Aperture Variable Microadjusting/Autofocus during DOF preview Aspherical Aberr

[PabloMagno]

Ok, I know that in the not posible to do post theres a reference to MA but it wasnt that clear to me.

So why am I proposing this? Theres a growing interest on shallow depht of field photo, however the greater the aperture, the more prone a lens is to present aspherical aberrations. What these aberrations do, is to basically screw up the focus planes in ultra fast lenses eg: 1.4, 1.2, ect. These effects tend to manifest as shifting backfocus and front focus. For example at 1.4 your lens may exhibit front focus and at 2.8 it will back focus, therefore any traditional 1 point microadjusting will not solve the issue.

What happens is that lenses always focus wide open, but at such large apertures, the light get into the lens in a different way than in let say f4. So the image might look focused, but when stopping down the lens to 2.8 or narrower, the plane of focus will move.

Therefore, the most common request to fix this issue is to enable multiple microadjusting points, do you think it is possible (obviously implemented in already microadjusting capable boddies). Do you think it may solve the issue?

Another solution that i thought (and havent read of, even after a lot of google) is enabling autofocusing during depth of field preview mode (I know at f8 phase detection is going to have trouble to focus but at f2.8  f4 f5.6 there shouldnt be a problem focusing). . What this mode allows is to step down the lens so you can have an idea of the DOF you will get when pressing the shutter. However if we were able to focus at stopped down settings, so when the photo is taken, the focus plane shouldnt move. This is an entire speculation of my part, and i dont know if it is already posible since im out of city and i dont have my 6d with me rigth now.

I know i am simplifying really complicated physics matters, and if im wrong in something, pls state it without being a flamer, im just trying to find a solution for all the people with large aperture lenses. Also sorry for the grammar and stuff, english is not my main language.

[Francis]

Sounds like it would be possible in theory since ML can already set MA values, at least on cameras that already have AFMA. As you suspect, AF is not possible while DOF preview button is held down.

You would have to manually determine the MA value for each aperture of each lens and have them stored in some sort of configuration file but I'm not sure how you would do that since something like the DotTune AFMA calibration tool would only run with the lens wide open. You'd just have to guess at MA values.

Have you run into this issue very much in practice? It seems like the increased DoF from stopping down should cover any minor shift in the focal plane unless you are working at macro distances. If it is so extreme that it is noticeable, you might just have a bad apple.

[Audionut]

You can trick an EF lens to keep it's aperture blades closed by stepping down the aperture, holding the DOF button and slightly detaching the lens.  Good for extension tubes 

AF doesn't work, but does it still give focus confirmation?  I don't have my camera body handy to try.

[Francis]

Nope, no focus confirmation since the chip is not connected. Behaves just like a fully manual non chipped lens.

[PabloMagno]

I dont have thise issues right now since i shoot mostly with 70-200 for Shallow dof. But planificado no buying an 85 and a 50 for that. Sorry for the late reply and thanks for yours. If ml developed that, it would be. Great fix for photo graphers using 50 1.2 and sigmas 50 1.4 among others...