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F stop vs. T stop

f stop t stop aperture exposure lens

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#1 Christian Schonberger

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Posted 02 April 2016 - 09:55 AM

Hi group,

 

Well I was always wondering: Why don't lenses provide the F stop which exactly corresponds to the light actually hitting the film emulsion or sensor (not accounting for lens flare and other random factors, just the glass inside the lens)? So in reality: if I get an F stop value from, say, a light meter, I most likely will get it in T stop. Otherwise the "exposure triangle" would make no sense (to me that is). Looks like some lenses are reducing as much as one stop while others perhaps a third of a stop. There is some information for some lenses regarding the actual T-stop, but many don't come with that information - and even with that information: what to do? Now add certain filters and you're into some serious math....

 

The only thing that remains the same (F stop/T stop) is of course the depth-of-field/focus.

 

Fully automatic digital cameras with 100% matching lenses (or built-in ones) most likely take all that into account (to varying degrees of success IMHO). But what about using different lenses with no electronics and a 100% manual exposure? With film, once it's in the camera (and you don't have a variable shutter angle) your only way(s) to control exposure is/are lighting, filters and F stop.

 

Please let me know how you go about the F stop vs. T stop issue and avoid under exposure.

 

As always: any reply highly appreciated.

 

Christian


Edited by Christian Schonberger, 02 April 2016 - 09:56 AM.

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#2 Mark Dunn

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Posted 02 April 2016 - 10:23 AM

You use the T-stop. Cine lenses are usually marked in T-stops but it's only really relevant for zooms or complex lenses with a lot of elements. If necessary, you find it out by testing.

Even for a fairly long zoom it's usually only about 1/3 stop.


Edited by Mark Dunn, 02 April 2016 - 10:24 AM.

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#3 Brian Drysdale

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Posted 02 April 2016 - 10:27 AM

You may find they're you're getting ramping of the exposure at the longer focal lengths on zoom lenses. It's unusual to get a one stop difference between f and T stop on modern glass, on old, 1960s zooms it could get into that range.


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#4 Christian Schonberger

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Posted 02 April 2016 - 11:22 AM

Mark and Brian,

 

Thanks for the information!

 

Christian


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#5 David Mullen ASC

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Posted 02 April 2016 - 12:14 PM

A T-stop is an f-stop mark corrected for actual transmission (exposure).  The only reason to use an f-stop mark instead is for depth of field charts because depth of field is still determined by the size of the aperture, not the amount of light reaching the film.

 

Besides more accurate exposures from T-stops, when you use multiple cameras and set them all to the same T-stop, you are likely to get closer to the same exposures than if the lenses only had f-stop marks.

 

Now in reality though I've found that even T-stop marks are not completely accurate, something you wouldn't have spotted shooting film but you can see on digital cameras.  For example, shooting on the three lightweight Angenieux zooms, 15-40mm, 28-76mm, and 45-120mm, I've always found that the 45-120mm image was slightly brighter than the other two if they all had the same T-stop.


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#6 Christian Schonberger

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Posted 02 April 2016 - 01:06 PM

A T-stop is an f-stop mark corrected for actual transmission (exposure).  The only reason to use an f-stop mark instead is for depth of field charts because depth of field is still determined by the size of the aperture, not the amount of light reaching the film.

 

Besides more accurate exposures from T-stops, when you use multiple cameras and set them all to the same T-stop, you are likely to get closer to the same exposures than if the lenses only had f-stop marks.

 

Now in reality though I've found that even T-stop marks are not completely accurate, something you wouldn't have spotted shooting film but you can see on digital cameras.  For example, shooting on the three lightweight Angenieux zooms, 15-40mm, 28-76mm, and 45-120mm, I've always found that the 45-120mm image was slightly brighter than the other two if they all had the same T-stop.

Thanks for the information, David. You are very likely referring to focal lengths applied to a 35mm (type) sensor with 50mm being the "mid point" standard. So 15-40 would be a wide angle zoom, then a mid range zoom (around 50mm) and the brighter 45-120mm would be a mid range (standard) to moderate telephoto zoom, correct?

 

Thanks again,

Christian


Edited by Christian Schonberger, 02 April 2016 - 01:07 PM.

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#7 Carl Looper

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Posted 03 April 2016 - 05:50 PM

Yes, as David pointed out the f/stop is useful for depth of field calculations. I've been recently writing software for a film out setup in terms of what allowable error range there could be in lens focus before the definition of the result became unacceptable. And for this the f/stop, rather than the t/stop, became the important number - along with the "circle of confusion" number one wanted to use in defining the range of acceptable softness either side of the optimum focus point.

 

C


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#8 Christian Schonberger

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Posted 03 April 2016 - 08:18 PM

Yes, as David pointed out the f/stop is useful for depth of field calculations. I've been recently writing software for a film out setup in terms of what allowable error range there could be in lens focus before the definition of the result became unacceptable. And for this the f/stop, rather than the t/stop, became the important number - along with the "circle of confusion" number one wanted to use in defining the range of acceptable softness either side of the optimum focus point.

 

C

Thanks for the information. I always wondered how the depth of field is actually behaving. It is very likely some sort of distorted bell curve with the focus rolling off on both sides (obviously steeper on the near side and shallower on the far side - and the curve widening as the focal point moves farther away). I'm sure there are graphics showing how different lenses behave at different apertures and with different focal points, just like frequency and directional response curves for microphones. Would love to take a look, just to get a basic idea. I'm sure this is very complex taking lens aberrations and lens blur behavior/distortion (the good old bokeh) into account - not to mention what is considered truly acceptable (here come-in sensor pixel resolution, noise and data compression artifacts - and film grain structure and light bouncing artifacts within the emulsion layers in color film).

 

Cheers,

Christian


Edited by Christian Schonberger, 03 April 2016 - 08:23 PM.

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#9 Carl Looper

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Posted 03 April 2016 - 10:58 PM

The f-number is the result of dividing the focal length of the lens by the diameter of the iris. Or reciprocally, the diameter of the iris is the result of dividing the focal length by the f-number.

 

When we write an f-number (N) in the form: f / N, we're literally representing the diameter of the iris: focal length (f) divided by the f-number (N)

 

For example, when we write:

 

f/5.6

 

we're literally expressing the diameter of the iris. All that remains is to replace f with the focal length of the lens and perform the division.

 

For example, with a 25mm lens, and an iris setting of f/2.8, the expression f/2.8 literally means: 25mm / 2.8, which means: 8.9mm (the diameter of the iris).

 

https://en.wikipedia.org/wiki/F-number

 

Given the diameter we can calculate depth of field:

 

https://en.wikipedia...he_DOF_formulae

 

C


Edited by Carl Looper, 03 April 2016 - 11:09 PM.

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#10 Christian Schonberger

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Posted 04 April 2016 - 02:19 AM

Thanks a lot for the reply. Yep, I knew about the f-stop formula and that it is literally using focal length and aperture (I just couldn't remember what is divided by what). Thanks also a lot for pointing out the article. Excellent images to see how the blur appears gradually when the depth of field is very narrow. I only know extremely narrow depth of field from 150 year old metal plate silver stills photography: the eyes are sharp and the eye lashes are already blurred ;-)

 

Thanks again,

Christian

 

Christian


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