Summary
Lenses that let in a lot of light can allow the camera to use fast shutter speeds. This is why lenses with (maximum) wider aperture (small f-numbers) are often considered fast lenses. The wider the aperture (lower f-number) can go the faster the lens is supposed to be. A lens is fast means it is fast at shooting photos. A lens that can go f/1.4 is supposed to be faster than a lens that can at best open up to f/2.8.
Because light has to go through different lens elements, lenses with same aperture value can be faster / slower than each other. T-Stop is used for measuring the actual light gathering capability (and thus the speed) of a lens.
When I first started reading and watching about photography, one of the terms that kept repeating was – “fast lens”. Bad low light performance? Get a fast lens. You want to shoot some sport events? Get a fast lens. Being an absolute beginner, I sometimes pondered – what makes a lens fast? Is it fast in performing auto focus? Does this lens speed up the light that goes through the glass elements? (Light can get slower through some lens, in case you didn’t know). I started digging and finally found the answer. And I put the answer briefly on top of this post so you don’t have to read through it to understand the basics.
Shooting Fast
There are scenarios when we need to use fast shutter speeds. For example – you’re trying to shoot photos of a sports event or may be you’re shooting birds. May be you want to take a photo of a bird in mid air? Or do you want to freeze motion of a moving train? In such scenarios, your shutter speed needs to be fast, really fast, depending on the motion of your subject.
May be you saw a humming bird flapping it’s wing – if you want to freeze those wings in motion, you need to adjust your shutter speed accordingly. You gotta go fast! But making shutter speed fast has an additional side effect. When you expose your sensor to light only for a tiny fraction of a second, it can’t collect a lot of light. As a result the photo can be underexposed – dark. But we have to use fast shutter speed, what are our options to properly expose for this shot? We can open our aperture or we can crank up our ISO. We know ISO can produce noise. Opening aperture is the better solution in such cases.
A lens that can open up wider (the maximum aperture value aka f-number is lower) would let more light in and allow us to increase the shutter speed without making the photo darker. Because with a wide aperture lens, we can shoot faster, we call the lens “fast”.
F-Stop vs T-Stop
A wider aperture does let more light in. But this light then have to travel through a number of glass elements before it can reach the sensor. The amount of light that enters through the entrance pupil of the lens is not the same amount of the light that reaches the end on the sensor.
This is where the T-Stop measurement is used. The T stands for Transmittance. It tells us about the actual amount of light that the lens can transmit. It is calculated by dividing F-Stop by the square root of transmittance percentage of a lens. A f/2 lens with 80% transmission efficiency would have a T number of – (2.0 / √0.80) = 2.23.
The T-Stop or T-Number is a more accurate representation of the light gathering capacity of a lens. But in real word scenarios, we often don’t see the T-Stop being advertised as much. People focus on F-stop in most cases and thus it has become the kind of de facto standard while comparing the speed of a lens.
Lowlight Advantage
We have come to know that a “fast lens” lets us make our shutter speed fast that’s why they are called fast lenses. But how do they help in low light?
The answer is simple if you think about it. A fast lens gets us more light. In lowlight situations, if we don’t increase the shutter speed but open up the aperture, we would get more light, right? Yes, that’s it.
A lens is fast means it can bring in more light – you can use it to shoot fast while keeping the photo well exposed, or you can keep the same shutter speed but use the light to lighten up the scene. This is why in many places, we see people suggesting “fast lens” for better lowlight photography.
Side Effects
Fast lenses have very wide maximum aperture. This can lead to really nice background blur and shallow depth of field. If you’re using a fast lens wide open (using it’s maximum aperture), be extra cautious about your focus. On some really fast lenses, the depth of field can be so narrow that it may become difficult to control and easier to lose focus.
Then consider low light when your camera’s auto focus sensor may struggle and make focusing even harder. But nothing to worry about, once you get used to using them, you will eventually learn to avoid these negative side effects and enjoy the lens.
And on the bright side, I always love the bokeh effect from faster lenses.
How fast is fast enough?
Now once I understood what a fast lens is, the next question that came to my mind is how fast does a lens have to be for being treated as a fast lens? I think it depends on the people who use it. Do you want to shoot wildlife or sports events? How fast do you want your shutter speed to be? How well lit will the scene be? Or do you want to shoot in lowlight? How much extra light do you need? It all comes to your own unique needs for a shoot.
But in general, to me, I consider any lens that is f/1.8 and better (lower f-numbers) to be fast enough for my needs.
Lens Choices
Choosing a lens just based on aperture is not a wise decision. Thats why we have covered several factors to consider while choosing a lens in our blog post – Lens Buying Guide. In the aperture section, we have recommended getting fast lenses. Because they simply offer better performance in low light and can help in freezing motion. And at the same time, you get nice background separation.
But how do we make choices between similarly fast lenses? Should we buy a EF 50mm f/1.4 or a 50mm f/1.8? In such cases, we should checkout the difference between the maximum aperture of the two lens. In my book, f/1.4 and f/1.8 is quite close. If I don’t need the extra light or bokeh for any specific needs, I am fine with f/1.8.
Then again, say I see a zoom lens that is f/4-5.6 and I see a lens with same zoom range but f/2.8 – the aperture difference is significant to me. I would want to choose the f/2.8 if I can afford it.
And sometimes because of the quality of the lens elements, the f-stop doesn’t accurately represent the amount of actual light being transmitted. If you’re really concerned about the amount of light gathered by a lens, try and see if you can get some data on the T-Stop of the lenses. That should give you a more accurate idea.
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