SLR Focusing Accuracy

[Mr Bill]

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.

I agree with you. It seems intuitive that a split-image focusing aid would be more precise (and accurate), but I think that this is wrong. My practical experience indicated that ground glass is more reliably accurate, as per a focus test using about a dozen different people.

I won't go into details, other than they each focused a piece of test gear, one time using a split-image screen, then using only a ground glass. Everyone felt they had nailed it with the split image, whereas the ground glass seemed more uncertain. The results, based on marking the lens barrel position, showed the opposite to be true. The ground glass focus barrel marks were all tightly grouped, the split-image focus marks spread out more than twice as far.

The results were so surprising that we repeated the test (an industrial decision was riding on this). The results were the same. Clearly, something odd was going on with the split-image system. Nothing changed except the person doing the focusing. We presumed that some difference in eye position was probably to blame, but did not investigate further. (The test was over, for our purposes.)

This test was enough to convince me to use the ground glass portion, only, when best accuracy is needed. You never feel as certain that you have nailed the focus, but it seems that a split-image glass can give a false sense of accuracy. BTW, for practical purposes, only a couple of "outliers" would likely have shown up on film tests.

 

[Leigh B]

The only way you can accurately test focus on an SLR is with an auto-collimator. Eyeballs won't work.

The auto-collimator will measure any error that exists between the optical path from subject to film and from subject to GG.

The split-image rangefinder is coplanar with the GG surface by design and manufacturing process. It's correct by definition.

If your eyes do not render the resulting GG image sharply, you need to add a diopter.

- Leigh

 

[summicron1]

i've never been entirely sure of how a split image focusing dealey-bopper in an slr works -- is it dependent on separation like a rangefinder? Something else?

I do know that there's no substitute for a good rangefinder, especially in dim light. Leica said its rangefinder was so accurate that if you put two pins, one inch in front of the other, a meter away and focused on the one in front you would notice a separation of the images on the one in back.

With an slr both pins would look the same, but I've tried that with a rangefinder and it works. With a very narrow depth of field, that error can be critical.

So, long way of saying -- if I were you I'd assume the ground glass is accurate, not the split image.

ps...i see some people already tried this with an slr and say they can see the difference there, too -- ok fine but i bet its a lot quicker in dim light with a rangefinder

 

[Mr Bill]

The only way you can accurately test focus on an SLR is with an auto-collimator. Eyeballs won't work.

The auto-collimator will measure any error that exists between the optical path from subject to film and from subject to GG.

The split-image rangefinder is coplanar with the GG surface by design and manufacturing process. It's correct by definition.

If your eyes do not render the resulting GG image sharply, you need to add a diopter.

- Leigh

The Gokosha autocollimators we used project a fairly narrow beam, so only uses glass relatively near the axis of the lens. If a wider-aperture test lens had, for example, some spherical aberration, it seems to me that the best effective focus point won't necessarily agree with the autocollimator. So although an autocollimator (the ones I'm familiar with) is much more precise than anything else I have, I think the final arbitrator of image focus has to be an actual image on film or sensor.

I can come up with possible explanations of how a split-image screen might have focus discrepancies compared with either an autocollimator or an actual image, I'm just not sure how to explain the significant variation in the test I mentioned, where everything but the viewer stayed the same.

A ground glass seems to me to be the best way to mimic film (or sensor) response because it accepts image-forming light from all zones of the lens. Assuming that the ground glass position matches that of the film plane. This would certainly be a good test application for an autocollimator with focusing lens.

 

[Steve Smith]

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.

Yes it will. Best focus for the film plane will coincide with best focus for your eyes, it just won't be as good as it would be with perfect sight. It can only get more out of focus either side of the in focus point.


Steve.

 

[markbarendt]

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

 

[BMbikerider]

As a rule of thumb, if your camera and particular lens combination will focus with the Split Image perfectly at infinity then I have always found that the remainder of the focusing range will be correct.

I had an Olympus OM1 a good few years back and the 28mm lens would not focus at infinity and it was out of focus all the way down to the closest focus point. It was distinctly 'soft'. I had an optical engineer look at it and set it up on a collimator and it was fine from there on.

 

[Steve Roberts]

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. .

Particularly problematic are elderly SLRs (as most of ours will be now) where the mirror rest is made of rubber or some other sound deadening/damping substance which degrades with time and use. The mirror stop in a Pentax MX I have is, in effect, a metal edge and a constant focussing error evident when using shallow DoF/long lenses turned out to be due to the edge having worn its way into the thick black crackle finish on the underside of the mirror, changing the mirror's position relative to the viewing screen. (Earlier Pentaxes had a flat metal pad that spread the load over a much wider area and doesn't seem to have caused a problem). A good way to worry yourself is to set up a tripod with an SLR body and lens on it and focus on an object, say, 15 feet away. Note the reading on the focussing scale precisely, then repeat using the same lens and a different body. Even allowing for the fact that the camera tripod bush location may vary relative to the film plane, the differences between indicated distance on the same lens but different bodies can be alarming!!!

Steve

 

[Leigh B]

The Gokosha autocollimators we used project a fairly narrow beam...

That's odd. The Gokosha that I used had an objective perhaps 75mm in diameter, large enough to cover any regular lens.

My military auto collimator has a 4" diameter objective, and will cover any lens I've encountered.

Are you sure the tool is appropriate to your usage?

This would certainly be a good test application for an autocollimator with focusing lens.

I don't understand that comment. What is an autocollimator 'with focusing lens'?
I've never seen an adjustable lens on an autocollimator. That would defeat the purpose.

- Leigh

 

[Leigh B]

...set up a tripod with an SLR body and lens on it and focus on an object, say, 15 feet away. Note the reading on the focusing scale precisely, then repeat using the same lens and a different body. Even allowing for the fact that the camera tripod bush location may vary relative to the film plane, the differences between indicated distance on the same lens but different bodies can be alarming!!!

If that's true, you have some seriously broken cameras.

Point #1: The distance marked on the lens is from the subject to the film plane, not to the lens.
The film plane is marked on all (decent) cameras by a small circle with a line running through it, parallel to the film.
The line is the actual film plane location, and is used for all distance measurements.

Point #2: The distance from the lens mounting plane to the film plane should be exactly the same in every camera using the same lens.
For Nikon F-mount lenses this distance is 46.5mm with a very tight tolerance (IIRC ±0.05mm but don't quote me).

Combining these two points places the lens very accurately within the optical system of the camera,
which should result in very slight variation in reading from one body to another.

I just tried this test using several different Nikons, from an F2AS through a D300S, and found no focus shift.
I cannot speak to the performance of other camera/lens brands since I have no experience with them, nor examples to test.

- Leigh

 

[Mr Bill]

That's odd. The Gokosha that I used had an objective perhaps 75mm in diameter, large enough to cover any regular lens.

Hmmm, equally odd to me. C.R.I.S. Camera was the US distributor, but I never knew of anything there other than the types I used. They were current products up to a handful of years ago, even though they look like 1950's sci-fi ray guns.

What is an autocollimator 'with focusing lens'?
I've never seen an adjustable lens on an autocollimator. That would defeat the purpose.

- Leigh

Like this one: http://www.flickr.com/photos/29504544@N08/6781981746/in/photostream/
It just extends the capabilities a bit.

On this model, the focusing barrel has micrometer-like scales, with zero at the infinity-focus position.

One use would be with respect to checking how well a zoom lens holds focus. If you refocus the Gokosha unit then read the offsets from the barrel, you can calculate the focus error of the zoom lens. Or, if the backfocus distance is in error, you could take a handful of readings, then calculate shim corrections in the lens mount.

I don't know of any way to do these things with a pure autocollimator.

 

[Leigh B]

One use would be with respect to checking how well a zoom lens holds focus. If you refocus the Gokosha unit then read the offsets from the barrel, you can calculate the focus error of the zoom lens. Or, if the backfocus distance is in error, you could take a handful of readings, then calculate shim corrections in the lens mount.
I don't know of any way to do these things with a pure autocollimator.

OK. The Gokosha in the Flickr photo is WAY smaller than the unit I used.
This one seems tailored for digital sensors. Is it designed for use with camera optics or directly on the sensors?

The one I mentioned predates digital imaging. I used it about 30 years ago.

The military auto-collimator I have now has several internal targets, at calibrated ranges, for checking close focus.
Nothing is adjustable by the user. The target positions are set during instrument calibration by a lab tech.

The camera (focal plane) position is adjustable using a micrometer drive.

This is quite a monster, with a 48" focal length. It weighs over 75 pounds and fills a six-foot-wide workbench.

