Basic Photography | What is a Fast Lens?

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Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is a fast lens because it delivers more light intensity (illuminance) to the focal plane, allowing a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is “slow” because it delivers less light intensity and requires a slower shutter speed.

A lens may be referred to as “fast” or “slow” depending on its maximum aperture compared to other lenses of similar focal length designed for a similar film format. Lens speed given by the minimum f-number, or alternatively maximum aperture diameter or maximum numerical aperture, is a useful quantitative way to compare similar lenses.

Source : Wikipedia

If you are still confuse about “Fast Lens”, let me shed some light for you in plain English. Basically “Fast Lens” refer to the lens that have big aperture at its maximum aperture. Usually any lens that have its maximum aperture which start at f/2.8 and below is consider a fast lens. The reason that a lens with a big maximum aperture is referred to as fast is that it lets more light in and therefore you can use faster shutter speeds even when there might not be much light around. It also make the focusing faster even in a dimly lit room.

Any lens that have aperture ranging from f/2.8, f/2, f/1.4 and f/1.2 is a fast lens, and there is even a faster lens than that which have its maximum aperture at f/0.95 or even lower but this lens is rare and very expensive and often out of reach for most people and usually this type of lens is design for certain purposed like 50mm f/0.7 Zeiss, Limited production lens built for the NASA space program.

Fast Lens is expensive and because they have big aperture, usually the lens also quite big and heavy due to the size of its glass is big and that contribute to the hefty price but not all fast lens is expensive. Among the cheapest fast lens that money can buy is 50mm lens, whether it is Nikon or Canon, most people can afford to own 50mm f/1.8 or f/1.4 and off course there is an expensive 50mm lens like Canon 50mm f/1.2 L lens.

Once you know what is a Fast Lens, you might also wanted to know the benefit of using a fast lens. Among the benefit of using a fast lens is, most of the times, fast lens is made from superb optics, the highest quality glass and as a result, they produce much more sharper image and have a lot of clarity or details.

Having a fast lens also allowed you to use faster shutter speed if you are shooting indoor where the light is very low like if you are shooting inside a monastery, where using flash is not practical or even prohibited because you will disrupted the people praying there. So having a fast lens is the answer where you can use fast shutter speed to prevent camera shake and get sharper imager instead of blurry one . Off course you can overcome this problem if you don’t have fast lens by using Higher ISO but you will have to sacrifice on the quality. Let’s keep in mind that increasing the ISO also increasing the “noise” even though some camera especially Pro Grade camera are very good at High ISO but nothing can beat the quality of Low ISO like ISO 200 and below. Using the Higher ISO is the last option when we can’t use Low ISO due to certain circumstances.

Have you ever wondered why Professional Sport Photographers shooting football use big heavy and expensive lens, like 300mm or 400mm f/2.8 instead of 100-300mm  f/4.5-5.6 or 75-300mm f/4-5.6? The answer is they need the speed, having big aperture allowed them to use much more faster speed to freeze the action without pushing the ISO higher and still maintain great image quality. Having big aperture also help the camera to focus much more faster because there is more light hitting the sensor through f/2.8 glass compared  f/4 lens. This is crucial especially when the sports happens at night or late afternoon where the light is not so bright. Using 70-300mm f/4-5.6 lens is “flexible” because you can change focal length easily but it is just not “practical” because if its slow aperture especially for sports photography.

Hopefully by now, you know what fast lens is, it is not necessarily a prime lens, zoom lens also can be considered as Fast Lens if they have big aperture like 70-200mm f2.8 and 24-70mm f/2.8, this zoom lens also in fast lens category but then again, not all photographers need fast lens depending on their chosen interest and genre. Fast lens is expensive because you pay for the speed and the quality. Fast lens is not for everyone but everyone should have at least own one…