Causes of focus problems

[arie_vandervelden1]

<p>Here's how to test: http://focustestchart.com/chart.html<br>

download the pdf, print it out, and follow the instructions.</p>

<p>Another way to test is to set the camera on a tripod, point it at something textured. AF with regular autofocus, then turn lens to MF. Turn on live-view with max magnification, then twist the focus ring and see if you can improve on AF. Repeat, repeat, and repeat again. If you can improve focus and the amount is consistent then you've got a front/back focus problem.</p>

<p>Why to test. Live-view autofocusing does have the feedback mechanism while traditional phase-detect AF does not. Live-view autofocus is deterministic while phase-detect is a more pragmatic approach: it calculates where focus should be, drives the focus there, but does not confirm. You'd think that half-pushing the shutter button multiple times would induce feedback, but strangely it does not.</p>

<p>My 17-55/2.8 IS front-focuses badly, and the above test confirms it. I fixed it with the AF microadjustment in the custom functions, and now it's much much sharper. The test is FREE, the only cost is a bit of your time. I say, get the max out of your optics you paid the big bucks for. It's worthwhile tweaking things if you can.</p>

[igor_jarm]

<p>Alec,<br>

You make these things sound way too complicated, 20 years ago cameras autofocused much better than my digital with this Sigma lens today. As far as I'm concerned their postdoctoral graduates could stop development in the 80s.<br>

I understand it's not in Canon's interest to make third party lenses focus better on Canon cameras, but I'm sure they could do it if they wanted.<br>

But this actually has little to do with the OP's inquiry, where I'm sure it's due to slop in the cheap lens mechanics. I also have this 50mm lens and sometimes it focuses ok, sometimes in front and sometimes in back. Not like my Sigma lens.</p>

[alec_myers]

<blockquote>

<p>You make these things sound way too complicated, 20 years ago cameras autofocused much better than my digital with this Sigma lens today. </p>

</blockquote>

<p>Igor, that's an interesting point. I had (still have) an EOS600, and I remember being very happy with the AF, at the time. However digital gets you into the habit of zooming right in to check... I recently dug out an old album of 8x10 prints, and guess what? The prints I was happy with 20 years ago now seem soft-focused to me.</p>

[igor_jarm]

<p>Arie,<br>

I don't believe there's no feedback in the auto focus process.<br>

There are two instances, where it's obvious, there's some feedback involved.<br>

1) when the Canon camera starts with severely misfocused lens, it tries first in the direction of closer focus, if it doesn't find focus there it goes to infinity direction<br>

2) my lenses sometimes "hunt" for the best focus back and front in progressively smaller steps (my guess, never more than 3 steps)<br>

If there's no feedback involved there, I really don't know what feedback is.</p>

[igor_jarm]

<p>Alec,<br>

Just for you I pulled out my old EOS 1000, which as you know was the budget camera in those days.<br>

Fortunately my Sigma lens mounts this camera and guess what? In my digital viewfinder I could correct the wrong close focus by the image I saw, and in this almost 20 years old camera focus looks ok in viewfinder.<br>

You'll just have to believe me this.</p>

 

[scott_ferris]

<p>This is interesting.<br>

 <br>

Igor, I would hardly call the eye/hand co-ordination and mental processing involved to manually focus a lens via split prisms trivial, and the computer power needed to do it electronically (way way faster than the best sport photographers ever have) and decide what exactly it, the AF, should be working on in both one shot and, unbelievably, predictive modes puts the achievements of the camera manufacturers into perspective, pretty amazing really. The fact that we can all zoom into our images at 100% and greater, far larger prints than almost anybody made back in the film and manual focus eras, puts the spotlight onto the focusing in a way that has never been done before. Take your back and front focused shots from your 15mp camera and print them to 5"x7", they will almost all appear sharp. Print your old manually focused film images to 24"x36" and they will mostly be out of focus too.<br>

 <br>

Expectations are crazy high now, as a reaction to this Micro AF adjustment is working its way down the camera ranges. When I got my first micro adjustable AF camera I was amazed at how far "out" my lens collection was and how much the AF performance improved after careful setting up.<br>

