Technical aperture question...

If you have 2 zoom lenses, one is a constant f2.8 and the other is a constant f4, and you take a picture with both lenses @ f4, will the f2.8 lens automatically let more light in since the diameter of the elements are bigger?


Hope this makes sense, just always wondered this

The 2.8 lens will use the bigger aperture for focusing but stop down to f/4 for the actual shot. So if all else is equal both lenses will give the same exposure @ f/4.

Thank you!

reefvilla wrote in post #18695960
If you have 2 zoom lenses, one is a constant f2.8 and the other is a constant f4, and you take a picture with both lenses @ f4, will the f2.8 lens automatically let more light in since the diameter of the elements are bigger?


Hope this makes sense, just always wondered this

It's a bit more complicated. An f-stop is a ratio between aperture size and focal length. The amount of light that will expose the sensor can change based on several factors, like lens transmission efficiency. T-stops is what measures the amount of light coming through.

However, from a more practical sense, ignoring lens and coating differences, f/4 should be really close between lenses.

If you had a 200mm f/2.8 lens and a 200mm f/4 lens, the diaphram diameter (not the front element size) is what matters, and would be 50mm in diameter at f/4 for both 200mm lenses.

Ignoring subtle differences in light transmission due to better coatings used on the air-glass surfaces, both lenses transmit virtually the 'same' amount of light

It would be a nightmare and end to photography as we know it if things didn't work this way.

It would be like having two cars, one with a bigger engine than the other, and if you set the cruise at 50, the larger engine car actually goes 60. That would be terrible and hard to monitor/learn.

TeamSpeed wrote in post #18696039
It would be a nightmare and end to photography as we know it if things didn't work this way.

It would be like having two cars, one with a bigger engine than the other, and if you set the cruise at 50, the larger engine car actually goes 60. That would be terrible and hard to monitor/learn.

I agree. I just figured all that out