Back/Front Focusing: a Lens Issue, not Body?...

[Khufu]

Whenever I catch people talking about f/b focus, the lens manufacturer always seems to have the fingers pointed at them... but I don't understand why!

I'm open to being schooled here, really, but if the camera has a sensor which detects that a subject is in focus at its distance from the glass, to me, it seems the image capturing sensor must be at a slightly more or less distant position than the focusing sensor if it's not receiving the same light distribution, right?! Anybody?

I LOVE my Sigma glass but they require so much MFA on my 5D3 (50mm f/1.4 @ +16!!) which I've lazily accepted, I couldn't use them on my (now sold) 60d, though I'm looking forward to maybe grabbing a 70D for wildlife, with the option of throwing my Sigmas on there... But investing in MFA cameras out of necessity seems... Unnecessary? I'd love to use these Sigmas with a 100D but there's no chance of it if I can expect focus to be off on all bodies.

Maybe I'm way off with the physics/mechanics and it makes sense when you guys exlain what's really going on

Thank you in advance!

[Lichtgestalt]

tolerances from body AND lenses can create issues.


read this:

http://www.lensrentals.com/blog/2008/12/this-lens-is-soft-and-other-myths

[ajfotofilmagem]

The auto focus when looking through viewfinder ( phase detection ) is very fast nowadays, and this makes it difficult to obtain extremely precise focus . Unlike the live view ( contrast detection ) that is very accurate and very slow in most cameras . When I bought a Canon 50mm F1.4 , I had to send to the service authorized to adjust the front focus that was constant. The adjustment made , determined the exact moment when the brake is activated in focus motor in the lens . For proper operation , there must be perfect sync between camera and lens. That is, the camera has to know exactly the current position of the lens , and the lens should know what the focus sensor in the camera is measuring. Sigma lenses try to fool the camera , pretending to be a genuine Canon lens , and this gives rise to larger errors than lenses manufactured by Canon . But why Canon lenses also make mistakes and need adjustment as my Canon 50mm F1.4 ? The quality control is not as good as it should be , and this is evident when we realize that Canon lenses as they pass by the authorized service repair usually get better than brand new lenses .

[Lichtgestalt]

But why Canon lenses also make mistakes and need adjustment as my Canon 50mm F1.4 ? The quality control is not as good as it should be , and this is evident when we realize that Canon lenses as they pass by the authorized service repair usually get better than brand new lenses .

keep in mind you have to control the combination of lenses and body... as both have tolerances.

as roger pointed out in the article i linked too, one pair of tolerances can work just perfect... another pair is crap. +3 +3 = bad -3 +3 = perfect.

you want to make the tolerances so small that any combination works fine?

well... are you willing to pay for such an improved quality control and improved manufacturing?

and this is evident when we realize that Canon lenses as they pass by the authorized service repair usually get better than brand new lenses

i only noticed that when i send the BODY + LENS to the service.
because then the lens is aligned to the body.

[candc]

i also like sigma lenses but i think it is a lens issue with the new ones. there should only be 2 variables, the mount to image sensor distance and the mount to af sensor light path distance. that remains constant so should cause consistent focus misalignment.  once you calibrate a sigma lens with the dock it makes changes to the focusing characteristics of the lens and does not consider any camera lens pairing. when i have done this the 2 lenses 18-35 and 120-300 work the same on the 3 camera bodies i have tried them on. i don't know all the inner workings but it seems more like the lens is just not calibrated right to begin with?

in order to really set the lens right with the dock you need to make 4 focal range x 4 distance corrections and they can be all over the place?

the 2 lenses that i have would be unusable by most peoples standards without the dock adjustments.

[Khufu]

Brilliant - thank you both!
Unfortunately my 50mm f/1.4 seems to be at the extreme end, if not way beyond, acceptable variation, though it hadn't occurred to me that it could be a "reaction time" issue, which makes a lot of sense, rather than it just being Sigma toying with the Universe's everyday physics.

I also really appreciate the link to the article, which makes a lot of sense and I recommend anyone who's read this far into the thread without clicking the link does so

Maaaybe I will pick up that 100D some time, send it off to Sigma with my 24mm & 50mm then MFA my 5D3 and yet-to-acquire 70D afterwards

Thank you, thank yooou...

[Khufu]

The auto focus when looking through viewfinder ( phase detection ) is very fast nowadays, and this makes it difficult to obtain extremely precise focus . Unlike the live view ( contrast detection ) that is very accurate and very slow in most cameras .

EDIT: no video on the 1000D, looks like I made that bit up - but I'm still considering it!

Here's a curveball... I'm considering throw-around, small APS-C EF shooters and the 400D/XTi and 1000D are about the same size as a pricier 100D, though the 1000D is a crap build and I'd maybe go 450D over 400D for Live View... and 1000D I think may have (crippled) video (sort of bonus)...

Anyways; 1000D has "slower" contrast detection AF - am I crazy to think it might nail focus where fast phase detection might not on mis-focusing lenses? I'm off on one now, you guys don't have to get too involved

[Random Orbits]

i also like sigma lenses but i think it is a lens issue with the new ones. there should only be 2 variables, the mount to image sensor distance and the mount to af sensor light path distance. that remains constant so should cause consistent focus misalignment.  once you calibrate a sigma lens with the dock it makes changes to the focusing characteristics of the lens and does not consider any camera lens pairing. when i have done this the 2 lenses 18-35 and 120-300 work the same on the 3 camera bodies i have tried them on. i don't know all the inner workings but it seems more like the lens is just not calibrated right to begin with?

in order to really set the lens right with the dock you need to make 4 focal range x 4 distance corrections and they can be all over the place?

the 2 lenses that i have would be unusable by most peoples standards without the dock adjustments.

How do the dock adjustments work on multiple bodies?  If you dock adjust the lens on body 1, will AFMA on body 2 get it to be accurate, or do you have to readjust all those values specifically for body 2?

[candc]

The dock just changes the lens. The 2 lenses that I have both were equally off on the 3 camera bodies I have tried them on, 2 70ds and a 40d. I Think that after you make the dock adjustments to the lens that it is within the range that is acceptable on the 3 bodies. If there were big variances between the bodies then I suppose you would have to make an afma adjustment on the body to compensate because the lenses does not as yet recognize and apply different settings depending on the body it is attached to, you might see that as a firmware update in the future?

[CharlieB]

For the OP,

Not totally unlike the operation of a modern vehicular engine, the Canon body and lens operate in a closed loop manner.  Both have processors, and those processors have a data bus that joins them, and.... importantly... its not really a finite system, but more of a fuzzy logic system. 

You can have issues with teh camera, the lens, or both.

Here's a weird one - I got an EOS 7D, and it had issues.  Additionally, I already owned a 28/2.8 that become a poison lens for the 7D.  Most lenses would focus ok, not great, but ok.  Mount the 28/2.8 and thenafter no lens would focus correctly...not even remotely so, except at about 4-6 meters, give or take a little.

The 28/2.8 had no issues on any other EOS body - EOS 5's (film), EOS 5DmkII, EOS 400.... all worked ok with this lens.  The 7D had a real issue with THAT particular lens.

I sent the camera to Canon... they changed lots of stuff (so they said).   It came back working fine.   And the 28/2.8 is no longer a poison lens.    Hell if I can say why.

[takesome1]

Whenever I catch people talking about f/b focus, the lens manufacturer always seems to have the fingers pointed at them... but I don't understand why!

Well I always blame the lens manufacture, because all my lenses are made by Canon.

[drmikeinpdx]

The whole lens/body autofocus topic has driven photographers crazy for years.  The only thing that saved my sanity was the purchase of a 5D3 last year.  All my old lenses that drove me crazy on my 5D classic, now work perfectly. Some need a little MFA, but not much.  Even my Sigma 50/1.4 works like a champ.

I can see why Canon doesn't put MFA on the Rebel bodies - most users mount zoom lenses with fairly small apertures, so focus errors are masked by the DOF.   Although i do recall adjusting the focus on my 450D with a teeny tiny Allen wrench one time in order to make it work with my old 50/1.8