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Thread: T-stop vs F-Stop

  1. #1
    Super Moderator Kayaker72's Avatar
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    T-stop vs F-Stop

    So, we always talk about lenses in terms of F-stops. I first came across the concept of T-Stops when looking at cinema lenses as they reported in T-Stops.

    There are a number of online explanations, but I generally think of f-stop or f-number as directly related to the physical size of the aperture. As the size of the aperture is the #1 factor limiting the light that hits the sensor, the photographic stills world almost always uses f-stops when discussing brightness. But really, the light that hits the sensor is a t-stop or t-number as light is lost passing through the various elements.

    T-stops are difficult to measure whereas f-stops are readily known and using f-stop to approximate light hitting the sensor usually isn't that big of a deal as usually there is only ~1/3 of a stop or less differences.

    Using DXOMark's t-stop numbers (only source I know of), a few quick examples:
    • 35 f/1.4 II has a t-stop of t/1.7
    • 24-70 II is really ~2.8 @24 mm, but 3.1 @70 mm
    • 135 f/2 has a T-stop of 2.3
    • etc


    So, t-stop is generally ignored.

    As I mentioned over in my thread on finding a travel super-telephoto, I noticed that shooting the same scene the 400 DO II was extremely consistently 2/3 of a stop lower ISO than my 150-600S (Tv/Av set, ISO allowed to float). But, I was shooting a subject on the porch where light could vary. So now that I have the 500 f/4 II, I wanted to test this under controlled conditions. The results:

    Lens DXO Reported T-Stop Observed ISO Diff Expected T-Stop
    400 f/4 DO II T/5 2/3 stop
    500 f/4 II T/4.5 1 Stop
    150-600S Not Reviewed T/10

    The 500 f/4 II was very consistently 1 stop lower ISO than my 150-600S at similar focal lengths and the exact same shutterspeed and aperture. So, this all lines up very nicely, the 400 DO II has a T-stop 2/3 of a stop higher than it's f-stop. The 500 f/4 II has a T-stop 1/3 of a stop higher than it's f/stop.

    However, this would indicate that the Sigma 150-600S actually let's light hit the sensor as if it is a f/10 lens. But I tend to shoot at f/7.1 to f/8 to get sharp images, thus I am shooting and getting the light as if I was shooting at f/11 to f/13 on an "ideal" lens.

    So, this may explain why I have always been fighting aperture/shutter speed and ISO with this lens.

    For what it is worth, the 100-400 II has a t-stop of t/6.3@400 mm. The 600 f/4 II, T/4.5. The 400 f/5.6 was not reviewed by DXOmark. Other Sigma lenses, those reviewed by DXO mark, are fine. 85 f/1.4 A, T/1.8; 120-300S f/2.8, T/2.8; 500 f/4, t/4.8; 150-500 t/7.3@500 mm.

    Of course, first question is if there is something very wrong with my 150-600S, even though recent images look fine. I've already left a message for Sigma.
    Last edited by Kayaker72; 02-20-2018 at 02:47 PM.

  2. #2
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    The only thing wrong is you are comparing a $1700 it to a $6000 and $10000 lens that are at the top of their class. Really IMO there not in the same category.
    No doubt they have better build quality, light transmission and wouldn't they start with more light since they have a larger front element.

    The 100-400mm II might be a better comparison. Then I think you look at number of elements, light transmission of the each element and build.

  3. #3
    Super Moderator Kayaker72's Avatar
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    Yep.

    Of course, there are going to be a number of trade offs with budget lenses. AF, build quality, IQ, etc. Looking through DXOMark's data, while this will vary from lens to lens, I would generally say some of the "budget" glass has T-stops 0.5 to 2/3 of a stop above their respective f-stop whereas the higher end glass is typically has t-stop's less than 0.5 above their respective f-stop.

    A quick example, the Tamron 150-600 G2 has a T-stop rating of T/7.9, which is ~2/3 of a stop higher than it's f/6.3 rating. If my numbers hold, the 1.3 stop difference with the Sigma 150-600S would be the largest difference I've seen.
    Last edited by Kayaker72; 02-20-2018 at 04:54 PM.

  4. #4
    Senior Member clemmb's Avatar
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    f-number
    noun
    PHOTOGRAPHY
    The ratio of the focal length of a camera lens to the diameter of the aperture being used for a particular shot (e.g., f8, indicating that the focal length is eight times the diameter).

    What is the definition of T-Stop or T-Number?
    Mark

  5. #5
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    noun1.a setting of the lens aperture on a camera calibrated photometrically and assigned a T-number





  6. #6
    Super Moderator Kayaker72's Avatar
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    F-stop is a theoretical number based on the calculation you describe. A T-stop is a measured value for the light hitting the sensor normalized back to be equivalent to a f-stop. Since I am using DXOMark numbers, here is there definition:

    https://www.dxomark.com/dxomark-lens.../#transmission

    But a few other references:


    https://www.youtube.com/watch?v=DzSlQRewS7Q
    https://www.youtube.com/watch?v=jYRJVRMlIe8

  7. #7
    Administrator Sean Setters's Avatar
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    Cinema lenses are rated in T-stops so that different lenses will produce the same exact exposure at equivalent exposure settings. It wasn't important for still photo lens exposures to be exactly the same across multiple (different) lenses, so the easier to ascertain f-numbers were used. F-numbers don't fully represent how much light will be hitting the sensor, but it's a good indication of what ballpark you're in.

  8. #8
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    ... but it's a good indication of what ballpark you're in... (Jeremy Clarkson voice) until now. (/Jeremy Clarkson voice)
    On Flickr - Namethatnobodyelsetook on Flickr | On the web - http://www.GrassStainedPhoto.com
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  9. #9
    Basically, a lens with a T-stop of 5.6 allows the same amount of light through as a theoretical f/5.6 lens with 100% transmission (0 reflection loss. 0 absorption loss).

  10. #10
    Senior Member clemmb's Avatar
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    Quote Originally Posted by Michael G. Clark View Post
    Basically, a lens with a T-stop of 5.6 allows the same amount of light through as a theoretical f/5.6 lens with 100% transmission (0 reflection loss. 0 absorption loss).
    First post? Welcome to the forum
    Mark

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