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SLR test / odd poor result
Old 06-19-2015   #1
CK Dexter Haven
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SLR test / odd poor result

I just made a comparison test, using four different film SLRs, to test six different lenses: 35 and 50mm. A mix of AF and mostly manual glass. All were shot on the same roll of portra 160.

All of the lenses tested comparably, except for one highly-acclaimed MF 50mm, which gave me super soft results. Blurry, really. Everything was tested at close range, and between f1.4 and 2.8. This blurry 50 was the only lens tested on this particular body.

What might be the cause of the problem? A lemon lens? I've had that experience once before, with a 50 Lux-ASPH. But this is an SLR lens, and the view through the viewfinder was the largest, clearest and most easily focusable of all the camera+lens combos I tested. Wouldn't I SEE a problem? If it's not a wonky lens, is there something in the body that could do that? Doesn't look like mirror vibration. It just looks like a bad or miso used lens, but all of the shots - 1.4, 1.8, 2, and 2.8 were equally bad compared to the others. And there were no vibration effects or camera movement problems seen on the entire roll.
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Old 06-19-2015   #2
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Focusing screen or mirror misaligned on that body?

Ronnie
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Old 06-19-2015   #3
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All of my dud SLR lenses looked perfectly fine through the viewfinder.
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Old 06-19-2015   #4
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Quote:
Originally Posted by ronnies View Post
Focusing screen or mirror misaligned on that body?
That. Or, if you did not check the negative to exclude it (or had a digital camera running on Auto ISO), severe over- or underexposure.
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Old 06-19-2015   #5
CK Dexter Haven
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@ronnies:
Is there any way for me to verify if the screen or mirror is misaligned?

@sevo:
All shots were made on one roll of film, with the same manual exposures. The negs are all pretty much equal, exposure-wise.
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Old 06-19-2015   #6
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Can you make a chart (spreadsheet) with lenses on one axis and f-stops on the other. Fill each cell with sharp or not. Post the PDFs, that will help us help you.
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Old 06-19-2015   #7
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Quote:
Originally Posted by CK Dexter Haven View Post
All of the lenses tested comparably, except for one highly-acclaimed MF 50mm, which gave me super soft results. Blurry, really. Everything was tested at close range, and between f1.4 and 2.8. This blurry 50 was the only lens tested on this particular body.
The 50 was only tested on SLR body "X"?
The other lenses were not tested on SLR body "X"?

Test another proven good lens on SLR body "X", and also the 50.
My guess is they will both be blurry - if so - SLR body "X" is broken, nothing else is really possible.

How did you move the film from body to body? There is a chance that got screwed up somehow.
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Old 06-19-2015   #8
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Quote:
Originally Posted by CK Dexter Haven View Post
@ronnies:
Is there any way for me to verify if the screen or mirror is misaligned?
If you have a ground glass (or a spare interchangeable viewfinder screen) then set a camera without film on a tripod, focus on an object, open the back of the camera, set the B exposure and lock it so that the shutter is open and you see through the lens. If you put the ground glass on the film rails at the back, your object should now be at a sharp focus on it; you may check that with a magnifying glass. If the object is at focus in the viewfinder and not on the ground glass, then the screen or the lens is misaligned.
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Old 06-19-2015   #9
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Quote:
Originally Posted by CK Dexter Haven View Post
@ronnies:
Is there any way for me to verify if the screen or mirror is misaligned?
...
With the camera mounted to a steady tripod, step up a shot looking down on a table top at about 30 degrees below level. Lay a yard/meter stick on the table with one end toward the camera. Place a pencil or pen pointing to one of the distance markings near the middle of the yard/meter stick. Now focus on the marking that the pen/pencil points to and take a picture with the lens wide open.

When you view the image you'll see if the camera's focusing system is properly aligned with the film. If the sharpest point in the picture is the marking indicated by the pen/pencil then things are aligned. If the mirror and/or the focusing screen are misaligned some other distance marking will be the sharpest.
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Old 06-19-2015   #10
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Originally Posted by photomoof View Post
The 50 was only tested on SLR body "X"?
The other lenses were not tested on SLR body "X"?