- Leigh

 

[Mr Bill]

Sounds like a comparison between a desk-size vs a desk-top unit. The little Gokosha's footprint is roughly like a laptop computer. It's surprisingly heavy, perhaps 20 pounds, which is mostly in the base.

My understanding is that they were intended for camera repair shops, in particular for 35mm cameras. The circular base, with 3 leveling screws, is large enough to hold a typical camera body. The normal routine would be to use a front-surface mirror at the film plane, but I've found the reflection from the surface of film to also be workable.

They definitely predated digital. Regarding digital, I don't see how an autocollimator would even be usable. A plain collimator, as an infinity target, yes, but digital has no proper reflective surface for an autocollimator.

I hope you got your military unit as surplus, even the little Gokoshas were priced around 5 or 6 or 7 thousand dollars in the mid-1990s.

 

[Leigh B]

The Gokosha looks like a nice unit. I've never seen one in that configuration, but that means nothing.

Mine is also designed to work with a first-surface mirror at the film plane, but works fine with real film.
You can also swing the reflex mirror assembly out of the way and use it as a regular collimator, with a ground glass at the film plane.

My comment about using it on the sensor itself was applicable only to the fact that photographic sensors have built-in optics.
That really doesn't sound like a productive usage; just a brain flash.

Yes, I got these surplus. Originally they were very expensive.

- Leigh

 

[qpzil]

This has been an interesting thread - I've always been curious about my lenses that are *slightly off* at infinity. The negatives come out looking fine, but the split prism is just barely off. If I recall, I've only noticed that on my old Canons and on my Pentax.

 

[Leigh B]

I've always been curious about my lenses that are *slightly off* at infinity.

Be very careful when evaluating lens performance.

Many lenses, particularly in longer focal lengths, have a mechanical stop on the focus ring
that's located past the normal infinity focus position.

This is done to allow for dimensional changes in the lens resulting from temperature changes.

- Leigh

 

[Steve Smith]

If the image in your viewfinder is in focus and your prints are equally in focus then you don't have a problem in the camera. Actual infinity focus might not coincide with the infinity marking on the lens but that shouldn't be a problem unless it is out excessively.


Steve.

 

[HansKerensky]

Like this one: http://www.flickr.com/photos/29504544@N08/6781981746/in/photostream/
It just extends the capabilities a bit.

On this model, the focusing barrel has micrometer-like scales, with zero at the infinity-focus position.

One use would be with respect to checking how well a zoom lens holds focus. If you refocus the Gokosha unit then read the offsets from the barrel, you can calculate the focus error of the zoom lens. Or, if the backfocus distance is in error, you could take a handful of readings, then calculate shim corrections in the lens mount.

I don't know of any way to do these things with a pure autocollimator.

The Gokosha on that photo is mine. It is model 24LT-2DTS which has a 193,5mm lens and is primarely designed to be used on 35mm cameras. There is a "big brother", the 32LT-2DTS which has a 300mm lens and no doubt would perform fine on MF cameras.
The 24LT has indeed a focusing lens which can be set between -20mm till +20mm. This feature is ideal for quickly establishing the amount of correction needed for perfect infinity focus and also for checking "in between" distances.

Sadly enough the seller didn't have a manual so i still have to find out all the details.

The unit came with a so called "Lens Micrometer M-3" add-on (article #901) which is an unit that can be attached to the collimators baseplate and consists of a mirror which can be varied in height by means of a micrometer.
A manual in its box descibes how to use it for Yashica Zoomtec 70 cameras. First you have to bring the cameras lens in the "adjust"mode with a special control box, then place it on the Lens Micrometer, set the collimators lens to -2.88 and then adjust the Lens Micrometer till you have a perfect collimator image. The adjustment of the Lens Micrometer should
then be within +/- 0.04 mm.

For MF cameras (i collect folders and TLR's) i use an old auto-collimator which was either build by Zeiss or Voigtländer. That one is twice as big as my Gokosha and has a fixed focal length.

Allways important when using these kind of eye-sight auto-collimators is that you first have to "calibrate" the diopter by using a flat surface mirror instead of a camera-lens just before you start adjusting infinity on a lens as your eye is part of the focal system.

 

[Steve Roberts]

>>>>>If that's true, you have some seriously broken cameras.

I'd stop short of "seriously broken", preferring "worn commensurate with age and use".

