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Why don't we use T-stop instead of F-stop?
- Thread starter fotos123
- Start date
rtex42
Leading Member
Hard to see how a ratio could be misleading, please explain.
I'm not sure I understand your question.
Maybe if it explain my question further it will help and forgive my less than perfect understanding of the specifics of either.
F-Stop is a standard that describes the mm length to diameter ratio of any given lens. AKA the Sigma art 50mm is a f.14 lens
T-Stop is the actual transmission of light for any given lens. Aka the sigma art 50mm T-stop is not 1.4
All lenses lose some light do to interference from the glass. Am I making any sens?
Maybe if it explain my question further it will help and forgive my less than perfect understanding of the specifics of either.
F-Stop is a standard that describes the mm length to diameter ratio of any given lens. AKA the Sigma art 50mm is a f.14 lens
T-Stop is the actual transmission of light for any given lens. Aka the sigma art 50mm T-stop is not 1.4
All lenses lose some light do to interference from the glass. Am I making any sens?
Oly Canikon
Veteran Member
f-stop does not take transmission loss into account so the light reaching the sensor can be somewhat less than a simple ratio would predict.
Sranang Boi
Veteran Member
No you are not making any sense. F stop is the standard definition. If it was changed, all old lenses and cameras would become obsolete since they would not comply to the T-stop you propose.
Personally I think that the weight of a lens should also be printed on the barrel somewhere.
MediaArchivist
Veteran Member
The T value for most lenses is known, so if that is more important (for you) the information is readily available. Lens manufacturers figured out quite some time ago that photographers tend to prefer f/stop when comparing lenses, but cinematographers (and videographers) tend to prefer T/stop. Their chosen designations follow their intended markets. Most lenses marketed as "cine" lenses are marked in T/stops.
I prefer comparing lenses according to their depth of field. Light gathering is a secondary consideration (for me), thus I fit the model that manufacturers expect for photographers.
I prefer comparing lenses according to their depth of field. Light gathering is a secondary consideration (for me), thus I fit the model that manufacturers expect for photographers.
That's my question. Why do we use a standard that doesn't tell us much about transmission. We all go crazy about how sharp lenses are how colour correct they are we want to know everything down to the last detail. But how much light they transmit (other that vignetting) we don't seem to care. So we use a model that doesn't reflect any given lens's actual characteristics.
Yes a get your point but in reality the difference (in still photography) between say 1.4 and 1.5 is close to non existent in reality.
By that I mean that if you take a shot at 1.4 T and then 1.5 t you will not know which is which.
By that I mean that if you take a shot at 1.4 T and then 1.5 t you will not know which is which.
Agreed I was just wondering if anyone knew why we use one over the other.
of the top of my head I can see how F-Stop could be useful for focus plane and bokeh and follow focus but then that makes me wonder why they use T-stop in film. With F-Stop you have standardization and with T-stop you don't.
MediaArchivist
Veteran Member
F-stop is measurable to a degree, it gives the photographer an idea of how shallow the DoF will be. For stills, a small change in shutter speed or ISO value is (usually) less important than the DoF desired for the shot. For film, particularly movie cameras that use a revolver-type set of primes, it is more important to have them all match in T/stop so the cuts are easier to edit together later.
Leonard Migliore
Veteran Member
No, because the f/stop controls the depth of field. This is often quite important in the way a photographer sets up a shot.
The T-stop is just a measure of the light going through a lens. With through-the-lens metering, which all digital cameras have, the exact light transmission is not particularly important. If you expose for 1/90th of a second instead of 1/100th, it won't make any difference in the final image. So the f/stop is more important than the T-stop.
In cinematography, it's important to have the same brightness when you change lenses during a scene. This is why T-stops are preferred for cinema lenses.
It is conceivable that one would look at the transmission of a lens before buying it; it can be a selection factor. I would be reluctant to buy a lens that lost half a stop. But then, once you've bought it, you can forget about it.
So continuity. That explains why film needs T-stop and also explains why photo uses F-stop but it doesn't explain why photo ignores T-Stop.
I know I'm being annoying at this point. ignore me if you want.
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tko
Veteran Member
Not really, at least not by itself. Subject magnification is more important. So is viewing size. F/stop, taken by itself, is only useful for the same format and is not the primary determination of DOF.
I guess you could say for the same subject and same sensor and same viewing size, f/stop can be used the modify the DOF.
You might think I'm quibbling, but there are always these posters who think because they've have an image of a bird with a blurred out background, their lens exhibits shallow DOF. No, it's more due to the high magnification.
tony field
Veteran Member
The camera reads the light falling on the sensor or really the metering system. Therefore the camera does not need knowledge of the lens t stop since the lens absorption of light is implicit in the light meter reading. The only reason the camera is concerned about the f-stop is that the metering system is usually working at full lens aperture and the exposure system has to compute and set the actual shooting F stop.
Think of it this way... If you put a 3 stop neutral density filter on your F 1.4 lens, you logically have a T4 lens. The camera automatically takes this into account and could care less about the fact that you put on a 3 stop filter and have an effectively T4 lens.
--
Charles Darwin: "ignorance more frequently begets confidence than does knowledge."
tony
http://www.tphoto.ca
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tkbslc
Veteran Member
Guy Parsons
Veteran Member
For the usal stills photographer the transmission factor simply does not matter.
They are more interested in the f/stop because of expected depth of field, or known sweet spot of the lens. The camera takes care of transmission differences by altering shutter speed and/or ISO to make up the difference. Simple.
Regards...... Guy
Guy Parsons
Veteran Member
[edit.... I went stupid and replied instead of edit that post above, I was just trying to fix a typo and I went berseko.]
"Usual" instead of "usal" is what I wanted to correct.
"Usual" instead of "usal" is what I wanted to correct.
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