Alley in Pisa, Italy

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If I put a Nikkor 35mm f/2 FX lens on a DX camera, is it still 35mm f/2, or has the effective focal length or minimum aperture changed, because it is being used on a smaller format than what it was designed for?

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And photo.stackexchange.com/questions/103 and photo.stackexchange.com/questions/139 , in different aspects. –  mattdm Jan 3 '11 at 1:53
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2 Answers 2

up vote 16 down vote accepted

Aperture is unaffected.

The field of view becomes 35mm x (the crop factor of your sensor), which is 1.5 in the case of Nikon DX cameras (It's also 1.5 for Pentax and Sony, 1.6 for Canon and 2 for Panasonic & Olympus).

So your 35mm Nikon lens has a 35mm x 1.5 = 52.5mm "35mm equivalent" field of view.

Note that the perspective doesn't change, just the amount of 'stuff' you can fit in the frame. The image will still look like a 35mm image as shot on a full-frame sensor, but with the sides cropped in to give you the field of view of a 52.5mm lens on a full-frame sensor.

(Most people round that to 50mm "35mm equivalent" field of view, regardless of 1.5 or 1.6 crop factor)

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Good explanation. I'll add another way of explaining it (for my own benefit :-) to help make it more clear? Another way of saying "just the amount of 'stuff' you can fit in the frame" changes is that the DX sensor takes the standard image you get on an FX sensor and crops for you. When you think about it that way, it becomes clear that if you were to print the (equivalent) DX and FX images on the same size paper, the DX image would look blown up because it's as though you cropped the FX image and stretched it out to the same paper size. –  Tom Jan 3 '11 at 3:38
    
Thanks, I appreciate the detailed explanation. –  PaulH Jan 3 '11 at 14:37
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The post by Scott is a nice summary, but it contains a common misunderstanding - that the perspective doesn't change, which it does. The angle of view of the lens is multiplied with ca 1.5 (or 1.6 if you use Canon, or 1.3 if you use an expensive Canon), which also changes the perspective as much as if you'd used a 52.2 mm lens.

I used a Nikkor 35 mm f2 extensivly as a 'normal' lens, and it was really nice. Then I bought a Nikon 35 mm f1.8 and it was even better.

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Perspective is a function of vantage point, not lens focal length or field of view. The confusion generally arises from the fact that people compare pictures taken with 35mm@FX to pictures taken with 35mm@DX and say that 'see, the perspective changes'. The trick is that they move around in order to fit the same amount of foreground to the frame, which in turn causes perspective change. –  Mart Oruaas Jan 3 '11 at 10:30
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The lens does not know which body its mounted on. All the optics are unchanged. What is cropped is the sensor, so the image is smaller, but if you are at the same place, taking the same photo, its exactly as if you had a FF sensor and cropped the image. –  Pat Farrell Dec 24 '12 at 18:59
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