In the article on lens apertures I mentioned the difference between big heavy lenses with big maximum apertures, and cheaper lenses with smaller maximum apertures. And I explained that a ‘stop’ is a halving or doubling (depending on which way you are going) of the light entering the camera compared to the value of the adjacent ‘stop’
The standard stops (called ‘f-stops’) run something like f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22
And it is common for lenses to be made with a maximum aperture of f1.8, which is a half-stop position.
The ‘problem’ with lenses with fast apertures, especially long focal length lenses is that there is more glass of greater diameter in them at the front end – and glass is heavy. Here for comparison are the weights of a couple of 300mm lenses. Of course, the f2.8 lens has more light-gathering power compared to the f4 lens, but that’s at the cost of size and weight.
AF-S NIKKOR 300mm f4E PF ED VR 755g (26 ounces)
AF-S NIKKOR 300mm f2.8 G ED VR II 2,900g (102 ounces)
Let’s suppose we are shooting an APS-C (crop sensor) camera – with a 300m lens (that’s the true focal length, not the 35mm equivalent), and an aperture of f2.8 and a subject distance of 50m.
The depth of field will be 3.871m, which is plenty of depth for almost anything one would wish to photograph. Of course the depth of field for the f4 lens will be greater (it’s 5.539m) but the point is that there is no downside at all to using the f2.8 lens except for the weight (and the price). And the advantage of the f2.8 lens is that it lets in twice as much light. And in failing light or in the early morning, or in among trees, that can make all the difference between no shot and a world-beating shot.