Distance for testing and setting microadjustment?

I've searched and found good information about setting microadjustment for lenses (I have a 50D) but, I still have questions. What is the best distance to use for checking each lens? I'm thinking that the minimum focus distance is best but my only basis for that is that the DOF would be smallest. Can the focal plane change as the distance changes? I.E. back focus at 18" and front focus at 15'. If so, then then what is the best way to procede? Any help would be appreciated.

Bob

Hi Bob, I've been wondering about this too, for a slightly different purpose. I think once infinity focus is set, the rest of the distance numbers will be correctly aligned.

My purpose is for some lens performance tests. I wanted specific distances. So far I've got closest-focus, infinity, and the distance which is double the last printed distance before infinity, plus some dof tests to see where the dof markings on the lens appear in a photo of a larger region.

The best distance to MF adjust is 50x the focal length of the lens. There is a good post Chuck Westfall from Canon has describing tips to do MFA.
http://www.pixel-shooter.com/forum/show​thread.php?t=21099

You can only adjust for one particular distance.

Fifty times the focal length is a reasonable rule of thumb, but if you know you tend to use a particular distance with a particular lens (as portrait photographers might do), then do your adjustment at that distance.

LensAlign recommends 25X the focal length. Has worked well for me.

50x focal length, or the distance you use it most often.

Thanks all. Very good info.

Bob

Well, I spent a couple hours the other day working on the micro-adjustment of my Sigma 70-200mm f/2.8. This was certainly an exercise in compromise. I can't pick a focal length that I use most on this lens so I had to find the best adjustment that would work for all. I ended up at +15. I started out with the 50x focal length which got me pretty close but I ended up doing final adjustment at about 100x. I used to think this lens produced pretty decent results when I used it with my XT. Now on the 50d I was amazed at how far off it was. In comparison, my 10-22mm Canon needed no adjustment at any focal length. Thanks for all the help.

Bob

Okay. don't know if this is right. So I set my focal length at 28 adn times it by 25 and that should give me my microadjustment?

doublehmom wrote in post #8426647
Okay. don't know if this is right. So I set my focal length at 28 adn times it by 25 and that should give me my microadjustment?

I don't know what you're doing there. Use either 50x the focal length or a distance that you most often use the lens. I would dispute the Lens Align recommendation of 25x the focal length.

I use a 24mm fairly often for environmental portraiture--usually at a distance of 10 to 12 feet from the subject. If I went by the 25x thumb rule, I would be doing the micro-adjustment at 600mm...not even a yard. If I use the 50X thumb rule, that gets me closer to "real world."

OTOH, I use my 70-200 zoom on portraits also at about 10-15 feet. The 50X thumb rule would be okay at 70mm, but not at 200mm.

So I'd recommend testing at the distance you most often use the lens, or failing that use the 50X thumb rule

doublehmom wrote in post #8426647
Okay. don't know if this is right. So I set my focal length at 28 adn times it by 25 and that should give me my microadjustment?

You can only make one MA for each lens so if it's a zoom it is recommended that you make the adjustment for the length you use the most. If that is 28mm then 28 times 50 gets you 1400mm which is about 4.5 feet. That is the distance you want the target from the camera for your testing. However, as pointed out, the actual distance you use the lens at is going to be the best distance.

Because I use my 70-200mm at all focal lengths, I did the MA process at three different focal lengths and then set the final adjustment at the average of the three. It might not be perfect at any length but it is a lot closer at all of them.

The link provided earlier in the thread, http://www.pixel-shooter.com/forum/s...​ad.php?t=21099 , has a very good recommendation for determining actual adjustment.

Bob

gjl711 wrote in post #8372034
The best distance to MF adjust is 50x the focal length of the lens. There is a good post Chuck Westfall from Canon has describing tips to do MFA.
http://www.pixel-shooter.com/forum/show​thread.php?t=21099

I believe that the 50x FL recommendation is to ensure a decent DoF to minimise possible errors caused by the target not being exactly parallel to the focal plane. At 50x FL you've got a DoF of 29 cm @f/2.8 - so, being Merkin, I guess they decided to go for a DoF of 1 foot.

Right from the article, right from the man himself, and I've been vainly trying to get this point across every time it comes up:

" Do not attempt to autofocus on an angled chart,"

While improbable that the 50D BODY is that far off, it is CONCEIVABLE that the lens was correct, and that your Xt was correct!!!!!!!!

I wish I could recall where I was told this, being a while back, but it was along with instructions to adjust the camera/lens using a computer screen and moiré. The distance to adjust by should be just about the lens' closest focusing distance. Zooms should be adjusted at the widest focal length. I used it, works for me.