focal length

flogger · 14 Sep 2006 at 19:10

can any1 explain how different focal lengths affect dof or post a link to an explanation, thxs in advance

olv · 14 Sep 2006 at 20:15

Focal length does not have a direct affect on the dof. dof of field is controlled by the aperture of the lens.

hoodmeister · 14 Sep 2006 at 20:23

focal length does have a direct effect on DoF. I can't give you any hard equations, but the longer the focal length, the less depth of field.

a 15mm lens @ F/2.8 will give you tons of depth of field - a 150mm lens @ F/2.8 will give you quite little.

rpstewart · 14 Sep 2006 at 20:25

olv said:
Focal length does not have a direct affect on the dof. dof of field is controlled by the aperture of the lens.

The DoF is dependant on focal length. While the aperture provides the main control the DoF at a constant aperture is not the same at different focal lengths.

ie at a constant f/8 for 35mm film and a 10ft subject distance the DoF is:

50mm lens - 6.28ft
200 mm lens - 0.34ft

The actual reason for this escapes me at the moment...

If you want to see how focal length etc effects the DoF then check out this DoF Calculator

Wile E. Coyote · 14 Sep 2006 at 20:27

Was just looking for that link to post it, beat me to it.

rpstewart · 14 Sep 2006 at 20:33

Found the reason, all thanks to Bryan Peterson's understanding exposure - is there nothing that book can't do

Anyhoo, at the aperture is noted as f/8 or the like, this means that the actual diameter of the diaphragm opening is an eighth of the focal length of the lens. Therefore for a longer focal length the actual opening size is greater than for a wide angle lens at the same aperture and as we know, smaller hole = more DoF.

There's a bit more to it than that in terms of compensating for the reduced light capturing ability of long lenses due to their narrow field of view but the gist is as above.

Gungnir · 14 Sep 2006 at 20:37

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Another link worth a look at

flogger · 14 Sep 2006 at 21:21

thxs guys, very useful reading, i think some of it will be trial & error :eek:

olv · 15 Sep 2006 at 20:21

i phrased my original post very badly. i suppose i was reffering to the simplest case of a fixed focal length.

Belly · 16 Sep 2006 at 18:07

rpstewart said:
The DoF is dependant on focal length. While the aperture provides the main control the DoF at a constant aperture is not the same at different focal lengths.

ie at a constant f/8 for 35mm film and a 10ft subject distance the DoF is:

50mm lens - 6.28ft
200 mm lens - 0.34ft

The actual reason for this escapes me at the moment...

If you want to see how focal length etc effects the DoF then check out this DoF Calculator

I'm sure it was easier years ago with film, when we focused on a subject, read the details from the barrel of the lens, set the camera to the shortest distance, had a look through the viewfinder to see what was in focus and then did the same again with the longest distance, and then set the camera?? I'm sure there is a diaphram locking feature on my Canon AE1 to show the actual DOF?

Thanx for the post for DOF (and Gangir too), maybe someone could manufacture a small pocket sized electronic version about the size of a small pocket calculator for those of us who own non (DSLR) digi cams.