Why do some lenses have front/back focus issues?

I've recently bought a Sigma 30mm f/1.4 and in reading up on it beforehand, it appears that these lenses are prone to front/back focus issues. Fortunately mine doesn't seem to be affected, but I'm a bit confused as to why any lens should be.

From what I understand about the AF system, it adjusts the focus to make the image at the AF focus sensors in focus.

This being the case, I can't quite get my head around why, for some lenses, the AF sensors register correct focus but the image projected on the sensor is consistently slightly front or back focussed. Does anybody know why this happens?

As in all manufacturing processes, nothing is absolutely perfect. All manufacturers set tolerances for the goods they manufacture. If you buy something with all the tolerances to their minimum, you get an outstanding thing. On the other hand, if you have all the tolerances at their maximum, you have a lemon. And there is everything in between...

So, if you get a lens with some front or back focusing, count your blessing, it can be fixed easily.

So many things can go wrong in manufacturing fine precision equipment, it's still amazing lens manufacturers give such high quality material.

but if it's out of focus at the Main sensor, then the info at the Focus sensor should also be out and an adjustment made !? as both light sources come through the same lens ?? is this not so ? I'm confused with this too !

links to more info required please

I agree. For normal "closed loop" control, the camera would adjust the lens until it was in focus. Hence the only calibration would be that the focus sensor and the actual photographic sensor were the same distance optically.

However, my logic tells me the fact that one has some lenses front focusing and some back, means the focus system does not work this way. Indeed, there needs to be some "feed forward" system to speed up the focus algorithm. A standard PID controller would be too slow.

What is missing from my understanding is exactly how they manage the focus algorithm. Perhaps this is a closely guarded secret. Do you want quick focusing or accurate focusing? (I could cynically say Pentax has neither, but fear the flames!)

Er - really?

The focus sensor is located below the mirror box, and the light is reflected to the focus sensor trough a secondary mirror located behind the main mirror. If any of these components are out of adjustment, the focus sensor won't "see" exactly what the sensor "sees".

When it comes to focus accuracy, Pentax is not worst than the "others". Actually, Canon should have taken lessons from Pentax when they did the AF on the EOS 1D Mk III. For a "pro" camera, that one was highly inaccurate.


edit.: Just noticed this is my thousandth post. If I keep on going, I might catch up with Stephan in about a thousand years.

But ALL those components are in the Camera body !! So how can the lens make any difference ?? It will be the same error for ALL lenses ?? No ?

Lenses also suffer from manufacturing variations. The focus sensor might not see the light exactly as the image sensor does. Add to that the color fringing of some lens, diffraction and reflections, color temperature, and you have quite a few things that can confuse the AF sensor.

I personally think the success rate of the AF system on most cameras is very good, given all the variables.

Just found this, I think it answers the question

AF cameras don't "keep adjusting" - they take one measurement of how far apart the images on the line sensors are, calculate how far the lens needs to shift, then shift that far. They don't then check to see if the image is in focus.

That makes it quite clear and explains why we need to adjust focus on a per lens basis


Tks for help !

Don't know where you got that (dis)information from but it's complete nonsense.

So-called experts on the web !

Several times from different people !?


Explanation ?? or links ??

Two seperate logical fallacies back to back ...

Appeal to Authority


Appeal to Popularity


Well I know from actually using a K10D, K20D and 3 K-5s that it is nonsense as they all take more than one attempt if the first one doesn't achieve it. And that's with the regular phase AF system.

Further the contrast AF system has to hunt as it doesn't know in which direction the subject is initially out of focus so by definition it takes a guess and then refines from there.

Then why do we have a FF/BF problem ?? if it thinks it HAS achieved it ?

You still haven't explained how the system works, or provided links to justify your rash statements !?

Bit of a tangent, but when using the focus chart, I had to dial in +12 to get the desired result, but ...
After spending hours doing the above, trying a "real" image on a stationary object, I changed this back down to +4

Are they really front and back focusing or is it our perception more than anything ?