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Old Mar 28, 2008, 12:38 PM   #1
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I was taught to compose pictures such that the subject matter more or less fills the frame. Does this make sense when taking macro photos where DOF is often an issue?
Assume you have two APS-C digital cameras. One has 3 MP and the other has 12 MP.
Assume pictures are viewed on a 2 MP display (Computer / HDTV) so 3 MP is sufficient.
Assume pixel noise is non-existant. (ISO 100 - full sunlight or flash)
Assume an excellent macro lens which can make full use of the 12 MP sensor.
Assume you take one photo with the 3 MP camera with subject filling the frame.
Assume you take another photo with the 12 MP camera with the subject only filling 1/4 (area) of the frame. (by doubling the subject distance compared to the previous photo). You then crop this photo to 3 MP.

Would the second (cropped) photo not have more DOF since the subject distance was twice that of the photo taken with the 3 MP camera? Am I missing something here?

Maybe I am just trying to justify getting a 12 MP camera :-)
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Old Mar 28, 2008, 2:04 PM   #2
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hingarfi wrote:
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Would the second (cropped) photo not have more DOF since the subject distance was twice that of the photo taken with the 3 MP camera? Am I missing something here?
Yes.

How to set your aperture for greater depth of field (use a smaller aperture, represented by a higher f/stop number) for any given focus distance, so that you wouldn't need to shoot from further away to get better depth of field.

If you use your approach (shooting from further away and cropping), you have fewer pixels representing your subject, with lower detail. If you need more detail for the print sizes you want, I'd get a higher resolution camera and still shoot from the desired subject distance, versus relying on cropping to get what I want.


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Old Mar 28, 2008, 5:57 PM   #3
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JimC wrote
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How to set your aperture for greater depth of field (use a smaller aperture, represented by a higher f/stop number) for any given focus distance, so that you wouldn't need to shoot from further away to get better depth of field.

If you use your approach (shooting from further away and cropping), you have fewer pixels representing your subject, with lower detail. If you need more detail for the print sizes you want, I'd get a higher resolution camera and still shoot from the desired subject distance, versus relying on cropping to get what I want.
Thanks jimC:

Glad you agree with my theory that the DOF is improved by the 12 MP camera shooting at twice the distance.
Regarding appropriate aperture, I had planned in my original post to add:
"Assume f-stop of 11 or 16 to avoid diffraction effects while achieving good DOF".
I removed this assumption since it added nothing to my scenario. At f16, I regularly would like much better DOF.

Regarding the photo detail, both scenarios achieve 3 MP so I do not believe any detail is lost. As I mentioned, I will be typically viewing these images on a 2 MP display (as opposed to large print size)
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Old Mar 28, 2008, 8:30 PM   #4
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It isn't clear to me you would get an improvement. When calculating the depth of field, you would have to use a circle of confussion half the size since you are using half the sensor (linear measurement - a quarter of the pixels).

But the real answer is . "Try it and find out". You don't need two cameras: pretty much all digicams allow different resolution settings so you can effectively reduce the resolution by close enough to a factor of four to test. You could go do the calculations, but I favor simple experiments instead. The experiments often end by showing that as a practical matter there is no difference between two techniques.

In this case, if there is no noticable difference in DoF shooting from further back is probably the way to go. Then you are less likely to disturb the bee on the flower, or the ant on the stem, or the elf under the mushroom.
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Old Mar 28, 2008, 10:58 PM   #5
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Bill:

You are correct suggesting the trial runs. I just wanted to see if someone would give an answer indicating my theory is NOT true. So far no one has definitely shot me down so I have a 7D and can shoot at 1.5 MP and 6 MP and will give it a try.
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Old Mar 29, 2008, 1:49 PM   #6
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Bill:
Took your advise and ran some tests. Using my Sigma 105 macro lens on my 7D, I photographed a stainless steel cm scale at 45 degrees to the lens axis. I manually focused on the 25 cm vertical bar (using a right angle viewer on 2.5 magnification)With the camera horizontal resolution at 1500 pixels, I framed the 23-27 cm range on the scale and took shots at f4, f8, f16. Then with the camera horizontal resolution at 3000 pixels, I framed the 21-29 cm range (while still focusing on the 25 cm bar) and took 3 more shots at f4, f8, 16. In all cases, I used 2 sec delay MLU and had SSS turned off. I used ISO 200 and used shaded daylight for illumination. Camera and target were both resting on a sturdy table.
Assuming that I only need A MINIUM NUMBER OF PIXELS TO VIEW ON MY 2 MP DISPLAYS, the images taken at twice the distance and then cropped to 1500 pixels horizontally were equal in IQ and far superior in DOF to those taken at the closer distance filling the frame. I am convinced that all my macro shots (where DOF is an issue) will be shot from a greater distance from subject and then cropped for display. Of course, if one were planning on making large prints and needed lots of pixels,and if DOF were not an issue, one would frame traditionally. Where DOF is an issue and lots of pixels are required, a compromise seems prudent.
If anyone is interested in seeing these test images, send me an email and I will email them to you.
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Old Apr 2, 2008, 8:09 AM   #7
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Jim is almost always right, but it is always a bad idea to take anyone's advice/answer without testing when a simple test is possible. There are so many ways for a typo or misunderstanding to lead to a wrong answer that everyone's response should be treated with a bit of doubt about it.

Forums like these are self-correcting to a fair exent. If some advice is given that is flat out wrong, normally there will be a quick shower of other responses contradicting it. But don't count on that happening.
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