T Factor
- Thread starter Barney
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- Kerry
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I don’t think the number of elements matters, but I asked a similar question once about different focal lengths - for example, a 100mm lens at F/4 will have an aperture width of 25mm, whereas a 16mm lens at the same f-stop will have an aperture width of 4mm, so in theory it will let in less light and need a slower shutter. The majority of answers I got said that it didn’t really matter, but I did find out that cine lenses have t-stops instead of f-stops, based on the amount of light they let in rather than just the aperture size.
I’m not entirely sure I’ve answered what you were asking there!
I’m not entirely sure I’ve answered what you were asking there!
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- Wayne
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But more food for thought Kerry, thanks!
sk66
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It's T-stop.
And no, even a single element f/5.6 lens won't transmit more light than a 12 element f/2.8 lens. An uncoated lens element may lose 2% so after 12 elements you would be down by ~ 28%; about 1/2 of a stop. Whereas f/5.6 is down by 75% (2 stops).
And no, even a single element f/5.6 lens won't transmit more light than a 12 element f/2.8 lens. An uncoated lens element may lose 2% so after 12 elements you would be down by ~ 28%; about 1/2 of a stop. Whereas f/5.6 is down by 75% (2 stops).
sk66
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It's light density, not necessarily the amount of light.
A 16mm lens will have a smaller aperture so it transmits less light from any given source (i.e. a street light bulb). But that bulb is also much smaller at the image plane. And therefore that less light is concentrated into a smaller area, thereby keeping the exposure of the source the same.
But what does matter is image area... I.e. if you use a longer FL lens at the same aperture in order to record the same composition on a larger sensor, then it does record more light than a shorter FL on a smaller sensor.
StephenM
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Since we seem to be in the realms of what obscure variables are there that can affect the absolute exposure, may I just call attention to Ansel Adams who calibrated all the apertures on his lenses because not all were accurate...
StephenM
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Actually, being pedantic, a 5 element lens will let in more light (strictly, pass more light) than a 12 element one at the same f stop. And two different 5 element lenses may differ in the exposure depending on aberration correction.
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- Graham
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As has been explained, aperture numbers are just a mathematical calculation based on the ratio between the size of the iris/aperture and the focal length.
But various other things can affect the amount of light actually reaching the film. The type of glass used for the lens elements, the number of lens elements, the lens coatings and the characteristics of the inner lens barrel. This is what T-stops measure, but the differences between t-stops and apertures are usually very small ( the lenses I've used have been marked with both)
Lenses used in the cinema industry usually have t-stops, because if you are cutting and splicing between film shot from multiple cameras of the same scene, you need the exposure from all those cameras to be "exactly" the same. Or switching lenses on the same scene, you need the exposure to be the "exactly" the same between lenses. Even tiny differences are noticeable and difficult to correct in post.
For most of us, even if T-stops are known, we can probably ignore them, especially if you are using TTL metering.
But it's still interesting to compare t-stops between lenses just to see how much the design of lens A compares to lens B, in terms of light transmission. especially comparing a high-quality prime lens with an equivalent focal length in a low quality zoom.
excalibur2
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Well I've always mentioned, in the past, why buy an expensive exposure meter to use with a film camera when considering manufacturing tolerances for lenses and shutter speeds (esp old used cameras) and there is no guarantee a film is exactly say 200 iso..ok if lucky they might all cancel out, but if they don't, then your exposure wont be correct....So use a good cheap exposure meter with an old film camera and let the neg film latitude compensate. For postive film using an old film camera you need a bit of luck?
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- Ian Grant
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The number of air/glass surfaces, particularly internal is far more critical than the number of elements.
If we take 3 classic lens designs, the Dagor, Tessar, and Celor/Eurynar (Dialytes), all have 4 elements. The Dagor has 2 cemented cells and only 2 internal air glass surfaces, the Tessar has one cemented cell and 4 internal air/glass surfaces. The Dialytes have 4 air spaced elements, and 6 internal air/glass surfaces.
Some tears ago I test these lens types using macro bellows on a DSLR, all in excellent optical condition. As expected the Dagor was the best, the Tessar not bad, and the Dialyte noticeably lower in contrast. Of course these were uncoated lenses, but the light loss is not significant, less than 1/3 of a stop.
As soon as you add coating the difference becomes insignificant, contrast similar. However, when you multiply up the air/glass surface that changes.
Transmission Stop is used in Cinematography where complex zoom lens designs may have around 20 - 25 elements. Cooke did make uncoated Zooms bofore WWII, but they only really became practical with lens coatings. Cameramen needed precision for exposure, particularly when changing lenses, or multiple camera shots, so lens light transmission was measured rather than calculated, hence the T stop.
We take a lot of terms for granted, take Focal length, at one time a lens might be sold or marked as as E.F. (Effective Focal Length) or Equiv - Equivalent Focus (Focal Length). The easiest way to explain this is Telephoto lenses. I have an ex-military 17" f5.6 Telephoto lens, it had no f-stop markings, to do that I needed to calculate the Effective Focal Length, an easy calculation as it's - f-stop x Aperture diameter, in this case around 10".
My 360mm f5,5 Tele Xenar needs approx the same bellows extension as my 210mm f5.6 Symmar S, so in reality the Tele-Xenar has an Equivalent FL of 360mm nut an Effective FL of around 210mm, checking a Schneider data sheet the lens has a Flange focus of 210.3mm.
It gets more complex with many wide angle lens designs, for SLRs, and then zooms, In practice a great many lenses are marked with their equivalent focal lengths, which is all we are interested in as photographers
Ian
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As Weegee (AKA Ascher Fellig) is so famously claimed to have said: "F8 and be there".
Personally, I've used everything from f64 to f1.2 at different times with some success...
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Yes, but I don't think he was actually giving advice on aperture choiceAs Weegee (AKA Ascher Fellig) is so famously claimed to have said: "F8 and be there".
Personally, I've used everything from f64 to f1.2 at different times with some success...
