# Thread: Question on f-value vs. T-Value, luminosity, DOF

1. ## Question on f-value vs. T-Value, luminosity, DOF

Some lenses, particularly adapted motion picture lenses, such as Speed Panchros or Kinoptik, show both the (for us) usual f-Value, as the T-Value. For instance, my 25mm Kinoptik shows an f/value of 2.0 and a T/value of 2.5. On the diaphragm setting ring, only the T-values are indicated. Same for a 18mm Cooke Speed Panchro f/2.0 and T/2.2

Kinoptik 25mm /f=2 /T=2.5

If I understood it correctly, the T-value takes into account the actual light transmitted by the lens, not the theoretical (geometrical) relation between diameter and focal length of a lens.

I would assume that the DOF is computed using the f-value, not the T-value. Correct?

Another question: when we invest for instance in an expensive Leica f/1.4 lens, we do not know how much light really reaches the sensor. Two lenses with the same f/ value may have very different effective luminosity. Correct? In lens tests I read in publications, I have never seen this subject is usually tackled.

Thanks in advance for any answers on this subject and practical consequences when buying / using lenses on the G1 or other cameras.

Peter

2. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by petermcwerner
If I understood it correctly, the T-value takes into account the actual light transmitted by the lens, not the theoretical (geometrical) relation between diameter and focal length of a lens.
Yep!

I would assume that the DOF is computed using the f-value, not the T-value. Correct?
Right again!

Another question: when we invest for instance in an expensive Leica f/1.4 lens, we do not know how much light really reaches the sensor. Two lenses with the same f/ value may have very different effective luminosity. Correct?
Yes and maybe . The reality is that for expensive fast primes the difference between T-stop and F-stop is very small. The T-stop primarily would come into play in older lenses, particularly zooms with many elements, that don't have modern coatings to improve transmission/reduce reflection. Really, for all practical purposes with a modern lens, especially a prime, the difference between the T-stop and F-stop is so small as to not be worth considering.

As a side note, so why T-stops on lens for motion pictures? My understanding is that this was primarily begun back when many motion picture cameras had multiple lenses (primes usually) on a rotating wheel. By using T-stops the operator could ensure there wouldn't be an apparent brightness change when they switched from one lens to another (no auto-gaining CCDs back then) which would be very noticeable to a viewer. (Keep in mind for TV work they might be switching between cameras which all need to have the same exposure for continuity or in film work they'll be splicing the different sequences from different lenses together and they don't have any modern editing tools to work with to correct exposure differences). Of course this is irrelevant in still photography where the exposure is calculated for each shot and there is no continuity between shots.

Ken

3. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by petermcwerner

Another question: when we invest for instance in an expensive Leica f/1.4 lens, we do not know how much light really reaches the sensor. Two lenses with the same f/ value may have very different effective luminosity. Correct? In lens tests I read in publications, I have never seen this subject is usually tackled.
Erwin Puts does actually discuss this in a round about way. He always measured and reported light fall off (sometimes ~2-3 stops from the center).

Two lenses with the same f number having different light transmission can only be found when they are actually compared side by side under standard conditions.

Expense does equate to quality (image) or even the number values.

A Navitron 75/1.3 (c-mount) is better on the G1 than the Summilux 75/1.4.

Computar-TV 25/1.3 is "better" (size, cost, performance,...) than a 'lux.

That said, none of the c-mount lenses can be used on an M9 and they will fail miserably there in every aspect.

4. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Thank you Ken for your explanation. Let me however comment on the following statement:

Originally Posted by kwalsh
The reality is that for expensive fast primes the difference between T-stop and F-stop is very small.
For the 25mm Kinoptik, an expensive prime, f/=2.0 T=2.5. That is the difference between a 50mm Summicron and a Summarit or ~35% in terms of list price. Not that negligible after all.

Cheers
Peter

5. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by Vivek
Two lenses with the same f number having different light transmission can only be found when they are actually compared side by side under standard conditions.
Thank you Vivek for your comments. Question: How do cine lens makers who put the T-values on their lenses compute this value? There must be an objective way of measuring it.

Cheers
Peter

6. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Peter, As a connoisseur of very many esoteric lenses, I think you may benefit by buying a book on optics.

I highly recommend the one by Sidney F. Ray (Applied Photographic Optics, ISBN 0 240 51540 4).

It is pricey but worth every penny, IMO.

7. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by Vivek
Peter, As a connoisseur of very many esoteric lenses, I think you may benefit by buying a book on optics.

I highly recommend the one by Sidney F. Ray (Applied Photographic Optics, ISBN 0 240 51540 4).

It is pricey but worth every penny, IMO.
Thank you Vivek.

8. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by Vivek
Erwin Puts does actually discuss this in a round about way. He always measured and reported light fall off (sometimes ~2-3 stops from the center).
That refers to vignetting due to optical or physical constraints and not to f vs. T values. They are not even related.

T values are measured only on axis.

f values are of course measured geometrically, as everyone has agreed and also only apply on axis.

9. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Henning,

Originally Posted by henningw
T values are measured only on axis. f values are of course measured geometrically, as everyone has agreed and also only apply on axis.
Cheers Peter

10. ## Re: Question on f-value vs. T-Value, luminosity, DOF

Originally Posted by henningw
That refers to vignetting due to optical or physical constraints and not to f vs. T values. They are not even related.

T values are measured only on axis.

f values are of course measured geometrically, as everyone has agreed and also only apply on axis.
Vignetting due to optical and physical constraints- f values- geometric measurements.

f values- not that simple. Even the shape of the aperture has an impact.

I suggested a book (I own one and actually read it as well).