What Maximum Aperture Opening Do You Need?
An important question to ask when selecting a lens for purchase or for a shoot is "What maximum aperture opening do I need?"
Well ...
How fast is the subject moving?
What is the range of lighting conditions going to be?
What is the maximum ISO speed setting acceptable?
How much DOF (Depth of Field) is required?
A wide aperture does a couple of things for you.
Probably the most common reason for requiring a wide aperture is to get a high shutter speed.
High shutter speeds stop movement - subject movement and photographer/camera movement as well.
Another benefit (or disadvantage sometimes) of wide apertures is a narrower DOF (Depth of Field).
The distracting Background can be blurred more easily at wide aperture settings.
However, part of the subject might be out of focus if there is not enough DOF.
Lenses with a maximum aperture of f/2.8 or higher allow some Canon EOS camera bodies to focus faster and more accurately.
Even when you are not using the wide open f/2.8 aperture, the lens still opens to f/2.8 prior to the exposure.
The camera's autofocus system takes advantage of the wider opening/extra light.
The brighter viewfinder will be welcomed by everyone.
Using extenders reduces the max-available aperture by 1 or 2 stops.
A fast lens will retain autofocus capabilities when paired with an extender while a slow lens will become manual focus-only.
So how wide of an aperture does one need?
Good question - with lots of answers specific to individual situations.
I'll make an attempt at some broad guidelines ...
If you are shooting indoors without a flash (or with a fill flash only), you will probably want an f/2.8 or wider aperture.
If your subject is still, image stabilization can be a substitute for a wide aperture, but image stabilization does not help stop subject action (IS Mode 2 panning not considered).
The wider the aperture available to you, the lower the ISO setting you will be able to use to get your desired shutter speed.
If what you are shooting is indoor action (such as sports), you will likely want an f/2.0 or wider aperture unless your lighting is excellent (and this is unusual).
An f/2.8 lens is often used in these situations, but generally ISO 1600 or 3200 is required to get close to action-stopping shutter speeds.
An f/4.0 maximum aperture is generally good in medium to good lighting.
An f/5.6 maximum aperture requires good lighting or image stabilization.
And again, image stabilization does not help stop subject action.
Adding a flash makes most lenses functional at most apertures as long as the subject is within range of the flash.
But, a lens with a wider aperture can capture more ambient light than a lens with a small maximum aperture yielding a more pleasing flash photograph.
