Originally Posted by Livesteamer
Let me say Thank You to All who have offered thoughtful suggestions. I'm one of the others who has a similar problem.
I have a Nikon F2 that was working fine but halfway thru a roll things got blurry. I suspect the mirror is not set properly so this thread has given me a lot to think about. It's not a high priority as I prefer my old Nikon F or an FM2 more but it's been educational. Thanks to All. Joe
You might be right. The mirror and its associated drive are alone in SLR optics, in having to move rapidly and frequently. How stable its calibration may be can vary depending on a number of factors, but it's possible for it to require correction, particularly after any sort of impact. Last year I had an ALPA 11si here (an expensive, high quality SLR, if ever there was one), and despite looking as new externally, the mirror needed to be re-calibrated as the finder focus would not hit infinity. This was first confirmed by mounting several lenses to it that were focusing OK on another, known good body, and also inspecting the film focus with a ground glass.
Bennett Sherman wrote an interesting article about focus accuracy comparing rangefinder systems to single lens reflexes in the May 1965 issue of Modern Photography. Towards the end of it he makes some points about the problems involved in maintaining accurate, repeatable mirror alignment you may find interesting. Thanks to the protean scanning stamina of Marc Bergman, you may read the complete article here
Theoretically, your screen could also be at fault, its installed height directly impacts the point of precise focus as it forms part of the overall focus distance from the lens. It is always worth checking this as it usually only takes a few moments to ensure it is correctly located. But I do have to stress the point I made in post four—and which Phil so kindly detailed how to do, step by step in post thirty—which is, to first verify the focus at the film plane is good.
As I have already said it's unlikely to be problematic, however the focus of the lens to the film obviously directly impacts the viewfinder focus—the lens is the one optical component shared by focus points of both film, and viewfinder screen. The key to calibrating an SLR correctly—or even a TLR, or rangefinder for that matter, since their various finder systems are all "driven" by the lens to film focus in different ways, even if the components are very different—is first verifying that the lens-to-body to film relationship is sound.
Why is this so vital? Apart from providing peace of mind, it aids identifying the precise cause of your problem. After verifying that (ideally, several) lenses focus correctly to the film plane at infinity, you may be reasonably sure that your problem must be located somewhere between the reflex mirror and eyepiece (although the latter tends, usually, to be rather apparent if a non standard magnification has been substituted).
It is from the reflex mirror upwards, that the light path deviates from that which forms the negative image. You're trying to pinpoint an unknown problem, initially. First checking the film focus, establishes the accuracy of the basic lens/body relationship. Because the lens --> body--> film relationship "drives" the focus system, you can think of it as the master calibration, with the finder accuracy as a secondary adjustment.
If possible, it’s not a bad idea to check the film plane with several lenses, and if you happen to have a compatible, trusted, body, so much the better. Why? When troubleshooting a fixed lens camera of some sort, one must only be concerned with ensuring the lens and its body focus accurately together. But it’s a little more complicated with an interchangeable lens camera body. Not only should your standard lens focus correctly when fitted to it—any other lens made for the system must be able to be focus accurately also. This is dependant on correct setting of the lens register, sometimes referred to as back focus, Ie the distance between lens flange to the film rails.
The easiest way to check the register is to place the camera body on a surface plate and using a depth gauge, measure across the rails on all sides. This has the advantage of not only verifying the register is to spec, but will also betray any alignment issues with the body or lens mount, should the various measurements differ excessively. I have a surface plate and depth gauge myself which I have used from time to time, for just this reason.
If you’re bereft of this equipment, all is not lost. Should several different lenses mount and focus accurately at the film rails, you can still be fairly confident the body is OK, and the more lenses that check OK, the greater your certainty. It is however always a plus if you can check more than one lens. Otherwise, you might establish that there is a focus error at the film plane, but, you won’t know if it is because the lens is out of adjustment, or the body itself. It’s more likely to be the lens, unless the camera has been badly worked on, has been damaged, or (less commonly) has a badly worn bayonet, perhaps (35mm Exaktas will suffer from this eventually). But again, that is not really the point, because you get best repair outcomes by eliminating as many potential faults as possible.
In this context, that means checking the body, checking the lens, and finally, checking the viewfinder, after you’ve eliminated the first two items as the cause of your problems. Hence, checking the film plane with several lenses permits you to be fairly confident that the calibration of both the lenses and
the camera body is sound. If you have several lenses, it’s not a bad idea to do a blind test with the camera body kept at a fixed close range focus distance, either.
Why check the focus at closer distances and
infinity? It's a reasonable question, given that the focus adjustment of a lens is nearly always set by its infinity stop, not
its close focus. Well, depth of field can be great at infinity, even with longish lenses. The image on a ground glass, even viewed with a high powered loupe, can still look pretty good if is out just a hair. But that hairs breadth can be better spotted at four or five feet on a contrasty target (black and white fridge magnets on my white refrigerator make for crisp close targets).
The distance scale markings on a lens manufactured by a first quality maker should be reliable. If you think the film focus of your camera at infinity looks bang on but the distance scale suggests the actual distance to your target a few feet away (always measured from the film plane, parallel to the target plane) is a centimetre or two off, it might just be the markings. If three or four lenses all deviate by a similar amount, well, best to critically check the infinity accuracy again. It might be just a bit off.
For the same reasons I also strongly recommend doing a blind test between the film plane focus and the viewfinder at close range. I can’t tell you the number of times I have, for example, painstakingly dialled in the two lenses of a TLR at infinity until both looked good, only to find, after cross checking them at four or five feet, that they still diverge slightly. Depth of field works against you spotting calibration errors at distance—at close range, it helps to make them more apparent. When you really have absolutely nailed
the infinity calibration of your viewfinder (whether it is a TLR finder with its own lens, or a SLR with mirror and pentaprism), it will agree with the film plane perfectly near minimum distance and, hence, at any other focusing distance.
It’s also worth noting, Joe that other problems are inherent in a reflex focusing system and, whilst they may not present themselves regularly, it’s as well to at least be aware of them in order to sidestep them, if they are apparent.
One such can be discrepancies between the focus points of a lens when set with a SLR split wedge rangefinder, compared to the ground glass of a screen. A ground glass will rarely lead you astray even if it may be a little harder to pinpoint precise best focus with. Split wedge rangefinders are fairly trustworthy for the most part but it is possible for them to result in focus inaccuracies, and I have experienced this first hand once or twice. (As an aside, it is also why, if a trusted SLR is being employed to autocollimate another camera, I always advise using the ground glass for inspecting the film plane of the test camera, in preference to the split wedge of the one being used to inspect the focus).
You can read about why, in certain situations, a split wedge can throw your focus off, in this column by Herbert Keppler
in the March 1965 issue of Modern Photography. If you want to try to see this for yourself and happen to have an old teleconverter handy (the worse quality, the better, in this case, a 1970s 3 x comes to mind) then, this will help to exacerbate the effect which can nevertheless occur with some lens and screen combinations in isolation.
Perhaps surprisingly, your camera manufacturer may have set you up for some disappointment in the way they have calibrated your cameras focusing screen, by trading off absolute best centre focus accuracy for better results across the image frame. Once again, Keppler discussed this issue
, in August 1968, this time.
As you no doubt appreciate, there’s more to the process of getting a single lens reflex to focus really accurately than may initially be apparent. It’s a very interesting topic, this post is but a brief overview of some of the things which can impact your results on film but hopefully will help things along when you examine your F2.