t stops

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#1 Kevin Mastman

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Posted 27 December 2007 - 03:36 PM

Greetings,

I perfectly understand what is meant by f stop. I was hoping that someone could explain to me what a T-stop is and how it is used on set; how it translates to fstops.

Thanks a lot.

Kevin
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#2 Stephen Williams

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Posted 27 December 2007 - 03:59 PM

Greetings,

I perfectly understand what is meant by f stop. I was hoping that someone could explain to me what a T-stop is and how it is used on set; how it translates to fstops.

Thanks a lot.

Kevin

Hi,

F stop is calculated but take no account of light absorbed in the glass of the lens. A T (transmission) stop is the amount of light that would actually hit the film.

If a lens has T stop markings use them, if only F stops use them instead.

To complicate matters, Depth of field is calculated using F stops.

Stephen

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#3 Chris Keth

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Posted 27 December 2007 - 09:14 PM

You can, for most purposes, think of T-stops as "accurized" F-stops. Properly, F-stops are still used to calculate depth of field but in practice, on the set, lenses usually only have T-stops marked so those are used in that application as well.
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#4 Mike Lary

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Posted 27 December 2007 - 09:16 PM

T-stops are determined by taking light measurements from the bottom of a given lens, so any lens that has t-stop measurements has the potential to have more accurate markings than one that has f-stop markings.
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#5 Stephen Williams

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Posted 28 December 2007 - 02:50 AM

T-stops are determined by taking light measurements from the bottom of a given lens, so any lens that has t-stop measurements has the potential to have more accurate markings than one that has f-stop markings.

Hi,

That is true for expensive motion picture lenses, don't expect the same from cheap new lenses marked in T stops. Made in England is not a guarantee of quality.

Stephen
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#6 Michelob Fedusenko

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Posted 28 December 2007 - 04:29 AM

F-Stop = Fractional Stop - The ratio of the the size of the aperature over distance of the rear element to aperature.

I.E. Let's pretend we are shooting at an F-5.6. That means that if the rear element of the lens is a 1 inch from the aperature then the aperature opening is .17857143 inches wide.

T-Stop = Transmission Stop - How much light passes through the lens to the film plane while also taking in account how much is lost traveling through the glass elements. This is done individually for each lens for each lens is slightly different depending on the material used (the purest quartz in the world found in Australia).

The reason that DoF is not measured using T-stops is because DoF is caused by the mechanical functions of the lens:
F-stop
Focal distance
Focal length
Film Size

DoF is independent to the amount of light hitting the film plane.

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#7 Saba Mazloum

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Posted 28 December 2007 - 06:52 AM

F-Stop = Fractional Stop - The ratio of the the size of the aperature over distance of the rear element to aperature.

I.E. Let's pretend we are shooting at an F-5.6. That means that if the rear element of the lens is a 1 inch from the aperature then the aperature opening is .17857143 inches wide.

T-Stop = Transmission Stop - How much light passes through the lens to the film plane while also taking in account how much is lost traveling through the glass elements. This is done individually for each lens for each lens is slightly different depending on the material used (the purest quartz in the world found in Australia).

The reason that DoF is not measured using T-stops is because DoF is caused by the mechanical functions of the lens:
F-stop
Focal distance
Focal length
Film Size

DoF is independent to the amount of light hitting the film plane.

So the more F-stop more the DOF, less the F-stop shallower the DOF. Right? and when i say shallow , im meaning when you only have the actor in focus and the background is out of focus.. right?

Thanks
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#8 Stephen Williams

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Posted 28 December 2007 - 11:46 AM

So the more F-stop more the DOF, less the F-stop shallower the DOF. Right? and when i say shallow , im meaning when you only have the actor in focus and the background is out of focus.. right?

Thanks

Hi,

As F stops are fractions 1/2, 1/2.8, 1/4, 1/8, 1/11, 1/16 the fractions are actually getting smaller (as is the hole) as the DOF increases!

