 Lynn R. Powers |
Depth of field when shooting with a digital camera the depth of field seems to be considerably greater at all lens fstops than with a film camera. Does the effective fstop increase at the same ratio as the 35mm effective focal length? For instance does a Canon 20D with a 2.8 (1.6x) lens give me an effective depth of field of a f3.5 while shooting wide open?
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Christopher A. Vedros |
No, not as I understand it. Take an image with a 50mm lens set at f/2.8 on a film camera (or full-frame digital SLR) and enlarge the image 1.6 times and crop it to a 4x6. Then take the same image with the same lens set at f/2.8 on a Canon 20D, and print it to a 4x6 without cropping. Your two images should be identical. The perspective and depth of field should be the same. In my experience, the increased DOF with digital applies to compact cameras, where the lens is physically smaller than lenses on an SLR. These cameras typically result in much greater depth of field at a given f/stop than you would get with an SLR camera. By the way, this is not really a digital phenomenon - the same has always been true with compact point & shoot 35mm film cameras. I'm not really sure how this applies to Canon EFS and other "digital SLR only" lenses. Since these lenses are constructed to have a smaller image circle, is the DOF at a given f/stop increased? I would have to research it some more. Has anyone looked into this? Chris A. Vedros
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Jon Close |
As Chris noted, the difference in depth of field has to do with the actual lens focal length. Use 50mm at f/2.8 and the same focus distance and one gets the same depth of field regardless of whether the lens is mounted to medium format, 35mm film, 1.6x DSLR, 110 film, or 1/2.5" compact digicam. But comparisons are rarely done that way. Usually the comparison is for the equivalent field of view and focus distance. So 85mm f/2.8 on medium format has less DoF than 50mm f/2.8 on 35mm film, has less DoF than 30mm f/2.8 on 1.6x DSLR, has less DoF than 10mm f/2.8 on 1/1.8" digicam, has less DoF than 8.5mm f/2.8 on a 1/2.5" digicam. Image circle diameter has no effect on depth of field..
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Alan N. Marcus |
Hi Lynn, Lens properties film camera vs. digital camera: The acceptable depth-of-field zone of a lens remains the same. The functioning f/stop remains the same. Stated another way, it makes no difference if the lens is being utilized on a film camera or a digital camera; the depth of field and functioning f/stop will be the same so long as: In both applications the lens is focused at the same object distance. As to the 1.6x factor: You might like to know that all lenses project a large circular image. This image is quite large and the edges are vignetted (reduced brightness) and fuzzy. Only the central portion is suitable for photographic purposes. Thus all cameras mask off this projected image. The inside dimensions of the mask are the image we know and love. Each format size and ratio has a lens focal length considered “normal”. If the lens is longer than normal, the image is said to be magnified (telephoto). If the lens is shorter, the image is of a reduced size (wide-angle). The formula used to determine normal is the diagonal measure of the mask. In the case of the 35mm camera this measurement is 43.3 mm. (camera makers round us this value to 50mm). In the case of your camera the diagonal is 27mm. Note that 27mm x 1.6 = 43.2. This is the way the 1.6 factor is derived. Image size is determined by focal length and so is depth-of-field. The longer the lens the greater the magnification (image size) and the shallower will be the depth-of-field. Conversely, a short lens produces a smaller image and the zone of depth-of-field is expanded. However, depth-of-filed is strictly a function the magnification used to view the image. If the image is reproduced large the depth-of-field diminishes. If viewed small, the depth-of-field zone appears to increase. I think you came to your conclusion based on the way you view your images. Prints on paper have far higher resolution than a computer monitor so I am advising that valid judgments can only be made by side-by-side comparisons, all images on the same media, all at the same magnification, all at the same lens settings etc. This response many not satisfy you as to why? Anyway, it will spark other inputs.
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Lynn R. Powers |
Thanks for your replies. I did make the specific tests that were recommended insuring that the digital lens was set to the same equivelant focal length as the film camera. Since I do not have a scanner the film was taken to the local lab , processed and enlarged to an 8x12. Then I had my digital image enlarged to the same size on the same processing machine. I used negative film and not slide film. Perhaps this makes a difference. At home I enlarged the digital image to an 8x12 on my Canon i9900 on glossy paper. No, it doesn't make any sense to me since the same f stop was used on both pictures. I'll just open up an extra stop when I want to blur the background more. Lynn Powers
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