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<blockquote data-quote="BF Hammer" data-source="post: 844629" data-attributes="member: 48483">The answer is that it is 100mm for both size sensors. A DX format lens is projecting a smaller circle of light to cover a smaller sensor, but the physical size of the image is still for 100mm focal length. FX 100mm lens projects the same size image but the circle of light is larger.Micro Four Thirds lenses also work like this, but the crop factor is 2. You buy a 100mm MFT lens, you get a photo equivalent to 200mm full frame in a more compact package.This confused me also years ago when I would shop for lenses. But imagining the lens projecting on a wall with 2 different sized screens is how I learned to resolve that. The size of the subject is identical on the wall, but you are only seeing a limited part of the image framed on those screens. Then you can imagine an even larger screen for medium format, and a very tiny screen for a cellphone camera. But that lens is still the same focal length in each case as it never moved.</blockquote>

[QUOTE="BF Hammer, post: 844629, member: 48483"] The answer is that it is 100mm for both size sensors. A DX format lens is projecting a smaller circle of light to cover a smaller sensor, but the physical size of the image is still for 100mm focal length. FX 100mm lens projects the same size image but the circle of light is larger. Micro Four Thirds lenses also work like this, but the crop factor is 2. You buy a 100mm MFT lens, you get a photo equivalent to 200mm full frame in a more compact package. This confused me also years ago when I would shop for lenses. But imagining the lens projecting on a wall with 2 different sized screens is how I learned to resolve that. The size of the subject is identical on the wall, but you are only seeing a limited part of the image framed on those screens. Then you can imagine an even larger screen for medium format, and a very tiny screen for a cellphone camera. But that lens is still the same focal length in each case as it never moved. [/QUOTE]

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