Effective Aperture vs. Computed Aperture

[Paul W]

As I understand the f number, it is calculated based on the diameter of the aperture and the focal length of the lens. Given this, and lens of say 100mm focal length should have exactly the same diameter opening at a given f number.

The question is, with different lens constructions (number of elements, coatings, glass types, etc) the amount of Transmitted light will vary from lens-to-lens: how is that compensated for in the aperture setting? In other words, a very high quality lens may transmit 95% of the light entering the front element, and a poor quality lens may only transmit 80% of the light. Yet both would have the same aperture diameter as found using the aperture equations. The poor lens would result in a reduced exposure compared to the high quality lens.

Thank you

[tclune]

I think what you are looking for is the difference between f/stops and T-stops. This short article explains the terms: http://thephotoletariat.com/f-stops-...ops-explained/ A T-stop is apparently used in cinematography, but never really caught on in still photography. By and large, the difference in transmission of lenses isn't very important in still photography, or at least not as important as the difference in depth of field (which T-stops ignore). If you use a buit-in light meter, you'll automatically account for transmission differences of different lenses anyway. FWIW

[Paul W]

Yes, that explains it. The question came up because I was comparing exposure settings on cameras with different lenses in preparation for a presentation. We are interested in f stop numbers to control depth of field, but if we use the in-camera meter to adjust for normal exposure, there can be a different "correct" f stop for a given image using different lenses.

Thanks for the help

[rpcrowe]

In the days when all professional photography and videography was shot with prime lenses, there was just enough difference between some lenses within prime lens sets (quite often used on a turret) that the exposure with one lens might be a tad different from the exposure of another lens at the same f/stop. This did not make a difference in still photography but often, in cineamtography, when cutting segments shot with one lens in with segments shot with other lenses using the same aperture; the difference could make a noticeable difference.

The solution was to use a set of T-stop lenses. The T-stops of these lenses were based on the actual transmission of the lenses and thus, T-8 on one lens would match exposures exactly with a T-8 on another lens within that series or set. The T-8 on a lens from one set, however, would not match up exactly with the T-8 exposure from a lens from another series or set. Since these lenses were calculated individually to match within the set.

Remember, the cameras used did not have through the lens exposure control based on the amount of light reaching the film plane. All exposure was computed manually with hand-held exposure meters. The cinematographers or directors of photography in those days were very accurate when it came to exposure and they wanted their lenses to reflect that acuracy.

The T/stop lens sets were extremely expensive and pretty much fell out of fashion when the industry began to use zoom lenses on a regular basis.