helicoid has quite a lot of travel as you might expect, in other words at close range it has pushed the shutter a long way out from the standard, and you almost have to reset focus to infinity to close the door again. I'll have to investigate more, I was a bit wary of taking things off. The shutter leaves really want cleaning but I was unsure which 2 of the 3 screws I had to remove to get the escapement out, to then split the shutter, so had to settle for swabbing them instead. It looked a bit like the escapement was going to come apart, I don't want that job again. It's a Seikosha MX shutter and all speeds are close enough to warrant leaving it alone.
Regarding distances, I normally collimate at fairly short range such as 5ft/1.5m as it can be measured directly, and the small DoF means it's easier to get it right. The longer distances should line up as well, plus they have more natural DoF to sort out any inaccuracy. So far it's worked well. I use an SLR focus screen with a split circle, and I've made a holder that fits on the film plane so the front of the focus screen sits where the front of the film would be (negates the thickness of the holder).
I'm surprised how little info there is out there on this camera, in terms of using and repairing. It has auto film advance, which I can only presume works by loading the film and advancing to the pair of white arrows part way across the back, then closing the back and winding on. I can also only presume this will work fine with the thickness of Ilford FP4 today instead of 1955!
This is a very long read but I hope it's both clear, and helpful.
Correct Lens Calibration Procedures
I don't want to seem argumentative for the sake of it, but whilst you can certainly match lens focus and rangefinder together at a close distance, this won't ensure the infinity focus is optimised. Of course the lens and rangefinder ought to, hopefully, track together right out to far distances. But using the method you've described it's less likely that when the focus ring hits the infinity stop, it will be keyed at the precise point of sharpest infinity focus. If you're lucky it might. But it's more likely to either stop just short of infinity, or run just past it.
Yes, depth of field means you can potentially live with either of these scenarios because if RF and lens focus are always tracking together, what you "see" with the patch will be what you get on the film. But you don't have to live with a lens that is a little off in either direction at its maximum focus setting. Because by locking in the best possible infinity sharpness with the focus ring at its stop, you'll be avoiding any potential problems that might occur, if DOF can't mask the deviations between what the focus scale shows at infinity and where the lens focus actually is.
I acknowledge that in reality perhaps DOF will be ample enough that you might never notice, on your films, that the lens at its infinity stop is slightly out. The point, though, is that you can either run with that and hope it's of no consequence, or you can use best practice and set focus at infinity—in which case, it's a non issue.
In a perfect scenario (perfect distance scale, perfect alignments of your target plane at five feet and your film plane, perfectly measured five foot distance between the target and the film plane, etc) your system should see also see the lens focus fall in perfectly at infinity.
But, what if your measurements are slightly off, and five feet is really 4' 11 3/4"? Or if the planes of test target and film are slightly angled? Etc. At the infinity stop, you could be very close, but you may still be a bit off. Setting the focus at infinity, whilst a bit harder to nail, really is best. It's what the factory did, and it's what you should do, too. OK?
Calibrating a lens focus at infinity really well does get easier with practice and experience. I've been doing it for years and, depending on the camera and lens, how bright or dim the ground glass image is, and so on, I might spend a good half hour patiently fine tuning the lens image to get it spot on. I suspect the time needed is partly getting my eye acclimatised to the view through the glass, so that it can begin to spot differences in sharpness that are positively minute. Sometimes I can nail it in five minutes, but, various camera/lens combinations are easier to make miniscule adjustments to than others, so, it can often be easier to see the exact setting you want, than to get it, if your adjustment increments are too coarse (perhaps because of the friction, or other peculiarities of the method by which the lens focus is affixed to the focusing ring).
I'll labour the point here, because it might better clarify the scale of accuracy you're typically dealing with, if your goal is to obtain a level of focus calibration equal to what your camera maker originally achieved.
Here’s a photo of my usual target I use for infinity calibration. A dead gum tree on the ridge across the valley in which I live here in rural Tasmania. It’s nearly a kilometre distant as near as I can judge it. I’ve used few lenses, regardless of focal length, that don’t hit their infinity stop to get it sharp.
Here it is a bit closer.
I find the tree to be an excellent calibration target. Not just because of its distance. The main trunk, its offshoots and their subsidiary branches become smaller and smaller, and hence progressively harder to lock into clear focus from long range. Getting a sharp image of the trunk and its branches through a loupe and ground glass is typically a painless process. Once I can begin to differentiate the smallest twigs visible in the above image (taken with a cheap Fuji super zoom digital) by detecting a visible change in their sharpness, I’m well in the ballpark. It’s then a case of tweaking the focus back and forth in minute stages until its rests at the point those twigs are as sharp as I can differentiate. The next challenge will be maintaining that setting as you lock off the focus adjustment without disturbing it, often easier said than done.
