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What is the difference between f stop and t stop


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#1 Arni Heimir

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Posted 09 January 2006 - 11:49 AM

What is the difference between f stop and t stop? If my light meter says F 2.0 7: Would I then set the aperture to T2.0 III?

Thank you for reading this post.

Árni Heimir
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#2 Stephen Williams

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Posted 09 January 2006 - 12:10 PM

What is the difference between f stop and t stop? If my light meter says F 2.0 7: Would I then set the aperture to T2.0 III?

Thank you for reading this post.

Árni Heimir
Iceland



Hi,

An F stop is mathematical. A T stop takes care of any light losses through the glass. On a zoom lens thy may be
1/3-1/2 stop loss through the glass. On a prime somewhat less.

F 2.07 is very close to F2.

Stephen
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#3 Arni Heimir

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Posted 09 January 2006 - 12:48 PM

Interesting, so the rule of thumb is that if you light meter says F2.8 you would then set the t stop at 2.5?
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#4 Scott Fritzshall

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Posted 09 January 2006 - 01:35 PM

The T-Stop is the F-stop after factoring in light lost to optical elements in the lens. If your lens is marked in T-Stops, treat them as though they were F-stops. So if your light meter reads 2.8, you set your lens to 2.8, regardless of whether it's T or F.
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#5 Dominic Case

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Posted 09 January 2006 - 06:53 PM

Treat your light meter reading as a T/stop setting.

If you then need to know about deoth of field, you must use the F/stop that the lens is actually set at.

So if your meter says 2.8, set the lens to T/2.8. If there is an F/stop scale on the lens too, then you can read it off to find out the depth of field from a set of tables. It will probably be around F/2.5.

Still photographers don't seem to use T/stops, and tend to overlook the actual loss of light in the lens. I have no idea why this is.
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#6 Tim J Durham

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Posted 09 January 2006 - 08:41 PM

Treat your light meter reading as a T/stop setting.

If you then need to know about deoth of field, you must use the F/stop that the lens is actually set at.

So if your meter says 2.8, set the lens to T/2.8. If there is an F/stop scale on the lens too, then you can read it off to find out the depth of field from a set of tables. It will probably be around F/2.5.

Still photographers don't seem to use T/stops, and tend to overlook the actual loss of light in the lens. I have no idea why this is.

Polaroid backs. I've never had a problem shooting to f-stops with my Hasselblad. As for 35mm, they've had TTL metering for 4 decades or more.
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#7 Dickson Sorensen

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Posted 09 January 2006 - 09:51 PM

Still photographers don't seem to use T/stops, and tend to overlook the actual loss of light in the lens. I have no idea why this is.
[/quote]

Remember it this way T (stop) is for Transmission F (stop) is for Focus

I believe that the reason still photographers are not as concerned is that they are not trying to match different lenses to exactly the same exposure for cutaways and closups. They do not use Zooms with as many elements where the light transmission loss becomes significant. In still photography they have had much more control of contrast and exposure in the darkroom.
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#8 Tim J Durham

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Posted 10 January 2006 - 09:54 AM

Still photographers don't seem to use T/stops, and tend to overlook the actual loss of light in the lens. I have no idea why this is.

What do you mean "overlook the actual loss of light in the lens"?

Any still photographer that's been doing it for more than a month knows the relationship between his meter and camera. Plus, nearly ALL 35mm still cameras now meter exposure OFF OF THE FILM PLANE and have for many years. Light loss is compensated for at every step if you understand how your camera works.
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#9 Dickson Sorensen

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Posted 10 January 2006 - 11:14 AM

What do you mean "overlook the actual loss of light in the lens"?

Any still photographer that's been doing it for more than a month knows the relationship between his meter and camera. Plus, nearly ALL 35mm still cameras now meter exposure OFF OF THE FILM PLANE and have for many years. Light loss is compensated for at every step if you understand how your camera works.


