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My friend has a Nikon 35mm DX lens that I want to use on my FX camera. If I use his lens on my camera in DX mode, does the effective focal length of the lens become 35mm * 1.5 = 52.5mm, or does it just take a crop of the center with an effective focal length of 35mm?

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2 Answers

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Nikon does this thing where when you mount a DX lens it (automatically if the Auto DX Crop option is enabled) uses a reduced portion of the sensor — the "DX mode" mentioned in the question. This addresses the image-circle issues raised in the other answers here.

Both Sony and Nikon offer this sort of compatibility mode, but because of physical limitations, you can't even mount the crop-factor Canon lenses on full-frame cameras. (And Pentax doesn't have a full-frame offering.)

Anyway, the lens is a 35mm lens and it always stays a 35mm lens regardless of sensor format, but the field of view on a DX (APS-C) camera is the same as the field of view through a 52.5mm (1.5×) lens on a FX camera. So good so far.

When you use DX mode on your FX camera, the edges of the sensor are ignored, just as if they were cropped away. (In a way, this is exactly the same as it is on a DX-format camera, except on those it's permanently away.) Since the sensor size is now for all intents and purposes the same as in a DX camera, this gives the same narrower field of view, again equivalent to an FX-format 52.5mm lens across the whole FX sensor.

If you use an FX-format 35mm lens to take a picture, and then crop out the middle 2/3rds (that is, 1/1.5 — the crop factor!), the field of view of the resulting crop is that same 52.5mm. Or, if you force DX mode on with that lens mounted, same thing. No magic is happening — you're just discarding the edges of the field of view.

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Does this mean that, for example, if I take a wide angle lens and crop out a very small portion of it in the center, it will have the same field of view as a telephoto lens, but not the same perspective? – Daniel T. Jan 28 '11 at 21:21
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Unless you physically move, that center portion will even have the same perspective! Zooming and cropping are practically interchangeable, except that of course cropping loses resolution. This is how the "digital zoom" offered by many point & shoot cameras works — they just crop. – mattdm Jan 28 '11 at 21:25
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Different focal lengths give different perspective simply because to make an object the same size you have to stand at a different place. To fill the frame with an object with a wide angle lens, you have to be very close, emphasizing the effect of perspective. – mattdm Jan 28 '11 at 21:28
So does this mean that, assuming you only need a 640x480 image for web use and you have a camera and lens with sufficient megapixels and resolving power, you can potentially turn a 50mm into a 200mm simply by cropping the center? – Daniel T. Jan 28 '11 at 21:46
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Perspective is dictated by camera-subject distance, and only by camera-subject distance. Wide-angle lenses invite you to move closer to the subject, tele lenses invite you to move further back. This movement will affect perspective but is not something that is inherent to the focal lengths as such, it is an indirect effect. – Staale S Jan 29 '11 at 0:38
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First off, focal length is focal length is focal length. The "crop factor" is totally unrelated to focal length. It's just been confusing to us newcomers since DSLRs (well..digital cameras in general) hit the market with a myriad of different sensor sizes.

You have to realize that the 1.5 crop is just just cropping the middle out of what would otherwise be an larger image circle. A DX lens just makes a smaller image circle. On an FX sized sensor, there will be heavy vignetting where the image circle doesn't fully cover the sensor. You can crop this image out, but probably (though not always) at much lower resolution that it would be on just a DX sensor.

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