Last weekend I was shooting in studio and tethering to laptop as well.
First I measured everything and shot the portrait with a
24-70mm lens at
Then I switched to
85mm lens and didn't change anything and made sure I am still at
f/2.8 and shot the same thing.
But on laptop screen I noticed the light on the background seemed to be increased! But I didn't change anything, I just changed lenses.
So that made me think does the length of the lens affect the exposure even in these short lenses? When I do Large Format photography the length of bellows affects the exposure but hadn't seen such a thing for digital.
First - When doing large format photography does the length of the bellows effect exposure?
When imaging we focus by moving the lens away from or towards the film or digital sensor. When imaging distant subjects the lens to image plane is most short, when imaging close, the lens to image plane is elongated. Any elongation of this lens to image distance causes the f-numbers to become invalid. If significant, under-exposure will be the result. For subject distances greater than 1 meter (1 yard) this phenomenon called “bellows factor” is infinitesimal. As we focus closer and closer “bellows factor” becomes significant. When we get close enough to achieve “unity” (life-size or 1:1), the f-numbers are 2 f-stops off. Thus “bellows factor” becomes significant at about 2 feet (600mm) and under exposure is likely. That is why the macro lens is popular, its design mitigates “bellows factor”. Additionally, modern cameras read exposure through the lens. Thus the exposure will be predicated on the camera’s ability to reverse the effect of “bellows factor”. We are talking the same danger, digital vs. film, large format vs. miniature camera.
I switched to 85mm lens and didn't change anything and made sure I am still at f/2.8 and shot the same thing. But on laptop screen I noticed the light on the background seemed to be increased!
The beauty of the f-number system is they take into account the focal length of the lens and the working diameter of lens (aperture). Thus the f-number system is universal, f/2.8 delivers same exposure regardless of focal length or working diameter. So why did you see the background change? Likely when you switched to the 85mm you repositioned yourself, i.e. you changed subject distance. That could account for this change especially if you were using a flash. Now different lenses will pass more or less light based on the number and coloration of the glass elements that comprise the lens. Most likely the lens switch changed the angle of view, you moved and recomposed. These interactions changed the way the background looked.
F#/F-ratio are not exact in terms of efficiency/transmission. There is usually a difference between the F-ratio and the T-ratio (measured transmission) of a lens and it varies by make/model. It doesn't usually vary more than the 1/3 stop accuracy of the metering/exposure system, but it happens.
Additionally, if you changed distance in order to keep the subject the same size you negated the increased magnification at the subject, but not at the BG. This changes the angular extent (size/area) of what BG is included and the etendue (light/light density) of the BG area. The result is more light from a smaller area covering a larger sensor area, but the exposure should not change.
you don't use any metering with studio lights [on the camera at least] so if the distance between the subject and the lights does not change, the exposure does not change. This is why it's easy to shoot fashion. what probably did change in your particular situation is the amount of the background in the frame is much smaller with a longer lens, so if your lighting is not constant across your backdrop, it may look brighter. you do need to compose your lighting for the particular angle of view you are using...