So many younger cinematographers today have come up during the DSLR revolution, and as a result have never used lenses that feature T-Stop markings. DSLR lenses are of course intended for stills photography first and foremost, and the standard measurement of aperture in photography has always been an F-Stop. In film however, aperture is measured in T-Stops which is why authentic cinema lenses will always have their markings based around a T-Stop, not an F-Stop. Read on to hear why this is the case, and to understand the big differences that it can make while shooting.
I had a recent conversation with an up and coming DP who was shooting on cinema prime lenses for the first time, and he asked me why some cinema lenses appear to be slower than their still lens counterparts. More specifically, he wanted to know why his 35mm Zeiss Superspeed lens only opened up to 1.5, while a Zeiss stills lens at the same focal length can hit 1.4. I quickly explained to him that he was not making an accurate comparison as the cinema lens is measured differently than the stills lens, and pointed out the T-Stop markings on the Super Speed. I realized after this exchange that this is a very common question amongst filmmakers (especially those that came from a DSLR background), and felt that it warranted a blog post to give some clarity to anyone else who might not understand the differences.
T-Stop Vs. F-Stop
Before I explain the simple (but important) difference between the two, let’s briefly discuss the science behind what an F-Stop really is.
We all know that F-Stop represents the aperture setting on the lens, and ultimately controls how much light is hitting the sensor. A wide open, fast lens might open up to F1.4 and close down to F22, meaning of course that the iris is able to open op to a very wide setting, and close down significantly. Where things get a bit more mathematical however, is when you start to look at how an F-Stop is calculated as there is a scientific relationship between aperture and focal length. Put differently, an aperture value is a result of the size (or diameter) of the lens opening when 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 opening of 50mm will have an F-Stop value of F4.0 (200/50 = 4). And an 85mm lens with a lens opening diameter of 50mm would have an F-Stop of F1.7 (85/50 = 1.7). The point is that aperture doesn’t simply refer to a universal value on any given iris, but rather refers to the relationship of that specific iris to the focal length of the lens that it is affecting.
For casual shooting, it doesn’t really matter whether or not you understand the science behind F-Stops, as you can trust that the lens manufacturer is giving you relatively accurate numbers to work with. That said, when you are shooting professionally, it is very important to understand at least the basics of the science behind aperture so that you expose your image properly – especially when shooting on DSLR lenses.
The first thing you need to know is that not all F-Stop settings are created equal. As I mentioned above, an F-Stop value is dictated by mathematics alone (the relationship between the focal length and lens opening), however there are other variables that come into play when considering your F-Stop setting – namely light absorption. Every lens absorbs light to some degree, but unfortunately not all lenses absorb the same amount of light. Based on the build of the lens, the optics and all sorts of other factors, a certain amount of light will never actually make it through the lens to hit the sensor. This means that if you have an F2.8 lens that is absorbing F0.3 of light, you are really shooting on an F3.1 lens, or at least that’s the true value of how that lens performs. That’s where T-Stops come in…
A T-Stop is no different than an F-Stop in the sense that it is also initially measured by the relationship between the focal length and lens opening, however it’s value is further adjusted to reflect the light absorption factor. So for instance in the example above (using the F2.8 lens), on a proper cinema lens it would actually be marked as T3.1 as opposed to F2.8. This is absolutely critical for professional level productions for so many reasons, and why every proper cinema lens (including the Schneiders pictured below) are marked with T-Stops.
Many professional film shoots will consist of very intricate and specific lighting set ups that are set to a light a meter. Exposing an image based on light metering is naturally dependent on the accuracy of the lenses, so as you can imagine things could get pretty messy and inconsistent when using lenses that aren’t adjusted for T-Stop values. In other words, on a professional set you might have your lights set in a way that would allow for an F-Stop of 2.8, but as we’ve already gone over every lens is going to absorb light differently. So if you swap out your Tokina F2.8 lens for a Canon F2.8 lens, that exact same lighting setup may very well now be inconsistent between the two lenses – even though they both have the same F-Stop value.
Even if you were just shooting on a single lens for your entire shoot, you would still want to know what your true aperture value is (especially if you’re using a light meter) as your light meter will give you a specific value (let’s say 2.8), but if your lens is really a 3.1, then you are going to underexpose your image. For these reasons and more it is always preferable to shoot on Cinema lenses whenever possible. Although most cinema lenses are significantly more expensive than DSLR lenses, there are now lots of great budget-friendly options including the Rokinon Cine Lens Kit which is pictured below.
There are so many reasons why it is critical on a professional set to have accurate and consistent aperture values. from matching two different cameras, to swapping lenses, to keeping your exposure in check. In an ideal world, it is always best to shoot on cinema lenses (when possible at least) since they are measured according to the actual value of light that’s hitting the lens, and not simply the ratio of the focal length to lens opening. If you need to shoot on DSLR glass as so many of us do, that’s perfectly okay… Just be aware of the inconsistencies that DSLR lenses can have, and be very careful about when you are setting your exposure and light levels – especially if you’re using a light meter.
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