Today, many young photographers came out during the DSLR revolution and therefore have never used a lens with the T-Stop marking. Of course, digital SLR lenses were first used for still photography, and the standard measurement of aperture in photography has always been the aperture value. However, in film, aperture is measured in T-stops, which is why real cinema lenses are always labeled in T-stops rather than F-stops. Read on to learn why this happens and the huge difference it can make when shooting.
Recently, I was talking to a budding DP who was shooting with cinema primes for the first time, and he asked me why some cinema lenses seemed slower than stills. More specifically, he wanted to know why his 35mm Zeiss Superspeed lens could only be opened to 1.5, while a Zeiss still lens of the same focal length could be opened to 1.4. I quickly explained to him that he wasn’t making an accurate comparison because cine lenses are measured differently than still lenses, and pointed out the T-Stop markings on the Super Speed. After this exchange, I realized this was a very common question among filmmakers (especially those coming from a SLR background) and thought it necessary to write a blog post to educate others who might not understand this Difference people provide some clarification.
T-Stop Vs. F-stop
Before explaining the simple (but important) difference between the two, let’s briefly discuss the science behind F-stops.
We all know that the aperture value represents the aperture setting on the lens and ultimately controls how much light hits the sensor. A wide-open, fast lens might open to F1.4 and close to F22, which of course means the aperture is able to open to a very wide setting and then close significantly. However, things get more mathematical when you start looking at how to calculate aperture, as there is a scientific relationship between aperture and focal length. In other words, aperture value is the size (or diameter) of the lens opening compared to the focal length of the lens. The equation for this relationship can be thought of as: F-Stop = focal length/diameter. This means that a 200mm lens with a lens aperture of 50mm has an aperture value of F4.0 (200/50 = 4). An 85mm lens with a lens opening diameter of 50mm has an aperture value of F1.7 (85/50 = 1.7). The point is, aperture doesn’t just refer to a universal value on any given iris, but rather the relationship of that particular iris to the focal length of the lens it affects.
For casual shooting, it doesn’t matter whether you understand the science behind aperture values because you can trust the lens manufacturer to give you a relatively accurate number. That said, when you’re shooting professionally, it’s important to understand at least the basics of the science behind aperture so that you can properly expose your image – especially when shooting with a DSLR lens.
The first thing you need to understand is that not all aperture settings are created equal. As I mentioned above, aperture value is determined solely by mathematics (the relationship between focal length and lens opening), but there are other variables that come into play when considering aperture settings, namely light absorption. Every lens absorbs light to some extent, but unfortunately not all lenses absorb the same amount of light. Depending on the construction of the lens, its optics, and various other factors, a certain amount of light will never actually pass through the lens and reach the sensor. This means that if your F2.8 lens can absorb light from F0.3, you are actually shooting with an F3.1 lens, or at least that’s the true value of that lens’s performance. This is where T-stations come in…
T-Stop is not unlike F-Stop in that it is also initially measured through the relationship between focal length and lens opening, but its value is further adjusted to reflect the light absorption coefficient. For example, in the example above (using an F2.8 lens), on a proper cinema lens it would actually be labeled T3.1, not F2.8. This is absolutely crucial for professional-grade productions for a number of reasons, which is why every suitable cine lens (including the Schneider lens pictured below) is marked with a T-stop.
Many professional film shoots involve very complex and specific lighting setups, with one meter of brightness set per meter. Metering-based image exposure naturally depends on the accuracy of the lens, so as you can imagine, things can get very confusing and inconsistent when using a lens that isn’t adjusted for T-Stop values. In other words, on professional equipment you might set up your lights in a way that allows for an F-stop of 2.8, but as we’ve already discussed, every lens will absorb light differently. So if you swap your Tokina F2.8 lens for a Canon F2.8 lens, the exact same lighting settings between the two lenses will most likely now be inconsistent – even if they both have the same aperture value.
Even if you only shoot with a single lens for the entire shoot, you still want to know what your true aperture value is (especially if you use a light meter) because the light meter will give you a specific value (let’s e.g. 2.8), but if your lens is actually 3.1, your image will be underexposed. For these reasons and more, it’s best to shoot with a theater lens whenever possible. Although most cine lenses are much more expensive than DSLR lenses, there are now many affordable options, including the Rokinon cine lens kit pictured below.
This 3-lens kit is only $1349.00 at B & H
in conclusion
Having accurate and consistent aperture values in professional cameras is crucial for a number of reasons. From pairing two different cameras, to swapping lenses, to controlling exposure. In an ideal world, it’s best to shoot with cine lenses (at least when possible) because they are measured based on the actual value of light hitting the lens, not just the ratio of focal length to lens opening. If you need to shoot on DSLR glass like many of us do, that’s totally fine…just be aware of possible inconsistencies in DSLR lenses and be very careful when setting exposure and light levels – esp. If you’re using a light meter.
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