Does a fast lens still give a light-gathering advantage when stopped down to the same f-number?
Asked 8/18/2010
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If two lenses are used on the same camera under the same lighting, with the same focal length and the same shooting aperture, do they transmit the same amount of light during the exposure? For example, if I compare an f/1.8 lens and an f/4.5 lens, and I shoot both at f/4.5, is the exposure effectively the same? If so, does that mean the fast lens only gives its speed advantage when used at wider apertures, while its other benefits are mainly things like a brighter viewfinder or better autofocus before the shot is taken?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
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In theory, both lenses will allow the same amount of light to pass through at f/4.5, so yes, when stopping down, you lose the advantage of fast lens. The actual amount of transmitted light depends on the lens design and is measured in T-stops.
But the fast lens will still make a difference when you look through the viewfinder and your autofocus will also gain from it as the lens is only stopped down to the aperture you set for the shutter actuation time.
Originally by user75. Source · Licensed CC BY-SA 4.0
user75
16y ago
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Generated from our catalog & community — verify before relying on it.
Yes—if both lenses are set to the same f-number, they should give essentially the same exposure in theory. So an f/1.8 lens used at f/4.5 does not keep a light-gathering advantage over a lens whose maximum aperture is f/4.5.
The practical caveat is transmission: real lenses differ slightly in how much light they actually pass, which is why cine lenses use T-stops. But by f-number alone, same aperture means same exposure.
A fast lens still offers important benefits even when you shoot stopped down:
- brighter optical viewfinder, because the lens stays wide open until the moment of exposure
- autofocus can be faster or more reliable, since the AF system also sees the lens wide open
At the moment the photo is taken, if both are set to f/4.5, you’ll get the depth of field and exposure you’d expect for f/4.5. So the extra “speed” benefit applies when you use the wider apertures; otherwise the main advantages are in viewing and focusing before the shot.
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