ferider
Veteran
I thought that it might be useful for others to briefly describe what I do when getting yet another classic lens, to adjust focus for use on my M240. I roughly follow what Dante describes in http://www.dantestella.com/technical/leicadjust.html, but with my own twists.
This is the latest example: a very clean Canon 85/1.8 that I feel I stole from ebay Japan and that arrived Fri:
A 50 year old lens, which was (or not) used by others on LTM or M film cameras, possibly calibrated to these, and even when coming out of the factory in the early 60s, it wasn't compliant to the focus tolerances required by a digital M.
Unless you also change the focal length of the lens (as Brian documented once on another copy of the 85/1.8), you have 3 parameters that can be tuned and that are easily accessible:
(a) - focus shims in the lens
(b) - the RF cam
(c) - for lenses other than 50mm, the M mount, or the LTM adapter
For all 3 you can remove (with sandpaper, for instance) or add space (copper tape, or shim material).
This is how I typically test a new lens:
(1) check infinity RF alignment and performance
(2) check close-up performance
(3) check performance for a target that is "almost at infinity"
For (2), in contrast to most, I don't care about center focus. I care about focus in the field, since I almost always focus and recompose. And while most important for fast moderate wides (see also http://www.rangefinderforum.com/forums/showthread.php?t=148485 ), even for a long lens like this fast 85, say at 1m focus distance, the cosine effect can back shift your focus point by 0.5 - 1.5cm or so, depending on where you focus.
In the case of this 85/1.8, with a Leitz 9cm adapter, the RF was overshooting just a tiny bit, but infinity performance was good; also, the lens front-focused in the center by more than two inches.
This is what I did for the 85/1.8:
- grind down RF cam very slightly to achieve perfect inf RF alignment
- reduce the thickness of the main lens shim to adjust close-up performance
- check (1) and (3) to see if I like the achieved compromise.
Final test pictures tell more than thousand words:
Min focus:
Infinity (you can see lens coma, BTW):
"Almost" infinity:
Besides showing what I'm doing, I wanted to convey that it's always a compromise, depending on how you want to use the lens, and depending on how the lens performed out of the box. For this one, focal length was just a little longer than the spec. And remember, for whoever owned the lens before you, it was also a compromise, possible different from what you want.
That's all folks. Cheers,
Roland.
This is the latest example: a very clean Canon 85/1.8 that I feel I stole from ebay Japan and that arrived Fri:
A 50 year old lens, which was (or not) used by others on LTM or M film cameras, possibly calibrated to these, and even when coming out of the factory in the early 60s, it wasn't compliant to the focus tolerances required by a digital M.
Unless you also change the focal length of the lens (as Brian documented once on another copy of the 85/1.8), you have 3 parameters that can be tuned and that are easily accessible:
(a) - focus shims in the lens
(b) - the RF cam
(c) - for lenses other than 50mm, the M mount, or the LTM adapter
For all 3 you can remove (with sandpaper, for instance) or add space (copper tape, or shim material).
This is how I typically test a new lens:
(1) check infinity RF alignment and performance
(2) check close-up performance
(3) check performance for a target that is "almost at infinity"
For (2), in contrast to most, I don't care about center focus. I care about focus in the field, since I almost always focus and recompose. And while most important for fast moderate wides (see also http://www.rangefinderforum.com/forums/showthread.php?t=148485 ), even for a long lens like this fast 85, say at 1m focus distance, the cosine effect can back shift your focus point by 0.5 - 1.5cm or so, depending on where you focus.
In the case of this 85/1.8, with a Leitz 9cm adapter, the RF was overshooting just a tiny bit, but infinity performance was good; also, the lens front-focused in the center by more than two inches.
This is what I did for the 85/1.8:
- grind down RF cam very slightly to achieve perfect inf RF alignment
- reduce the thickness of the main lens shim to adjust close-up performance
- check (1) and (3) to see if I like the achieved compromise.
Final test pictures tell more than thousand words:
Min focus:
Infinity (you can see lens coma, BTW):
"Almost" infinity:
Besides showing what I'm doing, I wanted to convey that it's always a compromise, depending on how you want to use the lens, and depending on how the lens performed out of the box. For this one, focal length was just a little longer than the spec. And remember, for whoever owned the lens before you, it was also a compromise, possible different from what you want.
That's all folks. Cheers,
Roland.
