Understanding Lenses: Aperture, F-Stops & T-Stops, by Caleb Ward

[Andreas Thaler]

This is an issue that becomes particularly relevant when working with camera testers. F-stops do not account for the glass used in lenses, which blocks some light. This is where T-stops come into play,

A comprehensive, easy-to-understand guide.

Understanding Lenses: Aperture, F-Stops & T-Stops

Lens apertures may seem confusing but with a little practice you'll be an f-stop master.

www.premiumbeat.com

 

[John Salim]

T-stops should have been made standard decades ago !

John S

 

[ic-racer]

I can't say I have ever had a situation that required t-stops. Even all my cine equipment is calibrated in f-stops.

 

[lumoStoria]

I can't say I have ever had a situation that required t-stops. Even all my cine equipment is calibrated in f-stops.

I think the only real use of T-stops is in filmmaking to ensure the same exposure when frequently swapping lenses. Don't know if this is still important when using a hybrid or completely digital workflow.

Anyway, the difference between f-stop and T-stop isn't that big when you use a lens with good light transmittance. For example, if the light transmittance is 0,9 (= 90%), the T-stop corresponding to an f-stop of 1.4 is just 1.48 (1.4 / sqrt(0.9)).

 

[Chan Tran]

This is an issue that becomes particularly relevant when working with camera testers. F-stops do not account for the glass used in lenses, which blocks some light. This is where T-stops come into play,

A comprehensive, easy-to-understand guide.

Understanding Lenses: Aperture, F-Stops & T-Stops

Lens apertures may seem confusing but with a little practice you'll be an f-stop master.

www.premiumbeat.com

So do you have a formula for calculating the light intensity with T/stop? For example if a surface has a luminance of 100 nits what's the illuminance at the film plane for a t/1.0?

 

[Dan Fromm]

I can't say I have ever had a situation that required t-stops. Even all my cine equipment is calibrated in f-stops.

Agree, but ... The Angenieux 8x8B on my 4008ZM should have been calibrated in T-stops. F/1.9 wide open, more like T/3. The AE unit failed while I was on a shoot in Costa Rica. At the time I had no idea how much light was lost in the lens, had to go to my backup camera, which wasn't as capable as the Beaulieu. Bummer.

Funny thing is, although the 6-66/1.8 Schneider on my 4008ZM2 and the 6-70/1.4 on my 5008S-MS were marked in f/stops when the exposures the camera wanted were compared with a good hand-held meter it seems that the lenses lost no light internally, i.e., T/stops to be used with an external meter were the same as f/stops. The cameras' shutter speeds were on spec.

 

[Andreas Thaler]

So do you have a formula for calculating the light intensity with T/stop? For example if a surface has a luminance of 100 nits what's the illuminance at the film plane for a t/1.0?

Please refer to our experts @Dan Fromm ; this isn't my area of expertise.

The only overlap I have with the camera tester is checking the aperture function of my lenses in T-stops. The intervals should be as consistent as possible. I don't have to do any calculations.

 

[Dan Fromm]

Please refer to our experts @Dan Fromm ; this isn't my area of expertise.

So you'll know, after I returned from CR I shot calibration footage. ISO 25 Kodachrome, known correct frame rate/shutter speed, marked f/stop and incident meter reading with a recently calibrated meter for a range of f/stops and frame rates, then picked the combination that gave best exposure. AE turned off. This last was estimated visually, not properly measured.

I did the same with the newer cameras after I got them.

 

[Petrochemist]

T-stops should have been made standard decades ago !

John S

No thanks.
T-stops don't quite relate to DOF, hyperfocal focus etc. Uses that are relevant to me.
For stills use TTL metering made the actual light transmission less relevant . Even without this f-stops get close enough for most situations, underexposing by 1/5th a stop is hardly ever an issue

With some lenses/applications transmission is not constant.
For example
A radioactive lens that has gone yellow over time.
Using the lens to shoot beyond the visual range, some of my lenses block quite a lot of NIR & the UV transmission is even more variable.


Having now viewed the article I have to say I've never heard photographers say 'stopping up' the usual phrase for the opposite of stopping down is opening up.

 

[RLangham]

T-stops should have been made standard decades ago !

John S

There’s reasons enough that it hasn’t.

To me if the difference is big enough to matter then it’s not a lens I want to use in general photography. Either it’s from 120+ ago or it’s in rotten shape, or it’s a simple box camera/toy lens or it’s some extremely long telephoto that I’m not the target audience for.

Every SLR 50mm, 28mm, 35mm and 135mm I’ve ever owned, including some that were radioactive and yellowed, some that were moderately fungused and some that were decoated, has shot within a fraction of a stop of true in visible light. If that’s a difference of .9 or even .8, it’s good enough for my purposes. Meanwhile f/stop, not t-stop, is relevant for DoF.

So while it’s definitely a useful metric for certain specialized tasks its relevance to general photography seems almost nonexistent. I would certainly be scratching my head if every small prime lens had a different aperture scale.

 

[Chan Tran]

For most lenses the f/stop and t/stop are not that much different but the Sony FE 100mm f/2.8 STF GM OSS has 2 stops difference between the f/stop and t/stop.

 

[_T_]

So do you have a formula for calculating the light intensity with T/stop? For example if a surface has a luminance of 100 nits what's the illuminance at the film plane for a t/1.0?

Wouldn’t the calculations be exactly the same as the idealized calculations for f-stop, the ones that don’t account for transmissivity, since the t-stop takes that into account for you?

So you would just plug the t-stop number into the calculation in place of the f-stop and the rest is taken care of for you by the manufacturer when they measured the t-stops.