Canon 40D AF Issues - What are my options?

[gkmusser]

Hi Bob,

I purchased the 40D back when you did when it was first announced and you could pre-order on Amazon.com.  I remember discussing with you what new features the 40D would have when it came out...and when they announced 6.5fps, a new AF system, etc....was very excited since I shoot a lot of sports!

But since I've owned the camera, I've often noticed my pictures being slightly OOF in situations where I expected them to be tack sharp!  Now I'm well aware of the DOF issues with fast lenses (I have the 70-200 f2.8, 85 f1.8, 24-104 f4, 100 f2. at large apertures, along with my own potential user error issues...but again...I've specifically tried to take all those factors into account, yet I still feel like there is an issue.

For example, I recently took some portraits where there were two people standing side by side, except one person was slightly behind the other (maybe an inch or two tops).  I was shooting about 5' from the two people at 47mm at f4 in good light.  When I reviewed the picture on my PC, I noticed the two AF points that activated were on the person's face slightly closer to me (not sure why AF points weren't on the other person as their faces were about the same size)...and that person's eyes were tack sharp!  However, the other person (positioned 1-2" behind) was slightly OOF, yet with the shooting info I mentioned above, the DOF calculation should have been approx. 6-7" I think.  Assuming approx. 3" infront and 3" behind...I would think the DOF should have easily covered that difference...but it didn't.

Along with this example, I have many other examples where i used just the center focus point on a subject and noticed the image was not tack sharp.  So...I'm coming to the conclusion (I've been in denial) that I might have an AF issue with my camera (front focus issue), since I seem to notice the issue, regardless of the lens I use.  I've really wanted to test my theory on this before I just send my camera into Canon and have them tell me "it is within a tolerable range"...and it comes back not being any better.  Based on some reading, I've heard that the new LensAlign product (http://www.rawworkflow.com/lensalign) is highly recommended to determine if you have any AF issues with a specific lens and/or camera body, so I've been thinking about purchasing it just so I can determine if I have a consistent problem...or if it confirms I'm the issue!

Obviously, the 40D does not have an AF micro adjustment like the 50D does, but I'm thinking I could test my equipment and then send in pictures (maybe on a CD), along with my camera body/lens to support any conclusion I have regarding my body or specific lens AF issue.  If I did send that in highlighting the problem, what would Canon do?  "Recalibrate"? I'm way past my warranty, so I'm concerned this is going to cost me a fortune to get fixed.  I live in northern NJ & was hoping there was a Canon repair facility near me that I could drop this off, rather than having to ship my camera body...and/or lenses.

Any thoughts or advice on this whole issue?  I would certainly appreciate your comments.  I could email you the one picture I mentioned above if you think that would help.

Thanks in advance for your help! 

Jerry

[Bob Atkins]

Things like this are difficult to diagnose, but there are a few points to consider.

First, the DOF limits are the limits of "acceptable" blur. They are not the limits of perfect focus. There is only one plane of perfect focus. Things at the DOF limits will look less harp than those in perfect focus if you examine the image at 100% on a monitor. The DOF limits are actually based on viewing an 8x10 print from a distance of about 18". 100% on a monitor is more like viewing a 24x36 print from 12" away, a much more critical test. While you may have a 6" DOF based on the 8x10 print criterion (which is what most DOF calculators are based on), for critical viewing on a monitor at 100% your total DOF might be closer to 3" for critical sharpness.

If you let the camera pick the AF point, it will lock onto what it thinks is the subject closest to the camera. Obviously it has to pick something and Canon figured that you'd probably be most interested in whatever was closest.

The focus accuracy specified by Canon is "within the DOF" for most AF zones, but is "within 1/3 of the DOF" for the center zone when a lens that's f2.8 or faster is used. It's not guaranteed to be perfect or within 1/10th of the DOF, so just because your DOF isn't exactly equally distributed about the focus point doesn't mean the camera or lens is out of spec. In fact for anything other than macro shots, with perfect focus you will get slightly more DOF behind a subject than in front of it. That's the nature of optics.

I'm sure the LensAlign stuff works, though you really don't need to spend $80 on a test target. You can do it for free with a little work (see http://www.bobatkins.com/photography/technical/focus_testing.html and http://www.bobatkins.com/photography/technical/lens_sharpness.html)

What is "tack sharp"? When viewed at 100% on a monitor, few images are "tack sharp" and often a lens needs to be stopped down a couple of stops from full aperture to get maximum sharpness.

The main Canon facility is in Jamesburg, NJ. I don't know where you are, but that's pretty central for NJ and it's close to the Turnpike. You can drop off equipment for repair there.

So there are really two possibilities here. First, you could have a camera/lens issue. It's always possible that something is slightly out of adjustment.  The second possibility is one of expecting too much (perfection) and looking too closely at the images (rather than prints) and finding "faults" that are not an indication of a problem but general issues of just how much sharpness (and focus accuracy) you can reasonably expect to see. It will never be perfect and that's inherent in the system. Nothing is ever perfect!

[gkmusser]

Bob,

Thanks for the reply and great info.  I will probably try to build my own AF test kit like the LensAlign product...but using your materials and see what kind of results I get.  I'm hoping that I'm just being too critical.

I definitely agree with your points about not expecting perfection....which is exactly why I haven't done anything up to this point.  But the example I gave you in my original post was viewed at a normal "fit to screen" view...not at 100%...and I noticed the difference immediately.  I think my term "tack sharp" is probably to general.  What I would say is that any person (even someone not into photography) would look at the picture I described and say that the person closest to me looked "normal"...meaning it was in focus...but the other person looked slightly OOF.  I guess the total DOF could have been less than I realized.

Having said that, if you are interested in taking a quick look at the picture, my email address is gkmusser@optonline.net.  Let me know and I can reply directly to you with a copy of the picture without going thru the forum.

I live in Rockaway, so Jamesburg is not that far...but I certainly don't want to drive down there if the reality is they can't do anything to my camera.  However, if my camera would happen to have an issue...what would/could Canon do? Can they "recalibrate" it?

Thanks again for your comments.

Jerry

[Bob Atkins]

Canon can (and do) adjust AF on camera bodies and/or lenses. Whether it will make any difference is unknown. If it's way off they will make it better. If it's within spec and just not quite as good as you'd hope, then it might come back pretty much as it was when you sent it.

You're welcome to email me an image. I'm not sure I'll be able to tell much from it, but I can take a look.