Recently a video by Matt Granger has been moving between photographic sites and working its way into conversation like a politician works a room. It’s about the relationship between F-Stops and T-Stops, and as you can all imagine, anything mentioning an F-Stop is enough to get photographers to re-adjust their glasses. The video is very good and gives a basic overview of what F-Stops and T-Stops are, explaining the basics of their function and how they are measured.
It would seem prudent, however, to stand on the back of this, and take it a bit further. For those of you who want a very short and summarized version, the video really simply states that while an F Number may suggest how much light may pass through the lens, it’s not entirely an accurate measure of how much really gets all the way through to the sensor due to light absorbance and reflection etc..
So while, for example, a lens’ aperture may be open to 1.4, the actual measure of light hitting the sensor may, in fact, be equivalent to 1.7. That actual number, 1.7, is the T-Stop, and some lenses, typically cinema lenses, will be rated as such. Also worth noting is that price is not necessarily a good indicator of how close a lens’ aperture will match the T numbers. You can check many of these on DxO mark.
So that’s the general gist of the video. To really understand how this works, it’s beneficial to understand a bit more about F-stops and T-Stops, from the ground up. Yes, some of you will be more than versed in the information to follow, but without a doubt, there are lots of photographers who make their living as such who have little-to-no real understanding of this.
Aperture is measured via f-stops. It is the f-stop that give us a definitive standard say on how wide the aperture is – aka how wide open the iris created by the lens blades is. It’s expressed as a ratio, hence on your camera you’ll often see it expressed as 1:1.4 or something of the sort. The ratio is of the focal length to the lens’ aperture.
The f-stop is measured by a simple equation then which is focal length over diameter of the (current) lens opening. For example, a 200mm lens with an iris opening width of 50mm would be expressed as an aperture of 200/50 = 4, therefore having an f-stop of f/4. Simple.
In relation to actual shooting and everyday use, we all pretty much know what manipulation of an f-stop does. Increasing the aperture makes the iris wider and is reflected by a reduction of the f number. When this occurs, more light can pass through the lens, and the depth of field reduces. So the visual effect on the exposure is that it will be brighter and have a greater defocused area. The relation to exposure, however, is more logarithmic than it is linear. Meaning, each full stop represents a doubling up or down, of light. So if you go down a full stop, say f/5.6 to f/4, you will double the amount of light that goes in. If you increase the iris further by another stop, to say f2.8, the amount of light would have increased by a factor of 4, then 8, 16 etc.
Why is that important? Well, from a practical standpoint, depending on your scenario and environment and subject, if you’re shooting a scene or person, or thing for that matter, and the light on it, or part of it, is overexposed by one stop, you’ll know that you need to halve the light hitting that area. Then, of course, this is useful for manipulating other camera and lighting settings.
Simply rephrased from what we can gather from the opening paragraph, is that a T-stop is the measure of light that actually arrives at the sensor. Why is there a difference? Light doesn’t cleanly arrive at the sensor as some of it is reflected, and absorbed by part of the lens etc, and the T-stop accounts for this. So a T-Stop is, in effect, a reflection of the real speed of the lens.
It’s important to understand here however, that if you have a lens set to f/1.4 and it has a T value of 1.6, the T value has no bearing on depth of field. The fact that the F-stop is based on a physical measurement means it is constant.
Do T-Stops Matter to You?
I wouldn’t know, you’d have to be the judge. I would go so far as saying 90% of you fine people will have little-to-no use for them. While a T-stop is a better measurement of light hitting the sensor, it can play tricks with you if you don’t understand when it should be used in photography.
If you’re old school or just accurate and use a dedicated light meter when you shoot, then knowing the T value of your lenses will surely be of important because the light meter is going to give you the ACTUAL amount of light required (F-stop) to hit the sensor for the right exposure, and not just what your lens says the F-stop is. If you take the reading of the dedicated light meter, and adjust your camera to those numbers not taking into account the T-value of your lens, your picture, will be underexposed.
When you shoot and use the light meter inside your camera, it does the metering as it measures the light making its way onto the sensor, in effect, judging the T value of the lens. This is why I feel most of you won’t need to know the T-value of your lens, since most these days meter in camera. If you do want to know, you can call your lens manufacturer to see if they’ve even done the necessary pricey calibration, or see if DxO mark has done it themselves.