samsdad
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gmwyatt wrote:
. . . plus your 50 mm acts like a 75 mm because you're shooting DX. . . .
Glen
That's not correct.
It's a frequently used bit of "advice" that is usually (mis)stated as, "A 50mm lens on a DX camera becomes a 75mm lens."
There is nothing that a camera can do to any lens to change its focal length. A 50mm lens on *any* camera is always a 50mm lens. A 35mm lens is always a 35mm lens. A 105mm lens is always a 105mm lens. All that changes is the angle of view that the *sensor* can record.
(It's assumed that any lens we're talking about can cover both formats. A FF 50mm lens can fully cover both FF and DX sensors. A 50mm DX-only lens has an image circle too small to fully illuminate a FF sensor, so it's out of the discussion for reasons having nothing to do with this explanation)
Using the image diagonal of a DX camera, vs the diagonal of a FF camera as a reference, a 50mm lens on a DX camera gives an angle of view about equal to that *of a 75mm lens on FF*.
Angle of view changes.
That's all.
Nothing changes about the focal length. Or anything about optical characteristics.
Nothing.
If you define an area in the center of the FF sensor exactly the same size as a DX sensor, that area will contain an image exactly the same as the one that would fall on a DX sensor. The FF sensor simply has additional image outside that area. It is able to capture the wider angle of view that the DX sensor can't. So the lens is more of a "wide angle lens on the FF than it is on the DX. Or stated the other way, the same lens has a more narrow angle of view on the DX than it has on the FF. But it's not a 75mm lens on the DX.
The confusion happens when the DX image and FF images get enlarged to the same physical size. Imagine both images in "landscape orientation" being displayed or printed so that their short dimension is 10" (a monitor or a print). The FF image will be enlarged 10X and the "wider image area" that it contains will fill the 10" dimension, leaving its central "DX sized portion," smaller.
The DX image will be enlarged 15X, filling the 10" dimension and will appear to be a "crop" of the FF image and will look larger (as though it was taken with a longer lens).
The FF image could just as easily be enlarged 15X, loosing the "extra field of view" beyond the 10" dimension, resulting in an identical image.
Compare the image from the DX sensor, to the image from the "DX-sensor-size-area-of-the-FF-sensor" and all optical attributes will be exactly the same. For instance, DoF. If the pixel density of both sensors are the same, the two images will be identical. If the pixel densities are different, that factor may be noticeable, but all optical characteristics of the images will be identical.
Rich
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