SLR Focusing Accuracy

newcan1 · Jun 30, 2012

I have a bunch of Nikkormat Ftns and a couple of EL's, an EL2, and an FE, among others. Recently I have been wondering if I can just assume that the cameras focus accurately. I noticed in some instances that where the camera has a split screen focusing circle in the middle of the screen, that it will show the image to be in focus at a point where, looking at the surrounding ground glass, the focus point is slightly different on the ground glass. Sometimes I feel that my images could be a bit sharper. My lenses aren't the greatest, but include things like a Nikkor 35-135 and a Nikkor 50mm f1.8 -- not too shabby.

I suppose I could do some critical focusing tests. But if any of the cameras are off, what would be the reason, and is corrective action possible? I guess I just don't know how the focusing mechanism works on an SLR. The lenses are all manual focus.

BobD · Jun 30, 2012

Your camera can only focus to one particular distance at a time. Different areas of the scene may appear somewhat out of focus because they are at difference distances from the film plane than the point you've focused on.

Also, with most SLRs, you are focusing at the widest lens aperture regardless of the aperture setting on the lens. The widest aperture has the least amount of depth of field (focus) so some objects may appear somewhat out of focus in the viewfinder even though they may be in focus in the final image.

LJSLATER · Jun 30, 2012

What Bob said.

One quick thing you can try is using the split prism to focus on something at infinity. If the lens and the split prism agree, you're probably good. If you notice a discrepancy, be aware that there are many possible causes, including the lens, mirror, focusing screen, diopter, and your own eyes!

Personally, when I get a picture that’s out of focus or otherwise unsharp, it’s almost always because of an error on my part.

markbarendt · Jun 30, 2012

LJSLATER said:
Personally, when I get a picture thats out of focus or otherwise unsharp, its almost always because of an error on my part.

Me too.

benjiboy · Jun 30, 2012

LJSLATER said:
What Bob said.

One quick thing you can try is using the split prism to focus on something at infinity. If the lens and the split prism agree, you're probably good. If you notice a discrepancy, be aware that there are many possible causes, including the lens, mirror, focusing screen, diopter, and your own eyes!

Personally, when I get a picture thats out of focus or otherwise unsharp, its almost always because of an error on my part.

In mirror reflex cameras focusing is dependant on the eyesight of person using the viewfinder because the eye becomes part of the optical system even split image prism screens in perfectly adjusted cameras can be apparently in focus to the user if their eyesight is defective when the image at the film plane is not in focus which is not the case with optical range finders.

E. von Hoegh · Jun 30, 2012

markbarendt said:
Me too.

Me three.

newcan1 · Jun 30, 2012

BobD said:
Your camera can only focus to one particular distance at a time. Different areas of the scene may appear somewhat out of focus because they are at difference distances from the film plane than the point you've focused on.

Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.

Pioneer · Jun 30, 2012

benjiboy said:
In mirror reflex cameras focusing is dependant on the eyesight of person using the viewfinder because the eye becomes part of the optical system even split image prism screens in perfectly adjusted cameras can be apparently in focus to the user if their eyesight is defective when the image at the film plane is not in focus which is not the case with optical range finders.

I did not know that. You are saying that aligning the split image in my SLR prism is somehow less accurate than aligning the images in a rangefinder because of the acuity of my eyesight? Somehow that does not really make sense to me. In both cases my eyesight is involved. I should be able to align to blurry halves of an image in my SLR viewfinder as well, or not as well as the case may be, as I can two blurry ghost images in a rangefinder spot. Hmmm.

E. von Hoegh · Jun 30, 2012

Pioneer said:
I did not know that. You are saying that aligning the split image in my SLR prism is somehow less accurate than aligning the images in a rangefinder because of the acuity of my eyesight? Somehow that does not really make sense to me. In both cases my eyesight is involved. I should be able to align to blurry halves of an image in my SLR viewfinder as well, or not as well as the case may be, as I can two blurry ghost images in a rangefinder spot. Hmmm.

If your eyesight is poor, you cannot focus properly with either system. My limit of sharp vision is about 10" from my eyes, so I use my glasses or a corrective diopter on the cameras. The only time I remove my glasses is to compose on the groundglass of a view camera; fcussing on the GG is done with a loupe.

