Aperture
Aperture is the iris of your lens. You can regulate the area that lets light pass, just like the iris in your eye. The larger the area, the more light it lets through. Typically on a modern camera, you can regulate the aperture via controls on the camera, but often you can also operate the aperture manually directly on the lens.
Just to confuse things, the aperture is measured in f-stop numbers, and the larger the f-stop the smaller the aperture.
Fast and slow
A fast and a slow lens. This is not a reference to sending your lens to the race track, which of course would be pointless, but rather it is about how much light your lens lets through to the sensor under a given set of conditions. Fast = lets through a loft of light. Slow = less so. Using a fast lens at open aperture can be used to highlight a subject due to the background blur that is achievable with a fast lens. So we all want fast lenses in the best of worlds. But there is no such thing as a free lunch. It comes at a price, literally. And not only your wallet feels the weight, also your camera bag will be burdened with more glass if you insist on a fast lens. And more so if you do so with a full frame camera body. But let’s have a look at your lens speed and what the impact is on your photography.
Aperture measures
Aperture is measured in f-stop numbers, so that the lower the f-stop number, the more light is let through. A very fast lens has an aperture of f/1.4, whereas a not-so-fast lens has an aperture of f/5.6. The aperture numbers are odd and difficult to remember, but it is a good thing to understand the scale, as walking one step up and down the scale is referred to as “stopping down one stop” and “stopping up one stop”.
An amazing fast lens has a minimum f-stop of f/1.0. The next f-stop on the scale is f/1.4. If you just remember these two numbers, and that the f-stop scale is a doubling of these numbers for every stop, then you can remember the entire scale. The f/1.0 series is hence:
f/1.0 f/2.0 f/4.0 f/8.0/ f/16 f/32
Similarly the f/1.4 series is:
f/1.4 f/2.8 f/5.6 f/11 f/22
If you then weave these two series together you get the following
f/1.0 f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8.0/ f/11 f/16 f/22 f/32
The series can be extended beyond the above, i.e. some lenses are below f/1.0, but it is rare. On an old lens like this Nikon 135mm, you typically find the aperture scale in the bottom of the lens (closest to the camera) with a ring to turn in order to change the aperture. Notice how the numbers on the lens match the f-stop numbers above. Here the lens is set at f/8:
If you try to look into your lens with the aperture wide open, you typically get a very good idea about how fast your lens is. This 135mm lens can stop down to an (relative to the length of the lens) impressive f/2.8, and as you can see, this lens is all glass, that uses every inch and mm of the inside of the lens to let through light:
Ambient light
Ambien or available light is important if you shoot in low light conditions where a flash or artificial light would ruin the scene or seem odd to those in the picture. Take this picture from evening in the summer cottage – the scene would simply not have been the same with a flash light added:
So in low light situations a fast lens is key.
If you are so lucky to have lots of light available to your photography, no matter if it is ambient light created by the sun, or artificial light created with flashes, steady light, reflectors and what not, then the speed of your lens is less of a concern. But only less. If you have a slow lens, then artificial light can get sufficient light to your sensor so that your Shutter speed is fast enough to prevent both camera shake and subject move without raising your ISO to a “grainy” level.
However, a solid depth-of-field effect where your subject is clearly separated from the background (“pop out”) can be difficult to achieve with high f-stop numbers, as the depth of field increases as you reduce the aperture.
If you want to play with the options, this depth of field calculator shows you exactly how narrow your depth of fields gets when you work with a fast lens and get close to your subject:
So a fast lens is more important for ambient light shooters than artificial light shooters, but faster is also better in terms of background separation. If you are willing to pay the price and weight penalty that comes with the a fast lens is of course entirely up to you.
Your lens and the speed of it
Either on the front of the lens or on the side of it, the fastest aperture it will go to is clearly written on the lens. The picture shows a lens that will go down to f/2.8 as the fastest. You may also be able to see that this is a prime lens (it cannot zoom, the length is fixed), and that it is a 135mm lens.
The longer the lens is, the more difficult it is for manufacturers to construct the lens so that it lest through a lot of light. Physics simply dictate that a long lens requires more glass – this is also why long fast lenses (that sports photographers are so dependent upon) are super expensive.
Non-linear relationship
Now, you may think that the difference between a lens that is f/2.8 and f/1.8 is a relative measure indicated by the size of the numbers. It is not. If you stop down from f/2.8 to f/2.0 you have reduced the light that hits your sensor to 50%!! Every time you stop up or down one step on the aperture scale, you double or half the amount of light! That is why the speed of your lens is so important – we are not talking a marginal improvement as as a consequence of the last stop – we are talking an overwhelming doubling of the light. And that is why serious photographers can get a heated debate about how fast their nifty fifty (50mm prime lens) should be: f/1.4, f/1.8 or f/2.0?
As you can see, I found the f/1.8 version to be appropriate for my budget and need, but of course I have a dream to get a 50mm f/1.4 lens some beautiful day!
Now, just to illustrate how dramatically the amount of light is reduced when you stop up in the f-series, here is an illustration of how fast the amount fades upwards:
So, in my mind, the speed discussion is very fair – we are at the important end of the scale!
Questions and comments
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