Lens manufactures put markings on still lenses because there are many situations a professional still photographer (even with a through the lens meter) will calculate his exposure with a hand held meter. Some amateur cameras don't provide markings on the lenses as they just confuse the user.
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#10 Tim J Durham

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Posted 10 January 2006 - 04:53 PM

Lens manufactures put markings on still lenses because there are many situations a professional still photographer (even with a through the lens meter) will calculate his exposure with a hand held meter. Some amateur cameras don't provide markings on the lenses as they just confuse the user.

I was taking umbrage with your assertion that still photographers were somehow unaware that the lens elements caused some degree of light loss before the film plane.

None of this explains WHY film lenses use T stops and stills lenses use ? stops. So why do film cams use T-stops? I have no idea. You need to know how much light a cine lens absorbs as well, right?
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#11 Dominic Case

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Posted 10 January 2006 - 06:32 PM

I was taking umbrage with your assertion that still photographers were somehow unaware that the lens elements caused some degree of light loss before the film plane.

It was originally my assertion, not Dickson's - he just quoted me.

But please don't take umbrage. It doesn't work.

It's intriguing that still and motion picture photographers use such different ways of setting up exposure. Some of the reasons are coming out in this discussion - and the difference between professional and consumer equipment is another factor of course. So it is natural that people who have learnt either amateur photography OR professional still photography will be confused when they come up against T/stops for the first time.

Of course any photographer (still or motion picture) develops their own relationship with meter readings, whetehr they are hand-held, or through the lens/off the film plane. The variation between one person who prefers a denser image and another who doesn't, or between one who reads the actor's face and another who reads his or her hand is likely to be as great (if not greater) than the difference between an f/stop and a t/stop on a given lens. So ultimately, you rise above the technical minutiae, and do what works.
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#12 David Mullen ASC

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Posted 10 January 2006 - 10:06 PM

I think one reason it matters more with motion picture images is that they are edited in sequence, so matching between lenses & exposures becomes much more critical. Generally still photographs aren't displayed in a sequence and enlarged to a 50' screen, so minor mismatches between lenses & exposures are not so critical. Generally a still photographer isn't going to shoot the same subject with ten different lenses, primes and zooms, and then cut them together side-by-side. It's possible, but it's rare. But with film images, it's their nature to be juxtaposed against each other in a scene without changes to the image other than screen size & perspective.
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#13 Dan Goulder

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Posted 10 January 2006 - 10:46 PM

What is the difference between f stop and t stop?
Árni Heimir
Iceland


T stops are generally taken around mid afternoon on the set of British features.
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#14 Tim J Durham

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Posted 10 January 2006 - 11:46 PM

I think one reason it matters more with motion picture images is that they are edited in sequence, so matching between lenses & exposures becomes much more critical. Generally still photographs aren't displayed in a sequence and enlarged to a 50' screen, so minor mismatches between lenses & exposures are not so critical. Generally a still photographer isn't going to shoot the same subject with ten different lenses, primes and zooms, and then cut them together side-by-side. It's possible, but it's rare. But with film images, it's their nature to be juxtaposed against each other in a scene without changes to the image other than screen size & perspective.

I accept that, and if T-stops are more accurate, why doesn't Hasselblad (Zeiss) use them on stills camera lenses? Or Leica (Zeiss again) or Nikon or Canon? No one except high end product photographers measure depth of field using any sort of ?-stop algorithm (that I know of, and I know quite a few) so why not make ALL lenses with T-stops?

I'm not trying to be thick, I had never heard of T-stops until I joined this forum, being a TV (and stills) guy and all.
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#15 David Mullen ASC

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Posted 11 January 2006 - 12:17 AM

I accept that, and if T-stops are more accurate,


What do you mean "if"? Of course they are more accurate, that's the entire reason why the whole thing was invented. They are basically "corrected" f-stop marks, corrected for actual light transmitted.