Test another proven good lens on SLR body "X", and also the 50.
My guess is they will both be blurry - if so - SLR body "X" is broken, nothing else is really possible.

How did you move the film from body to body? There is a chance that got screwed up somehow.
I only have the one lens for that particular body.

I simply rewound the film from each body, inserted it into the next camera, and with the aperture at its smallest (16 or 22), and lens covered, and shutter speed set to the fastest available speed, I fired off enough frames, plus 3, to convert the previous camera's exposure count.
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Old 06-19-2015   #11
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Sounds like the focusing screen..? Focus the 'bad' lens and body on something (window blinds) a known distance away (tripod). 2 or 3 meters works well, it's always marked on every lens. Note the distance the lens says when it's in focus. Do the same with a known good camera and a lens of the same (ish) focal length. Compare the distances the lenses say. If they're the same, lens is not so good. If they're different, body is not so good. The screen in AF bodies is rarely in the right place these days...
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Old 06-19-2015   #12
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Maybe the pressure plate out of alignment..has been known to happen..quick fix..remove and reattach. [Reading glasses used? Just a thought.]
Best of luck, Tom S.
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Old 06-19-2015   #13
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Go through this thread:
http://www.rangefinderforum.com/foru...d.php?t=137012
You will see, that what you see is not (necessarily) what you get and how to test focus accuracy. In my opinion this has to do with the angle at which the rays hit the mirror. More telecentric lenses ( longer fl or designed for digital) will create fewer issues. The ultimate solution is to get the Nikon F3 and try various focusing screens and/or get a dedicated body for the lens you want.
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Old 06-19-2015   #14
Sarcophilus Harrisii
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Quote:
Originally Posted by CK Dexter Haven View Post
I just made a comparison test, using four different film SLRs, to test six different lenses: 35 and 50mm. A mix of AF and mostly manual glass. All were shot on the same roll of portra 160.

All of the lenses tested comparably, except for one highly-acclaimed MF 50mm, which gave me super soft results. Blurry, really. Everything was tested at close range, and between f1.4 and 2.8. This blurry 50 was the only lens tested on this particular body.

What might be the cause of the problem? A lemon lens? I've had that experience once before, with a 50 Lux-ASPH. But this is an SLR lens, and the view through the viewfinder was the largest, clearest and most easily focusable of all the camera+lens combos I tested. Wouldn't I SEE a problem? If it's not a wonky lens, is there something in the body that could do that? Doesn't look like mirror vibration. It just looks like a bad or miso used lens, but all of the shots - 1.4, 1.8, 2, and 2.8 were equally bad compared to the others. And there were no vibration effects or camera movement problems seen on the entire roll.
Hi,
Because you only made images with one lens fitted to that body, you can disregard any comparisons between other combinations. There's no direct benchmark. So: this is simply a matter of doing some basic focus checks.

I don't know how much you know, sorry, so bear with me. Except for scale focus cameras and their ilk, generally speaking, any camera that features a calibrated focus system enabling the photographer to precisely set the focus of the taking lens to a particular focal point, is actually comprised of a couple of sub systems.

Eg: a rangefinder has its lens focus set by, the focus stop of the lens (or body in the case of, say, the Contax). But it also has a rangefinder focus patch, and the mirror/pivot/prism/ is adjusted to calibrate this to the lens.

A TLR has two lenses: the taking lens is set correctly to infinity by (usually) the stop on the focus lens. The viewing lens is turned on its threads so that it agrees with the taking lens.

In both the above examples, the cameras employ optical components that are independant of the primary lens to enable accurate focus to be set. They must however always be calibrated to match the primary lens and are always secondary to the primary lens focus adjustment.

