Difference between f-stops and t-stops

Discussion in 'Open Discussion' started by speedandstyle, Feb 16, 2014.

  1. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    In another thread it became obvious that some people do not understand the difference between f-stops and t-stops. The page from wikipedia is actually pretty good on the subject and so I will reference it.


    f-stops are calculated via a mathematical formula of focal length divided by the aperture diameter, t-stops are actually measured via light transmittance through the lens. No lens has 100% transmittance of light so a t-stop is always higher than an f-stop. Different lenses can vary a fair amount from others in how much light actually transmits through it.
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  2. fortwodriver

    fortwodriver Mu-43 Top Veteran

    Nov 15, 2013
    Toronto, Ontario, Canada
    Thank you sir ... Everyone should read this and understand the difference between focal ratios vs. actual transmitted light.
  3. T N Args

    T N Args Agent Photocateur

    Dec 3, 2013
    Adelaide, Australia
    call me Arg
    So... f-stop is the true guide for determining depth of field, and t-stop is the true guide for determining exposure?
  4. Reflector

    Reflector Mu-43 All-Pro

    Aug 31, 2013
    A small note to add on: Some lenses can have t-stops close to their f-stop number, like f/1.2 with t/1.24, which you might see as getting rounded down to t/1.2. A good example of this is when you see a f/1.8 lens getting a t/1.8 on something like DxO (They are not so reliable in their tests, but still) because they round hundredth.

    You can see this in action with the Sigma 18-35 f/1.8A:
  5. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    There are more factors than just f-stop that affect DOF so even it isn't "the" guide.

    I will refer you back to wikipedia since the DOF info there is also good.
  6. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    That is correct. There are some lenses that have very high transmittance of light. And yes often numbers get rounded - the f-stop numbering system rounds the numbers!
  7. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Thanks for pointing this out. I've always thought that many traditional f-stops were actually rounded. For example, assuming that f = 4.0 is exact, then exact f-stops at +/- 0.5 EV would be:

    1 -- 1.22 -- 1.41 -- 1.73 -- 2.00 -- 2.45 -- 2.83 -- 3.46 -- 4.00 -- 4.90 -- 5.66 -- 6.93 -- 8.00 -- 9.8 -- 11.31

    which would naturally be rounded (2 significant figures) to:

    1,0 -- 1.2 -- 1.4 -- 1.7 --2.0 -- 2.5 -- 2.8 -- 3.5 -- 4.0 -- 4.9 -- 5.7 -- 6.9 -- 8.0 -- 9.8 -- 11

    The only one that's "off" from the conventional series of f-stops is f/5.6.
  8. dwig

    dwig Mu-43 Top Veteran

    Jun 26, 2010
    Key West FL
    Nope, most of those are also rounded.

    The progression is such that if you start with exactly f/1.0 then only every 2-stop increment from f/1.0 can be expressed without rounding. This is because the light intensity varies with the area of the aperture and focal ratios (f/stops) reference the diameter. Since A = Pi @ r^2 (where A is the area and r is the radius) and d = 2 * r (where d is the diameter) it follows that doubling or halving the light intensity (area of aperture) change the diameter (and therefore f/stop) by an increament of the square root of 2, which is an irrational number and can't be written with numerals without rounding.
  9. Reflector

    Reflector Mu-43 All-Pro

    Aug 31, 2013
    I'll add to this as well:

    It isn't very hard to pull off very thin DoF: Move the subject closer to MFD and the background away from them. Sometimes this is missed and then there's discussion of a "full frame look" in reference to the ultra thin DoF.

    Equally you can achieve that "cameraphone look" by having a wide, wide lens and having everything near infinity then firing away with a small aperture.

    But what can I say, there will be always funny conversations like that.
  10. fortwodriver

    fortwodriver Mu-43 Top Veteran

    Nov 15, 2013
    Toronto, Ontario, Canada
    Yeah, it seems like any time aperture is discussed, it's always in the name of ultra thin DOF. I suppose people have gotten used to trying to take photos in places with poor background choices. So having a razor-thin DOF lets them be reckless with their backgrounds. Someone could be peeing against the wall in the background and you'd never know.

    That had nothing to do with the whole t-stop vs f-stop discussion. That came about where someone felt the Oly 25 looked "brighter" than the PL25. Of course, more than a few people thought that was simply not possible, although it certainly is possible.

    But hey, razor-thin DOF FTW! I think. ;-)
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  11. Reflector

    Reflector Mu-43 All-Pro

    Aug 31, 2013
    I just wanted to add that for the benefit of anyone reading this thread and scratching their head at why someone can make a f/11 shot that still has thin DoF.

    I'm actually really curious about the results of the O25 and PL25. Here's to hoping someone here on mu-43 owns both and can do the defocused white wall with a fixed light source shutter speed test to validate what's the real case behind this.
  12. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    Hyperfocal distance - http://en.wikipedia.org/wiki/Hyperfocal_distance


    This is a very old technique for landscape. You get a very deep DOF!

  13. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    I just found this tutorial video from B&H on depth of field. It clearly shows the best ways to adjust your DOF, one way being aperture{f-stops}.
  14. fortwodriver

    fortwodriver Mu-43 Top Veteran

    Nov 15, 2013
    Toronto, Ontario, Canada
    Ah Hyperfocal. The thing that sucks about Hyperfocal is that it's not really compatible with any thing other than native-sized frames with most lenses. Even so, many lenses have poor (or downright incorrect) depth scales if they even have a focus distance indicator.

    The ones that do have them are calibrated to 35mm. The only camera I still use that technique on is my medium format stuff and my film cameras - because I don't own a "full frame" digicam.

    For a time Canon and Minolta had AF Depth programs. I don't remember what it was called on the Minolta system but on Canon it was called "DEP" and later "A-Dep". You'd point your focus points at everything you wanted to have in focus, and the camera program would calculate the correct distance and aperture to get it all in. Most of the time it was relying on hyperfocal distance calculation unless things were really close by.

    Good times...
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