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Forum: Pentax SLR Lens Discussion
12-14-2017, 06:04 AM
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Nice!
I'd bet that if you put the lens with the 32 mm disk on a 110 format camera, the image would have a very large blob fully obscuring the image circle of the Q (6.17 x 4.55 mm frame) but there would be some image in the edges and corners of the larger 110 format frame (13 mm × 17 mm).
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Forum: Pentax SLR Lens Discussion
12-13-2017, 04:31 PM
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There's one sure way to tell if a part of a lens is being used: create a opaque disk the size of the front element and punch a small hole in the part you think is not being used. If the result is a totally black image, then that part of the lens is not being used by the image circle of the camera. If there's any image at all, then that's the contribution of that part of the lens to the image.
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Forum: Pentax SLR Lens Discussion
12-13-2017, 02:47 PM
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That, in theory, a 200 mm f/2.8 absolutely must have at least a 71.4 diameter front element which leaves little room for mounting structure and lens barrel. In reality, such a lens needs a larger front element to avoid vignetting.
There's also the issue that the lens might say "200 f/2.8" but actually be 195 mm f/2.9 (and needing only a 67 mm front element).
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Forum: Pentax SLR Lens Discussion
12-09-2017, 11:10 AM
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Focal length and maximum aperture determine the minimum size of the front element because those two variables determine the diameter of the physical opening of the lens. No 300mm f/2.8 lens can have a front element smaller than 300/2.8 = 107 mm.
Yet often the front element must be larger than this minimum size to accommodate a stack of corrective optical elements in front of the aperture iris. Each of those elements is progressively larger the further it is from the aperture and the wider the angle of the lens. That is why high-quality wide angle lenses have such large front elements.
Generally speaking, high-quality, large aperture lenses have big front elements although that does not imply that all lenses with big front elements are high quality.
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