Lens for dog agility / action

[Richard Lane]

Hi David,

I would generally stay away from a variable aperture lens like this one for fast moving subjects.

To start off the 70-300 f/4-5.6L IS is going to be:
70-103mm = f/4.0
104-154mm = f/4.5
155-228mm = f/5.0
229-300mm = f/5.6

Here's a couple more points:

So, from 105 and up you will be getting a slower lens and from 155 to 228 you will be at f/5 and so on. At f/5.6 this lens will be 4 times slower than the f/2.8 lens, since for every 1 stop loss the lens will let in half as much light. So, from f/2.8 to f/4 the "lens speed" will be twice as slow and from f/4 to f/5.6 the "lens speed" will be twice as slow once again, that's 4 times slower shutter-speed than f/2.8.

For a fixed aperture lens like the f/2.8, the lens will always open up to f/2.8 to focus quickly even if you are at f/4, and then once the lens focuses it will close down to f/4 for the shot. So, you always get the benefit of faster focusing at f/2.8 even if you are at f/4 or narrower for DOF.

The other point is that f/5 to f/5.6 will further limit your shutter-speed if there is less than optimal light, so, you may be looking at higher ISO's or blurry shots. Also, if there is ever an event indoors, then your screwed.

The last point is that the f/2.8 lens provides a nicer blurring out of the background for better subject isolation, especially when their are people standing around on the sidelines or cars in the background, at these type of events. It will take some time to getting used to the narrower DOF, but once you do, you will greatly appreciate the f/2.8.

I hope this helps,

Rich


EDITED: Changed "focusing speed" to "lens speed".