View Full Version : F-stop and image size

01-23-2009, 06:34 AM
When the f-stop increases, the aperture diameter decreases and less light is let through. Now my question is: The bundle of light that is passing through the aperture, is it collimated or like a cone? Because if it is collimated, it seems to me the image should decrease as well i.t.o. projection area. Am I looking at this right?

01-23-2009, 07:31 AM
I am going to guess that the nature of the light is largely dependent on the design of the surrounding glass elements, not the aperture itself. Although smaller apertures do begin to exhibit diffraction effects, so there is some impact on the behavior. Some manufacturers may set design standards. The 4/3rds rules are setup for what they call 'telecentricity'; light at the lens exit is supposed to be perpendicular to the sensor surface. This would seem to imply some degree of collimation.

01-23-2009, 02:22 PM
I believe that the light enters the lens collimated, as you put it, and is then bent by the lens elements into a cone to pass through the aperture. There is naturally an opposite cone on the other side of the aperture, and yet more lenses bend the light back into a collimated beam to hit the sensor. Just a guess based on what I know of lenses. Obviously it's more complicated but I think that's the jist, though correct me if I'm wrong.

01-23-2009, 05:15 PM
Do you get variable aperture prime lenses, or only with variable focus?

D Thompson
01-23-2009, 08:20 PM
Do you get variable aperture prime lenses, or only with variable focus?

Primes are not variable aperture as there is no zoom. Lenses are typically variable focus, or at least I sure hope so.

01-23-2009, 08:21 PM
what exactly is an f stop? I'm new to cameras

01-24-2009, 12:43 AM
I found this wikipedia entry that explains it quite nicely: