So, we always talk about lenses in terms of F-stops. I first came across the concept of T-Stops when looking at cinema lenses as they reported in T-Stops.
There are a number of online explanations, but I generally think of f-stop or f-number as directly related to the physical size of the aperture. As the size of the aperture is the #1 factor limiting the light that hits the sensor, the photographic stills world almost always uses f-stops when discussing brightness. But really, the light that hits the sensor is a t-stop or t-number as light is lost passing through the various elements.
T-stops are difficult to measure whereas f-stops are readily known and using f-stop to approximate light hitting the sensor usually isn't that big of a deal as usually there is only ~1/3 of a stop or less differences.
Using DXOMark's t-stop numbers (only source I know of), a few quick examples:
- 35 f/1.4 II has a t-stop of t/1.7
- 24-70 II is really ~2.8 @24 mm, but 3.1 @70 mm
- 135 f/2 has a T-stop of 2.3
- etc
So, t-stop is generally ignored.
As I mentioned over in my thread on finding a travel super-telephoto, I noticed that shooting the same scene the 400 DO II was extremely consistently 2/3 of a stop lower ISO than my 150-600S (Tv/Av set, ISO allowed to float). But, I was shooting a subject on the porch where light could vary. So now that I have the 500 f/4 II, I wanted to test this under controlled conditions. The results:
Lens DXO Reported T-Stop Observed ISO Diff Expected T-Stop 400 f/4 DO II T/5 2/3 stop 500 f/4 II T/4.5 1 Stop 150-600S Not Reviewed T/10
The 500 f/4 II was very consistently 1 stop lower ISO than my 150-600S at similar focal lengths and the exact same shutterspeed and aperture. So, this all lines up very nicely, the 400 DO II has a T-stop 2/3 of a stop higher than it's f-stop. The 500 f/4 II has a T-stop 1/3 of a stop higher than it's f/stop.
However, this would indicate that the Sigma 150-600S actually let's light hit the sensor as if it is a f/10 lens. But I tend to shoot at f/7.1 to f/8 to get sharp images, thus I am shooting and getting the light as if I was shooting at f/11 to f/13 on an "ideal" lens.
So, this may explain why I have always been fighting aperture/shutter speed and ISO with this lens.
For what it is worth, the 100-400 II has a t-stop of t/6.3@400 mm. The 600 f/4 II, T/4.5. The 400 f/5.6 was not reviewed by DXOmark. Other Sigma lenses, those reviewed by DXO mark, are fine. 85 f/1.4 A, T/1.8; 120-300S f/2.8, T/2.8; 500 f/4, t/4.8; 150-500 t/7.3@500 mm.
Of course, first question is if there is something very wrong with my 150-600S, even though recent images look fine. I've already left a message for Sigma.