E. von Hoegh · Jun 30, 2012

newcan1 said:
Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.

That's probably beacuse by moving the camera you have placed the object a different distance from the filmplane. A few inches will make a differnce if the object is nearby and the lens is wide open.

snederhiser · Jun 30, 2012

Hello;
Make sure that the ground glass is cut to fit on the inner film guide rails, not over the outer rails. There is a difference of about 0.015 to 0.020 thousands. Steven.

Peltigera · Jun 30, 2012

How do pictures come out? If they are fine I cannot see anything to worry about.

If you are experiencing the same thing with different cameras and lenses - it is you, not the kit.

ic-racer · Jun 30, 2012

newcan1 said:
I suppose I could do some critical focusing tests. But if any of the cameras are off, what would be the reason, and is corrective action possible? I guess I just don't know how the focusing mechanism works on an SLR. The lenses are all manual focus.

Most of the focus issues I have encountered with fixed-focus screen 35mm SLRs are from the mirror. The mirror is a likely suspect because it moves with each exposure and needs to always come back to the exact same position that it was when it left the factory. The fixed-focus screen 35mm camera system I use has an easily adjustable (without camera disassembly) mirror for both angle and up-down.

In cameras with interchangeable focus screens, in addition to the mirror being off, there can be a host of additional focus issues relating to focus screen placement from tampering etc.

In cameras in which the mirror is not adjustable, sometimes it is easier to shim the focus screen. Sometimes you have to both adjust the mirror and shim the focus screen to avoid inducing parallax error between what the screen shows and what the film sees.

Diapositivo · Jun 30, 2012

My hypothesis: the split-image rangefinder or the microprism crown rangefinder are not influenced by the defects of your sight, focusing on the ground glass is, your sight is not optimal, which makes you focally-challenged when using the ground glass.

For instance, when the split-image rangefinder shows a perfectly aligned edge in the two halves, the image is in focus. If you have sight defects you will not see the image perfectly in focus, but you will see that the two halves are aligned, or are not, so that the indication of the split-image rangefinder will be correct regardless of your sight quality.

Focusing on the bare ground glass on the other hand is influenced by your sight. If your sight is not the one which is presumed to be by your viewfinder (normally 0, sometimes -1 dioptre, it's indicated in the camera specifications) the entire focusing system will act as a correcting lens for your eye, and the best focus for your eyes will not coincide with the best focus on the film plane.

A problem with the camera mirror would normally affect both the split-image rangefinder and the bare ground glass, I suppose, and so would a problem of geometry between ground glass and focal plane.

Three remedies to the problem above:

- Only use focusing aids (split-image rangefinder, microp-crown rangefinder) without relying on the ground glass (not satisfactory);
- Use your glasses when using your camera (your camera will scratch them unless the glasses are made of real glass, or you put some sort of rubber ring around the ocular);
- Mount on the camera ocular a lens of the correct power so that the sum of the power of the viewfinder and the power of the additional lens equals the power of your glasses.

For instance, supposing you have myopia and in your right eye, which you use for focusing, you need a lens with -2 dioptres, and supposing you camera, as it comes out of the factory, has a viewfinder with a -1 dioptre, you need to add a -1 dioptre in order to reach -2.

As far as I understand there is a little trick here to be kept in mind.

If you buy the original make dioptre for your camera, in the case above, the dioptre marked -2 will actually be a -1 dioptre, because the original manufacturer will probably take into account the power of the viewfinder and give you a dioptre marked with the "total effect".

If you buy a universal dioptre (or have it made by your optician) you have to apply a -1 dioptre in order to obtain -2.

Fabrizio

250swb · Jun 30, 2012

newcan1 said:
Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.

It will be, you moved the camera. When you tilt it up or down you are changing the spot from where you focused to somewhere else.

