Distance scale off?

Lucius · Mar 12, 2022

I'm very new to film photography, but I've got a few m42 bodies and lenses. Today was my second outing, first with my Fujica ST 801. Towards the end of the film, I noticed that, when the view was in focus, the distance on the lens scale was way off the real distance. I came home, and set out to experiment. I chose a target about six metres away, and it seems that, although most other cameras seem to be in focus at approximately the correct distance (according to the lens scale), though there seem to be some discrepancies between different bodies and lenses, but the Fujica was in focus at about 10 metres on the lens scale, with different lenses. I don't recall this issue being mentioned, and I wonder whether the fault lies with the cameras calibration, or with the viewfinder, in other words wether I should trust the viewfinder or the distance scale. I guess I'll know when the film is developed, but would appreciate if anyone has similar experience to share.

RalphLambrecht · Mar 12, 2022

Lucius said:
I'm very new to film photography, but I've got a few m42 bodies and lenses. Today was my second outing, first with my Fujica ST 801. Towards the end of the film, I noticed that, when the view was in focus, the distance on the lens scale was way off the real distance. I came home, and set out to experiment. I chose a target about six metres away, and it seems that, although most other cameras seem to be in focus at approximately the correct distance (according to the lens scale), though there seem to be some discrepancies between different bodies and lenses, but the Fujica was in focus at about 10 metres on the lens scale, with different lenses. I don't recall this issue being mentioned, and I wonder whether the fault lies with the cameras calibration, or with the viewfinder, in other words wether I should trust the viewfinder or the distance scale. I guess I'll know when the film is developed, but would appreciate if anyone has similar experience to share.

I'd trust the viewfinder first and verify if that will give me in-focus exposures. During the actual exposure the film plane will be where the viewfinder is during focusing unless something is misaligned.

MattKing · Mar 12, 2022

Welcome to Photrio.
It would be worthwhile to check your results on film.
If the camera and/or lens have been damaged or improperly serviced, it is possible that:
1) the distance scale on the lens will indicate the wrong distance; or
2) the viewing and focusing system will show a part of a subject at one distance to be in focus in the viewfinder whereas a different part of a subject at a different distance will be in focus on the film.

wiltw · Mar 12, 2022

Simple enough to

put the focal plane mark on the body at one end of a yardstick,
place a box at the other end of the yardstick
Focus on the box and see if 36" is indicated on the focus scale

A manual focus SLR should be accurate, unless the focus screen was somehow mis-positioned (jarred loose by impact) or (for an SLR with user changeable focus screens) someone inserted the focus screen wrong side facing the eyepiece.
If focus scrren and film both agree about accurate focus, but the lens distance scale is wrong, likely the lens was not reassembled properly after servicing (like to remove oil from aperture blades). Manual focus lens distance scales are very close to their end of range of motion when at the Infinity mark, although AF mechanisms could mandate a bit of margin from Infinity mark in their motion range

Sirius Glass · Mar 12, 2022

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Welcome to ~~APUG~~ Photrio!!

Lucius · Mar 13, 2022

Thanks everyone! But now I'm really confused. As wiltw suggested, I tested the Fujica with a yardstick - and it does get in focus at three feet, both with its own Fujinon lens and with a Takumar, as does the Spotmatic, likewise with both lenses. (I guess my initial confusion was caused by estimating distance from the lens's outer element, not from the focal plane.) But when I move the target away to about twenty feet, the Spotmatic focuses at the correct distance, whereas the Fujica thinks it's thirty feet away (again both cameras with both lenses)... I guess I'll have to wait till the film is developed to see what's going on.

Don_ih · Mar 13, 2022

The mirror may not be in exactly the correct position. At least on Contax cameras, the adhesive that holds the mirror is pretty bad and the mirror can slide down and end up giving you incorrect focus. Maybe it's the same with your camera. It's also possible someone played with the mirror (flipped it up with their finger a few times) and bent something.

