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# Thread: light and f stop

1. ## Better to"f" than"T"?

Originally Posted by kaiyen
Claire,
The size of the aperture opening is a fraction/ratio, but the "f" in f-stop stands for focal length, not fraction. So it is a fraction, but not for the reason you state.

To be more accurate, it's the "/" in the more correct "f/stop" that means fraction. not the "f" itself.

allan
I've always thought that the T stop system used on zoom lenses in the cine industry is probably a better system as an indication of an optics actual light transmission ability rather than a geometric system, and from some of the experiences I have had with long multi-element zooms f8 is not the same on some lenses, probably due to the reflective index of the glass elements , and the type of coating used in it's manufacture.

2. Originally Posted by Dan Fromm
But seriously and all kidding aside, do the arithmetic.
Do the arithmetic correctly and there will
be no argument.

Take this case: The allowed circle of confusion is
.01 inch or .25mm. What focal length lens will allow a
near distance of 8 feet and a far distance of infinity
at an aperture of f8? Dan

3. Originally Posted by dancqu
Do the arithmetic correctly and there will
be no argument.

Take this case: The allowed circle of confusion is
.01 inch or .25mm. What focal length lens will allow a
near distance of 8 feet and a far distance of infinity
at an aperture of f8? Dan
Cute, but irrelevant.

According to the standard DOF formulas I copied from Cox, you can do it with an 85 mm lens focused to around 7.4 m. I'll be happy to send you the spreadsheet by e-mail.

That's nice, but you can't get an acceptably sharp contact print with a CoC of 0.25 mm, let alone an acceptably sharp enlargement. And if you're shooting MF I doubt you're contact printing.

Dave, do you contact print your 35 mm (I assume that's what you shoot wildlife with, if I'm mistaken, sorry) wildlife shots?

So why bother? Why pick an absurd CoC? It makes you look sillier than I'm sure you are.

Cheers,

4. Originally Posted by Bentley Boyd
I've always thought that the T stop system used on zoom lenses in the cine industry is probably a better system
just curious. what is the t stop system?

allan

5. "just curious. what is the t stop system?"

Allan,

T stops are transmission numbers that correspond to the theoretical transmission of an f-stop of the same number - they take losses into account, which f-stops don't because they are simply the ratio of the focal length and the entrance pupil diameter. Most ciné lenses are marked in T-stops and f-stops, the difference being about a quarter or a third of a stop for primes and half to two-thirds of a stop for zooms. So you have a nice fast f/0.95 25 mm lens that is really only a slow old T 1.1, or an f/1.1 16 mm to 44 mm zoom that barely makes it to T 1.3, for example. Most of the losses occur at air-glass surfaces, so the lenses with a lot of elements lose out.

Strictly speaking, the f-stops are used for focus calculations and the T-stops for exposure.

Best,
Helen

6. It gets even more fun with old uncoated lenses: If each glass/air interface loses 4% of the light to reflections, an 8-surface lens like a Rodenstock Eurynar will only transmit 60% of the light. But there will be more light in the shadows, as some of the light loss ends up on the film as flare. So an Eurynar at f:8 might be said to give T 10, but the shadows might well be lighter than with a multicoated lens at f:5.6...

7. So.... what this is leading to is... that... bohica WAS onto something? Or am I misunderstanding where this discussion is heading?

And also - is there a definite, fixed relationship between a T stop and an f stop? My understanding of this so far is that it strictly depends on a particular lens (or even type of glass used?) - is that correct?

I am trying to grasp this, but I am afraid my grasp of the technical aspects of this is a little bit lacking...

8. The relationship between T-stop and f-stop is not fixed - it does depend on the lens so you understand correctly. Strictly speaking, this does mean that f/2.8 on one lens does not necessarily 'let in as much light' as f/2.8 on another lens, but in practice the difference is negligible, especially between two prime (ie non-zoom) lenses.

For point sources of light, like stars, the diameter of the entrance pupil (absolute aperture) is important, rather than the f-stop (relative aperture). For example, a 100 mm f/2 does let in more light than an 80 mm f/2 in this case, and there is a fixed relationship, because the 100 mm has an entrance pupil 50 mm in diameter and the 80 mm has one 40 mm in diameter. But this only applies to point sources of light, like stars.

Best,
Helen

9. You're right, gnashings. The T stop at a given f stop depends on the type of lens, glasses used, and sometimes even age of lens (like the radioactive APO-Lanthars - mine loses a third of a stop to yellowing).

So there is no fixed relationship. That's the great advantage of TTL measurements, and the reason noone notices that their superduper f:4 IS superzooms are really T 7.2!

10. Originally Posted by Ole
You're right, gnashings. The T stop at a given f stop depends on the type of lens, glasses used, and sometimes even age of lens (like the radioactive APO-Lanthars - mine loses a third of a stop to yellowing).

So there is no fixed relationship. That's the great advantage of TTL measurements, and the reason noone notices that their superduper f:4 IS superzooms are really T 7.2!
Um, Ole, my first Beaulieu was a 4008 ZM that had an 8-64/1.9 Angenieux zoom. The lens t-stopped around t/3.3, and TTL metering or no I was painfully aware of how slow it was.

Cheers,

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