Are the f-stops wrong on 1.6x crop cameras?

[henry_minsky1]

If the F-stop is the ratio of aperture diameter to focal length, then

when you use a 1.6x crop sensor, isn't the apparent f-stop different

than when you use a full frame camera?

<p>

 

For example, say you used a 80mm lens on a full frame camera, with an

aperture of 40mm, to get f/2.0. You focus your camera at a subject in

a scene, covering a certain angle of view <b><i>A</i></b>, say

standing a distance

<b><i>d</i></b> from your subject. Let's call the depth of field

<b><i>l</i></b> at this time.

<p>

If you put a 50mm lens on a 20D, and move your viewpoint so that the

field of view you have is the same as in the first picture, and you

have an aperture of 25mm, to get f2.0 again, would you still have the

same depth of field <b><i>l</i></b> as you had in the first image?

[steve_dunn2]

<p>The focal length of the lens is a property of the lens itself and does not depend on the size of the sensor. Thus, a 50mm lens is still a 50mm lens whether it's on a 35mm camera, an APS camera (which has a roughly 1.25x cropping factor), a 1D/1D II (1.3x cropping factor), or one of the 1.6x crop bodyes. Since the f-stop is the size of the aperture relative to the lens' focal length, it too does not change; 50mm f/2 is 50mm f/2.</p>

 

<p>DOF depends on the circle of confusion, which in turns depends on how much the image is to be magnified (and a cropped image needs to be magnified more to produce the same final print size). See <a href="http://www.photo.net/learn/optics/dofdigital/index">the photo.net article on digital DOF</a> for a more in-depth treatment.</p>

[les]

I am sure that better answer are available, but:

 

(I am assuming that by f-stop you refer to the lens aperture)

 

the f-stop in other words is a light gathering ability of the lens. The amount of light entering the lens is proportional to the square of the diameter of the aperture.

How could that be changed by the type of the camera attached to the end of the lens ?

 

Think about current f-stop as amount of light entering the lens. For a given lens design (35mm format for instance) the INTENSITY of light falling onto the image circle (at the focus plane) is determined by the current aperture (and the level of ambient lighting of course).

 

Now, you can put a sensor (of any size) within this circle, and the INTENSITY of light falling on the sensor does not change.

 

Which means that f-stop is a property of the lens, not of the sensor, or film onto which the image is projected.

[anupam]

I think the real confusion here is between focal length and angle of view. The focal length of a lens does not change no matter what the sensor/film size, the angle of view does.

 

When someone refers to a 100mm lens as 160mm equivalent on a 1.6 crop sensor, all they are saying is that if one quite arbitraily chooses the angle of view for 35mm film as the standard, the new one would be equivalent to a 160mm lens. Of course that same lens would be a widish angle on a large format camera.

 

-A