17-55mm front-focusing...how bad is it?

[alex]

Hi everyone,


I've had suspicions for a while now that my kit isn't performing as well as it should. I see 100% crops on TDP from Bryan and from many of you, and I've always thought to myself, "wow, that's way sharper than my shots ever are." I always thought there that maybe my issues with sharpness were mostly due to my own inexperience (too long shutterspeed, not using tripod, etc.), but the times that I have done everything in my power to get a sharp shot (mirror-lockup, remote activator, tripod), I've still never really achieved the sharpness that I feel I'm supposed to be getting. The 17-55, by all accounts, is about the best lens you can get without the <span style="color: #ff0000;"]L <span style="color: #000000;"]designation. So at the end of it all, I<span style="color: #000000;"] finally decided to check out the focus on my 17-55 connected to my XSi.


I set up a measuring tape on the coffee table, the XSi is on my Manfrotto 190XB locked down well, the camera was positioned close to directly above the end edge of the table. The camera was about 8 inches above the plane of the table.


There are four shots here. I took 8 shots total using exactly the same settings, one after the other. The first seven all were taken on center-point AF, mirror-lockup, IS off, f/2.8, ISO800, either 1/40 or 1/50 sec exposure. The last one used all the same settings, except I used LiveView MF. I was shocked at the variance between all the AF shots. I have selected three of the AF shots that are the most different from each other, and the last one is the MF shot.


These are all nearly 100% crops, but not quite (maybe 90%? I don't know how to do an exact 100% crop, actually). [:P]


For all of them, the red light from the center point was aimed at the intersection of the horizontal and vertical lines right next to the number 4.


Number 1, the very first shot that I took


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Number 2, the second shot I took (look how DIFFERENT the focus is!!!!)


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Number 3, somewhat in between one and two


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and this last one is the one in which I used MF using LiveView. I tried to focus right on the horizontal and vertical line intersection next to the 4.


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This seems pretty bad to me. Unfortunately, as stated above, I have an XSi, so no AF microadjust. I only have one lens, so I guess I don't know if this problem is the lens or the camera, right?


So how bad is this, and what are my options?


As always, I really appreciate everyone's thoughts!


alex

[Sean Setters]

1) Send the lens in to Canon

[Richard Lane]

Hi Alex,


I think that it's great that you have identified the problem, that's the first step towards recovery. Sean has provided you with some wonderful suggestions, there's really not much more to do. I would think the best thing to do would be to send the body and lens to Canon for calibration, especially since you only have one lens. If you buy other lenses in the future, then you should check their AF right away and exchange them if they are not spot on.


I guess your next body will definitely have AFMA. That's why a lot of people were surprised that the new 60D didn't have that feature.


For now you should manual focus your important shots, or select an Auto Focus point that is approx. 2 inches behind what you want to be in focus (since your setup appears to be front focusing by about 2 inches).


Good Luck,


Rich

[ddt0725]

Boy, can I relate to this problem! I currently have two lenses and my camera at Canon Repair because I was having problems with achieving sharp focus with the 135L and I had all I could take with trying to micro-adjust it.Right before this I returned a Sigma 85mm for another copy do to the same problem. I wasn't sure if the problem was the camera or the lens, so off they went. It's the first time I have ever sent anything to Canon and hopefully I will have them back next week. Here's the link I started regarding my experience ...


community.the-digital-picture.com/.../5497.aspx

Your lens is clearly front-focusing. It also looks like focus is pretty sharp with this copy, it's just not focusing where you want it to so if I were you, I'd send the camera and the lens off to Canon.

Good Luck! Let us know what you decide to do and how it turns out!

Denise

[alex]

Same time reply, Denise! Thanks for the thoughts, I will continue to look at your thread to see what happens. I

[alex]

Thank you Sean, and Rich, for the info. I was afraid those were the only options available. []


I've been reading Denise's long saga with her 7D and 135L with interest, so I finally decided to just test the lens and see what I've got. Needless to say, I was a bit let down.


I think the thing I was most surprised about was the inconsistency in the different shots. I didn't think that was possible.


I've been lusting after the 7D for over a year now, and I'm no closer to getting it now than I was then (student with family = no money!!!!). But you're right, Rich, there is no way my next body wouldn't have AFMA. I can't believe they dropped it on the 60D. I suppose that might have helped the resale market for 50Ds.


So you would both say that these are pretty definitive results?


thanks,


alex

[neuroanatomist]

I think the thing I was most surprised about was the inconsistency in the different shots. I didn't think that was possible.


So you would both say that these are pretty definitive results?

I would not say these are definitive results, IMO. The problem is the set up of the test. You may know that you are intending to focus at the number 4 (and with Live View you certainly can), but how does your camera know that? The actual AF point is larger than the little box in the viewfinder - so, the image element that's being used by the AF system is not the, "intersection of the horizontal and vertical lines right next to the number 4," but rather it's the two vertical edges of the measuring tape against the coffee table, passing through the entire AF point. The fact that those edges are at an acute angle to the camera, and the fact that there are actually two parallel edges, is a challenge for the AF system and that's what accounts for the inconsistent focus you're getting. Most AF testing methods get around this issue in one of two ways - the free 'paper' target which Denise has used recently (focustestchart.com) has a focusing target that is a single line with sufficient blank space around it, and that line is oriented horizontally on the page, so the angle of the target relative to the camera doesn't matter. Other testing systems (e.g. the LensAlign that I use) have a focus target with starbursts that is oriented parallel to the sensor - so, the ruler is at an angle but the focus target is not (an early version of the chart from focustestchart.com actually has a cut-out part that you taped to the current flat paper to provide that parallel-to-the-sensor target). Also the starburst targets (like concentric circles targets) work with AF sensors regardless of sensor orientation.


Denise - thinking about this issue brings a potential confound to mind with your testing as well. Like Alex, with the 135L you're using a lens faster than f/2.8, meaning that you're able to use the high-precision capability of the center AF point. However, I don't think you're actually using it with that chart. The focus target line is horizontal, meaning it's going to optimally activate only the horizontal line-sensitive part of the AF point. Your 7D's f/2.8 sensor is a pair of diagonal lines, meaning a thin black-on-white line oriented horizontally is not going to activate that sensor. Likewise, Alex, if you were to use that chart, despite your XSi also having a high-precision f/2.8 center AF point, you wouldn't be using it either - for your camera, the f/2.8 sensor is a vertical-line sensor that ignores the horizontal line on the chart. So in all cases, that horizontal line is only going to activate the f/5.6 horizontal line-sensitive part of the cross-type sensor in the center AF point. You may be wondering - why did the guy make the chart like that? I would guess it's because he designed it to test AF on Nikon cameras, specifically the D70. Nikon bodies don't have the high-precision AF point capability - their cross-type sensors are f/5.6-sensitive in both orientations.


Alex,bottom line is, if you have a problem, the test you did isn't the best way to show it. You could probably easily put together a modified setup on your coffee table, leaving the ruler but adding a target. Try printing something like this image, then propping it up so it's approximately parallel to your sensor (taped to a building block like my daughter plays with, for example). Just make sure you know which line on the ruler the target is centered on (you might need to prop up the ruler so it's height is aligned to the target as well). Or, you could use the older version of the test chart with the cut-and-fold template, and tape that starburst image to the focus target on the folded piece.


If you do have an AF issue (which seems likely based on your statements about elss than desirable sharpness with your images), Sean correctly ID'd the possible steps - your best bet is to send body and lens into Canon.


Hope that helps...


--John

[alex]

John, thank you so much for taking the time to construct this informative response! I will attempt to redo my test using some combination of methods you mention above. You