 Roy Blinston |
Apertures on most Digital cameras? Why is it that most "fixed lens" Digital Cameras (not SLR's with interchangeable lenses) tend to have a narrow Aperture range (ie: 2.8 to f5... and not the full range like a normal lens... ie: f.28 to f16 or more). See examples of specs from a Samsung Camera I have just browsed...
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Justin G. |
I'm not exactly sure but I'm going to take a stab with other information that I know (or think I understand). If i'm correct the hole's diamter on a lens determines the aperture as compared to the focal length. for example a 50mm lens at f/2 would have a 25mm opening. 50mm / f.2 = 25mm. so in order to have small apertures you need to have very small openings. a 50mm lens at f/22 would have an opening of 2.27mm. well most fixed lens P&S digis have very short focal lengths like 8mm - 17mm is close to the one I own. so at 8mm f/8 would be an opening of 1mm. if you could imaging having f/22 on an 8mm lens it would be a 0.36mm opening, probably nearly impossible to get the aperture curtains to go that small and be accurate so I hope this helps and I hope i'm right. justin
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Roy Blinston |
Thanks for your effort, unfortunately I am still confused with all the numbers etc. If they can make a removable lens to have an aperture range of say f1.4 to say f16 or more, then why don't they have this same range on fixed lens Digital cameras. Something must be stopping this happen as it is very common on digital cameras. I have a Fuji Finepix 7000 (aperture range of f2.8 to f8) which just about covers most needs - but not all (obviously). Some newer models have a range much less than this (though the cameras look interesting to buy). Is this one of the benefits of having a Digital SLR with normal aperture range on your lens?
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Justin G. |
basically what I was saying that with a 50mm lens @ f/22 you have to have a hole 2.27mm wide. as you know 2 milimeters is VERY small so with this in mind.. an 8mm lens (lens on the P&S digi) an f-stop of 22 would require a opening of .36mm wide, which is probably impossible.
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robert G. Fately |
Roy, Justin's response is okay as far as it goes, so just to add some thoughts: Justin's calculation of f-stop is correct, it is essentially the focal length of the lens divided by maximum diameter of the aperture (in the same unit of measure, like mmm). The lower limit starts to hit when the aperture must be very small - you start to get light diffraction effects. To get f32 on a lens of focal length you'd need a 1/4MM aperture - and that is both small enough for diffraction to set in as well as mechanically very difficult to execute. At the fast end, the main issue that crops up is distortion. WHile you've obviously seen 50mm f1.4 lenses, you may not know that there are some f1.2 lenses (and for a while years ago Leitz made an f0.9 lens!). The thing is, these bad boys are very expensive. So, while technically they could be produced, the question becomes would you have purchased that Finepix if it cost $1500 and weighed almost twice as much as it does now? And even if you said yes - how many others would feel similarly? So, from a marketing perspective, manufacturers don't see high speed lenses as being viable.
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Roy Blinston |
The lenses I am referring to look like 50mm lenses (same size etc) not those little lenses you get on compact digital cameras. My Fuji fixed lens is f2.8 to f8 and is the same physical size as a removale 50mm lens... therefore why doesn't the aperture range go down to f11, f16 and even f32. I can understand it with a tiny lens (the opening would be infinitely small).
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robert G. Fately |
I'm not sure what lens you are talking about, Roy - if you mean one of the mega-zoom lenses (like the Panasonic Lumix FZ20 with 10x zoom) that externally look to be about the same size as a 50MM SLR lens, then again, to be fair you'd have to comare it to a 10X zoom for an SLR (like an 18-200 or something), which is somewhat bigger. In fact, the 18-200 lenses available for SLRs are 1-2 stops slower than the built-in digizoom (the Lumix is f2.8 and the Nikon 18-200 is f3.5-5.6. Again, more speed requires more glass, better glass, and more optical correction - all of which add up to more cost, which don't make sense on a relatively inexpensive digital. As for giving the lens smaller apertures - the formula is the focal length divided by the aperture. Your Fuji (and my Lumix) lens is not really a 38-380 (or whatever) - that's it's equivalent in terms of full frame 35MM film. The smaller the imager size, the shorter the focal length required to achieve a given angle of view (which is the other way to measure "wide-angle-ness" or "telephoto-ness"). This is why on a 35mm camera a 50MM lens is "normal", while for a 6x6 medium format camera aan 80MM lens is "normal" and on a 4x5" camera a150MM lens is "normal". Going the other direction, for a format the size of a small postage stamp, a 30MM lens could be "normal" (even though 30MM is wide angle for 35MM film, ultra-wide fo medium format, and impossible to achieve for 4x5). So, in real optical terms, that Fuji lens might be an 8-80MM - I don't know exactly - and if you divide that by 22 or 32 you start to see how tiny the hole would have to be.
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