Fstop to Tstop

We just zoomed in. Great for checking both grain and focus.

Jimmy, yes i also think that's what he means, but i think the difference in t-stop is too small to explain any significant difference. I think the difference in field of view makes the camera expose differently since it doesn't see the exact same thing, even with a supposedly perfectly smooth light source. Other issues are flare and vignetting but no lens should really have much of either at f/4...
 
Except that he uses a closet light, so the difference in FOV shouldn't be all that different. I'm betting it's flaring.
 
the difference in field of view is huge no matter what the light source. even if it covers the entire fov and looks even to the eye it's definitely gonna vary a lot if you see it all or just the center of it. i can adjust the shutter by simply panning across an evenly lit wall, go figure.

/matt
 
well, that's true. It's probably an uneven light source, no matter how much we diffuse it. The light would be simply too close to the diffusion for it to work that way...
 
"Doesn't work like that. F/2 on 50mm is going to be brighter than F/2 on the 85 and 28."

> No it is not.

"Say my first shot was with my 50mm 1.4 and I stop it down to 4, what would be the equivalent stop on my 20mm and 85mm?"

>It would be f4
 
F-stops are purely mathematical so that's simply not true when it comes to exposure. t/4 is t/4 though, which is why he's asking. My answer is that the t-stop in relation to the f-stop varies very little.
 
I think it's really weird when people post false claims like that. I would never post something unless i knew it to be true. Of course everybody makes mistakes and i'm always ready to admit mine, but this guy is clearly just making things up. Does he even know the difference between f and t? If i didn't i'd see that as a huge sign that maybe i need to listen a bit more before i post. :-)
 
Car3o said:
I have a 28 f/2.8 wide open it's not nearly as bright as my 50mm f/1.4 stopped down to 2.8.
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how did you measure that? Again, the difference should be very small unless they are very old and also completely different brands. Can you take a picture of a grey card with both and post here? Bracket the shot and i'll calculate the t-stop difference for you.
 
Shooter said:
"Doesn't work like that. F/2 on 50mm is going to be brighter than F/2 on the 85 and 28."

> No it is not.

"Say my first shot was with my 50mm 1.4 and I stop it down to 4, what would be the equivalent stop on my 20mm and 85mm?"

>It would be f4
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Are people saying that "shooter" is getting it wrong? Because, by all theory, he's exactly right. A stop is a stop is a stop.

When you're lighting with a light meter, you don't ever have to input what focal length lens you're using; you just put in the ISO of the film and the shutter speed, and the light meter tells you what to set the aperture at. F/8 on a 16mm lens should be providing identical exposure to f/8 on a 28mm lens, to f/8 on a 50mm lens, to f/8 on an 85mm lens.

This will not necessarily be the case with a zoom lens, as zoom lenses typically have t-stops that are quite a bit slower than their nominal f-stops, but with prime lenses the f-stop and t-stop should be very much the same.

The very widest-open f-stop may not deliver an equivalent to its t-stop, but any point stopped down some should be the same. Otherwise, how could one work with those lenses in any sort of external metering condition?
 
I totally understand the difference between an f-stop and a t-stop. As a D.P for over 30 years I have lived with t-stops for all my career. The variation between t and fs in my experience as marked on cine lenses is minimal. (less that a 1/4 stop)

Unless the OP uses lenses with T-stops marked ...he will never get the "accuracy" he is seeking. A stills lens is not marked in t-stops and is also incredibly difficult to set in small increments between f-stops. eg a line closed on F5.6 and one half .

The OP does seem to be quoting the wide aperture of the lenses as if this makes some difference to what he is trying to achieve. His use of the term "brighter" in reference to this puzzles me.
 
I tested 8 Nikkor prime lenses (on a Letus DOF) in the set up as shown behind me here:

http://www.dvxuser6.com/uploaded/3075/1233697045.jpg

I was testing color match of the lenses, sharpness and distortion. Framing of the set up was full frame color and rez chart for each lens. Centre frame was a grey scale. Exposure was f4 for every lens. I used a waveform monitor so knew exactly where the grey scale sat on the IRE scale.

All clips were examined in Premiere Pro and on a HD monitor. The scopes were viewed in Premiere and were consistent with external waveform monitor

The variation in the grey card scale on the scope between lenses was absolutely minimal (1-3 IRE units - ie 1-3%) and could easily have been attributable to "lens contrast/quality" as well as the fact some of these lenses are 20-30 years old mechanically. Would report also that the lenses color matched surprisingly well.

Biggest difference was "lens contrast". Best glass is contrasty glass.
 
"I totally understand the difference between an f-stop and a t-stop. As a D.P for over 30 years I have lived with t-stops for all my career."

Hehe I think matt made a mistake.

Do most cine lenses have a de-clicked aperture ring?
 
I do not recall ever seeing clicks on a cine lens aperture ring but that does not mean they do not exist.
 
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De-clicking the aperture ring on nikon primes might be a good option especially considering we can't fine tune exposure via ISO/shutter control. I'm finding the difference between stops too great to set the right exposure.
 
Seems to be a major "flaw" with the D90 and Canon camera (D5?) . I can not believe they were both released with this limitation.
 
Barry_Green said:
Are people saying that "shooter" is getting it wrong? Because, by all theory, he's exactly right. A stop is a stop is a stop.
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No he isn't. In practice he's not so far off but in theory he's completely wrong.
with prime lenses the f-stop and t-stop should be very much the same.
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define very much. Never exactly and for older lenses the difference can be significant.
Otherwise, how could one work with those lenses in any sort of external metering condition?
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Because the difference is small and if it isn't you compensate. Sometimes the t-stop is even indicated on the lens.
Come on guys. This is *really* basic stuff.
 
Shooter is saying that f/4 on one lens is going to give equivalent exposure to f/4 on another lens.

That's a fundamental observable law of exposure. How can you say he's "completely wrong"?

As for T-stops, again, in zooms they can vary by maybe 1/3 of a stop or so from the F-stop, and that's why you'll actually occasionally see t-stops marked on the lens. But in primes?

Come on guys. This is *really* basic stuff.
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That's why I assume there must be some misunderstanding here, because the concept of f/4 = f/4 and f/5.6 = f/5.6 is so incredibly basic and ingrained, I can't see how anyone would disagree with it?
 
Shooter said:
The variation between t and fs in my experience as marked on cine lenses is minimal. (less that a 1/4 stop)
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which is what i said and you didn't. In fact isn't it even worse to knowingly post false information than to be ignorant? :-)
His use of the term "brighter" in reference to this puzzles me.
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Why? If one lens has a lower t-stop than another it's brighter at the same aperture. I know what you're trying to do. I have a tendency to give advice that's way too complicated but simplifying to a point where it becomes misleading is not a good idea imo. It's like when people say that a 50mm "becomes" a 75mm on dx. This might be a useful way of thinking for a lot of people but it's *wrong*. :-) btw i didn't think you didn't know the difference, i've read your posts before, i was just trying to provoke a clarification, which worked.
 
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