Get your minds out of the gutter, we are talking about aperture size here! Recently one of my friends saw my 300 f2.8 and not knowing what it was asked what size it was. He was shocked when he found out it was “only a 300mm” since he had one that was much smaller and lighter. He is an enthusiastic amature and I really expected him to know the difference and why I used that lens verses a smaller one of the same focal length. I’ll recap the explanation here since there may be others who also do not realize why someone who shoots sports needs a wider aperture.
The main reason a sports shooter needs a wide aperture is speed, or to put it in terms of aperture, more light. If more light can enter the lens then one can shoot at higher shutter speeds or lower ISO’s. During a sunny afternoon shooting outdoors is a piece of cake, you can use that variable aperture lens at f5.6 and have 1/1250 of a second shutter speeds no problem. Step onto a high school football field at 8pm on a Friday night and it is a whole different ballgame (pun intended). You will need 1/1250th of a second to freeze the athletes and at f5.6, it just is not going to happen unless you go to a crazy high ISO like 10,000. Unless your shooting a new pro model Nikon or Canon those images will look horrible. Variable aperture lenses are a compromise between cost, weight and performance. For everyday shooting by Joe consumer they are great lenses at a good value. I personally have a 70-300 AF-D f4-5.6 Nikon that is a wonderful lens, I take it on trips where light weight is the most important factor. I also own a Sigma 120-300 f2.8 for sports, where speed is the most important factor.
The other major factor for sports shooters is being able to isolate the subject. Isolating the subject is done by selecting a wide aperture, which provides us with a shallow depth of field. Variable aperture lenses usually will not be wider than f5.6 at the long end (300mm), and as a result will not have a shallow enough depth of field to really separate the subject from the background. If you look at Sports Illustrated and check out the action shots from any game you will notice how the subject is separated from the background, i.e. the background is really out of focus and just a mush of color. Think about it for a minute, if the stands in the background are in focus your attention will soon move from the subject to the background. When I shoot sports I set the lens to f2.8 and that does not change, it stays wide open.
I put the Nikon 70-300 f4-5.6 AF-D variable aperture lens beside the Sigma 120-300 f2.8 fixed aperture lens for a little comparison. Obviously, as the photos confirm, the Sigma is MUCH larger than the Nikon, especially considering they have the same maximum focal length. The extra size is all because of the f2.8 maximum aperture. Too let in all of that extra light there has to be concessions made, here it is the size. Sigma was not concerned about the size as much as they were about the ability to get in lots of light and maintain f2.8 through out the zoom range. Too give you an idea about the difference in size lets compare the front element filter size: the Nikon uses a 62mm filter whereas the Sigma a 105mm filter.