Is Sigma problems created by Canon?

I got a sigma lens that work perfectly on a 20D body. 28-70mm f2.8. A really good quality lens, sharp and fast focussing. However I've just tried it on my t3i body and the lens is back focussing quite a lot. 10cm at least. Nothing to do about it.

I've seen other stories like this;
https://photography-on-the.net …/showthread.php​?t=1156870

Is Canon pulling a Apple on us and try to sabotage the competition, slightly changing settings on newer bodies?

No.

Earwax69 wrote in post #14911534
I got a sigma lens that work perfectly on a 20D body. 28-70mm f2.8. A really good quality lens, sharp and fast focussing. However I've just tried it on my t3i body and the lens is back focussing quite a lot. 10cm at least. Nothing to do about it.

I've seen other stories like this;
https://photography-on-the.net …/showthread.php​?t=1156870

Is Canon pulling a Apple on us and try to sabotage the competition, slightly changing settings on newer bodies?

Oh that was my post.

Created by EVERYONE in the manufacturing process.
Imagine one tiny error can actually have a butterfly effect.
I actually got rid of the T2i body and Sigma 30mm and went with 5Dc + 50mm 1.4 (for almost about the same price.)

Canon lenses have focus issues too, the first copy of 50mm 1.4 was soft (a while ago), which was the reason I got rid of it, at first I thought it was supposed to be soft wide open and I couldn't stand it. Then now I pick it up again and it turns out to be very sharp on a 5D. I am not 100% sure if my first copy had a focus problem because I wasn't aware at the time so I didn't do any rigorous testing with it.

The problem is rather complex because I have no engineering/programmin​g knowledge of the actual lens manufacturing process. I can't really say which contributes more to the problem. Many people have made posts about it and they just say lens and bodies have variation which is like saying nothing. It's obvious there is variation but which one is the major contributor?

As I understand it, Canon do not disclose their autofocus algorithms. Sigma, Tamron et al have to reverse engineer a solution for their lenses to work on Canon bodies. It's unlikely that all of the nuances can be determined this way, resulting in potential for some incompatibility.

The one thing that does puzzle me is that the autofocus seems to not be properly closed-loop when it comes to confirming focus. If it were truly closed loop, front or back focus would surely be eliminated?

Instead the system seems to use something like a look up table approach for each lens, which might explain differing compatibility issues for different bodies. This may be twaddle though, I have no direct knowledge myself of how the system works.

As long as you have 2 different physical systems in a body where light is deflected one way to the AF sensors, and when you shoot the pic, that light follows a different path to be recorded, you have tolerances that cause these issues. Canon should get tighter on the manufacturing of the AF sensor plane, the mirror, the lens mount, and the sensor so that light travels an identical distance either way.

Geejay wrote in post #14911806
As I understand it, Canon do not disclose their autofocus algorithms. Sigma, Tamron et al have to reverse engineer a solution for their lenses to work on Canon bodies.

afaik sigma does reverse engineering, while tamron pays for licensing.

This is mighty interesting! News for me. That explain why tamron dont get all this front focussing nonsense.

Just different tolerances. Another t3i body might be spot on or even front focus. Having siad that are you testing things properly. 10 cm is almost 4 inches and I don't think too many people have reported a lens being off that much. In addition, be realistic about your expectations. If you are shooting mostly landscape photography who cares? If you shoot a lot of close up stuff then yes you care.

Geejay wrote in post #14911806
The one thing that does puzzle me is that the autofocus seems to not be properly closed-loop when it comes to confirming focus. If it were truly closed loop, front or back focus would surely be eliminated?

Instead the system seems to use something like a look up table approach for each lens, which might explain differing compatibility issues for different bodies. This may be twaddle though, I have no direct knowledge myself of how the system works.

The main reason for front and back focus is that the light path used for the AF (non-live view) is not the same one that will be used to expose the actual image. Light is diverted by a mirror to a separate sensor where it is evaluated and the AF amount calculated. If there is any variance between these light paths then the AF will not be perfect.

As to why things can change, no piece of hardware can be perfectly manufactured. There are tolerances a +/- amount that is considered within specs. A perfect lens and body will both be +/- 0 but this is near impossible to achieve. If the lens is off a little to the + side and so is the body, then thing are real close but if the lens is a little to the + side and the body to the - side the differences are additive and it can be farther off the either alone. Those with micro focus adjust and FoCal, an automated MFA adjust tool, clearly see this as nearly all lenses require some adjustment to be spot on.

It depend on the distance from the subject. Very close it's more like 1 inch, at 3 meter it's quite 4 inches.

It's an old lens. I bought is new in 2003 but it was already replaced by a 24-70mm. I guess it's what they say about Sigma lenses not being compatible with newer bodies.

kendon wrote in post #14911876
afaik sigma does reverse engineering, while tamron pays for licensing.

I've read that several times here, but no one offers any source of the information. Is this real, or is this POTN Legend?

RTPVid wrote in post #14912100
I've read that several times here, but no one offers any source of the information. Is this real, or is this POTN Legend?

I don't even think this is the issue at hand, but it seems to be the general internet perception this is the case.

The issues will most likely be more physical construction than software issues, the Sigma mount, any sloppiness in lens group movement, the camera tolerances, etc. Software is a great tool to be used for adjusting for these tolerances, either MFA, or Sigma calibrating a lens, etc.

RTPVid wrote in post #14912100
I've read that several times here, but no one offers any source of the information. Is this real, or is this POTN Legend?

A quick peak at Wiki indicates that Tamron is not licensed.
http://en.wikipedia.or​g/wiki/Canon_EF_lens_m​ount

Earwax69 wrote in post #14911936
That explain why tamron dont get all this front focussing nonsense.

No

Expanding on what John from PA and gjl711 and said:

With regards to focus, imagine 0 as perfect - which, as gjl711 noted, is quite difficult to acheive. Camera and lens manufacturers can't trash every lens and camera that isn't perfect, so they build within tolerances, let's say -20 to +20.
If your camera body is -8 and your lens is +8, you'll have, essentially, a perfect combo. Even if it's -9 and +12, it will still be pretty good. But when you start getting high-value same-sign numbers, like +17 and +19 or -20 and -16, you'll find noticeable front- or back-focusing. Just remember, each copy of every lens and every body can and will be different.

So your 28-70 might be a -10 lens, which is within tolerance. Your 20D might be a +8 body, which is also within tolerance. Together, they focus pretty damn well. OTOH, your 600D might be -10. Suddenly, you start noticing that the focus isn't quite where it should be.

Note: all numbers are arbritrary and for illustration purposes only.

There is a review at http://www.the-digital-picture.com ….8-EX-DG-Lens-Review.aspx of the Sigma 28-70mm EX DG lens. Is that what you have? If so it is stated, near the bottom...