Camera guide for Action Photography
Vlad and David recently wrote a reasonably comprehensive camera buying guide - start here. This short guide is intended to give additional content for action photography.
There are two common types of autofocus found on camera, phase detection and contrast measurement.
Don't let the image scare you, I'm not going to give you a physics lesson. I created the image to give a crude representation of phase detection autofocus. Phase detection is found in SLR cameras, it takes light from two parts of the lens and compares the light (phase) difference. Depending how far out of sync these two different samples are the phase detection can calculate (in principal) the exact direction and distance change to obtain the correct focus. This is why the AF on an SLR is so fast.
Contrast measurement is what you typically find on mobile phones, compact consumer cameras and is also in most mirrorless interchangeable lens cameras (e.g. Sony NEX, Olympus PEN). My image simplifies the problem massively. In this system the camera is trying to find the highest contrast image, the higher the contrast the better the focus. It does this with a trial-and-error approach, unlike phase detection it can't quickly calculate direction and distance. Of course there is plenty of clever software behind the systems and they aren't as bad as this sounds.
Clearly phase detection AF is the superior system for action photography. When time is critical phase detection gives you the best chance of capturing the moment. For this reason it's hard to recommend using a mirrorless camera. SLR is king for action photography, but don't let that fool the more budget conscious - a good compact camera with contrast measurement AF can still get cracking action photographs.
ISO & Noise
Boring photographers care far too much about noise in images. When it comes to action photography, it's about capturing that exciting moment. Most non-photographers won't even notice the noise of a high-ISO shot.
This image was taken indoors in a dark and smoky hall. I had a choice of lenses but chose a 70-200 mm f/4.0 L. I had available an f/2.8 but it's a heavy beast, so instead I just cranked up the ISO. The ISO here is several thousand, on the full size image it's really noticeable but I simply don't care - my set got published and make a few quid. Those arsing about with low ISO's (hundreds, low thousands) got rubbish results and made no money. My noise advice is simple - get the lowest noise camera you can afford. However, it doesn't matter what you end up with if you're not a pansy and use high ISO when required.
Autofocus Features & Settings
I hope you're not bored with autofocus yet. The original guide by The Verge touches on the difference between cross-type AF points as opposed to linear points. In short, you want as many cross type AF points as you can afford. I won't mention specific camera as the advice would be irrelevant in a few months with new releases.
My Photoshop skills know no bounds as demonstrated above... if only! What I am trying to demonstrate with this wakeboarding image is a photograph gone wrong. On my camera (Canon 40D) there are 9 AF points, widely spaced out. The 40D doesn't do a good job of automatic selecting a point for action photographs. I usually lock the camera to focus on the centre AF point only (coloured red on above image) - this is very limiting. When I get to use a 1D I am much more happy as there are many more AF points and ability to group them together. These features are making their way down to lower level cameras such at the 7D too.
A 9 point-AF would be more than adequate for a boring photograph of a flower, it's not the best fit for action. I hope you see a theme here, AF is very important for action photography. Pick a camera which gives you great control over all AF settings.
Limiting to a centre AF doesn't mean photographs have to be centre framed. Here
I have the centre AF points on the drivers head (approx).
Viewfinders are covered well in the camera buying guide. To reiterate a point - you will want to be using your viewfinder. SLR's that offer live-view - this is like shooting a gun from the hip. When in live-view your camera will focus using contrast detection (or will not focus at all), don't use it! The size of the viewfinder is important too. Having used a 7D over my old 40D the difference between the 100% 0.63x size on the 7D and the 0.58x on the 40D is huge. This isn't action specific, just worth taking into account.
SLR or SLT
An SLR is Single Lens Reflex camera, SLT is Single Lens Translucent (as on some Sony cameras). What do those terms mean? Take a deep breath, I have been photoshopping again:
I have kept this diagram very crude. SLR's have been around for many years. A mirror reflects light up to the prism and through the viewfinder (also light metering unit). The mirror also focuses light downward to the AF sensor where the phase detection takes place. When taking a photograph the mirror flips up out of the way (hence the sound) and the light can pass unimpeded to the image sensor. As we look though the same lens as the sensor we know exactly how our photograph will be framed, it's the reason for the Single Lens part of the name. Reflex is because the mirror moves.
An SLT has a fixed mirror. A majority of the light passes through to the sensor whilst a small amount is reflected up to the auto-focus sensor. This has a major advantage in that the camera can keep on focusing even while a photograph is being taken. Let's take an example. I am photographing a mountain bike doing 10 m/s (~20mph), it's quite dark and I am using 1/100 sec shutter speed. 1/100 is 0.01 second, thus the bike would move 10 cm in the period of the photograph. On an SLR if there is any lag this could be longer, and the whole period the mirror is up the AF is blind - thus your camera isn't ready when the mirror comes back down for the next shot. On an SLT the mirror never moves, the camera always knows where the subject is - even when capturing video. This allows for some great burst speeds, ideal for action photography.
Of course there are drawbacks. You HAVE to use an electronic viewfinder and/or LCD panel, though an OLED EVF can be fine - it's a real dividing issue. The translucent mirror is always blocking some light, making your images darker and for an action photographer this can be bad if you already maxed out on ISO and can't afford faster glass. The best camera with SLT currently is the Sony A77, to quote dpreview "it is perfectly capable most of the time, but in terms of its tracking abilities it can't quite keep up with the systems in competitors like Canon's EOS 7D and the Nikon D7000". Camera Labs say something similar "put the A77 up against a Canon EOS 7D in a sports environment, and you'll realise why the latter is a better choice". I think that says it all. SLT is perhaps something to keep an eye on for the future (A Sony SLT full frame could be a killer cam) but for now it's about equal with SLR for an action photographer, with its advantages there are drawbacks.
If you can think of anything I have missed why not leave a comment/reply to this post and I will edit/update my mini-guide. I hope this of use to somebody one day :-)
Notes & Caveat
I am not a professional photographer or camera expert. I am just an enthusiast sharing my knowledge. As such don't treat this guide as gospel. All images are self created and all photographs used are my own work.