 Robert A. Staub, Jr |
Lens fstops how do you know if a lens fstop, say f/2.8 is f/2.8 throughout the focal length?
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Alan N. Marcus |
Hi Rob, I think I understand where you are going and what you are asking. I have been wrong a thousand times however: The aperture settings engraved on the lens barrel (f/numbers) are calibrated to be correct only when the lens is focused at the infinity position. When you focus on objects closer than infinity, an error is induced. The result is, the f/numbers are no longer correct. Most cameras permit close focusing until the error reaches 1/3 f/stop. A macro lens is designed to counter this error known as “bellows factor”. As you focus a macro lens, the working aperture (iris diaphragm) is geared to the focusing mechanism and the aperture is caused to open up and compensate for “bellows factor”. The formula is BF=M+1². BF=bellow factor – M = magnification. The error is a four (4) fold decrease in image brilliance at unity which is magnification one (1) or life size. M = 1 at unity thus (1+1)² =4. This value is handled just like a filter factor i.e. 4x reduction in brilliance = 2 f/stops as each f/stop = a 2x change in light. A zoom lens by definition changes focal length as you zoom. A 2 time (2x) change in focal length (say 50mm to 100mm) results is a change in image brightness at the image plane by a factor of 4 (4x or 2 f/stops). No need to worry. The zoom lens aperture (iris diaphragm) is geared to the zoom mechanism. The gearing adjusts the effective aperture ensuring that the image brightness at the image plane remains constant through the zoom. Additionally, all modern cameras now meter through the lens and the light sensors are positioned to read light levels at the image plane. This instrumentation corrects exposure using chip logic. So, there is no need for you to worry about these technical details. Best regards,
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anonymous A. |
If your question relates to knowing what your lens specs mean so that you know if the maximum quoted f stop is available throughout the zoom or focal length range, you will see the min/max f stop quoted as either 2 single values or as a single value and a range. Actually, many lens makers have given up quoting the minimum f stop, which is a pity. Anyway, if the max aperture range is quoted as f/22 to f/3.5 then f3.5 is available throughout the focal length range. If however it is quoted as f/22 to f/3.5-5.6 (two values) then the larger value (representing a smaller aperture) is the biggest opening available at the longest focal length of the lens
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Christopher A. Vedros |
Rob - Alan gave you the science, here's the simple answer: Lenses are almost always described by their focal length and maximum aperture (minimum f/stop). Example: 300mm f/2.8 (a lens that I want, but can't afford) Most zoom lenses have an aperture that varies with the focal length, so two max apertures will be given. Example: 28-300mm f/3.5-6.3 (at 300mm, your max aperture is f/6.3) Higher-end zooms (pronounced "more expensive") sometimes have a constant max aperture throughout the zoom range. In this case, only one max aperture will be listed. Example: 70-200mm f/2.8 If you're ever in doubt about a specific lens, just ask. Chris A. Vedros
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Robert A. Staub, Jr |
Thanks for the thorough responses. As you all may have noticed through some of my posts, I am researching new lenses, and I want to make sure that I get a lens that maintains the aperture through the focal length if I get a zoom. I am beginning to understand. Is there any merit to renting a lens from Zip lens to see if the "high end" lens that I want is truly what I need? Do you have any experience with these rental places? Thanks again!!
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Eddie Lagos |
So, Alan in other words the polarity of -2(h + i) = v minus 5> B+L then take 4/3 32%
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Alan N. Marcus |
Hi Eddie L, "I think I should understand that better, Alice said very politely, if I had it writer down: but I can’t quite follow it as you say it” Alice In Wonderland by Lewis Carroll
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