lens aperture vs how much light actually passes through it

claudiu · Joined: 15 Apr 2008 Posts: 28 Location: Romania

Maybe lots of you already know but maybe some who haven't read

http://www.dantestella.com/technical/fast.html

In brief it says that aperture is not the only determining factor for the amount of light getting to the camera sensor(film). Optical design has also great influence. For example how much air-glass alternations, coatings... determine how much light will get through.

I observed this while using 2 of my lens:

Flektogon MC 20 2.8 and zebra 20 4. The exponometer was indicating 1/2 to 1 stop difference more for MC version (same texp and f/4 aperture). Taking pictures showed the same result. I will try soon a test with other lens I have. From now on I'll be more careful about huge maximum apertures. It gives no guarantee that the light passing advantage over a "slower" lens is as important as in theory.

Rob Leslie · Joined: 20 Mar 2007 Posts: 1103 Location: UK Swindon

This is very true. before the days of TTL metering you used to get to know your lenses and make exposure adjustments for this. it is not uncommon for a lens to be a stop lower than stated.
Today with digital and TTL metering it is quick and easy to test your lenses. This may sound pointless and in some cases I would agree but there are many so called f5.6 long lenses which are nearer f8 and with a 300 or 400mm lens that light loss is important.
For example I know my Tamron SP f5.6 300mm is what it says and it is pin sharp wide open. I know of a f4 300mm lens which on my camera gives me the same shutter speed as the Tamron for the same shot yet it is wide open at f4, this is confirmed by the histogram. Add to that the lens is no good wide open and needs to be stopped down for any sharpness and I am looking at a big f4 lens, twice as big as the Tamron but is in reality a F8 lens that still isn't as sharp as the Tamron at f5.6.

The aperture of a lens as stated by the manufacturer is a number that is calculated. It is never the real light gathering power of the lens. also Manufactures are allowed a percentage leeway in their calculations. I know for a fact that there are many 'Fast' lenses out there which are not anywhere near as fast as their users think. this is often a shame but in some cases it is almost a con as speed costs money. If your expensive 100-300mm f4 zoom is nearer to f5.6 at 300mm IMO you have been conned and you are carrying a lot of size and weight for no reason.

claudiu · Joined: 15 Apr 2008 Posts: 28 Location: Romania

Thanks for the information, my info on this topic is quite limited. Also, I don't see it it discussed too much.

Yes maximum aperture is If I recall front lens diameter/focal length ratio so not telling too much.

Anyway, I will try to make some comparison tests between my MF lenses and some new AF lenses to see really how "fast" are them. Maybe someone could also try this.