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01/11/2006 01:03:06 AM · #1 |
As a newbie to DSLR I'm a little confused about comments I've heard regarding effective apertures with DSLR being different to the same aperture in 35mm SLR. Is this the case? For example, will f/4.5 in a 35mm SLR produce a shallower DOF than f/4.5 with a DSLR? (Or am I just a complete gumby, who has got it all wrong?)
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01/11/2006 01:09:53 AM · #2 |
Originally posted by AdrianQ:
As a newbie to DSLR I'm a little confused about comments I've heard regarding effective apertures with DSLR being different to the same aperture in 35mm SLR. Is this the case? For example, will f/4.5 in a 35mm SLR produce a shallower DOF than f/4.5 with a DSLR? (Or am I just a complete gumby, who has got it all wrong?)
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I think you may be thinking of effective focal length, where most DSLRs have a focal length magnified over that of a film SLR. |
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01/11/2006 01:26:05 AM · #3 |
F/stop is the ratio between physical diameter of the aperture and the focal length of the lens. A 25mm aperture on a 50mm lens is f/2.0 ΓΆ€” the same 25mm aperture on a 100mm lens is f/4.0...
DOF is dependent on the physical size of the aperture, not the f/stop; a wide angle lens has more DOF at f/8 than a telephoto does, and this is true because the physical size of the aperture is smaller.
You dSLR has a smaller sensor than a piece of 35mm film. I'm not sure what the conversion factor is, so I'll use my Canon 20D as an example; the Canon has a 1.6 "crop factor". This means that my 10mm lens on the canon is the equivalent of a 16mm lens on a 35mm full frame camera. THIS means that the f/8.0 on my ultra-wide is a smaller physical aperture than the f/8.0 on your ultra-wide of the same angular coverage on a 35mm film SLR.
So, yes: for the same angular coverage you get more DOF at a given f/stop from a cropped-sensor dSLR than from a film dSLR. HOWEVER:
If you use the same lens interchangeably between the two cameras (it's done all the time) the DOF at a given f/stop remains the same; all that changes is that on the dSLR you getting a cropped portion of the image circle.
Hope this helps.
Robt. |
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01/11/2006 01:53:47 AM · #4 |
thanks very much for such a detailed answer Robert. i figured it had something to do with the difference between sensor size/35mm in the same way that my 200mm zoom is worth around 300mm in 35mm equivalent. i also appreciated your refresher course on aperture vs. f/stop. i hadn't given this any conscious consideration for quite some time.
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