An SLR configuration is a little different. As it's name (single lens reflex) implies, there is only one lens, the primary lens which records the image. The system used to view the focus screen and set your desired focus diverges from the primary system at the reflex mirror that is dedicated (in the case of non-pellicle designs) to reflecting the image into the focus screen. So, what you need to work out is whether the primary focus of your lens on the film plane is good, first. Knowing that you have a good sharp view through the finder doesn't tell you much, in the first instance, until you've worked out whether the film plane is actually sharp when the lens is set to infinity. Then, and only then, can you think about inspecting and adjusting the sharpness of your viewfinder image. The finder must match the film plane. Not vice-versa!

To check the film plane, you can use a ground glass and a loupe to examine the formed image, view camera-fashion, through the lens. Obviously the shutter must be set to "B" or "T" (a lockable cable release in the socket makes life that much easier if you have one).

Alternatively, you can fashion a usable target to be used in conjunction with a second, collimating SLR out of a spare piece of unexposed, developed film. Eg. the first couple of frames of a processed roll of black and white often has some unfogged portion you can snip off, and very gently scribe some cross hairs into with a sharp knife. With this target film placed across the film rails, (emulsion side to the lens, naturally) and sandwiched by a clear piece of glass resting on the pressure plate rails, with the lenses of both camera set to infinity, you may visually inspect the focus of the camera under test at the actual film plane once you've set the shutter to Bulb. You'll want some natural light or another light source behind the camera to prpvide some illumination, of course.

Both techniques have their merits and I have used both on numerous occasions with very good results. Actually, I've done precisely that, today, to calibrate a Voigtlander Vitessa that has needed a lot of work. Many times, in fact, because of the need to pull the top cover off that particular camera and put it back on, every time a adjustment is made! But that's another story...

If using another SLR as a collimator, there are two important points worth noting. Firstly, the camera used must be definitively trustworthy. If you're not sure of its accuracy, using it to set another camera is likely to be a frustrating exercise. So think about the kit you have, and select the piece you have the most confidence in as to its accuracy.

Secondly, accuracy is improved greatly by mounting a lens to the collimating camera that has a substantially longer focal length than the lens fitted to the one being tested. If you're examining an SLR fitted with a 50mm lens, you can inspect the focal plane using a 35mm lens, but, don't expect a lot of accuracy if you do. You should really be using a lens with a focal length of at least 100mm (2x), ideally more, as the additional magnification provided will afford you a much more precise view of the patient cameras film plane, and even the smallest focus error in your target markings will be clearly visable. When setting the Vitessa mentioned above, which has a fixed 50mm Ultron lens, I used my Hasselblad with its 250mm f/5.6 Sonnar (more magnification) and its 150mm Sonnar f/4 (brighter view) to inspect the film plane focus and adjust it accordingly. Given a choice, use a long, fast, telephoto as the most accurate and easiest lens with which to collimate.


Assuming you have used either of the above techniques to inspect the primary lens focus at your film plane, you will now know, definitively, whether or not the focus of the lens is correctly set. If it's not, you'll need to adjust this before proceeding further, and the procedure for this varies a lot, depending on which camera/lens you're discussing.

If the film plane focus is really sharp and crisp, you're over halfway there. You've worked out where your problems aren't, and that's good. So if you're not getting a good, accurate focus image through the viewfinder of the camera, you know it's not the lens at fault. Any discrepancy between what you, the photographer, is seeing, and what the lens (film plane) sees, can only result from either the position of the reflex mirror or the focus screen itself. (A non-standard viewfinder eyepiece dioptre is a third possibility I suppose, however if this does not match your eyesight your view of the focus screen should be unsharp at any distance with any lens, which ought to be fairly obvious, so we can probably discount this one).

To sum up:
Check the film plane focus at infinity. Adjust if necessary.
Check the viewfinder focus at infinity. Adjust if necessary. Usually this will be via an adjuster that changes the angle of the reflex mirror, however you might, in some cases, need to remove the top cover and adjust focus screen mounting clips or shims.