Stephen
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#9 Kevin Mastman

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Posted 28 December 2007 - 12:16 PM

thank you everyone for the responses. I am still confused as to how the exposure is translated in tstops. Do tstops also take in to account various filters that are put on the lens that take away light from the film? When the gaffer takes reading of the exposure does he or she communicate to the camera team in fstops and then the camera team converts it to tstops or does the gaffer communicate in tstops?
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#10 Stephen Williams

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Posted 28 December 2007 - 12:33 PM

thank you everyone for the responses. I am still confused as to how the exposure is translated in tstops. Do tstops also take in to account various filters that are put on the lens that take away light from the film? When the gaffer takes reading of the exposure does he or she communicate to the camera team in fstops and then the camera team converts it to tstops or does the gaffer communicate in tstops?

Hi,

Where I come from the DOP is the one who decides what T stop to shoot at, I often expose differently from the no my meeter reads depending on how I want the scene to look. Any filters must be taken into account (or not).

There is no need to convert F stops to T stops

Stephen
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#11 David Mullen ASC

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Posted 28 December 2007 - 01:15 PM

T-stops and f-stops are the same thing in practical terms on a set, the term is used interchangeably. Meters read in f-stops, lenses are (usually) marked in T-stops. So if a DP calls out a stop, that's the stop.

Gaffers don't tell camera crews normally how to expose the scene, unless he's taken over temporarily from the DP. But a Gaffer may ask the DP, sometimes the AC, what ASA rating he should be setting his meter to. ND's normally don't come into play except outdoors in daytime when the Gaffer isn't normally taking meter readings.

Anyway, if there are ND's or Polas on the camera, normally an AC will remind the DP when the stop is called out. If I say "T/2" or "f/2", the AC will say "that's with the Pola, right?" or "we're still at a 90 degree shutter".

Some DP's don't compensate for different frame rates or shutter angles or even filters and let the AC's do that. I don't work that way though.

But sometimes when running several cameras on a big stunt and some are running at different speeds and I have to take a quick reading of the sun before they all roll, I tell the AC's that my reading is for 24 fps, let's say, so when I call out the stop over the walkies, anyone running off speed should make the correct adjustment in exposure. Saves time over calling each camera crew over the walkie and giving them individual stops. Don't want one camera mishearing an instruction for another camera.

You compensate for filter loss either mentally or by changing the ASA rating on the meter. For example, if you put an 85ND9 on the camera, which is 3 and 2/3's of a stop loss, if your base ASA rating is 200 ASA, you can either leave the meter set to that, take a reading, and then remember to open up 3 and 2/3's of a stop from that reading, or you can set your ASA rating on the meter to 16 ASA instead of 200 ASA.
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Posted 29 December 2007 - 09:43 AM

Anyway, if there are ND's or Polas on the camera, normally an AC will remind the DP when the stop is called out. If I say "T/2" or "f/2", the AC will say "that's with the Pola, right?" or "we're still at a 90 degree shutter".

Some DP's don't compensate for different frame rates or shutter angles or even filters and let the AC's do that. I don't work that way though.

In the UK they refer to this a Base Stop shooting or Shooting Stop. Just like David does, I give them the shooting stop for every new setup. But I know many DP's that give the AC's a base stop and let's the AC adjust the stops according to what filters and speeds are used etc.
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#13 Stuart Brereton

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Posted 29 December 2007 - 10:14 AM

I know many DP's that give the AC's a base stop and let's the AC adjust the stops according to what filters and speeds are used etc.

I don't think this is very fair to your AC. It's the DP's responsibilty for exposure. Leaving the maths to someone else seems like asking for trouble.
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#14 David Mullen ASC

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Posted 29 December 2007 - 12:12 PM

I don't think this is very fair to your AC. It's the DP's responsibilty for exposure. Leaving the maths to someone else seems like asking for trouble.

Like I said, the only time I did it was on "Astronaut Farmer" when I had four cameras in remote locations filming the doors of the main barn being blown-up. One camera crew was hidden in the air stream trailer, one hidden in the house, one camera was buried in a hole next to the doors, and one was on a hilltop far away -- and all had different speeds, some were shooting through tinted windows, etc. And with the sun going in and out of the clouds, I had to take a reading and call out a stop just before we rolled.

