# Lens's Diameter

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1. Dec 7, 2014

### walter1998

Hello guys, I was wondering if you could guess or even calculate the diameter of a camera lens with a picture of a ruler.

2. Dec 7, 2014

### Staff: Mentor

Not in any realistic way. An ideal camera will make the same picture independent of the size of the lens, and real cameras are quite close to that (and for the tiny deviations, you have no idea which part of the camera lead to them).

3. Dec 7, 2014

### walter1998

This question is asked in a famous competition in my country: "Approximately calculate the diameter of the lens which this picture is taken with.(for measurements consider lens as a flat object) "

4. Dec 7, 2014

### Staff: Mentor

Ah well, you also have some background here that can be useful. And I think you still need some additional assumptions (like no additional elements that block light).

5. Dec 7, 2014

### sophiecentaur

I think this is to do with probable f number and focal length. There are some clues to help you but the main bit of information that is totally missing would be the size of that image on the film / sensor assume it is an APS sensor?? perhaps.. Firstly, the ruler looks pretty straight, unless the lens is very expensive, we are not dealing with a very short focal length. The window frame is probably not far from the ruler and is badly out of focus so the aperture must be fairly wide (poor depth of focus).
The left side of the ruler is a bit soft so that could suggest either a poor lens or a wide aperture. You can make assumptions about the probable width of the window frame and then guess how far it is behind the ruler. That would give a clue about how far the camera is away from the ruler (draw some sketches).
I could suggest you take a camera of your own and try to reconstruct a similar picture (with buildings in the distant background - or is it a fence, only a few metres away?) and note the focal length and f setting. That will give you the width of the internal aperture but it cannot tell you the diameter of the lens without a lot more information.
There is no way you can get an accurate estimate but, like estimating the number of sweets in a jar, you can avoid giving really wrong estimate.