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Old Oct 16, 2002, 10:17 AM   #1
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Default DOF and closeup lenses

I have just bought a Minolta 7i and this weekend I set up some tests (Both inside with natural light and outside) to check the DOF. I set the camera lense at 50mm, Aperture Priority, and used all of the aperture settings from 3.5 to 9.5. I didn't find much difference in the DOF. Someone told me I could get a shallower DOF by using closeup lenses. Is this the best way or am I doing something wrong with my camera?
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Old Oct 16, 2002, 10:48 AM   #2
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The small size of the CCD, compared to the frame size of a 35mm camera, also effects the DOF. Even at f 3.5, the depth of field is much greater at any focal length than with a 35mm. I expect a close up lens would help. Using macro mode certainly reduces the depth of field.
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Old Oct 17, 2002, 2:39 AM   #3
Don
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Default Depth of field for the 7 series

Depth of field depends inversely upon the diameter of the lens opening, but don't forget that the smaller the "f" number the larger the opening. This is because the f number is the inverse ratio of the lens opening to the focal length - if you have a f1.0 lens then the apperature is the same as the focal length, at f22 the focal length is 22 times the apperature opening (diameter).

Depth of field also depends inversely on focal length. If you take a picture useing a 50 mm focal length the picture will have 4 times the depth of field of one taken with a 200 mm focal length lens FROM THE SAME POSITION.

If you check the specs of the 7 series you will find that the real focal lengths of the zoom lens are onely 7.2-50.8 mm. Coupled with the small CCD the angles of coverage are the same as those of a 28-200 mm lens on a 35 mm camera. Transport this lens to a 35 mm camera and you would get fantastic depth of field, but in combination with the small CCD you should get exactly the same depth of field as with a 35 mm camera and equilalent focal length lens.

The last factor which is important is distance from the subject. In the previous example you have four times the depth of field with a 50 mm focal lenght vs. a 200 mm focal length, but only if you take the picture from the same position. If you move into the subject until you cover the exact same image with your 50 mm focal length as you did with the 200 mm at a greater distance you will have exactly the same depth of field. In other words, the closer to the subject the less the depth of field for the same focal length.

Hope this helps.
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Old Oct 17, 2002, 12:16 PM   #4
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I agree with most of what Don says except that the D7 lens and CCD produce the same depth of fields as an equivalent 35 mm lens. Long experience with the tiny CCDs on digital camcorders has shown me this, where often dust specs on the filter are brought into focus. For an excellent eplanation of why your digital camera has DOF many times that of a 35mm, check out this site:

http://wrotniak.net/photo/dof/
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Old Oct 18, 2002, 8:18 AM   #5
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Thanks for the help everyone. I'm going to try a set of close up lenses this weekend.

Barry
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Old Oct 21, 2002, 12:29 AM   #6
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Default DOF of digital cameras

Charlibob, thanks for the reference to the depth-of-field article. I can agree with everything said except for one very important point. The lens manufacturers are producing incredible lenses but I wonder if they are truly capable of producing a circle of confusion not larger than 0.006 mm? That is the value needed in order for the Olympus C-30x0Z, according to the article, to match the resolution of a 35 mm camera (See the web page reference in charlibobís post for details).

:?: In order to truly answer the question of the DOF of any camera it is necessary to run resolution tests. The photo magazines used to do this for various new camera/lens combinations but I donít know if they still do this or have done this for any digital cameras. I may have a set of standard resolution charts the US Air Force used to publish, but if I do they are in a storage shed half way around the world from me. I return to the U$A in Feb. and Iíll check for them and run some tests then if I still have the charts. If anyone knows of published resolution tests please let us know. I, for one, would be very interested in the results.
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