What's the difference between an f-stop and a t stop?

[panblue]

What's the difference between an f-stop and a t stop?

:study: :camera:

 

[2WheelPhoto]

the difference for me is I don't use t stop. but if I wanted to know about it for any reason, I'd google it =)

 

[2WheelPhoto]

goggle told me this: F-Stops Vs. T-Stops Explained - The Photoletariat

 

[MLeeK]

I was always confused by this, but this is my understanding. F-stops are a physical measurement of aperture. T stop is the measurement of the actual amount of light that reaches the film or sensor.
Because of whatever factors you could have an f/2.8 lens by PHYSICAL MEASUREMENT, but the actual amount that reaches the sensor could be totally different.

It is also my understanding (and don't live by what I do or don't understand, cuz it could be totally jacked up) that this is why primes are better and sharper in low light than zooms. The f/ and T are closer to the same because there isn't as much in the workings of the lens where you can lose actual light.

 

[Derrel]

After an f-stop, you feel satisfied. After a T stop, you need to go pee. This has been my experience. YMMV.

 

[fjrabon]

T-stop accounts for light absorption in lenses with tons of elements. This is a bigger deal in motion picture cameras with zoom lenses, which have a lot more elements in them and are plain bigger, and thus there's more material to absorb light.

f/stop is purely just focal length divided by diameter of the hole the light is passing through.

Essentially T/stop takes that and tells you jsut the amount of light that would be passing through if this were a theoretically perfect lens that lost no light in its workings.

So, if say a lens lost half its light and was a mathematical f/2.8, then it would be a t/4, because it would be the f/stop of 2.8, but losing another stop of light to the glass (which would be awful, but the example is just chosen to make the math easier).

That's my understanding of it anyway.

 

[amolitor]

T-stop is far more important for the movie industry, where you used to have tons of footage all shot at different times and different apertures and so on, where the exposures had to MATCH after standardized processing, so when you spliced a scene together the light didn't jump up and down. It used to matter a lot in the analog film days, since adjusting for missed exposures in post was either impossible or very difficult. I suppose you COULD have printed an intermediate film with differing exposures to compensate, but that sounds like a pain in the ass, and expensive. They wanted to get it right in camera, within a very very small margin.

Nowadays with most post being digital anyways it probably doesn't matter nearly as much. Even so we have systems like RED chattering on about T-stops, either because they're talking to movie guys who expect T-stops, or because they're trying to look extra cool, I can't decide which.

 

[Derrel]

T-stops are VERY critical to cinematographers. One simple reason is that in motion picture scenarios, the shutter speed is not that easily varied...in fact, it can be an almost constant speed...and thus the EXACT amount of light the lens diaphragm is passing is very critical in setting the RIGHT exposure. It's not like a landscaper or sports shooter who can move the shutter speed through a huge,wide range of potential speeds, for creative effects; in full-motion cinema shooting (ie. not stop-motion, for example), there are no "long shutter speeds" like 1/2 second...the cinematographer cannot dick around...he wants to KNOW what the T-stop is. On complicated zoom designs, a maximum f/stop of f/2.8 might very well be T 3.5...and that is far too big of a reduction to just blow off. Second, when switching between lenses, having each lens calibrated in T-stops allows one to get the same exposure by using accurate, "real" values.

 

[Rick58]

I never even heard of a T stop. I was going to say 13 letters, but I guess that's not right.

 

[panblue]

Thanks all!

'just checking!' :hugs:

 

[panblue]

the difference for me is I don't use t stop. but if I wanted to know about it for any reason, I'd google it =)

You're right; I have; also Rockwell ..but posing these questions on TPF I can sometimes also get info from practical experience. And additional anecdotal info, which can be good to know.

 

[Helen B]

goggle told me this: F-Stops Vs. T-Stops Explained - The Photoletariat

Yet another article on the internet written by someone who doesn't know what they are talking about, not even fundamental things like what the f-number is.