 <br>

Doesn't help with the question, "how does it all work?" just a reality check on what we expect of our consumer electronic equipment nowadays.<br>

 <br>

Take care, Scott.</p>

[igor_jarm]

<p>Scott,<br>

I'm actually just arguing that grossly misfocusing certain lenses has nothing to do with how tricky or delicate the autofocus process is. In these cases I'd not even call it autofocus it's something totally useless. Photographing something from distance of 50cm with accuracy of less than 5cm is hardly high expectation, isn't it? The lens is optically and mechanically decent, yet the camera can't focus it. Someone is not doing their job right, and I think it is Sigma, but then again it is the camera that does the focusing not the lens.<br>

Again this is not the problem the OP is experiencing although it seems like it is.</p>

[scott_ferris]

<p>Igor,</p>

<p>Whilst I sympathise with your problem, if the lens always focuses 5cm forward or backward then there is obviously an expectation/communication breakdown in the lens body information flow. Some third party lenses are recognised in the microadjustment setup, obviously some third party manufacturers did a better job of reverse engineering Canons transfer protocols, you can hardly blame Canon for Sigmas errors. If the body has an expectation that the lens will focus correctly if the focus motor is rotated x degrees and it signals the focus motor to do that via a protocol that works for every one of the body manufacturers own lenses but does not for the third party manufacturers lenses, well, that is very clearly, in this case, the fault of Sigma.</p>

<p> This is similar to tolerance issues where micro-adjustment works well, the bodies expectation of what a lens will do when instructed to does not always mesh with the actual outcome, hence the need for adjustment in some cases. Don't forget that micro-adjustment comes in two flavours, global adjustments, that affect all lenses and is clearly there to adjust camera body tolerance problems, and lens specific micro-adjustment that can sort out combination's of lens and body tolerance issues. I made lens specific micro-adjustments, the global settings did not improve accuracy, the lens specific ones did and they were all over the scale from strong positives to negatives of the range of adjustment.</p>

<p> </p>

[mark soares]

<p>Wow this has become a very active thread. I'd like to contribute a bit more to it to clarify some of the things i said in my previous posts. Lenses (just like cameras) have certain specifications and tolerances. The manufacturers can only guarantee a certain degree of accuracy when manufacturing these units, and as such the vast majority of them have to be calibrated or corrected in order to be within specs. This is not to say that most lenses need adjustment - what i mean is that components can be physically moved to adjust focus, however, in order to perform minute adjustments you have to rely on electronic calibration. What this means is that most lenses have data within their circuits that works sort of as a focus shift or focus compensation in order to get the focus right. This is calibration data (micro adjustments or shifts) that the lens has embedded in its own circuitry. If lenses were all perfect from the assembly line, you would not require this compensation data or microadjustment info to be in the lens.<br>

What this means is that if the lens suddenly does not meet the original hardware specifications as when it was calibrated (and the data uploaded onto the lens) due to the barrels slightly shifting, or element groups becoming misaligned, etc etc, then the electronic data which is there to ensure the lens meets the manufacturer specifications is no longer applicable because it is telling the camera to compensate and microadjust for different hardware specifications. In that case you would need to have the lens recalibrated to meet it's current hardware specs. So in summary, most repairs on lenses that require full disassembly or element group repair/replace require an actual physical adjustment (by shifting elements/barrels, etc) as well as electronic adjustments (by connecting the lens to the respective manufacturer's software and performing re-calibration on it, which is essentially compensating for whatever hardware specification fluctuations may be present in the repaired lens). If a lens doesn't need new parts, just an electronic calibration is usually enough.<br>

In regards to the little "brushes" i mentioned - this is a component in the lens that resides outside the barrel and travels along a pathway (gold plated) and feeds back information as to where the current location of the element groups/barrel is. Because these brushes are a physical component - it is fully expected that if they become damaged in any way or go out of specifications that the lens will then suffer a focus problem. Anybody with access to a repair manual on most modern lenses will be able to see these brushes and the function they perform.<br>

Mark</p>