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Old Oct 19, 2008, 3:39 AM   #11
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Well if you are sure you are not re-composing after using the centre AF point, then clearly your body is front-focussing with those lenses.

Actually it's possible it's front-focussing with all your lenses, but because you are shooting with smaller apertures you are working inside the increased DOF so have never really noticed.

At this stage the best thing really is to do some controlled testing with a tripod and ruler/newspaper and find out definitively.

All cameras and lenses are calibrated within a set tolerance.

So if your camera is like this |-x--------0-----------| and your two 50mm lenses are like this |-------0-----x-|
then the combination can be out by an amount that is too great for comfort at wide apertures.

It may be for example that your other lenses are like this
|---x-----0-------| i.e. out in the same direction as your camera, so you don't see a problem.

If you are suffering from this situation then you just need to send the whole lot in to Canon for calibration. Or alternatively if you are still within your warrantee period with either the camera or the lenses just return them to the store and take pot-luck with the next one.

The great thing about the new higher-end Canon cameras is that you can micro-adjust the AF settings for your individual lenses! I for one will never buy another camera without that feature.

If you discover that you do need to send them to Canon for calibration you may actually find that it's cheaper just to upgrade to a 50D and then you can do the AF adjustment yourself!!
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Old Oct 19, 2008, 7:06 PM   #12
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that's a great explanation. I found an online procedure for testing the focus here:

http://www.focustestchart.com/chart.html

and using that setup it is not quite so conclusive. My sigma 17-70mm focuses either on target or back. The Canon 50mm f1.4 does focus on target, but less than half the time. The rest of the time it is front focus.

I've written to Canon and they said I can send the camera in for inspection, but I really don't want to if I don't have to. I've asked them to send me the specs so I can test and decide for myself.

Does it really cost that much to have Canon calibrate the camera?
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Old Oct 19, 2008, 9:56 PM   #13
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Interesting thread. A bit confusing... the calibration and |-x--------0-----------| stuff. Definitely a conversation between experts. But I'll have a look at focustestchart.com.

Question: What is "front-focusing?" I'll Google it after I'm done writing here, but if anyone has a quick explanation...

I feel like I'm also having a focus problem with my XSi. But like cr6707, I have to wonder if it has more to do with me than it does with the camera. I'm only using the center-point AF, but I still feel like the images I was capturing with my S2 IS were much better (sharper, better color, overall better focus). In fact, I did a few test shots this afternoon, just portrait-type shots of my wife standing in the kitchen. I always zoom in to the pupils of a subject's eyes to judge how well the image is focused... In nearly all the test images I shot today, the S2 produced better results.

I'm really hoping that I don't need to become a Photoshop scientist to get good photos out of my new DSLR.


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Old Oct 19, 2008, 10:45 PM   #14
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Boldstar wrote:
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Interesting thread. A bit confusing... the calibration and |-x--------0-----------| stuff. Definitely a conversation between experts. But I'll have a look at focustestchart.com.

Question: What is "front-focusing?" I'll Google it after I'm done writing here, but if anyone has a quick explanation...

I feel like I'm also having a focus problem with my XSi. But like cr6707, I have to wonder if it has more to do with me than it does with the camera. I'm only using the center-point AF, but I still feel like the images I was capturing with my S2 IS were much better (sharper, better color, overall better focus). In fact, I did a few test shots this afternoon, just portrait-type shots of my wife standing in the kitchen. I always zoom in to the pupils of a subject's eyes to judge how well the image is focused... In nearly all the test images I shot today, the S2 produced better results.

I'm really hoping that I don't need to become a Photoshop scientist to get good photos out of my new DSLR.

It could be you or it could be the camera. The S2 is going to seem sharper, better color, and overall better focus because of a larger depth of field and much more processing in the camera with saturation and sharpening. The reason the XSi is lower on sharpening and color is because the photographer is supposed to do it in post production. You can try different settings and styles to get the type of shot you want, but with sharpening, it's best to do it last. If you do it in camera then it's done early on and to the entire photo instead of the area you wanted sharpening the most.
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Old Oct 19, 2008, 11:11 PM   #15
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Thank you for the advice KALEL33. I've heard that about the S2,the in-camera processing stuff, etc. And to a certain extent, I understand the rationale... DSLR users wanting more control.

But that leaves me a little fearful. While I have some experience with Photoshop, and I use it for the odd bit of doctoring, I'm no expert. And I certainly don't have time to become an expert. Coming home from a Caribbean vacation with 500+ photosto fiddle with, for example, would be a Photoshop nightmare.

I wonder how the average amateur or hobbyist photographer finds the time to learn and work with Photoshop...

All that being said, I still have trouble understanding why my XSi doesn't seem to focus as well as the S2. Focus is not something that can be fixed, at least not as far as I know.

Also, if anyone can tell me what "front focus" is, that might help me to understand the beginning of this thread.
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Old Oct 20, 2008, 2:26 AM   #16
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Look at the picture of the baby above.

The shooter was using center-point AF and focussed on the baby's eye/face. But if you look at the point of sharp focus it is on the toy/hand.

So the camera said it had a focus lock on the face, but actually the image came out with the plane of focus a few inches in front of where it was supposed to be. That is called front-focussing. It can go the other way too, with the camera focussing behind where it is supposed to, that would be back-focus.

Erwin Puts has a very technical site (he is a lens/optics expert) but this article is reasonably accessible and addresses the issues at hand:

http://www.imx.nl/photo/optics/optics/page62.html

Cameras and lenses are manufactured within tolerances. The cheaper the lens or camera, generally the larger the tolerances allowed.

One of the big differences between a £70 50mm f1.8 and a £950 50mm f1.2 is that the tolerances allowed in the factory for the more expensive lenses are lower, and quality control is higher.

When you move to £3,000 Leica 50mm lenses, the tolerances are tighter again, every lens is hand tested before it leaves the factory.

So your camera's AF has a band around the optimum result in which it is considered to be working within spec. Same with the lenses. So in general you should assume that only a small minority of lenses and cameras are working at exact calibration, the problem comes because sometimes the lens and camera, although individually within tolerance, can have errors that when added together make the image unnaceptable, as we see above. On the other hand, sometimes you get lucky and the errors compensate for each other and make things work out nicely.

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Old Oct 20, 2008, 8:04 AM   #17
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Thanks for the front-focus answer peripatetic. I understand now, and I'll have to read some more online so I can test the camera/lens myself. I sincerely hope that I don't have to mail my camera anywhere to get it recalibrated. I'd blow my stack.

One thing I still don't understand... If the cameras and lenses are calibrated within a range based on price, why does my S2 focus perfectly every time? It's only about half as expensive as my XSi.
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Old Oct 20, 2008, 2:23 PM   #18
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Two things:

Firstly the DOF on the S2 is MUCH bigger than on the DSLR.

Secondly it does not have interchangeable lenses, so the lens and camera get calibrated once and that's the end of the story.
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Old Oct 20, 2008, 2:33 PM   #19
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peripatetic wrote:
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Firstly the DOF on the S2 is MUCH bigger than on the DSLR.

Maybe I don't completely understand what that means. This is a combination of apature and the lens capability, right? And my apature-priority F numbers go up pretty high... higher than my S2 if I remember correctly. Shouldn't a high F-number bring everything into focus?


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Old Oct 20, 2008, 3:31 PM   #20
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Just to illustrate by picking some numbers based on my picture. I believe I shot that at f1.4. If I had changed my camera to f11, then the DOF would have expanded to make my boy within the focus range (and probably the backdrop, too).

So if you shoot your XSi photo at f22, then the DOF should be very large/deep.
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