focus with lens wide open, off at infinity?

[richard_wrede]

<p>I just noticed today while putting different lenses on my camera (Sony NEX 3n) the focus is off at infinity with the aperture wide open. I tried 3 leica, 2 canon and a Nikkor, all M39 LTM mount with an adaptor. I never tested this with film, is this fairly normal? when stopped down to f4 or 5.6 it's fine.</p>

[SCL]

<p>Many inexpensive adapters aren't manufactured to tight tolerances. It sounds as if your is slightly too thick; stopping down uses the increased DOF to get to infinity. Is there perhaps a small adjustment screw on your adapter to slightly shorten it?</p>

[wharridge]

<p>Also keep in mind that the infinity stop on these lenses may to be accurate enough with the lens wide open even before you add that adapter. You can't rely on just using the focussing scale when the lens is wide open.</p>

 

[richard_wrede]

<p>using a FOTGA adapter, it had good reviews, focusing to the stop, not using the scale. I found that the Sony flange to sensor distance is 18mm, the LTM/M39 flange to film distance is 28.8 I need an adapter thickness of 10.8mm. I don't have but a ruler so not that accurate but it seems I have an 11mm thick adapter, would .2 mm make that much difference?<br>

It won't adjust</p>

 

[richard_wrede]

<p>just found another forum http://www.dpreview.com/forums/post/53698580 and it sounds like my adapter is indeed too thick. It suggests a Fotodiox brand adapter as being thinner and possibly going past infinity which I could live with. The only one on eBay is $24 from a questionable seller.</p>

<p>Is there a way to shave off some of the adapter? it is the two-part kind, dark anodized aluminum base, chromed inner ring. The inner ring is removable with 3 screws (rotatable). I was thinking of rubbing it on some sand paper on a flat surface but that could lead to a sloped uneven mount.</p>

[Ed_Ingold]

<p>Most adapters are cut a little short so that you can always focus at infinity despite manufacturing and temperature variations. That means the focus ring will be a little short of the infinity mark. This means you can't count on the infinity stop.</p>

<p>You should use a vernier (or dial) caliper, or micrometer to measure items of this sort. A ruler is not nearly accurate enough, even a machine engraved Starrett ruler. 0.2 mm is a lot when it comes to focusing a lens.</p>

[SCL]

<p>If the inner ring is removable, AND you are extremely careful, yes, you can shave it down. I don't know if the set screws go into a hole in the inner ring or are pointed and just rest in a groove. If I was doing this, I would use fine emery paper and finish up with a polishing grit of 600-800, which you can get from most automotive shops. Leave no burrs or sharp edges. Afterwards scrupulously clean everything, wash it to remove any traces of abrasive dust. I've done something similar with T mounts in the past. Just take lots of time.</p>

[Ed_Ingold]

<p>If you go the sandpaper route, put it on a sheet of plate glass (ground and polished) for best results, and use it wet or with oil. Aluminum is gummy, and tends to load the abrasive quickly.</p>

<p>Before going that route, make sure you aren't over-focusing at the infinity stop. If the adapter is too short (and most are), making it shorter won't help much.</p>

[richard_wrede]

<p>I got a Fotodiox adapter at Amazon (the eBay seller had horrible feedback) it works great. Just a tiny bit "past" infinity which is fine, I can now focus at infinity wide open. The other Fotga adapter will eventually be sanded down. Perhaps one of the fellas in the shop at work can machine it down.</p>

<p>But either way I now have two adapters and can quickly change from a wide angle to normal lens as I use wide open only about 1% of the time.</p>

<p>Thanks for the suggestions</p>