Site Sponsors
Results 1 to 10 of 10

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

  1. #1
    Senior Member petermcwerner's Avatar
    Join Date
    Jan 2009
    Location
    Geneva, Switzerland
    Posts
    511
    Post Thanks / Like

    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
    Peter Werner
    Leica M8, R9+DMR & Digilux 2; Nikon D700; Panasonic FX01, FX150 & G1; Samsung TL350 (WB 2000)

  2. #2
    Member kwalsh's Avatar
    Join Date
    Sep 2009
    Posts
    147
    Post Thanks / Like

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

    Quote Originally Posted by petermcwerner View Post
    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.

    Hope that helps. Short answer, don't worry about it!

    Ken

  3. #3
    Super Duper
    Senior Member
    Vivek's Avatar
    Join Date
    Apr 2008
    Location
    Netherlands
    Posts
    13,604
    Post Thanks / Like
    Images
    21

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

    Quote Originally Posted by petermcwerner View Post

    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. #4
    Senior Member petermcwerner's Avatar
    Join Date
    Jan 2009
    Location
    Geneva, Switzerland
    Posts
    511
    Post Thanks / Like

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

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

    Quote Originally Posted by kwalsh View Post
    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
    Peter Werner
    Leica M8, R9+DMR & Digilux 2; Nikon D700; Panasonic FX01, FX150 & G1; Samsung TL350 (WB 2000)

  5. #5
    Senior Member petermcwerner's Avatar
    Join Date
    Jan 2009
    Location
    Geneva, Switzerland
    Posts
    511
    Post Thanks / Like

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

    Quote Originally Posted by Vivek View Post
    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
    Peter Werner
    Leica M8, R9+DMR & Digilux 2; Nikon D700; Panasonic FX01, FX150 & G1; Samsung TL350 (WB 2000)

  6. #6
    Super Duper
    Senior Member
    Vivek's Avatar
    Join Date
    Apr 2008
    Location
    Netherlands
    Posts
    13,604
    Post Thanks / Like
    Images
    21

    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. #7
    Senior Member petermcwerner's Avatar
    Join Date
    Jan 2009
    Location
    Geneva, Switzerland
    Posts
    511
    Post Thanks / Like

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

    Quote Originally Posted by Vivek View Post
    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.
    Peter Werner
    Leica M8, R9+DMR & Digilux 2; Nikon D700; Panasonic FX01, FX150 & G1; Samsung TL350 (WB 2000)

  8. #8
    Member
    Join Date
    Apr 2009
    Location
    Vancouver, BC
    Posts
    221
    Post Thanks / Like
    Images
    1

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

    Quote Originally Posted by Vivek View Post
    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. #9
    Senior Member petermcwerner's Avatar
    Join Date
    Jan 2009
    Location
    Geneva, Switzerland
    Posts
    511
    Post Thanks / Like

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

    Henning,

    Quote Originally Posted by henningw View Post
    T values are measured only on axis. f values are of course measured geometrically, as everyone has agreed and also only apply on axis.
    Thank you for your clarification
    Cheers Peter
    Peter Werner
    Leica M8, R9+DMR & Digilux 2; Nikon D700; Panasonic FX01, FX150 & G1; Samsung TL350 (WB 2000)

  10. #10
    Super Duper
    Senior Member
    Vivek's Avatar
    Join Date
    Apr 2008
    Location
    Netherlands
    Posts
    13,604
    Post Thanks / Like
    Images
    21

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

    Quote Originally Posted by henningw View Post
    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).

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •