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07/22/2005 02:04:39 PM · #1 |
I get a lot of comments about my images not being totally focused, or sharp.
I decided to try a focus test shot. A ruler (on a level surface) was shot with the camera on a tripod, using remote - so there is NO camera shake involved.
Sigma 28-105mm lens had lens set at 50mm - the mid range which usually seems best with this lens.
f5.6 1/500 ISO 100
the upper left hand corner (from 7 to 8 inches) seems out of focus. It should not have been as I had all 7 AF points selected and the camera/tripod was level.
What would cause this and how do I fix it?
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07/22/2005 02:25:48 PM · #2 |
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07/22/2005 02:32:49 PM · #3 |
I don't know but it looks like from the left side to 8" it looks like the ruler is not flat like on the wood like the rest of the ruler. Looking at the shadow of the ruler below the 7" & 8" mark seems thicker. Could the light made a difference if it was not level/flat like the rest of the ruler.
I also think the best way to tell is a straight down look at the ruler not at an angel. I think it needs to be at an exact perpendicular angle. Can you try a shot like that and see what it looks like?
Message edited by author 2005-07-22 14:34:40. |
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07/22/2005 02:40:50 PM · #4 |
It looks like the ruler was sitting on the same plane to the camera - so the aberrations you see are lens issues, NOT focus issues.
A focus test requires a object (a ruler or similar) with a spot to focus on (of sufficient size and definiation that the camer will focus on IT and not something else). You must use the CENTER POINT only for focusing. so you KNOW what you are focusing on. you must take 3 or 4 shots, forcing the camerato changeit's focus each time (you are looking for an average overall ability to focus.) You must know the DOF at the target distance and lens used, aperture, etc. (as distance, lens choise, focal lenght, aperture change so will DOF and perhaps the lens' ability to focus correctly). There must be enough light. camera and target must be still (indoors, no breeze, tripod, etc) Use a timer or cable release.
see
//www.photo.net/learn/focustest/
I have noticed with my rebel that focus is fine - but seems more unreliable in M mode...still watching the issue.
In the basic modes the camera will choose which AF point to focus on - it does pretty good, but can be wrong. THe point that has focus will be lit up. also, remember camera shake - the shutter speed shold be no less than 1/focal length (on the kit lens that would be 1/30 to 1/125 for 18-55mm allowing for the 1.6 crop factor - which does matter unless you are very steady)
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07/22/2005 02:49:57 PM · #5 |
I think canon has some spec that as long as the focus is within 1/3 the DOF it is deemed acceptable...i read it on a Canon site - canon eurpoe i think, but i cannot find it now. Anyway, see //dofmaster.com/dofjs.html to calculate the DOF for any lens at any distance, etc.
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07/22/2005 02:50:41 PM · #6 |
I'll check out the tutorial and do a re-test. Won't be for a few days though as I work 12 hour shifts through the weekend. (It's usually to busy at the hosital to do the test during work hours)
Thank you for clarifying how a test is done properly
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07/22/2005 03:30:06 PM · #7 |
Looking at some of your photos, I wonder whether I might be so bold as to offer a little input.
Regarding "Uninhibited", it seems that the camera has focussed on the tree trunk on the right and not on the moose. I noticed that, in your focus test, you had all seven focus points enabled. If you had all seven points enabled when you took this shot, it is most likely that the camera would choose the trunk to focus upon. This, indeed, looks sharp enough to me. To avoid a repetition of this particular problem, I would recommend setting the centre focus point only.
Regarding "Beyond Retired", I notice that, for one thing you used f3.5. This should give quite a short depth of field, hence the comments that the left of the tractor looks particularly soft. In addition, I downloaded the image and applied quite an aggressive hit of USM. That certainly made a difference. In your post processing details, you list: levels, resize, USM. If that is the order in which you carried out these actions, then perhaps applying a little more USM would help. If, on the other hand, you applied USM before resizing, you could perhaps apply USM to best effect after downsizing, as a little sharpness always seems to be lost doing that.
Finally, regarding your focus test itself: from the image you posted, the left hand side of the ruler doesn't look particularly 'soft' to me, rather it seems that the lighting is a little uneven and, hence, misleading.
In short, I don't believe that these three images convince me that there's a problem with your camera or lens, rather I believe that in these three images, there were three rather small problems that would be easy to correct.
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