Once set, it is always worth using a ground glass to cross check the film plane focus and the viewfinder focus at close distance. If you're working with a quality camera, when both optical subsystems have their infinity focus accurately calibrated, at any close distance, they should also be absolutely bang on. If so, you can be very confident that your cameras focus adjustment is excellent. On the other hand, if the infinity looks good, but the minimum distance cross check is off, your infinity adjustment likely needs to be reviewed, and fine tuned.
Cheers,
Brett
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Old 06-20-2015   #15
photomoof
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Brett Rogers whew!!! Here is a cheap and dirty way for someone who has no ground glass, and no tools.

Assuming it is an SLR and you don't own any ground glass to put on the film back here is a quick way to check focus. You do however need a slide viewing loupe, or in a pinch a magnifying glass.

Buy a roll of frosted tape, and put it over your film plane.

Make sure the tape is on the film rails -- not on in between. Do this by putting the tape on in a diagonal. You do not need to cover the entire film plane for a quick check. Stretch the tape tightly, you do not want to stick it to the shutter.

Focus using the viewfinder. Focus at about 10 feet on a newspaper, or something with type.

Set the camera on B or if you have a camera with a T setting, to keep the shutter open and the mirror up -- so you can look at the image on the tape. You can use a locking cable release to keep the shutter open. My guess is you won't be able to read the type, even though you could in the viewfinder.

If the image on the tape is bad, have the camera repaired, or consider investing in the time to troubleshoot it as Brett has suggested.
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Old 06-20-2015   #16
Sarcophilus Harrisii
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Quote:
Originally Posted by photomoof View Post
Brett Rogers whew!!! Here is a cheap and dirty way for someone who has no ground glass, and no tools.

Assuming it is an SLR and you don't own any ground glass to put on the film back here is a quick way to check focus. You do however need a slide viewing loupe, or in a pinch a magnifying glass.

Buy a roll of frosted tape, and put it over your film plane.

Make sure the tape is on the film rails -- not on in between. Do this by putting the tape on in a diagonal. You do not need to cover the entire film plane for a quick check. Stretch the tape tightly, you do not want to stick it to the shutter.

Focus using the viewfinder. Focus at about 10 feet on a newspaper, or something with type.

Set the camera on B or if you have a camera with a T setting, to keep the shutter open and the mirror up -- so you can look at the image on the tape. You can use a locking cable release to keep the shutter open. My guess is you won't be able to read the type, even though you could in the viewfinder.

If the image on the tape is bad, have the camera repaired, or consider investing in the time to troubleshoot it as Brett has suggested.
I made a serviceable ground glass with nothing more than a glass cutter, a small piece of window glass, and a little cerium oxide from a local optical fabricator. It's nowhere near as hard to produce one as some might think. In fact I have two: a smaller piece to suit 35mm format, and a larger 6x6 one, for TLRs etc.

I haven't done it myself, but have read that some valve grinding paste may provide a reasonable substitute if proper glass polishing compound isn't readily accessible.

I also have a couple of similar sized glass pieces to suit film rails for when I want to backsight a scribed film piece using another camera. How much effort one goes to, depends of course on how frequently one needs to do these sorts of adjustments. As I do a reasonable amount of camera repair I felt it worth the effort: others may not...
Cheers,
Brett
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Old 06-20-2015   #17
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Quote:
Originally Posted by Sarcophilus Harrisii View Post
I made a serviceable ground glass with nothing more than a glass cutter, a small piece of window glass, and a little cerium oxide from a local optical fabricator. It's nowhere near as hard to produce one as some might think. In fact I have two: a smaller piece to suit 35mm format, and a larger 6x6 one, for TLRs etc.

I haven't done it myself, but have read that some valve grinding paste may provide a reasonable substitute if proper glass polishing compound isn't readily accessible.

I also have a couple of similar sized glass pieces to suit film rails for when I want to backsight a scribed film piece using another camera. How much effort one goes to, depends of course on how frequently one needs to do these sorts of adjustments. As I do a reasonable amount of camera repair I felt it worth the effort: others may not...
Cheers,
Brett
One important aspect of using ground glass is that it be placed with the "emulsion" side toward the lens. If the ground side is placed on the pressure plate side -- won't work.