>>>>> Point #1: The distance marked on the lens is from the subject to the film plane, not to the lens.

Indeed. I thought I'd covered the issue of the film plane point with my words "Even allowing for the fact that the camera tripod bush location may vary relative to the film plane,"

>>>>> The film plane is marked on all (decent) cameras by a small circle with a line running through it, parallel to the film.
The line is the actual film plane location, and is used for all distance measurements.

This is not news to me, though I'd hesitate to label a camera 'decent' or otherwise purely on the presence or absence of such an indication. IIRC none of my Pentaxes (inc. the LX) have this, though a more lowly Yashica TL Electro does!

>>>>> I cannot speak to the performance of other camera/lens brands since I have no experience with them, nor examples to test.

Likewise, I have no experience of Nikons!


- Leigh

Steve

 

[georg16nik]

....

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.

Leica III/IIIx series (up until M series) got rangefinder magnification factor of 1.5x, and You can easily distinguish between pins at 10 feet and even 20 feet.
Contax II or later Leicas M3, M2 etc rangefinder magnification factors are usually in the range 0.58x ~ 1.0x.. so their chances are remote.
Thats the price You pay when combine rangefinder and viewfinder in one.

 

[BMbikerider]

If the image in your viewfinder is in focus and your prints are equally in focus then you don't have a problem in the camera. Actual infinity focus might not coincide with the infinity marking on the lens but that shouldn't be a problem unless it is out excessively.


Steve.

I will go along with that, especially with some zoom lenses, the focusing marks on the barrels are pure figment of the engineers imagination.

 

[Diapositivo]

In my Tamron 500/8 catadioptric infinite is actually marked with a line, like an underscore __ so that the use doesn't worry about discrepancies.

 

[newcan1]

Well this thread has stimulated a great discussion and lots to think about, so thanks all. In my case though, I did enough shooting this weekend to convince myself that my problems are entirely due to operator error. I am lousy when I don't have a long time to focus, or when the light is low. My Nikons are mainly Nikkormats, some with split screen/microprism, some with microprism only.

Short of going the autofocus route, any suggestions for improvement? Do the F series cameras have significantly better screens, or are they just interchangeable but not qualitatively different? Are there any add-on devices that help any way, other than diopter correcting eyepieces? I really don't want to start again with autofocus backs and lenses so I'm open to suggestions on how to improve. Or maybe I'm not that different from the norm, hence the popularity of autofocus.

 

[LJSLATER]

I think the DG-2 magnifier will fit Nikkormats.... I originally bought one to use with my D200; the small viewfinder made it difficult for me to focus manually even with a Katz Eye screen installed. I eventually abandoned the idea since the magnifier only shows the center of the viewfinder and you have to flip it aside to compose. I still use the DG-2 for close-up photography though. I can say that my focusing accuracy has improved since I started making an effort, but I am still slow.

The F4, F5, and F6 all have electronic rangefinders with LEDs to aid in manual focusing. Some people don't like them, but I think they work great.

 

[Diapositivo]

Short of going the autofocus route, any suggestions for improvement? Do the F series cameras have significantly better screens, or are they just interchangeable but not qualitatively different? Are there any add-on devices that help any way, other than diopter correcting eyepieces?

I think focusing aids, such as green lights of AF devices, are really helpful only when there is a non-correctable sight defect. If your sight is fine, or if it has a defect that can be corrected with a diopter in front of the eyepiece (or contact lenses, or glasses) than I would rely more on traditional focusing methods.

For critical work you could apply one of those magnifying finders which can be easily turned over. When focus becomes critical you turn the focusing aid in front of the eyepiece, and in normal use you don't use it.

When doing portraits the focus point should be centred on the nearest eye. This is important. Focusing on the tip of the nose, or the furthest eye, will cause a sensation of defocus and you will not remember any more where exactly you were focusing. The focus point must be chosen accurately.

In macro work sometimes it is not easy to chose the focus point. For insects it is normally the eye (or the nearest eye), for flowers it is normally the tip of the pistil.

Don't rely on depth of field indications on the barrel for focusing. Acceptable focus is not the same as very good focus. Always choose which is the important focus point of your subject.

It might be that you focus accurately but don't think carefully about where to focus and when observing your results you feel a sense of focus deficit, when the problem was just in focus planning so to speak.

Generally speaking, focusing has its importance but shouldn't detract too much from other technical aspects of photography.