Micro Focus Adjust Question
/forum/topic/1071236/0



Langran
Registered: Jan 14, 2009
Total Posts: 567
Country: United Kingdom

I know that the discussion around this has come full circle hundreds of times but I'm really struggling with calibrating one of my lenses. I've got a sigma 30mm f1.4 that is fine when I'm shooting headshots (quite close) with the small ma adjustment but then focuses behind the subject for full body shots. If I were to correct this then I'd get a slightly off result for headshots. If the amount the lens is 'off' by varies along with the distance from the subject, what do I do to sort it? Is there anything?

I'd really like the lens to just work, it's really sharp, absolutely love it except for this one quirk. Sure it becomes un-noticable by f2.8 or whatever but I got it to shoot at f1.4-f2.0, everything I do pretty much is in that range.



Imagemaster
Registered: Feb 23, 2004
Total Posts: 34927
Country: Canada

I didn't know you could MA non-Canon lenses



BrianO
Registered: Aug 21, 2008
Total Posts: 8551
Country: United States

Langran wrote: ...I've got a sigma 30mm f1.4 that is fine when I'm shooting headshots (quite close) with the small ma adjustment but then focuses behind the subject for full body shots. If I were to correct this then I'd get a slightly off result for headshots. If the amount the lens is 'off' by varies along with the distance from the subject, what do I do to sort it? Is there anything?

Since depth of field decreases with decreasing subject distance, it seems to me that accurate focus for headshots would be the most important. Purhaps doing your adjustment at a test distance that's somewhat further than your normal headshot distance, but closer than your normal full-body distance would work by splitting the difference.

What distance were you using before? I usually try for 50X the focal length, so for the 30mm I'd use 1500mm = 60 inches = 5 feet as a starting point.



scalesusa
Registered: Sep 02, 2008
Total Posts: 2417
Country: United States

Unfortunately, some lenses do change focus accuracy for near and far subjects.

I do the ma at close to 25X focal length and full aperture. Thats where the depth of field is less. Then, at further distances, the inaccuracies are often hiden in the depth of field.

Otherwise, return it for adjustment.



Ian.Dobinson
Registered: Feb 18, 2007
Total Posts: 11880
Country: United Kingdom

Imagemaster wrote:
I didn't know you could MA non-Canon lenses

Yep you can. Any 3rd party lens should be ok. On some motors you may not be able to get the finest adjustment but that could also be the same for some of canons basic motor lenses.



Ian.Dobinson
Registered: Feb 18, 2007
Total Posts: 11880
Country: United Kingdom

I only had an MA capable body for a short time but I did have a lens that could show a difference close and far . I gave up with all the target stuff and just used a real world target at a set distance . And adjusted the MA setting until I had the sharpest shot. A set distance x focal length didn't seem to be the answer.

I sold the problem lens and MA body but found it would be easy enough to have a defaul 'alround use' setting dialed in which would give acceptable results (probably due to not using it wide open at these times) and when I wanted to use it for a specific subject I could dial in either the better near or far MA settings quite easily. I just had the MA feature on the my menu tab.

At some point we will probably get MA systems that can learn the lens and adjust differently for different focal lenngths. I did used to think you should be able to use contrast detect AF to get a true focus and the MA system should then be able to get the difference dialed in. But I did notice that contrast detect could come up with a quite large variance of sharpness (tripod mounted @10x zoom) on most of my lenses aimed at the same target and refocused a few times. So may be its not the answer after all.

We will probably end up with the hybrid systems that Nikon have put in the new 1 cameras