Real ground glass is not cheap from photo sources, and is usually sold as 4x5 inches. Fully frosted microscope slides work very well and can be purchased from Amazon, but they too are not cheap.

I find however that spray frosting works very well, available from Krylon and others at one's local hardware store. It does not take much to produce an image.

Cerium Oxide is also available on Amazon, but does not really work as well as etching solutions.

Armour Etch Glass Etching Cream, is available from michael's and other art supply stores.
https://www.michaels.com/armour-etch...M10208713.html

EDIT: ground glass must fit between the outer rails, and should only lie on the inner rails if there are double rails such as on the Nikon F. Many cameras only have one set of rails, so there is no problem.

Me - I just use Scotch 810 Magic Tape.
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Old 06-20-2015   #18
CK Dexter Haven
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Wow, Brett. Thank you.

I think I will first try moof's tape test. I do have a Hassy and mamiya screen, but they probably won't fit, and I don't want to cut them....I'll have an F3 screen next week.

Thank you, also, Marek....
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Old 06-20-2015   #19
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Oh you're so welcome, Dexter. Out of curiousity, has anyone found an slr to be out of register with the lens? How often would you say that that has been the case?
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Old 06-20-2015   #20
Sarcophilus Harrisii
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Originally Posted by Ranchu View Post
Oh you're so welcome, Dexter. Out of curiousity, has anyone found an slr to be out of register with the lens? How often would you say that that has been the case?
Ranchu this thread discusses that very point: I found it an interesting read at the time.
Cheers,
Brett
http://www.rangefinderforum.com/foru...d.php?t=143650
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Old 06-20-2015   #21
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That's a good point, I thought it was interesting too. I'm somewhat suspicious of the whole ground glass on the film rails test though. I'm not convinced that's where the film lies. The cool thing about that thread is the guy tested the film plane off of the film while it was in the camera, with the excellent machine.
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Old 06-20-2015   #22
Sarcophilus Harrisii
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Quote:
Originally Posted by Ranchu View Post
That's a good point, I thought it was interesting too. I'm somewhat suspicious of the whole ground glass on the film rails test though. I'm not convinced that's where the film lies. The cool thing about that thread is the guy tested the film plane off of the film while it was in the camera, with the excellent machine.
Hans Kerensky is my role model for DIY camera repair, apart from his unrivalled ability to sniff out stashes of NOS factory parts, he got himself a Gokosha autocollimator a few months back, something I would love to do myself. But quite apart from the purchase price the delivery cost to Australia isn't cheap.

Re: the ground glass, I think it would be important to be very careful if a camera focus screen was being used. They often have mounting frames or edges that are not in the same plane as the part that records the focus image so depending on the screen there might be register issues when using some of them. But with a plain, flat piece of ground glass, providing it is firmly seated on the film rails (and not the pressure plate rails) I can't see why it would be a problem. Not for 35mm at least. With 120, perhaps? Using an actual piece of film supported by a glass placed over the pressure plate rails and a trusted SLR to collimate the camera being tested is probably as close as those of us without access to an autocollimator can get to setting focus precisely (discounting the obvious option of shooting some test film, which will inform you whether you have a problem, but won't quantify the adjustment needed on the camera, per se).
Cheers
Brett
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Old 06-20-2015   #23
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Originally Posted by Ranchu View Post
Oh you're so welcome, Dexter. Out of curiousity, has anyone found an slr to be out of register with the lens? How often would you say that that has been the case?
The thread I linked is very interesting but, thinking about it, is really discussing viewfinder accuracy, isn't it? Whereas the point you raised, the register of the actual lens to body, is related but not an identical issue. Thinking about it, the answer is yes, I have found several SLRs that have had their primary lens focus off. But not in the normal sense that the problem occurs with most SLRs, because I can recall a few Contaflexes I've worked on that needed to have their focus corrected. But these are a strange design by SLR standards, with the shutter integral to the lens and the focus helical mounted in the body rather than the actual lens itself, which is a "permanent" part of the camera body, unlike conventional SLRs where the entire lens array complete with its own helical is interchangeable with other lenses.

