f stop numbers, why those exactly?

[Nigel]

I have an Angeneiux lens that I use as a back up on my Aaton--It is a 15-150 F2.8 or a
T of 3.1 and it does not stay consistant through its whole T/F range. So it is always wiser to use T-Stops. If they had no purpose they wouldn't have been marked.

[mattias]

yes, it's always wiser to use t-stops when calculating exposure, but f-stops should be used for dof calculations and lookups or you will get the wrong result. actually, on modern lenses only the t-stop is marked, since exposure is considered more important, and the difference in dof is so small when the two are very close.

[Nigel]

I use the T's on my lenses just as I would F's. I use the same DOF charts and I was always told that the DOF is exactly the same. I could be wrong.

[mattias]

sorry nigel, but you are indeed wrong. ;-)

the dof charts are calculated based on f-stops, and since f and t stops can differ quite a lot on a lens, you could get the wrong result. think about it: an nd filter inside the lens would change the t-stop dramatically, but the dof obviously wouldn't change. the difference is tiny though, so unless it's very critical you'll be fine.

[Actor]

The f/stop of a lens is its focal length divided by the diameter of its effective aperture. That's it. Period.

E.g., a 50mm lens with an effective aperture diameter of 50mm has an f/stop of 1.

The word "effective" is used in the definition because most practical lenses are in fact two lenses with the iris placed between them. Since the lens bends the light the effective diameter of the aperture is actually greater than the physical diameter of the iris.

Notice that the definition takes into account two factors that effect the amount of light reaching the film: (1)the area of the aperture and (2)the distanced from the lens to the film. Since the area of the aperture is proportional to the square of its diameter and the light is attenuated by the inverse square law on its way to the film the two squares cancel (sort of) and we deal only with the ratio of the diameter and the focal length. The down side is that we have to square the f/stop to know how much the light is reduced by "stopping down" the lens. Thus an f/4 admits 1/16 as much light as an f/1.

Notice that I wrote the f/stop as f/4, with a fraction bar. This is because (technically) the f/stop is a fraction and (technically) the definition above is upside down. An f/8 is actually an f 1/8 but lens makers have long written only the denominator on their lenses to save space. The numerator and the fraction bar are understood. The result is that most photographers have forgotten them entirely. It makes little difference but the down side is that a bigger f/stop admits less light, which can confuse beginners.

Why are these particular numbers are used? Well, start with an f/1. An f/1.4 admits half as much light (1 stop) as an f/1. An f/2 admits one quarter as much (2 stops) as an f/1, etc, etc, etc.

T/stops are not "true f/stops". They are in fact totally artificial numbers which introduce fudge factors to state exposure in terms of f/stop. This is why they should not be used to estimate dof since many factors that effect exposure do not effect dof. It is also dangerous to use t/stops with cameras other than the ones which the manufacturer intended since elements of camera design can effect exposure (pellicles vs rotating mirrors, variable shutter angles, etc.).

F/stops do not stop at one. Stanley Kubrick used an f/0.7 lens to film Barry Lyndon. Telescopes have extremely low f/stops. They don't need any dof because everything you look at with a telescope is at infinity.

[Nigel]

The reason I called TStops "...true 'f' stop for the lens." I sbecause the TStop varies from lens to lens depending on how much light gets gobbled up on it way to the film plane. It takes into consideration these "fudge factors" you speak of. TStops are the only way to set your lens. I will never set it using anything else. Now, if Matt and Actor are correct in what they say about the DOF charts--Then I stand corrected.