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Matt Figler

Need MATH help to determine PL mount FFD issue!

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Hello, I have a special little S16mm PL mount camera and I need to adjust the FFD. The FFD is a little too long not allowing me to hit infinity nor have my marks line up on wider lenses. 

I will need to either replace the mount or machine some metal off to make it work properly. I'm trying to calculate how much I would need to reduce the mount by to have it properly spec'd. Can anyone help me with the math required to determine the difference in FFD? I pointed the camera at a focus chart and have noted the discrepancies below. Thanks!

12mm Zeiss lens T1.3
Lens barrel marking sharp at 18 inches
Actual tape-measured distance 13 inches
Difference 5 inches
 
18mm Zeiss T1.3
Lens barrel marking sharp at 18inches
Actual tape-measured distance 15inches
Difference 3 inches

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Aapo is right, you really need a depth gauge with a flat backing block in the gate to properly measure and set the flange depth.

In theory your method could work, but it does require that the lenses you're using are perfectly collimated (ie back-focus set to the 52.00mm PL standard), and that the ground glass is perfectly set to the same depth as the gate (since you're checking the lenses through the viewfinder not at the gate itself). Then there is the question of how accurately you can judge focus on a wide lens. There may also be play in the lens threads adding further discrepancies.

From your measurements, using a test projector, I estimated the 18mm needs to seat about 0.25mm closer, while the 12mm needs to be about 0.20mm closer. The difference of 0.05mm is substantial in terms of flange depth, which we usually try to set within 0.01mm.

If you took the camera to a technician (at a rental house service department or repair facility) they could measure your flange depth and ground glass depth very accurately in about 10 minutes. The best way to set it is to machine the material about 0.10 mm under, and then shim back up to within 0.01mm.

I'm curious what your "special" little camera is..

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14 hours ago, Dom Jaeger said:

Aapo is right, you really need a depth gauge with a flat backing block in the gate to properly measure and set the flange depth.

In theory your method could work, but it does require that the lenses you're using are perfectly collimated (ie back-focus set to the 52.00mm PL standard), and that the ground glass is perfectly set to the same depth as the gate (since you're checking the lenses through the viewfinder not at the gate itself). Then there is the question of how accurately you can judge focus on a wide lens. There may also be play in the lens threads adding further discrepancies.

From your measurements, using a test projector, I estimated the 18mm needs to seat about 0.25mm closer, while the 12mm needs to be about 0.20mm closer. The difference of 0.05mm is substantial in terms of flange depth, which we usually try to set within 0.01mm.

If you took the camera to a technician (at a rental house service department or repair facility) they could measure your flange depth and ground glass depth very accurately in about 10 minutes. The best way to set it is to machine the material about 0.10 mm under, and then shim back up to within 0.01mm.

I'm curious what your "special" little camera is..

Thanks so much for the thorough reply Dom. Your numbers are very helpful to get a ballpark sense. I'll be bringing the camera into a house to get a true depth calc. I'm curious to see how the results compare, luckily both lenses were very recently collimated BUT, I realized that I actually rounded a little on the 18mm measurement which should put it close to the .20mm mark that you mention. here is the special little camera, I'd been wanting to do this for years but only recently figured out it was possible.  

IMG_20190529_115447.jpg

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