If however you are seeing a change in focus in your results rather than as implied on the screen, so you accurately focus one way or another, and then your neg shows the focus point is a fraction nearer or further away, you may be getting aperture related focus shift. In which case the lens is (say) calibrated for accurate focus wide open, but when you shoot the picture your lens automatically stops down and in many lenses as you stop down the focus point changes. This is something rangefinder users can get obsessed about because it is more difficult for them to know if it's their rangefinder out of adjustment or the lens has aperture related focus shift. A lot of lenses are calibrated to be accurate wide open, when DOF is most critical, and of course that is how you see things with an auto-diaphragm SLR, but focus may wander as you stop down, until you get to f/8 when DOF takes over and compensates for any focus shift. Because focus shift is a flaw in the design of the lens (given that no lens can be perfectly corrected), the only option may be to stop down after you focused and see if with the smaller aperture the focus point still looks crisp, but it can be hard or impossible to do. The other option is to know your lens inside out and compensate each time you think it may happen.

Steve

summicron1 · Jun 30, 2012

one thing nobody has mentioned is that the sharpness of focus also depends on how fine the ground glass is ground -- it can look sharp when it isn't really.

Leica used to say that the way to show how accurate its rangefinders are was to put two pins stuck into the table 3 feet away, one pin an inch in front of the other. If you focus with the rangefinder on the one in front, you will notice that the one in back already looks a bit doubled in the rangefinder.

An SLR, I wager, would make both pins look sharp except for the most excellent viewing screen around (leicaflex SL2???) but the real point is that, with a wide-open lens, both pins can look sharp in the viewfinder, but the image will show a difference.

this is why a good rangefinder is better in dim light, especially with wide-angle lenses. It may not be a defect in your slr, it may just be the way the system works.

markbarendt · Jun 30, 2012

summicron1 said:
An SLR, I wager, would make both pins look sharp except for the most excellent viewing screen around (leicaflex SL2???) but the real point is that, with a wide-open lens, both pins can look sharp in the viewfinder, but the image will show a difference.

this is why a good rangefinder is better in dim light, especially with wide-angle lenses. It may not be a defect in your slr, it may just be the way the system works.

Um, you'ld lose that bet. Just tried a facsimile of that test with my FM2.

The grind though can definitely affect the brightness.

E. von Hoegh · Jun 30, 2012

markbarendt said:
Um, you'ld lose that bet. Just tried a facsimile of that test with my FM2.

The grind though can definitely affect the brightness.

I tried it with a Nikkormat Ftn and 50/2 Nikkor H. No trouble focussing on one pin or the other.

Edit - If the Leica rangefinder could distinguish between pins at, say, 10 feet, I'd be impressed. I wonder if a Contax II could at 10 feet? Making that test at 3 feet is giving the rangefinder every advantage - not much of a test really. Roughly 3% accuracy.

newcan1 · Jun 30, 2012

I think I am going to do a test. A bit like the Leica test, but maybe 12 pins one inch apart. I can focus on the central one (say pin 5 or 6) from distances of 3 feet, 5 feet, 10 feet, 15 feet, and expose at full aperture for whatever lens I am using. I'll use b&w film, maybe ultrafine Xtreme 100ASA of Polypan F. I can do the test in the alternative, focusing using the split screen, where the camera has one , and the ground glass. I will do this for each of the cameras I use regularly. Give me a week or so to get through this all and I will report my findings.

My eyes are not what they used to be, but are pretty good with glasses. I usually use my glasses when focusing, except in the case of one of the Nikons that has a corrected viewfinder that seems to suit me.

newcan1 · Aug 20, 2012

I finally did a couple of tests. I took a 3 foot long strip of wood and nailed nails to it about an inch apart. I placed it about 3ft from the camera, focused on a nail in the middle of the strip and took a picture. Then I did the same with the strip about 10 ft away.

With the first of the above, the focusing appeared accurate. On the second, with the strip more distant, the nail I focused on was at the rear end of the depth of field.

I did some research, and I am led to believe the following:

1. My eyesight has nothing to do with what appears to be the best focus point on the ground glass screen that I am essentially viewing through the camera's viewfinder prism. The image is either in focus or not. Poor eyesight may make it harder to discern the best focus, but it could not cause an image to appear sharper when it is less sharp on the ground glass.

2. The point of focus in the split screen focusing aid in the center of the ground glass focusing screen must coincide with the best focus on the ground glass screen, absent manufacturing defect; the split screen prisms are embedded in the focusing screen. For more on how they work see:

http://dougkerr.net/pumpkin/articles/Split_Prism.pdf

3. Therefore, How accurately focus is depicted on the focusing screen should not vary with the distance of an object.

My conclusions: my focusing issues are either the result of operator error, it being harder to focus accurately on a smaller, more distant object; or there is a very small error in the camera that may cause focus to be off only a fraction of an inch up close but several inches at greater distance.

