Am I expecting too much off my camera?

[untenchicken]

Why is the marriage between many of my lenses (Canon, Sigma, Tokina) and camera bodies (7d, 1diin, 6d) such a tumultuous one?
I mean, front focus, rear focus, somewhere in between focus...! If this was real married life, it would be "divorce galore"

So can any of the more technically schooled CR members please tell me why it is seemingly impossible to calibrate a camera (i.e. the autofocus module) so that it can handle any kind of lens and make it focus properly? Who is telling who what is focused and what is not in the first place: the camera the lens or the other way around!?

I would think the 'computer' in the camera has been taught what "in focus" means before being wrapped up in a box and send off to make somebody happy, and there for, as long as it is getting a decent reading from one of the focus points, it would kick the lens (even a Sigma one) in 'gear' and makes it focus spot on.

Too simple? :-[

 

[Valvebounce]

Hi untenchicken.
Can we have some more info,
What Lenses, focal length, primes or zooms?
Have you done AFMA on all the bodies?
Is one lens front focusing on one body and rear focusing on another, or are you getting front and rear on the same body?
Are you getting front focus at one end, say near distances and back focus at far distances?
Light level?
Example shots?
This might give us a clue to be able to help, so far what you post is not really sufficient information.

Cheers, Graham.

 

[privatebydesign]

Manufacturing tolerances.

Bare in mind a grain of sand on the lens mount will push a lens well outside AFMA range, tolerances are that fine. Add in the fact that sensor to lens mount flange also has that same manufacturing tolerance and you begin to realise the problems, now enlarge everything to the crazy sizes we do and 100% crops and you get the picture.

The lenses and cameras could be manufactured to finer tolerances, but it would cost a lot more money.

Most DSLR AF systems are not a loop, that is, they don't say go that far, check again, go some more, check again, come back a bit. Most are open, they know how far they are out of focus and the body says 'lens move that much', if the lens or body is slightly mismatched then you get inaccurate focus.

 

[Mt Spokane Photography]

If you use live view, then AF is controlled by the computer in your camera, and focus is very accurate, but its slow. Otherwise, for phase detect AF, the camera sends a message to the lens and tells it where to autofocus. The computer and associated sensors in the lens is supposed to go there.

But, the computer in the camera also knows which Canon lens you are using, and takes that into account when it controls a lens. If you are using a 3rd party lens, the camera does not know anything about the lens, because the lens tells the camera it is a certain Canon lens, sometimes a very old Canon lens. Then it becomes difficult, because different camera models will handle the lens with a slight difference. Sigma created their dock in order to adjust a lens to a specific camera.

The end result is usually a lens issue, the lens needs to be adjusted to match the camera go to commands if you want it to be perfect.

As noted above, tolerances are so tight that even tiny variations have a effect on accuracy.

 

[Marsu42]

Too simple? :-[

I cannot say for your specific setup, but remember you can afma for *one* specific distance with Canon's system (and afaik unlike Sigma's usb dock solution). So unless shooting standard scenes or in a studio, with a very thin dof your afma cannot be "spot on".

Other than that, phase af has an inherent jitter. And esp. with (sub-)mediocre af system like the 6d that don't even have one (1! uno! einen!) real f2.8 double-cross point you're bound to have micro-misses with faster lenses on textures with little contrast. Last not least, focus & recompose does its share to screw the af up.

Unless we're getting mirrorless with precise af spots all over the place, even with midrange gear you have to accept that you'll have give important scenes more than one shot to expect a spot-on af lock on pixel level at ~20mp.

 

[SPKoko]

You may find some of the articles from Roger Cicala (lensrentals) related to AF accuracy very interesting. I certainly do!

https://www.lensrentals.com/blog/2008/12/this-lens-is-soft-and-other-myths
http://www.lensrentals.com/blog/2010/03/this-lens-is-soft-and-other-facts
http://www.lensrentals.com/blog/2014/02/why-you-cant-optically-test-your-lens-with-autofocus
http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy
http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-ii-1-vs-2-and-old-vs-new
http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-3a-canon-lenses
http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

 

[old-pr-pix]

It's simple: Realistic trade-offs between - Physics, Economics, and Time.

As photographers we expect our cameras and lenses to perform operations with a precision measured in microns (that exceeds every other consumer product I can think of) in milliseconds that involve moving multiple parts (that can be fairly heavy and wear over time) without using too much battery power for the cheapest possible price.

As explained above, the result is a mostly open-loop system without checking and feedback that actually works pretty darn well most of the time.

Could it be better? Sure, change any of the constraints. Use LV and focus manually (try that with the nifty-fifty and you will appreciate how good your autofocus really is!). Provide more power (1DX). Re-calibrate lenses and bodies both for initial manufacturing variation and as they wear. Be willing to pay a lot more money (a NASA sized budget could go a long way toward better autofocus).

 

[untenchicken]

Thanks all very much for the replies! Gave me some food for thought

 

[takesome1]

Why is the marriage between many of my lenses (Canon, Sigma, Tokina) and camera bodies (7d, 1diin, 6d) such a tumultuous one?

Because in a marriage it is best to only have one partner and stay faithful. Your Canon camera views your Sigma and Tokina lenses as cheating.

 

[Valvebounce]

Hi takesome1.
Funny, very very funny, thanks for the laugh.

Cheers, Graham.

Why is the marriage between many of my lenses (Canon, Sigma, Tokina) and camera bodies (7d, 1diin, 6d) such a tumultuous one?

Because in a marriage it is best to only have one partner and stay faithful. Your Canon camera views your Sigma and Tokina lenses as cheating.

 

[drjlo]

I mean, front focus, rear focus, somewhere in between focus...! If this was real married life, it would be "divorce galore"

Go mirrorless

 

[Halfrack]

Your 7d and 6d should be able to be tuned happy. The 1DIIn is not going to get better, adjustments didn't show up until the 1DIII (I think... i'm at 1 cup of coffee so YMMV).