Rather than confuse the airwaves and AC's over the walkies with "A is at a 5.6-8 split, B is at a 4, C is at a 5.6, D is at a 2.8-4 split..." or switch to individual channels for every camera crew, I decided I'd call out one stop and every AC was told what the compensation factor was.

But otherwise, yes, I do the math because I need to be reminded of any filters, shutter angles, etc. I don't want to be calling out a base stop all day and then find out I never took the camera out of a 45 degree shutter I intended to be used for only one shot.

When I call out a stop and the camera is set-up unusually I usually say this as well, like "T/2.8 at 48 fps" or "T/2.8 with a 90 degree shutter" so the AC knows I compensated for that. Otherwise he's just going to ask me anyway (which he should.)
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#15 Stuart Brereton

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Posted 29 December 2007 - 12:49 PM

Rather than confuse the airwaves and AC's over the walkies with "A is at a 5.6-8 split, B is at a 4, C is at a 5.6, D is at a 2.8-4 split..." or switch to individual channels for every camera crew, I decided I'd call out one stop and every AC was told what the compensation factor was.

In a multi camera setup like you describe, calling a base stop is the only sensible solution. I was referring more to DPs who call base stop all the time. Asking an AC to compensate for multiple filters & shutter variations just before the Camera rolls, while they're also trying to think about focus is risky. No-one is checking their math. If the DP calls a shooting stop, referencing any non standard setup (as you do) then both the DP and the AC know what's being compensated for.

One of my regular ACs here always raises an eyebrow and points at the 'POLA' sticker on the mattebox when I call a stop that he thinks sounds 'unusual'....
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#16 David Mullen ASC

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Posted 29 December 2007 - 01:05 PM

I was once screwed over when after moving from outside to inside in daylight, I shot an important crying scene of an actress only to discover that the AC had left the Pola in the camera and no one noticed (this was an old Arri BL3 so the darker eyepiece image wasn't a clue, especially since we had just come from outside and had gotten used to the darker image.)

This is why I am religious about making sure the AC's properly put filter labels on the mattebox.
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#17 Alexander McCarron

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Posted 25 June 2008 - 07:25 PM

I have a question about depth of field.

I have a Scoopic 16M that's only marked with t-stops. I know that t/2.5 = f/1.8 on my lens.

Should I just subtract 1.3 from any given t-stop to get the proper f-stop if I want to measure the depth of field?

Thanks.
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#18 Stephen Williams

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Posted 26 June 2008 - 04:18 AM

I have a question about depth of field.

I have a Scoopic 16M that's only marked with t-stops. I know that t/2.5 = f/1.8 on my lens.

Should I just subtract 1.3 from any given t-stop to get the proper f-stop if I want to measure the depth of field?

Thanks.

Hi,

No, you will be totally wrong. Allow 1/2 stop will be close enough.

Stephen
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#19 Jim Carlile

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Posted 10 July 2008 - 12:28 AM

thank you everyone for the responses. I am still confused as to how the exposure is translated in tstops. Do tstops also take in to account various filters that are put on the lens that take away light from the film? When the gaffer takes reading of the exposure does he or she communicate to the camera team in fstops and then the camera team converts it to tstops or does the gaffer communicate in tstops?

Like somebody else said T-stops are really just accurate F-stops that take into consideration the light absorption of the glass elements, which could be particularly a problem with big zooms.

So, if the meter says F/5.6, then just set the lens T-stop for 5.6. In the old days-- or if the lens isn't calibrated for T-stops-- you would have to figure, 'well, the meter says 5.6, but I know my big zoom absorbs at least one full stop of light, so I have to mentally compensate for that light loss, so I better open up the lens to something like F/4 instead to make up the difference...'

The T-stops take that into consideration. For many older primes such as Bolex and the like, there's not much of a difference between the two. Some T-stops calculations actually take into consideration the light loss through the camera path as well.

BTW, the T-stop concept was invented by the Twentieth Century Fox camera department in the very early 40s and the 'T' originally stood for the name of the studio. They even got an Academy Award for the development. Prior to that, DPs ran their own equipment all the time and knew the various idiosyncrasies of their gear. With the growing studio system, camera interchangeability was needed at a quick grasp.
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