Canon EF 100-400mm f/4.5-5.6L IS II USM Lens or other options?

Sandyld7 wrote:

3.I know I am going to look like an idiot for asking this, but I am going to anyway. Can the extra reach help lessen the need for larger apertures when shooting action shots? For example, if I am having to zoom at 200 mm on my 70-200 and it is needing f/2.8 to get a shot that I then crop in to, will it lessen the demand any if the lens is taking me "there" where the subject is at 400mm?

The whole point of f-numbers is that they allow for the focal length difference so that the brightness of the image is the same at any focal length. If you need f/2.8 at 200 mm then you will also need f/2.8 at 400 mm, to get the same result in terms of exposure settings.

That's the simple answer, but it's not the whole story. Cropping the 200 mm image is exactly the same as using a smaller sensor, and if you've read anything about 'equivalence', it applies here. By using only 1/4 of the image (1/2 width and 1/2 height) you are only using 1/4 of the light, and while that doesn't affect the brightness, it does affect the image noise. I'll skip all the explanation of equivalence as it has been well documented many time before, and just give the result, which is:

200 mm, 1/1000 sec, f/2.8 and ISO 400 cropped to the same image as a 400 mm lens (1/2 width and height) is equivalent to 400 mm, 1/1000 sec, f/5.6 and ISO 1600 uncropped.

Notice that what I've done is to increase the ISO speed by two stops to compensate for the slower aperture. Using the 100-400L II at 400 mm f/5.6, you can achieve the same noise level as the cropped image from the 200 mm f/2.8 while upping the ISO speed by two stops.

Why bother if it's only the same, not better? Well, it's the same in terms of noise, but you now have four times as many pixels so you have recorded much more detail; the greater magnification should also help you to choose your subject for focusing more effectively.

I hope I've understood the question (will it "lessen the demand") correctly!