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.