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<blockquote data-quote="Bob Blaylock" data-source="post: 359336" data-attributes="member: 16749">The actual focal length will not change.&nbsp; What will change is the field of view, due to the fact that where the lens focuses the image, there is a smaller sensor, capturing a smaller part of the focused image.&nbsp; You've probably come to be conditions to think of a 50mm lens as “normal”, anything much shorter than that as “wide angle”, an anything much longer than that as “telephoto”.&nbsp; These characteristics, though, are not absolute to the focal length of the lens itself, but also to the size of the sensor or film that is capturing the image.&nbsp; It has always been the case, even back in the stone-aged film days, that the size of the film or sensor is relevant to the field of view, and the focal length that it takes to achieve a particular view.&nbsp; Imagine, for example, an 8×10 inch view camera, equipped with a 380mm lens.&nbsp; This would be a “normal lens” for that camera, providing a field of view that approximates a human eye.&nbsp; It would be equivalent to about a 50mm lens on a 35mm camera.&nbsp; Of course, if you mount that same lens on a 35mm camera, then it's a rather long telephoto lens.&nbsp; The lens hasn't changed; just the fact that there's a much smaller piece of film capturing a much smaller portion of the focused image.&nbsp; So, if you mount a 70-200mm zoom on your DX camera, it doesn't “become” a 105-300mm lens.&nbsp; It's still 70-200mm.&nbsp; However, on your DX camera, it will give a field of view that is comparable to what a 105-300mm would give on an FX camera or a 35mm film camera.</blockquote>

[QUOTE="Bob Blaylock, post: 359336, member: 16749"] The actual focal length will not change. What will change is the field of view, due to the fact that where the lens focuses the image, there is a smaller sensor, capturing a smaller part of the focused image. You've probably come to be conditions to think of a 50mm lens as “normal”, anything much shorter than that as “wide angle”, an anything much longer than that as “telephoto”. These characteristics, though, are not absolute to the focal length of the lens itself, but also to the size of the sensor or film that is capturing the image. It has always been the case, even back in the stone-aged film days, that the size of the film or sensor is relevant to the field of view, and the focal length that it takes to achieve a particular view. Imagine, for example, an 8×10 inch view camera, equipped with a 380mm lens. This would be a “normal lens” for that camera, providing a field of view that approximates a human eye. It would be equivalent to about a 50mm lens on a 35mm camera. Of course, if you mount that same lens on a 35mm camera, then it's a rather long telephoto lens. The lens hasn't changed; just the fact that there's a much smaller piece of film capturing a much smaller portion of the focused image. So, if you mount a 70-200mm zoom on your DX camera, it doesn't “become” a 105-300mm lens. It's still 70-200mm. However, on your DX camera, it will give a field of view that is comparable to what a 105-300mm would give on an FX camera or a 35mm film camera. [/QUOTE]

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