Yashica MAT lens calibration and camera distance markers

[Ellyrion]

Hello all.

Just have some questions regarding TLR lens calibration and how it relates to the focus dial distance markers

I put some greasproof paper level over the film plane as a focusing screen and tested the focus on the camera - it seemed the taking and viewing lens were misaligned so i took apart the camera and, keeping the camera steady, adjusted the viewing lens to make sure it was aligned with the taking lens. I had to unscrew a small screw in the side of the viewing lens to get it to move freely.

I did this by positioning the camera 5 feet from a monitor displaying a focus testing image (with the radial pattern), focusing the taking lens using the focusing knob on the side of the camera and then adjusting the viewing lens to match focus.

Both lenses seem to be aligned and share focus, however it seems the distance dials on the focusing knob are not accurate. When focusing on an object 3.3ft away, the focus knob will read 4ft instead.

I have tried to adjust the focus knob itself, but when I move the dial so it shows an accurate distance (3.3ft in this case) I find that the dial will then no longer rotate all the way to the infinity mark.

My questions are:

Is this the best way to calibrate focus or should I be using an object at infinity as my reference point for the two lenses?

and

Do the distance numbers on the side of the focus knob need to be accurate? Will a deviation of 6" between the actual distance and shown distance to an object in focus have any effect on the final image or should I just be concerned with making sure the object is focus in the viewfinder?



I'd love to take this camera with me on a trip in a few days, I'll be shooting primarily 35mm and digital but it would be cool to test it out properly so any help would be greatly appreciated!

Thanks

 

[Dan Daniel]

Ideally, distance scales will be pretty close to actual. BUT they are luxury items. Like cup holders in a car- handy but not necessary to drive a car.

You have probably the most important thing down- getting the taking lens/film plane image match focus with the viewfinder image. With that in place. all the rest is gravy, except maybe getting to infinity.

I have no idea what you mean when you say that you 'took apart the camera.' That could be ominous, or it could be you did what was needed to adjust focus and nothing more.

I'll describe the 'standard' way to adjust TLR focus. This assumes that you have the camera disassembled in the following way- 1), the cover on the focus knob is off and the screw holding the knob in place can be loosened; 2) the lens shroud covering the viewing lens lock screw is removed. That's what you need- access to the knob lock screw and access to the viewing lens lock screw. Nothing more.

1) Have something in film plane, like your grease paper. Lock shutter open, open aperture.
2) Focus taking lens on INFINITY- distant object, collimator, another camera set up fro this (search rick oleson collimator hack)
3) Adjust focus knob so that it is at infinity stop when the lens has infinity object in focus.
4) Using same infinity target and with focus knob set to infinity, adjust viewing lens so that it is also focused at infinity on the focus screen. Lock it down.

Done.

Again, getting the focus screen and taking lens to agree is the key issue.

Not being able to turn the focus knob to infinity is concerning. At the least, if infinity is out of the question, I would use a target 10/20/30 feet away, not 5.

 

[Ellyrion]

Ideally, distance scales will be pretty close to actual. BUT they are luxury items. Like cup holders in a car- handy but not necessary to drive a car.

You have probably the most important thing down- getting the taking lens/film plane image match focus with the viewfinder image. With that in place. all the rest is gravy, except maybe getting to infinity.

I have no idea what you mean when you say that you 'took apart the camera.' That could be ominous, or it could be you did what was needed to adjust focus and nothing more.

I'll describe the 'standard' way to adjust TLR focus. This assumes that you have the camera disassembled in the following way- 1), the cover on the focus knob is off and the screw holding the knob in place can be loosened; 2) the lens shroud covering the viewing lens lock screw is removed. That's what you need- access to the knob lock screw and access to the viewing lens lock screw. Nothing more.

1) Have something in film plane, like your grease paper. Lock shutter open, open aperture.
2) Focus taking lens on INFINITY- distant object, collimator, another camera set up fro this (search rick oleson collimator hack)
3) Adjust focus knob so that it is at infinity stop when the lens has infinity object in focus.
4) Using same infinity target and with focus knob set to infinity, adjust viewing lens so that it is also focused at infinity on the focus screen. Lock it down.

Done.

Again, getting the focus screen and taking lens to agree is the key issue.

Not being able to turn the focus knob to infinity is concerning. At the least, if infinity is out of the question, I would use a target 10/20/30 feet away, not 5.

Gotcha - I appreciate the advice. Yeah I just took it apart enough to get to the screw to adjust the viewing lens

I'll give that method with infinity focus a go tomorrow, it seems that at the moment the two lenses line up when focusing at 5ft but the viewing lens does not focus to infinity anymore so I'll use your method! The focus knob turns to infinity just fine now, I only had issues when I adjusted it so it matched the taking lens at 4ft.

 

[Tel]

It looks like you're trying to work within fairly fine tolerances, which can be a good thing in optics if frustrating. I would suggest that you use a focusing screen other than greaseproof paper. You'll get a more accurate reading if your focusing surface is rigid, like glass or hard plastic. I use salvaged glass from a junk camera ground glass and I make sure it's exactly the right width to rest on the film guide rails in the camera, exactly where the emulsion side of the film will be. If you're confident in your ability to cut glass it's easy, but if not, any local glazier will likely do it for you.

And ditto everything Dan says--start with an infinity collimation and don't worry too much about what it says on the focus knob; if you're doing it right you're relying on the VF to focus anyhow. (I've got an old Arnold & Richter collimator that makes this stuff easier, but before I got it I used to use the upper branches of a dead tree about 100 yards from my front porch. Probably just as accurate.)

 

[bernard_L]

I would suggest that you use a focusing screen other than greaseproof paper.

+1 Assume 50µm circle-of-confusion diameter for MF (d/1500). And f:4 aperture. meaning a tolerance of 4x50µm=200µm. BUT... that is your whole budget, to be used preferably for depth-of-field in picture-taking, not for errors in the camera technical adjustment. So, if you embark into an adjustment of your camera's focus you should make sure you can deliver an accuracy of (IMO) 50µm; that is the accepted value for the diameter of a human hair (give or take). I don't think greaseproof paper allows you to achieve that.

Also agree with the sequence outlined by Dan Daniel.

And, for the Rick Oleson method, you don't actually need a ground glass (or an ersatz of that). Clear glass with a fine line scribed on the front-facing side; use a known good SLR with a split-image zone in the center of the GG. Just make sure the piece of glass is resting on the inner rails.