What happened with the Contaflexes is that the infinity height of the helical had either moved slightly as the fixing screws for the lens/shutter unit had backed off slightly or because of old, dried out grease no longer damping the helical. Or (and this is not unusual) someone tampering with the camera, not understanding how to put it back together correctly, and stuffing up the focus adjustment in the process.

It's not something I've really noticed with conventional SLRs, but although I work on the odd one I'm usually happier fixing a Contaflex, Rollei or some other German rangefinder and don't do many Eg. Japanese SLRs.
Cheers,
Brett
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Old 06-20-2015   #24
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Originally Posted by Sarcophilus Harrisii View Post
But with a plain, flat piece of ground glass, providing it is firmly seated on the film rails (and not the pressure plate rails) I can't see why it would be a problem. Not for 35mm at least. With 120, perhaps? Using an actual piece of film supported by a glass placed over the pressure plate rails and a trusted SLR to collimate the camera being tested is probably as close as those of us without access to an autocollimator can get to setting focus precisely
I'm willing to accept that you're right about that. But I'm still suspicious, or perhaps superstitious...
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Old 06-20-2015   #25
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Originally Posted by Ranchu View Post
Oh you're so welcome, Dexter. Out of curiousity, has anyone found an slr to be out of register with the lens? How often would you say that that has been the case?
I have never found one to be out on its own but I have repaired and re-adjusted a couple that the owner tried to modify himself for whatever reason.. Too many wannabe repairmen...
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Old 06-20-2015   #26
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Quote:
Originally Posted by Ranchu View Post
That's a good point, I thought it was interesting too. I'm somewhat suspicious of the whole ground glass on the film rails test though. I'm not convinced that's where the film lies.
The sprocket holes run on the polished inner rails. You do have to avoid the outer rails. That is why tape works so well, IMO better than glass, since many cut the glass too wide and it sits on the outer rails. Tape just "finds" the right place. Glass MUST fit between the outer rails, however microscope slides do.

Nikon F from: http://www.mir.com.my/rb/photography.../fcontrol3.htm

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Old 06-20-2015   #27
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I meant those rails. Brett's method here is the better way to go though, because of the distance between the film rails and the pressure plate..


Quote:
Originally Posted by Sarcophilus Harrisii View Post
Using an actual piece of film supported by a glass placed over the pressure plate rails and a trusted SLR to collimate the camera being tested is probably as close as those of us without access to an autocollimator can get to setting focus precisely
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Old 06-20-2015   #28
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Originally Posted by Ranchu View Post
I meant those rails. Brett's method here is the better way to go though, because of the distance between the film rails and the pressure plate..
The nikon F has two sets of rails, there are cameras with only one set. The frosted side (toward the lens) sits where the emulsion would normally be. On the Nikon F the rails almost appear to be the same. The difference is minimal, but enough to cause collimator problems.

Here is a good page on placement for the ground glass.

http://monopix.z0g.eu/collimator.shtml
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Old 06-20-2015   #29
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No, I understand. The pressure plate rails have to support the pressure plate above the film rails enough so the film slides freely (or less freely). The place the film sits on the pressure plate may not be the same place as the tape across the film rails. It's likely behind, imo..
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Old 06-20-2015   #30
photomoof
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No, I understand. The pressure plate rails have to support the pressure plate above the film rails enough so the film slides freely (or less freely). The place the film sits may not be the same place as the tape across the film rails. It's likely behind, imo..
Perhaps, but not on a Nikon F, it is not that sloppy. But you may be right the film may off if the emulsion is thin.

The rails do keep the pressure plate slightly above the opening, but not by much. Amazingly the F does not often scratch film.