Matching Film Plane Focus To Viewfinder Focus
I got onto this process when I was learning the fine art of repairing my first Rollei twin lens. I’d set the lens focus via the fixing collet nut around the focus shaft and focus knob. Once I had that as accurate as I could get it I’d then place a loupe inside the focus hood onto the top of the focus screen and repeat the process with the knob at the infinity stop until the ground glass image was at maximum sharpness. Off topic, but in the case of a Rollei the viewing lens being adjusted by backing off the securing screw or collar of the viewing lens to its threads in the lens board, and screwing it in or out independently of the focus knob, until sharpness at infinity is maximised (thus, locking viewing lens focus setting to that of the taking lens and assuring they would track together).
When I do a close focus cross check of my infinity settings to verify that the viewfinder matches the film plane, like you, I tend to set a contrasty target at about five feet. It’s close enough to absolute minimum distance to tolerate slight deviations at that minimum between the two (TLR) lenses, but far enough out that the infinity setting should see the lenses still locked in really well notwithstanding typical manufacturing tolerances of TLR lens matching, (or RF component precision, SLR mirror stop repeatability, etc.). On those rare occasions a factory manual might inform a specified distance for a close check you’ll most often find they will instruct this to be done not at the minimum focus stop of the lens, but at a range just a little longer than that.
I find that it is a good idea to do a blind test. What I mean by that is I’ll use my ground glass and loupe to set the sharpest possible image of the test target, but I won’t even look at the viewfinder, to see if it agrees with the ground glass focus. Despite one’s best intentions, it’s rather easy to see what you want to see. Yes, of course, you’ll notice gross discrepancies, but I’m talking about the most marginal of variations, here.
So, I’ll get the ground glass to maximum focus and then very carefully note the exact setting of the focus knob or ring. Maybe this will be against a number at the reference mark. It might be the alignment between one of the aperture markers and a numeral on the scale or knob. Point is, it will be a setting I can accurately note and then repeat as needed with extreme accuracy.
Only then will I check and adjust the viewfinder focus on the target for best sharpness. If subsequent inspection of the focus knob or ring reveals it’s exactly where it was set when the ground glass image was sharpest—it’s a pass. If separate tests of lens focus and finder focus give the precise same setting your infinity adjustments will be very good.
If they diverge slightly, the reason will usually be that your infinity check looked pretty good but was still just a bit off, and this does demonstrate that the distinction between an infinity image that is sharp, and one that’s absolutely bang on, can be extremely fine, indeed.
Originally the lens focus would have been optically calibrated by the manufacturer using auto-collimation, which makes the process both efficient and repeatable with a high degree of accuracy and consistency. Using a ground glass and loupes will take longer, but if you have a trained eye and the right aids can usually be just about as accurate, given a little patience.
Now, I mentioned this in the context of a TLR and its two lenses. But, it should be noted that it really doesn’t matter if you’re calibrating a SLR, TLR or a rangefinder. The schematics of the cameras may be very different, but the principle is exactly the same in every case. You’re setting the film plane infinity focus as perfectly as your eyes and equipment let you, then, dialling in the viewing system (whether that’s an SLR mirror and focus screen, TLR viewing lens and screen, or rangefinder focus patch of some sort) until the latter agrees with the former.
Split Wedge Focus Aid Considerations
May I also suggest that it’s safer to use a plain ground glass for calibrating lens focus than a screen with a split wedge aid? Whilst the latter are generally trustworthy, because they rely on the outer parts of the lens to work rather than the almost invariable better corrected centre, there can be instances when the split wedge won’t be as accurate as a plain ground glass. It’s true that a SLR made by a first quality manufacturer should, if its focus screen features a split wedge, almost always be reliable when it is used with the range of lenses the maker designed it for. Because you have been using a screen from a particular maker for calibrating other completely unrelated lenses, split wedge inaccuracy is a potential issue. I’m not saying it definitely exists with your set up—only that it’s possible. With a ground glass, it’s something you simply don’t need to worry about.
Herb Keppler wrote an interesting article about the little-known point of split wedge inconsistencies published way back in the March 1965 issue of Modern Photography, and, thanks to the protean scanning efforts of Marc Bergman you can refer to that
here for a more detailed explanation of my comments above.
None of this addresses the matter of how to actually adjust the lens of your Fujica so that its infinity focus will be optimum. But I know the answer to that after viewing your video, so I’ll address that in the following post.
Cheers
Brett
