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Old Nov 7, 2009, 12:50 AM   #1
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Default Focus issue with Canon XSi + Kit lenses

I've discussed in several places on this web site about my "Assumed Focus Problem" with my Canon XSi Camera and the kit lenses. Others that have responded to my posts have asked me to "Show me the Money", or in other words,... show me the images. After getting some education on how to do this from Hards80 I was successfull in getting the links to my uploaded images.

The quick story is that about a year ago I got my XSi just before Xmas. I had told all my kids and grandchildren all about it and made a long list of little extra things they could get me for Xmas. We had planned a very large family get together. When I got the camera and started shooting, my first subject of course where flowers, because that is what I do. I was shocked to see the images were not as good as my G7 or my older S3 IS. I started testing every which way I could with every focus chart I could find. I spend hours on the tech support line with Canon Photo experts.

My Xmas was not that good because I was very unhappy about the Camera and I wanted Canon to take it back. I even went as far as going down and testing a medium priced Nikon and every image I shot with that camera was great.

Canon asked me to send the camera back with all the lenses and I did. Three weeks later I got it all back with a very small note saying, "The camera and both lenses are within Specifications".

I've spend the rest of 2009 shooting half the time with my G7 and the rest with the XSi,.. trying to learn more about the Front Focus problem. At one time I even described it as a lack of ability to focus on a certain spectrum of colors, specifically the Magentas.

Recently I was sparked into new investigations by members of this web site, and I'm glad they did it. I've competed a new series of Focus testing and have confirmed, "The camera has excellent focus on all subjects that provide good to high contrast targets" However, If the target is any image that has poor contrast, I won't get good focus.

The point is I have learned that you can't put the single focus point (the little square at the center of the lens) totally on a low contrast target.
If you look at the first picture in the series I have posted I had set the target totally on the petals of the magenta vinca flower. These petals if you examine them very closely you will find THERE IS NO CONTRAST in these petal. It is not like the petal of a rose that has viens and some testure, these petals only refect a monochrome color of magenta. There is not even any contrast in the color, i.e., from light to dark or to shadows.

I learned when taking telephoto images of the moon, that you can't get focus on the moon where there are no craters. It's the same thing with this type flower, and many other flowers that have no contrast.

The Canon XSi is supposed to have a new "Phase Detection" Focus system, but I can't find where I read that or what it is or how it works. I do know that my older Canon G7 does a much better job at focusing on these low contrast images than my XSi.

You can go to my Flickr website to look at the images in 100% and I have loaded most all of them in the links below.
http://www.flickr.com/photos/trigger1937/

Unfortunatley all my comments for each of these images are still up on the flickr website. If you don't mind, just open up two windows and you can view all of the comments for each picture. If anyone feels they have a better understanding of why I get the Front Focus on certain images, I would really appreciate your knowledge.


This is the first flower that got me started.
































I hope all of these images have not caused a lot of problem on this web site, and I hope the members can provide some comments.
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Old Nov 7, 2009, 1:15 AM   #2
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Default Focus issue with Canon XSi + lenses

I guess this is another lesson learned... I thought that people would be able to access the EXIF data on each of these shots and know what the settings were and see the exact focus point,...just as I see everything in Canon's Zoombrowser EX software. I'm disappointed as these shots are not much more that pictures without the data.

Anyone one to add to my education.
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Old Nov 7, 2009, 8:58 AM   #3
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Flickr is probably stripping out the EXIF, as you can see some of the camera settings looking at image properties in your albums.

Check your focus settings. Flickr is showing Focus Mode as being AI Focus in some photos like this one (which means it can switch back and forth between single shot and AI servo). So, it may be detecting camera or subject movement and locking on something other than your intended target).

http://www.flickr.com/photos/trigger...82154080/meta/

Were you using a Tripod for tests? I'd use a tripod to test AF accuracy, switch to One Shot AF, make sure you've got a good lock with your selected focus point (with a half press of the shutter button, waiting for a good lock, before pressing it the rest of the way down).


Here's the menu you want for focus mode settings:

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Old Nov 7, 2009, 9:37 AM   #4
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P.S.