Edit: There is only so much play, emulsions cannot vary by much. But as you know the film is not absolutely flat, it is not a glass plate.
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Old 06-20-2015   #31
Sarcophilus Harrisii
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I'm willing to accept that you're right about that. But I'm still suspicious, or perhaps superstitious...
Actually, I didn't discount your comment out of hand, quite the contrary. For example, Rollei made certain Rolleiflex models including the Wide, the Teles, and some of the Es and Fs with the option of fitting their plane glass in front of the film, so that the film running through the gate would be literally sandwiched between the pressure plate and the glass. And Modern Photography's period test of the Tele Rolleiflex with and without the glass fitted, did, indeed, demonstrate that there was a slight but noticeable improvement in film sharpness with the glass fitted (at the expense of dust etc. according to most owners who have actually tried to use it, I should add). The reason for the improvement is because, unlike a rigid piece of flat glass, a piece of film will not necessarily sit perfectly flat across the gate. Hence my comments about it being less of an issue for 35mm than, potentially, 120. I have copies of that test report about the Tele including their photos so let me know if you'd like to see it. A while ago I went to the trouble of tracking down the plane glass for my own Tele, they're reasonably uncommon and perhaps were sometimes broken, but I found one cheapish. It's a beautiful piece of optical glass...

By using a glass across the pressure plate rails, placed behind a clear film piece with some cross hairs scribed into it, one is simulating as close as it is reasonably practical to do so, the situation of a loaded film sitting in the gate on which to calibrate the lens to. Which one hopes would also extend to any deformation of the film in situ. In the absence of an autocollimator, it is the best I can do to set focus as precisely as possible.
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Old 06-20-2015   #32
photomoof
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Originally Posted by Sarcophilus Harrisii View Post
The reason for the improvement is because, unlike a rigid piece of flat glass, a piece of film will not necessarily sit perfectly flat across the gate. Hence my comments about it being less of an issue for 35mm than, potentially, 120.
Indeed, there is always some compromise. Film is just not flat.

The early Nikon S uses a much more primitive film rail (a screwed on part) than the Leica M or the Nikon F, where the rail is part of the body casting.

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Originally Posted by Sarcophilus Harrisii View Post
By using a glass across the pressure plate rails, placed behind a clear film piece with some cross hairs scribed into it, one is simulating as close as it is reasonably practical to do so, the situation of a loaded film sitting in the gate on which to calibrate the lens to. Which one hopes would also extend to any deformation of the film in situ. In the absence of an autocollimator, it is the best I can do to set focus as precisely as possible.
Not sure I fully understand why place the film between the gate and the glass.

I am assuming that the emulsion is toward the lens, with the film base and some type of anti-halation backing against the pressure plate.

What am I missing here?
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Old 06-22-2015   #33
Sarcophilus Harrisii
Brett Rogers
 
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Indeed, there is always some compromise. Film is just not flat.

The early Nikon S uses a much more primitive film rail (a screwed on part) than the Leica M or the Nikon F, where the rail is part of the body casting.



Not sure I fully understand why place the film between the gate and the glass.

I am assuming that the emulsion is toward the lens, with the film base and some type of anti-halation backing against the pressure plate.

What am I missing here?
Sorry, perhaps I should have been clearer in my previous post. On the topic of film flatness I mentioned the example of certain Rollei models which could have a plane glass fitted in front of the film (IE. between the film and the taking lens) in order to improve film flatness.

My subsequent comments about using a glass across the film gate were in relation to using another camera in order to examine lens focus at the film plane. The film is usually held in place by the pressure plate ensuring it is flat (flat being a relative term, as has been discussed). In order to see the film it has to be lit from the back with the lens open, meaning of course that the back has to be open or off the camera. Without a pressure plate present the film may not sit as it normally would. Hence a small piece of glass placed on the pressure plate rails will both hold the film in place as a substitute pressure plate and also enable a light source behind the camera to illuminate the film so that the target markings on the emulsion side are easily visible.
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Brett
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