DOF on faster, larger lenses

[rbailey23]

If I were to compare 2 lenses, lets say 35mm 1.4 and a 35mm 2.8, The 1.4 is a bigger lens and takes in more light. Now lets say we are comparing both lenses at 5.6 aperture. Will the 1.4 lens allow more light at 5.6, and therefore be more useful for high DOF under low light conditions?

 

[Ashtun]

5.6 on one lens is the same as 5.6 on another.

 

[480sparky43 42]

If I were to compare 2 lenses, lets say 35mm 1.4 and a 35mm 2.8, The 1.4 is a bigger lens and takes in more light. Now lets say we are comparing both lenses at 5.6 aperture. Will the 1.4 lens allow more light at 5.6, and therefore be more useful for high DOF under low light conditions?

The 1.4 will 'let in more light' while you're looking through the viewfinder, but once you push the shutter button, it will close down to 5.6; as will the 2.8 lens. So both will result in identical DOFs. (assuming all else is equal, such as focus point, same camera, etc.)

 

[Mully]

Sometimes the lens element grouping can make a small difference but not enough to be concerned about.

 

[KmH]

The f in f/5.6 is the focal length of the lens. The f is used because it is a variable value in the ratio of lens focal length to aperture.
No doubt, prime lenses have a fixed focal length, but there are different focal length prime lenses, and of course zoom lenses.

35 mm / 5.6 = 6.25 mm. So, 6.25 mm is the diameter of the lens aperture of any 35 mm lens set to f/5.6 and they will all let in the same amount of light.

A 50 mm lens set to f/5.8 has an 8.93 mm diameter lens aperture.

 

[Patrice]

And just to add to the above discussion; any lens, regardless of focal length, will result in the same exposure when set to a specific aperture but DOF will vary with focal length and focus distance.

 

[KmH]

Interestingly -

If a 50 mm lens is set to f/5.6 and is at a focus point distance of 5 feet,
and a 4x longer focal length 200 mm lens is set to f/5.6 and a focus point distance 4x times further away - 20 feet,
the total DoF both lenses deliver is the same.
Note: Subject scale in each image would also be the same. - because of the difference in focus distance. The background in the image made with the 200 mm lens would be magnified more than the background in the image made with the 50 mm lens. consequently, the background in the photo made with the 200 mm lens will seem to be more out of focus when it is actually just magnified more making the blurry parts bigger in the image frame.