Curiously, I performed the same test on three different Nikkormats, with similar results. I also practiced with a D70 (sorry) with power off, then switched power on - each time the in-focus light was on, indicating that my focusing was accurate.

I can only think of one cause other than operator error, and that could be that over the years, wear could have caused the mirror to be off by a couple of thousandths of an inch. Rather than adjusting it and potentially making things worse, I will do a final test affixing some masking tape to the back of the mirror, to raise it slightly, and see what happens. But at this point I'm guessing that the problem is operator error.

Diapositivo · Aug 20, 2012

It should actually be easier to focus precisely a distant object than a near object. DoF decreases and the near object requires a more accurate focusing I would believe.

I think the result might be due to the focus shift of the lens with the stopping down of the diaphragm.
When testing the accuracy of the focusing system (alignment of focusing screen) I would only take pictures wide open: that way the focus shift with a different lens opening is taken out of the equation.

newcan1 · Aug 20, 2012

Good point re: focus shift -- I did the test outdoors and had to stop down partially. I'll do it again in lower light tomorrow.

I just looked through the viewfinder a few minutes ago, and when I focus and then press the depth of field button, the split prism (which works down to about f5.6) is way off. I say off, because it doesn't agree with the ground glass; and at long distances it would call for focus adjustments that are plainly absurd. I guess the plot thickens.

Leigh B · Aug 20, 2012

newcan1 said:
2. The point of focus in the split screen focusing aid in the center of the ground glass focusing screen must coincide with the best focus on the ground glass screen, absent manufacturing defect; the split screen prisms are embedded in the focusing screen.

Yes, the point of optimal focus must agree between the GG image and the split prism. That's how they're designed and built.

newcan1 said:
Curiously, I performed the same test on three different Nikkormats, with similar results. I also practiced with a D70 (sorry) with power off, then switched power on - each time the in-focus light was on, indicating that my focusing was accurate.

The "in focus" light just means that the image was focused within the acceptance limits defined by the system,
which means not terribly precise.

newcan1 said:
But at this point I'm guessing that the problem is operator error.

I'm afraid I must concur.

I repaired Nikons for several years, and never encountered a focusing error on any that were not obviously damaged.
In fact, I've never seen a focus problem on an undamaged Nikon in the 50+ years I've been shooting them.

The probability of finding the same problem on three different cameras, all of which belong to the same shooter, are zero.

- Leigh

newcan1 · Aug 20, 2012

Leigh, it is good to hear with someone with Nikon repair experience. I can concede operator error, but only on the assumption that as operator I assumed the split screen would give a greater accuracy than it does. Others have suggested that the lens may change focus as it is stopped down; I have noted that the split screen only agrees with the ground glass at the widest apertures; and some have suggested I really get to know my lenses to compensate for system limitations. I am beginning to think that the last point is important - whether it is me or the lens, I am going to have to focus slightly beyond the subject at medium distance/aperture, for optimum results.

Leigh B · Aug 20, 2012

The split screen should be correct. Make sure you eye is centered on the viewfinder.

The ground glass image on the other hand is subject to errors based on your eyesight.
That's why they make diopters (corrective lenses) for the viewfinder. These only affect the GG image, not the split screen.

The subject of focus shift is rather contentious, and has been discussed ad nauseum.
Some people think it always happens; some think it never happens; many are in between or undecided.

There's a very simple test for your lenses.
Set the camera on a tripod and a good focus target at an angle across the field of view.
Focus on the middle of the target using the split screen.
Take a series of photos at all available apertures, controlling exposure with shutter speed.

You could do this test initially at 5 (or 10) feet, then repeat at twice and four times that distance.

newcan1 said:
I am going to have to focus slightly beyond the subject at medium distance/aperture, for optimum results.

If that's true, there's a problem that needs to be corrected.

I've never encountered a Nikon camera nor a user thereof that required that kind of correction.

- Leigh

SLR Focusing Accuracy

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