Lenses with different largest aperatures - amount of light let in when both at f/8

[stacenator]

Will a 400mm f/2.8 lens let in the same amount of light as a 400mm f/5.6 lens when both are at f/8?

I am wondering because I am trying to balance shutter speed with the amount of depth of focus.

Any insight is appreciated. Thanks!

[VTphotog]

Yes.

[TCav]

Yes, absolutely. At the same focal length, f/8 is f/8, regardless of how it got there. On the same camera, you should have the same exposure using the same shutter speed and ISO sensitivity, and you should also have the same depth of field.

[Ozzie_Traveller]

Originally Posted by stacenator

Will a 400mm f/2.8 lens let in the same amount of light as a 400mm f/5.6 lens when both are at f/8?

I am wondering because I am trying to balance shutter speed with the amount of depth of focus. Any insight is appreciated. Thanks!

G'day mate

As others have said - the answer is 'yes' because ...
Aperture numbers [like shutter sheed numbers] are fractions, and 1/8 equals 1/8 regardless of the total size ~ if that makes sense to you

The aperture number you see is the bottom part of a fraction, that fraction being the diameter of the hole the light goes thru to reach the film or sensor. So a 400mm lens at F8 has an aperture hole size of 400/8 = 50mm [this does sometimes vary a bit with optical design, but the theory is as above]

Equally a 50mm lens at F8 has an aperture hole of 50/8 = 6-1/4mm diameter or thereabouts

The photographic fact is that exposure is a balance of shutter speet and lens aperture, and for the same shutter speed, you can swap between the 50mm lens set to F8 and the 400mm lens set to F8 and the light reaching the sensor will be the same

However - you are also asking about Depth of Field [not depth of focus ... that's inside the camera]
Depth of Field is the region in front of the camera where everything "is about the same amount of sharpness"

You can vary DoF via the lens aperture you use ... the bigger the aperture number, the bigger the depth of field

But by swapping lenses you change the image size. When you move forward / backward with the 'new' lens to make the image size the same as before, you will find that when the image size is the same, the DoF is the same

Hope this helps a bit
Phil