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'T' stop vs. 'F' stop


Daniel D. Teoli Jr.

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44 minutes ago, Daniel D. Teoli Jr. said:

Is the only benefit with a T stop vs an F stop is that you can swap lenses and be guaranteed a T5.6 on one lens is a T5.6 on another lens?

If not, what is the benefit with T stops?

 

That's more or less it, as I understand it. It corrects for any difference in transmission which would otherwise affect exposure if you set only f-stops.

I suspect that if you shoot non-critical video, and don't need the headroom for heavy post-production, the difference is pretty minimal, not to say negligible.

It goes without saying that the DoF will still be determined by the f-stop.

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The "T" stands for transmission.  F-stops are calculated mathematically for mass lens production so each stop is a bit of a generic measurement.  Transmission stops (T-stops) are literally measured lens by lens by shooting light through the individual lens in order to measure precisely where each stop is.  You treat F-stops and T-stops the same when calculating exposure.

G

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T stops take into account loss of light transmission through the lens, while f stops are a simple mathematical calculation from the focal length of the lens and the diameter of the iris.  Thus, the T stops are more accurate for measuring exposure.

That said, I don't believe T stops are measured for each individual lens and diaphragm setting.  And most likely, though I'd really need to google this, an overall correction for the lens design.

Way back, when I used to own lenses, I took my set to Panavision and had them measured for MTF and transmission at different apertures and I learned a couple things: 

1. Shadow detail did not improve between T2.0 and 1.4 due to increased flare.  Yes, there was less depth of focus, but no more shadow detail could be recorded.  So the exposure "advantage" of T1.4 was not nearly so much as I expected.

2. The T stop markings were only truly accurate with the lens wide open.  And, the spec for my Ziess lenses for iris accuracy was + or - 1/3 stop for each mark on the iris ring.  And I would expect the same for lenses marked in f stops.

In general, complicated lens designs such as zoom lenses will have the biggest difference between f stops and T stops regarding exposure accuracy.  And if one is concerned about setting the iris "just a hair" differently, in the end, it won't make any meaningful difference ?

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3 hours ago, Bruce Greene said:

.

In general, complicated lens designs such as zoom lenses will have the biggest difference between f stops and T stops regarding exposure accuracy.  And if one is concerned about setting the iris "just a hair" differently, in the end, it won't make any meaningful difference ?

It is also important to realize that with zoom lenses, the lens speed  (exposure) changes throughout the zoom range. Generally, they get slower towards the long end. 

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