The EXIF properties on Flickr for this photo show that AI focus was being used, and that 4 out of 9 focus points were being used for that shot (making it more likely either Auto AF points were being used, or that the camera switched to AI servo using more points for tracking, because it saw movement and focused on something other than what you thought you locked on):

http://www.flickr.com/photos/trigger...82154080/meta/

See page 60 and 61 of this .pdf manual for how to set both Focus Mode (and I'd use One Shot for stationary subjects), and how to make sure that you (and not the camera) are selecting the focus point:

http://gdlp01.c-wss.com/gds/0300000933/EOSRXSi-EOS450D_EN.pdf',%20'EOSRXSi-EOS450D_EN.pdf
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Old Nov 7, 2009, 6:41 PM   #5
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yes, I came to the same conclusion as Jim. I imagine that the Ai Focus + multi focus point is the culprit here.

repeat these exact tests using One Shot and the center focus point and post them the same in this thread.

-Dustin
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Old Nov 7, 2009, 7:27 PM   #6
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Default Focus issue with Canon XSi + lenses

JimC,... Thanks for your quick reponse, and I assume you moved this post to the correct Canon DSLR topic.

All of the initial photo where shot within 1 month of getting the XSi and I had not learned very much about the Camera. About 6 months into shooting I got it straight and switched the settings back to Single Point. The last 5-6 images are shot that way. However, most of the images that show considerable front focus where shot with only the center point focus, even though some of them were shot under AIServo.

As my post has pointed out, I am convinced that if I can locate a point with good contrast, the camera will focus correctly. However, I also believe I have demonstrated just what happens to the AutoFocus on low contrast subjects. The focus "Quits" somewhere allways Front focused.

I don't know how you looked close at these images since I could not find any EXIF data anywhere, and the largest size of any image I uploaded turned out to be 1/8th of the true pixel count of my original.

Hards80,... look at the last 5-6 images, they were all shot correctly. Also, even though the window sill images where shot with AIServo, there is no movement in anything that would disturb focus. This is just Front focus due to low contrast image. I can and will repeat the test and upload the images.
How were you able to view them at 100% or original size????

Last edited by trigger1937; Nov 7, 2009 at 7:29 PM.
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Old Nov 8, 2009, 3:22 AM   #7
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Okay, so a couple of things.

AI Servo is "shutter priority" it will fire even if it doesn't get a lock. One-shot is "focus priority", if it cannot lock it will not fire.

For action work most (but by no means all) people find the "spray and pray" approach of AI Servo to be more productive.

Secondly you are absolutely correct, with low-contrast subjects camera AF struggles. One of the main things you pay for moving up the food-chain in the camera lines is to get better AF sensors, algorithms and processors. Combined with AI Servo and you can expect a fair number of OOF shots.

Thirdly viewing at 100% will show you a fair number of shots that don't look as sharp as you would like, but are still within manufacturers spec. The algorithms will quit as soon as they think the focus is "within accepted DOF" for the subject distance and aperture for that focal length. This is mostly for speed, if the algorithms try to get it exactly on the right plane they will often end up "hunting" which adds to motor wear and slows down performance. Now "standard accepted DOF" is by no means good enough for 100% viewing on screen, it's good enough for an 8x10 print, but that is much less demanding than 100% viewing on screen with modern DSLRs. If the picture looks sharp at 25% on screen, that's about what the manufacturer is aiming for.

You can reduce the number of OOF pictures you get by using one-shot, because the camera will just not fire if it can't lock. You can reduce the number by focussing on edges or high-contrast objects in the same plane as your subject and recomposing if you can't lock on the subject. Finally of course you can reduce the number by getting a more expensive camera. :-)

Or in other words you can get better results by improving your skills with the tools available or by buying more expensive tools. Though beware - the more expensive the tools get, the more complex they get and demand more and more skill from the operator for optimum results.
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Last edited by peripatetic; Nov 8, 2009 at 3:32 AM.
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Old Nov 8, 2009, 9:34 AM   #8
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trigger1937

Frankly, I think you're probably looking for camera defects that don't exist, especially since you've already sent the camera and lenses into Canon for a checkup. ;-)

The older images demonstrating any major issue were taken using AI focus. You can see the EXIF information if you click on a photo in your album, and then click on the "More Properties" link under the text describing the image on the right hand side of the page.

Also, just because you select One Shot focus, doesn't mean that you have only one focus point selected. It could be an error in the way flickr is interpreting your camera's EXIF. But, I'm seeing 4 active focus points in most of your One Shot images, too. Make sure that you (and not the camera) are selecting the focus point (as described on page 61 of the .pdf manual I linked to). I'd make sure to select the Center Point for the greatest sensitivity.

Also, when taking Macro photos, you'll have a very shallow Depth of Field. So, if you lean even a tiny bit after locking focus with a half press of the shutter button, your desired target may be out of focus. That's one reason many macro shooters always use a tripod (which is what I'd suggest you do if you want to test focus accuracy) ;-)

As for the comment that the "focus "Quits" somewhere allways Front focused", that's the idea if you don't have enough light and contrast to lock focus (so that you don't get an out of focus image). It's not "front focus" (as you described it) if it can't get a good lock. It wasn't able to complete locking focus. ;-) If you have sufficient light, you may also be too close to your subject if it can't lock (you have to take minimum focus distance of the lens into consideration).

IOW, you need to make sure you get a good focus lock with a half press of the shutter button (watching for the focus lock icon to light), making sure that *you* are selecting the focus point, not allowing the camera to pick it, noting the focus point(s) selected in your viewfinder (which will momentarily illuminate with a half press).

Then, press the shutter button down the rest of the way (making sure that the camera and subject don't move any in the interim after a half press, since you'll have a very shallow DOF when filling the frame with smaller subjects).

Another comment... if you're using any filters, I'd remove them and see if your results improve. Anytime you have a lot of reflections from light sources, it can cause both Image Quality and Focus issues, and cheaper UV filters are known for scattering light. If you're using a Polarizer in dimmer lighting, that will cause problems, too (because you can lose a couple of stops of light, depending on how the filter is rotated, which means only about 1/4 the light gets through, impacting the AF sensors' ability to "see" well enough to focus). Dimmer lenses are going to have problems in borderline lighting, even without any kind of filters, especially if you zoom in much (since most are down to a maximum available aperture of around f/5.6 when zoomed in).
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Old Nov 8, 2009, 9:50 AM   #9
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P.S.

Again, as I mentioned in another thread you started, this article by Petteri Sulonen makes a good read for someone struggling with their first dSLR:

Don't Be A Bozo

This the part I'd focus on (no pun intended). ;-)

Quote:
"The right-hand path is harder. It requires you to draw a deep breath, sit down, relax, and read the manual through. Then read the parts again that concern what you're trying to do. Then read those parts more time, this time trying out each of the things described in it. Then go out and shoot pictures with the camera, in the way described in the manual. And after this, if the pictures *still* come out orange, soft, dull, flat, blown, or blurred, there's just a possibility that you misunderstood something."
For example, it's not uncommon for a user to misinterpret things like how focus point selection works (and how to make sure it's locking on your intended target with a half press of the shutter button, using the selected focus point), which is why I'm suggesting you review pages 60 and 61 of the .pdf manual I linked to. Canon has already checked your Camera and Lenses for problems from what you're telling us. So, my guess is that you are doing something wrong (or are misunderstanding how the camera is supposed to work in the conditions you're using it in, with the settings you have selected).
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Old Nov 8, 2009, 10:27 AM   #10
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One more suggestion. Use the Clear Settings choice in your Setup menu to reset everything back to factory defaults, in case you've changed a custom setting impacting how the camera locks focus on a half press and/or changed it to release priority where it will fire without a lock (as it may do by default using AI focus anyway, as pointed out by peripatetic).

Then, change your Focus Mode to one shot, making sure that you are selecting the focus point (again see pages 60 and 61 of the manual), making sure you have a good focus lock with a half press (noting the focus point illuminated to make sure it's the one you want to use), and making sure you have a good focus lock (AF lock icon will light steady) before pressing the shutter button the rest of the way down to take the photo.
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