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40D Focusing (1 Viewer)
- Thread starter tony.saw
- Start date
You can check the focus by taking shots of a static subject with the camera on a tripod and use MLU, however if you are happy with the results in real world photography then I don't see the point in doing this. You cannot adjust the focus on the 40D but if you feel your set up is a long way out then you can send it to Canon to calibrate the focus.
tjsimonsen
Well-known member
When I got my 40D I wasn't quite happy with the results. So I sat a ruler up at a 45 degree horizontal angle to the lens (a 100-400L), focused on a specific point and took five pics at f.5.6 and five at f8, both at 400mm and using a tripod, from approximately five meters. When I analyzed the results it became clear that there was considerable back-focus. So I repeated the process with my 100/2.8 macro (from a shorter distance) with the same result. I had the camera calibrated, and now I'm 100% happy with the focus of the 40D.
Thomas
Thomas
tdodd
Just call me Tim
Angled ruler tests are not reliable. The focus sensors are a lot larger than the focus squares in the viewfinder and you have no way of knowing which part of the sensor focused on which part of the ruler.
You need a focus target that is parallel with the sensor. By all means use an angled ruler as a scale to judge whether focus is correct, behind or in front of the target. The LensAlign tool is, in my opinion, vastly overpriced but it does present a foolproof test target.
If you can rig up something equivalent to that then you should be able to perform a reliable test. Here's a nice easy example, attached. I only set this up really quickly and it looks like I have some front focus here (the 40cm mark should be sharpest and I think I'm just a fraction forward of that), so I might need to get busy with some more thorough testing and possibly an adjustment, but hopefully you can see the principle. I focused on the game case, which is clearly parallel to the sensor, not angled at all (or not significantly at least) and the ruler provided a perfect scale from which to judge whereabouts the focus is actually ending up.
Note that the test should be done with the lens wide open and, for a zoom, at the long end of the focal length range. Ideally, for a general purpose test/calibration it is recommended that you set the focus target at a distance at least 50X greater than the focal length. e.g. for a 400mm focal length the focus target should be at least 20m away. If you routinely shoot at some other distance then by all means test and optimise your focus for your preferred distance. My example is a bit of a cheat because I used an 85mm lens but at only ~2m instead of 50X85mm = 4.25m. You should also test in good light - it is not helpful if the AF is struggling. Your test is to make sure that distances are set correctly, not whether the AF can work in miserable conditions.
What you should also note about my example setup is that the game case - my focus target - is huge, relative to the focus square and there can be no possibility of the AF picking the wrong thing. The target also has good contrast, thus allowing the AF perform with confidence, not hesitation.
It is also recommended that you focus, defocus and refocus at least three times in order to establish a reliable pattern and remove random quirks from the test results.
More info from Chuck Westfall on this page - http://www.digitaljournalist.org/issue0812/tech-tips.html.
You need a focus target that is parallel with the sensor. By all means use an angled ruler as a scale to judge whether focus is correct, behind or in front of the target. The LensAlign tool is, in my opinion, vastly overpriced but it does present a foolproof test target.
If you can rig up something equivalent to that then you should be able to perform a reliable test. Here's a nice easy example, attached. I only set this up really quickly and it looks like I have some front focus here (the 40cm mark should be sharpest and I think I'm just a fraction forward of that), so I might need to get busy with some more thorough testing and possibly an adjustment, but hopefully you can see the principle. I focused on the game case, which is clearly parallel to the sensor, not angled at all (or not significantly at least) and the ruler provided a perfect scale from which to judge whereabouts the focus is actually ending up.
Note that the test should be done with the lens wide open and, for a zoom, at the long end of the focal length range. Ideally, for a general purpose test/calibration it is recommended that you set the focus target at a distance at least 50X greater than the focal length. e.g. for a 400mm focal length the focus target should be at least 20m away. If you routinely shoot at some other distance then by all means test and optimise your focus for your preferred distance. My example is a bit of a cheat because I used an 85mm lens but at only ~2m instead of 50X85mm = 4.25m. You should also test in good light - it is not helpful if the AF is struggling. Your test is to make sure that distances are set correctly, not whether the AF can work in miserable conditions.
What you should also note about my example setup is that the game case - my focus target - is huge, relative to the focus square and there can be no possibility of the AF picking the wrong thing. The target also has good contrast, thus allowing the AF perform with confidence, not hesitation.
It is also recommended that you focus, defocus and refocus at least three times in order to establish a reliable pattern and remove random quirks from the test results.
More info from Chuck Westfall on this page - http://www.digitaljournalist.org/issue0812/tech-tips.html.
Attachments
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tdodd
Just call me Tim
Actually, I was so chuffed with that prototype above that I scaled it up for use in the garden with my 100-400. You can see that the focus target is massively larger than the focus point indicator and leaves no room for error. The rule clearly demonstrates the DOF zone. Here is the resulting image with a microadjustment of +8....
Focus should be sharpest at the 90cm mark, or around 35.5". It seems very close to that to me...
(Actually, to be really pedantic, as the top of the box is visible, it means the box is leaning towards me slightly, which means that my focus target is in fact positioned a little forward of the 90cm mark, maybe by 5mm or so, but that sits well with what appears to be a very small amount of front focus with this microadjustment setting.)
EDIT : Attachment 2 is the result of an equivalent test with my 40D. It looks pretty good to me, fortunately.
EDIT : The first two attachments are screenprints at 50% magnification. Here is a better view of the whole test rig at the far end of the garden - see attachment 3.
Focus should be sharpest at the 90cm mark, or around 35.5". It seems very close to that to me...
(Actually, to be really pedantic, as the top of the box is visible, it means the box is leaning towards me slightly, which means that my focus target is in fact positioned a little forward of the 90cm mark, maybe by 5mm or so, but that sits well with what appears to be a very small amount of front focus with this microadjustment setting.)
EDIT : Attachment 2 is the result of an equivalent test with my 40D. It looks pretty good to me, fortunately.
EDIT : The first two attachments are screenprints at 50% magnification. Here is a better view of the whole test rig at the far end of the garden - see attachment 3.
Attachments
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