Can't seem to find any info on this..
- Thread starter nho
- Start date
j1clark@ucsd.edu
Veteran
Look up T-stop. It is a calibrated with actual test measurements number. An F-stop is sort of 'in the ball park', but is not really calibrated and tested by the manufacturer.
Since the lens you are looking at is a PL mount that would be for a motion picture camera, and so, would have a T-stop. The difference being if you take lens 1 set it to T-stop X,
and take lens 2 and set it to T-stop X the actual light transmitted through the lens is 'identical' (within calibration tolerances...). This is important in motion pictures where one
may have a 'wide' lens for a master shot, and several different lenses for closeups or coverage shots, and there would be differences in the negative density (or digital capture...)
if the lenses were not individually calibrated for how much light they allowed through for a given setting.
This is not necessarily so with a F-stop marked lens.
Erich Ocean
Active member
F-stop is based on the size of the aperture opening, full stop. T-stop includes the contribution of the glass, which obviously varies between lens designs, so it represents the actual amount of light that makes it to the sensor.
I think f-stop says how much your aperture is physically open or closed, and t stop says how much light is transmitted. t=transmission. this could vary between lens and lens so i think because of the demanding precision of cine glass they use t. truer indication of how much light as opposed to how wide the aperture is open... hope i'm not wrong.
Gabel
Well-known member
At T 2.8 yes, but at F 2.8 there might be a difference. For example a 2.8 zoom usually has a T-stop of 3, meaning it looses quite a bit of the glass.
Don't worry you're right.
Ryan Patrick O'Hara
Veteran
F stop is a formula. A formula that doesn't and can't account for the light loss of the real world physics of light passing through glass.
Thus a T-Stop is a true measurement of the light transmission through the lens... T-stops are more accurate.
If a photography lens is listed as a F/2... expect it to behave more like a T/2.2 or 2.5... I can't tell you what it would be, since it depends on coatings and the number of lens elements, but I can tell you that T-stops are more accurate and F-stop lenses are really slower than they are advertised. By a half stop, quarter stop or less.
Thus a T-Stop is a true measurement of the light transmission through the lens... T-stops are more accurate.
If a photography lens is listed as a F/2... expect it to behave more like a T/2.2 or 2.5... I can't tell you what it would be, since it depends on coatings and the number of lens elements, but I can tell you that T-stops are more accurate and F-stop lenses are really slower than they are advertised. By a half stop, quarter stop or less.
Ryan Patrick O'Hara
Veteran
A F/2.8 lens will not let as much light through as a T/2.8 lens. How much entirely depends on the specific F-stop lens being evaluated.
Some lenses are worse than others... some modern and nice photography lenses say F/2.8 and they are just a T/3 in reality. However older lenses, or zoom lenses without good coating can be much less efficient.
Last edited:
Gabel
Well-known member
I know, I was in a bit of a hurry when I wrote it. What I meant was that using two F2.8 lenses might yield difference in brightness, but two T2.8 would yield the same exposure between them all. And as said, T-stops are brighter (and therefore have shorter depth of field) than the same number in F-stops.
Ryan Patrick O'Hara
Veteran
I suppose to compensate for light loss from light bouncing off elements, and the number of elements, a T-stop lens would have to mark the aperture accordingly, where as the F-stop lens is a formula that does not account for the lost light.
Therefore, I would assume that a T-stop lens at a T/2.8 would have a fractionally more open iris blade assembly versus a F/stop lens at F/2.8...
I don't know for certain, but thinking about it logically, that is my conclusion. Good point you make.
But also remember that I've found most differences are around a third or quarter of a stop... if that. Sometimes more... I think the most I've seen was a difference of half a stop, but that was a really really old lens...
Point being, I really doubt if you can tell the depth of field difference between a 2.8 and a 2.8 3/10ths. However, I suppose it is true. I suppose that is why T-stop lenses are so important. If striving for the highest level of consistency in an image, using different F-stop lenses could produce different exposures, albeit very slight in nature. I would assume this amount of variation can be almost nothing to very different depending on what lenses you are pairing together.
I've never thought of that before. Really cool point you made.
Therefore, I would assume that a T-stop lens at a T/2.8 would have a fractionally more open iris blade assembly versus a F/stop lens at F/2.8...
I don't know for certain, but thinking about it logically, that is my conclusion. Good point you make.
But also remember that I've found most differences are around a third or quarter of a stop... if that. Sometimes more... I think the most I've seen was a difference of half a stop, but that was a really really old lens...
Point being, I really doubt if you can tell the depth of field difference between a 2.8 and a 2.8 3/10ths. However, I suppose it is true. I suppose that is why T-stop lenses are so important. If striving for the highest level of consistency in an image, using different F-stop lenses could produce different exposures, albeit very slight in nature. I would assume this amount of variation can be almost nothing to very different depending on what lenses you are pairing together.
I've never thought of that before. Really cool point you made.
Last edited:
Gabel
Well-known member
Yeah, I actually came to think about it when ACing, as I had a DoF calculator in F-stops... But we were using Zeiss Superspeeds (T-tops) and that was when it hit me:
The corresponding F-stop would have a smaller aperature than the corresponding T-stop.
Take for example my LOMO lenses. I have a set of OCT-19 primes. All have an F-stop of f/2, while the T-stops are all rated at T/2.4. So when I set them on 2.8, the T-stop of 2.8 is closer to f/2 than f/2.8!