The F word

[UKBob]

This probably sounds like a silly question, but does a lens which has a lower F-stop number let in the same amount of light as a lens with a higher f stop number, when both cameras are set to shoot on the same number.

ie, does my 24-70 2.8 let in the same amount of light at f3.5, as the 18-55 at 3.5

Im guessing it does?

It dawned on me the other day (given that I do lots of close up work) I never shoot anywhere near f2.8, therefore that end of the focal range is wasted...


Another question, about this lens:
Canon EF 24-105mm f/4L IS USM

Does the F4 mean that at all times, the lens shoots at F4. ie, backgrounds will always be blown out? If so, why?? (from users as well as manufacturing points of view)

[te51cle]

UKBob said:

Does my 24-70 2.8 let in the same amount of light at f3.5, as the 18-55 at 3.5

The same amount of light will reach your sensor or film with each lens set to the same f number. f numbers are a ratio so they are consistent across all lenses. Wikipedia

UKBob said:

It dawned on me the other day (given that I do lots of close up work) I never shoot anywhere near f2.8, therefore that end of the focal range is wasted...

Even if you are not shooting at f2.8 it means that you will be able to compose and focus with much more ease as you will be able to see better because the image in the viewfinder will be brighter. Try looking through the viewfinder holding down your depth of field preview button with the aperture set to f5.6 when you are in the studio and you'll really notice the difference.

UKBob said:

Another question, about this lens:
Canon EF 24-105mm f/4L IS USM

Does the F4 mean that at all times, the lens shoots at F4. ie, backgrounds will always be blown out?

Nope, the f number quoted on lenses is the maximum aperture that can be used. You can use all apertures in half or third stop steps down to the minimum for each particular lens (f22 in this case) in whatever manner you wish.

Edited by te51cle on Saturday 5th August 12:28

[Imperialism2024]

te51cle said:

The same amount of light will reach your sensor or film with each lens set to the same f number. f numbers are a ratio so they are consistent across all lenses. Wikipedia

Ehhh I don't think that's entirely accurate, as it only applies to lenses projecting onto the same size sensor. f/2.8 on a full-frame 35mm lens is different from f/2.8 on a DX lens is different from f/2.8 on a 1/1.8" sensor, etc. But in general they are comparable within the same type of lens.