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Since I am considering to buy the Nikkor 105mm micro VR, I wondered if the cropped sensor does affect the closest focusing distance of 0.31m?

Thinking about this, the following additional (and theoretical) question came up: I have a Nikon D7000 which has a 1.5 crop factor, so for my understanding the focal length multiplies with 1.5. Lets say a lens at 100mm results in 150mm focal length. In the EXIF data of the picture however the value of 100 is saved. Shooting the same lens with a film or full-frame camera, the focal length is 100mm and also saved as such. I was wondering why crop-sensor cameras do not use and save the "real" focal length to the EXIF data, so the picture data is comparable without taking camera and lens into account?

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Matt Grum's answer is excellent as usual. I wanted to call out a key point, though: the value saved in the EXIF data is the real focal length. (This confusion is one reason it's much better to say "crop factor" than "focal length multiplier".) –  mattdm Jan 24 '11 at 21:59
    
Note, however, that there's another EXIF tag, FocalLengthIn35mmFilm, that does contain the "equivalent" 35mm focal length. I don't really like the name of this field, for the reasons Matt Grum described, but there you go... –  coneslayer Apr 13 '11 at 16:46

2 Answers 2

up vote 19 down vote accepted

Focal length is a measure of the lens's ability to bend light. As such this figure doesn't change when you use a smaller sensor. What actually happens when you use a smaller sensor is that your field of view narrows. Field of view is dependant both on the focal length and the format (the size of your film or sensor). The ubiquity of 35mm film among amateur photographers in the last half of the 20th century effectively took format out of the equation and lead to focal length being used to categorize the field of view.

When digital arrived and suddenly all sorts of different sensor sizes were being used the idea of a crop factor was introduced to people relate to the field of view they expected from a certain focal length on 35mm. This is not a problem so long as you realise the focal length doesn't really change, the crop factor operates only on the field of view (a crop factor of 2 halves the field of view). I agree that it would be nice to have the field if view stored in the EXIF data, seeing as the camera knows both the focal length and sensor size!

The concept of crop factors (and the term "full frame" which I avoid using at all costs) is only really used by small format photographers - no-one using medium format refers to their camera as having a 0.7 crop factor! Likewise if you were to mount a 50mm medium format lens on a 35mm sensor DSLR, it will act just like any other 50mm lens.

Similarly the minimum focus distance is a property of the lens (and distance to the sensor) and thus doesn't change when you use a different size sensor.

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Ah of course, ... Thank you for answering and enlightening me :) –  el_migu_el Jan 24 '11 at 11:10
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No problem, it's a very common misconception especially as focal length is always used to refer to lenses and FOV is rarely stated. It's very difficult to talk about different lenses without using focal length, so I don't think it's too much of a problem, as long as it's clear what format you're talking about. –  Matt Grum Jan 24 '11 at 19:11

"the focal length multiplies with 1.5"

This is incorrect. If your lens reads 50mm, it is a 50mm lens, even on a "crop" sensor. However the field of view is different on an APS-C sensor, compared to a "full frame" sensor. The only way to multiply the focal length of a lens is by using a teleconverter.

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