Flange focal distance verification methods

[Daimon]

What are the available methods/tools to verify proper flange focal distance (aka register) on L39/LTM mount cameras?

For open back cameras I could use depth micrometer together with ground plate resting on film rails.

For bottom loading camera that's not possible - only reference point is film pressure plate which is based on spring so not really good measuring point.

Other ideas, thoughts?

 

[Bill Jones]

What are the available methods/tools to verify proper flange focal distance (aka register) on L39/LTM mount cameras?

For open back cameras I could use depth micrometer together with ground plate resting on film rails.

For bottom loading camera that's not possible - only reference point is film pressure plate which is based on spring so not really good measuring point.

Other ideas, thoughts?

Diamon,

1) place the camera on a flat surface flange side up
2) remove the cap of the depth mic
3) with the cap removed the depth mic rod will float and stay in position for all readings short of the actual
4) with the mic set to a position where there is space between the rod and the thimble place it on the camera lens flange
5) set a test indicator on the top of the depth mic rod
6) back out y rotating the thimble to a shorter measurement., observing the test indicator. when the indicator starts to show rod movement, the
reading on your depth mic is the camera register.
I recommend testing several times and using a high resolution test indicator.

 

[shawn]

Do you have a lens that you know is correct? If so for open back cameras put that on and put the ground glass on the film rails with an X drawn on it. Use another camera (mirrorless is great for this) and with the other camera focused at infinity look at the ground glass through the LTM lens. If the X is sharp your flange focal distance is correct. For closed back cameras you could remove the body shell to do the same thing.

 

[Daimon]

That's the problem - I don't have a true "golden" reference (neither lense nor camera). I can verify how pairs work but not sure which one of them is true to spec.

Thought I could try to establish that by checking which one of my cameras has FFD closest to actual specification and then check lenses on that camera

 

[shawn]

How many cameras/lenses do you have? If you have a couple of bodies I'd maybe try one lens on a few bodies and see if you have consistency with the optical method of checking this. I'd think the odds of the body flange distance being off are lower than a lens being off. If several bodies with one lens all show infinity is good then the body and that lens is likely correct. From there you can use that lens to test other bodies.

If all bodies with one lens show a consistent out of focus it suggests the lens is off. Try again with a different lens. If one lenses varies on different bodies then it suggests the bodies are off.

 

[Daimon]

9 bodies and 16 lenses for LTM. I understand idea you mentioned but I'm more into more precise measurements. I already know that Canon 50mm f1.2 is a bit off compared to Voigtlander 50mm f1.5 (might be focus shift as well on Canon) and when checking those lenses they are a bit different between Canon P and Canon 7sz. I'd like to know which body is really true to spec and then follow with other adjustments.

Sounds like depth micrometer might be the only way now. I found some laser based tools but price is way out of my reach 🙂

 

[shawn]

Send a lens to DAG for a CLA. Then use it as your reference point. Or setup a jig to hold a LTM lens 28.8mm away from ground glass with the X to verify the lens.

Doing this optically is going to be more precise than trying to measure depth. Look at all the controversy about the Hexar RF and people trying to measure it's flange focal distance. Depth doesn't tell you if the system is calibrated, optically does. And doing it optically is the only way to check your lenses.

When you are comparing the lenses on the P vs the 7sz are you checking focus optically or just going by the rangefinder? RF just adds another variable. After you get a known good reference you then calibrate the RF to that.

 

[Daimon]

Send a lens to DAG for a CLA. Then use it as your reference point.

That's something I missed in previous post. I'm from Poland. So would need to find someone in europe (doubt we still have true RF specialists in Poland but I may be wrong).

When you are comparing the lenses on the P vs the 7sz are you checking focus optically or just going by the rangefinder?

That's result of body + lense pair, i.e.:
- camera RF is adjusted for infinity when lense meets infinity (actually this looks to be quite consistent with most of my japenese lenses)
- focus using RF on test target (either far away power line through my window or black/white checkered target on short distance) - by looking through RF
- check TTL focus using SLR split prism focusing screen with a loupe on film rails

In that kind of test (same lense, different bodies) I can be sure that there's difference between camera bodies but it could be either flange distance of mount or rangefinder linear calibration for short range or both or lense is actually off and all results are meaningless other than knowing they are not the same 🙂

I'm not 100% sure of any of my lenses or bodies.

Just trying to "true" one of variables so I can establish verified golden test unit and then use that as reference. Once I have verified flange distance I could verify one lense for infinity focus and this should give me my "golden standard". And yeah maybe my approach is overzealous but that's how I like doing those things - it's part of the fun for me when tinkering with equipment

 

[shawn]

That's something I missed in previous post. I'm from Poland. So would need to find someone in europe (doubt we still have true RF specialists in Poland but I may be wrong).



That's result of body + lense pair, i.e.:
- camera RF is adjusted for infinity when lense meets infinity (actually this looks to be quite consistent with most of my japenese lenses)
- focus using RF on test target (either far away power line through my window or black/white checkered target on short distance) - by looking through RF
- check TTL focus using SLR split prism focusing screen with a loupe on film rails

In that kind of test (same lense, different bodies) I can be sure that there's difference between camera bodies but it could be either flange distance of mount or rangefinder linear calibration for short range or both or lense is actually off and all results are meaningless other than knowing they are not the same 🙂

I'm not 100% sure of any of my lenses or bodies.

Just trying to "true" one of variables so I can establish verified golden test unit and then use that as reference. Once I have verified flange distance I could verify one lense for infinity focus and this should give me my "golden standard". And yeah maybe my approach is overzealous but that's how I like doing those things - it's part of the fun for me when tinkering with equipment

Regarding your test you should not be using the RF at all. Test at the infinity stop on your lens. It eliminates the possibility of the RF screwing up your results. The wider the lens the greater a different in flange focal length will impact focus. If one lens at infinity is sharp on multiple bodies you are likely looking at a good lens and bodies. If you get different results on different bodies you know your bodies aren't consistent. If you don't get any sharpness on any bodies your lens is off. Doing the optical test with a second camera (mirrorless zoomed in works great) is extremely precise and is much quicker and sharper than trying to look at the moon through the lens on ground glass. I've built many dozens of cameras from scratch using this method to calibrate infinity focus. It works very well and the negatives back up how sharp the calibration was.

I understand you are trying to true your variables but you are likely going to end up chasing your tail with measurement deviations, errors and tolerances on the cameras themselves.

 

[Phil_F_NM]

Use Shawn’s method aiming one camera into the one in question. It’s extremely precise when done correctly and how old motion picture film planes are calibrated then lenses matched to individual cameras.
As I mentioned in my redacted post, your RF patch will not be precise enough because you can move your pupil a fraction of a millimeter and focus will appear off. The key to doing all this calibration yourself is that you need to start with one known perfectly calibrated lens or camera. Otherwise, you’re just going around in circles. Your focus will be noticeably off even if your measurement to the film plane is half the thickness of film, so at least one lens or one body needs to be in perfect calibration. Other than that, stop down a bit more and enjoy shooting.

Phil Forrest

 

[Daimon]

Thanks Shawn and Phil. But I'm still left with this initial problem. I don't really know which of my bodies/lenses is really adjusted to spec - that's where idea about checking FFD with micrometer came from.

Even if it's not perfect maybe that's the best I can do now. Use it to find most true to spec camera from the ones which have removable/openable backs and then use the method you described to check lenses on it.