Javier González · Mar 13, 2022

If the focus is correct at infinity I wouldn't worry too much about the lens scale. If it is off at infinity it should be adjusted first. The procedure will depend on the kind of lens. Of course the proof will be the negatives

wiltw · Mar 13, 2022

Javier González said:
If the focus is correct at infinity I wouldn't worry too much about the lens scale. If it is off at infinity it should be adjusted first. The procedure will depend on the kind of lens. Of course the proof will be the negatives

If, as tested, OP confirmed that 36" distance focus was right in viewfinder with the lens scale also indicating 36", then it is very puzzing as to why Infinity focus would not have the index mark on the Infinity distance, too.

Lucius · Mar 15, 2022

I've got my negatives developed, and happy to report that, at least close-range, focus seems correct. I'm less certain abut infinity (I only ordered the lowest-definition scans, so it's difficult to see), but can't think of a reason why it shouldn't be, though I still can't explain why the camera/lens is in focus at 30 feet when the distance is in fact 20...

Sirius Glass · Mar 15, 2022

Lucius said:
I've got my negatives developed, and happy to report that, at least close-range, focus seems correct. I'm less certain abut infinity (I only ordered the lowest-definition scans, so it's difficult to see), but can't think of a reason why it shouldn't be, though I still can't explain why the camera/lens is in focus at 30 feet when the distance is in fact 20...

John Koehrer · Mar 15, 2022

1: don't bother with the focus scale.

2: check focus with a ground glass at the film plane. focus through finder and confirm focus on gg.

Javier González · Mar 16, 2022

I have checked several rangefinder cameras and SLR lenses and I can confirm that the distance scale is a gross approximation in several cases. All of them can focus just fine using the viewfinder or the rangefinder. As long as the lens is well collimated at infinity I think you can safely ignore the distance scale. Not so in a scale-foccusing camera obviously.

Lucius · Mar 17, 2022

John Koehrer said:
check focus with a ground glass at the film plane.

Thanks, that's a great idea! Have to figure out though where to get a piece of ground glass.

Lucius · Mar 17, 2022

Javier González said:
I have checked several rangefinder cameras and SLR lenses and I can confirm that the distance scale is a gross approximation in several cases. All of them can focus just fine using the viewfinder or the rangefinder. As long as the lens is well collimated at infinity I think you can safely ignore the distance scale. Not so in a scale-foccusing camera obviously.

I get that, what I don't get is why the same lens behaves differently (scale-wise) on different cameras, and at different distances. To stay with the two pairs I mentioned, at the distance of three feet, both my Spotmatic and Fujica ST801 are in focus at the correct distance (according to the lens scale) with both the Takumar and the Fujinon (i.e. each with their own native lens as well as crosswise). But when it comes to 20 feet, the Spotmatic focuses at the correct distance (with both lenses), whereas the Fujica 'thinks' it's 30 feet (again, with both lenses). If the viewfinder is reliable, this I guess is of purely academic interest, but still...

gone · Mar 17, 2022

It's an SLR, so go by what you see in the viewfinder and ignore the distance scale, which you will probably never use w/ an SLR anyway. There is always a slim possibility that something is off w/ your camera's focus screen or mirror, but if it takes sharp pics as it is then you're good to go.

MattKing · Mar 17, 2022

momus said:
It's an SLR, so go by what you see in the viewfinder and ignore the distance scale, which you will probably never use w/ an SLR anyway.

How soon we forget working with manual flashes and guide numbers.

__Brian · Mar 17, 2022

If the focus is correct on film and through the viewfinder BUT the focus scale is off: the problem is likely the Lens Mount of the camera is off. If the lens mount is too close to the image plane, then you will reach infinity focus with the lens before the focus scale indicates infinity. Many cameras use a Shim or some other adjustment to set precise distance between the lens mount and the image plane. If a Shim is missing, that can cause this problem.

Try the focus carefully at infinity as seen through the viewfinder and verify it is in true focus on film. Note the distance scale of the lens as you get infinity focus through the viewfinder. The difference in lens travel indicates the error for the film to flange distance.

My first 35mm camera is the Minolta Hi-Matic 9 with the Easy-Flash system. Just dial in the GN of the flash and the camera sets the F-Stop as you focus. I also have a GN-Nikkor 45/2.8.

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