I don't really know why still photographers don't use them but clearly the most likely reason has something to do with the inherent differences between still images and moving images. Since it's not movement, it must be editing, the fact that film images are laid out in sequence so even minor differences in exposure, color, etc. become more obvious.
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#16 David Sweetman

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Posted 11 January 2006 - 03:08 AM

I guess the question has already been answered, but the way it was explained to me is:

F-stops are the theoretical amount of light passing through the lens. T-stops are the actual amount of light. These measurings compensate for light lost while passing through the lens. They're basically the same thing, and can, for the most part, be used interchangably. If you read f/4 on the meter, you can expose at t/4, for example. Correct me if I'm wrong, but that is my understanding.
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#17 David Mullen ASC

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Posted 11 January 2006 - 10:00 AM

I guess the question has already been answered, but the way it was explained to me is:

F-stops are the theoretical amount of light passing through the lens. T-stops are the actual amount of light. These measurings compensate for light lost while passing through the lens. They're basically the same thing, and can, for the most part, be used interchangably. If you read f/4 on the meter, you can expose at t/4, for example. Correct me if I'm wrong, but that is my understanding.


Yes, a T-stop is an adjusted f-stop mark, although an f-stop is a physical thing, a mathematical ratio determined by the size of the aperture and the focal length, whereas a T-stop is determined by measuring the light transmitted by the lens. So technically, you'd use f-stops when calculating depth of field, T-stops for setting exposure.
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#18 Brian Drysdale

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Posted 11 January 2006 - 11:26 AM

I accept that, and if T-stops are more accurate, why doesn't Hasselblad (Zeiss) use them on stills camera lenses? Or Leica (Zeiss again) or Nikon or Canon? No one except high end product photographers measure depth of field using any sort of ?-stop algorithm (that I know of, and I know quite a few) so why not make ALL lenses with T-stops?

I'm not trying to be thick, I had never heard of T-stops until I joined this forum, being a TV (and stills) guy and all.


I suspect the use of T stops increased when zoom lenses became common on film cameras in the 1960s. For example the 9.5mm to 95mm zoom has a f stop of 2.2, but a T stop of 2.8 - that's quite a difference. With fixed focal length lens the difference is much smaller. Modern lens coatings reduce the transmission loses in zoom lens to about a 1/3 stop.

Perhaps the still lens manufacturer's feel that the difference won't affect any one but high end photographers, who will do test exposures (which will factor in the difference) and who also often bracket their exposures.

You don't really need to know the T stop of a video lens because you're not using an exposure meter to set the aperture. If you do set a ASA rating for the camera, you'd be already factoring in the lens transmission loses when you're determining the camera's rating. Studio TV camera's aperture are set at the racks by a vision control engineer using a waveform monitor, who doesn't need to know the T stop, just the f stop.
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#19 Stephen Williams

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Posted 11 January 2006 - 01:48 PM

What do you mean "if"? Of course they are more accurate, that's the entire reason why the whole thing was invented. They are basically "corrected" f-stop marks, corrected for actual light transmitted.

I don't really know why still photographers don't use them but clearly the most likely reason has something to do with the inherent differences between still images and moving images. Since it's not movement, it must be editing, the fact that film images are laid out in sequence so even minor differences in exposure, color, etc. become more obvious.


Hi,

For many years still photographers have bracketed their exposures. Thats not possible with normal motion picture photography. However I often bracket exposures when using a motion control and Nikon still camera lenses!

Cheers,

Stephen
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#20 Greg Gross

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Posted 14 January 2006 - 08:57 PM

I really enjoyed all the posts here on this f/t stop subject learned a lot. A lot of food for thought
here. Speaking of food, my girl friend is presently shooting a commercial on HD here and I'm in
charge of feeding the cast and crew. I want to direct but she won't let me. I've got meatballs and
sauce in crockpots,pasta,garlic bread,cheese cake. Saved some wine for the "Martini" shot. I'm
going to turn my crockpots down to t 2.07 now. I bracket 1/3 ev to 1/2 ev.


Greg Gross
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