1. ## aperture diameter

If the aperture diameter is given by focal length/f-number, won't a 105mm at f4 have roughly the same aperture diameter as a 50mm at f1.8? Does the aperture diameter give the size of the aperture such both lenses will be as fast as each other at that ratio?

3. ## Re: aperture diameter

Originally Posted by canalrat
If the aperture diameter is given by focal length/f-number, won't a 105mm at f4 have roughly the same aperture diameter as a 50mm at f1.8?
Yes roughly.

Originally Posted by canalrat
Does the aperture diameter give the size of the aperture such both lenses will be as fast as each other at that ratio?
No. The image that is projected onto the film plane decreases in brightness when the focal length gets longer due to optical magnification. In order to compensate for the lost of light due to longer focal length, the pupil size of the aperture (it's area) needs to get larger. That is why f-stops are expressed in terms of the ratio of focal length against pupil diameter. So f/4 at 105mm gives the same exposure value (EV) as f/4 at 50mm, and f/4 at 105mm does not give the same EV as f/1.8 at 50mm.

In other words, at f/4, 50mm : pupil diameter = 50mm/4 = 12.5mm. In order for a 105mm lens to have the same EV, pupil diameter now needs to be larger = 105/4 = 26.25mm. This is the exact reason why long lenses need to be bigger comparing to standard lenses at the same aperture (e.g. 300mm vs 50mm at f/2.8).

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•