If you are looking to buy a lens for a DSLR camera to shoot video with, there are a few things that are
worth looking out for.
First question: Prime or Zoom?
The answer to whether to buy a prime lens or a zoom, really depends on the type of work you do. Although we would all love to shoot everything we do on primes, with a tiny depth of field, how realistic is this? For certain jobs there is the time to change lenses, a camera assistant to help out and speed the process along, and for other jobs there is no time. As your rig grows, it is easy to end up with a matte box or flare hood, an ND fader, a follow focus etc etc and all of this kit slows down the amount of time it takes to change a lens.
Imagine setting up an interview on a 50mm lens and the director asks you to punch in a bit tighter, you can either reset your tripod moving forward and re bubbling it, or change to another lens – some shoots there is time for this, and some not. I find having the flexibility to move in a touch, often mid interview to show some emotion on the interviewees face is a big help. A standard zoom such as Canon’s 17-55mm for the 7d or the 24-70 for the 5d would be my first choice for lenses. Both of these lenses are nice and fast (2.8 throughout the range). If you are willing to sacrifice a bit on the speed of the lens you could get the 24-105mm on the 5D. You gain a bit of extra length here, but the sacrifice is dropping down to a slower 4f throughout the range.
Ok, so you have your 17-55mm or 24-70mm standard zoom, this is a great start but if you do the kind of work that involves a lot of interviews, I find 80mm (which is roughly what you’re looking at with the 17-55 on the long end with the 1.6 crop factor) is not quite long enough. For a standard MCU (medium close up) in an interview you need to bring the camera too close to the interviewee. Having a camera right in someone face, if they are not used to being interviewed, can be uncomfortable for them.
Longer zoom lenses such as the 70-200mm will solve this problem, but can be very expensive, so what are you paying for? At the top end of the Canon range you get a much faster lens (2.8f) this is very useful as it gives you more scope to shoot in low light conditions and has a smaller depth of field. It is also stabilised, which is really useful for video. I was surprised at how good they are when I first started using them . Canon EOS lenses have IS in their name to show they have this function.
Should I just buy a cheaper zoom lens?
There are a few issues with lenses as you go down the price range. Firstly, they are slower with apertures around the 4.5 to 5.6 mark, this means they are not as great in low light and don’t have such a narrow depth of field. Secondly, the build quality isn’t as good, often the cheaper lenses are in plastic casing rather than metal. Thirdly, and most importantly for shooting video, the aperture changes throughout the zoom range. Imagine you are shooting a tight shot on the long end of the lens, and then you zoom back for a wide and you’ll be pretty over exposed. This probably isn’t an issue for a photographer, but it could be if you are shooting video. The next point is a bit of a killer. If you take a zoom lens, zoom in to the subject and make sure it is sharp, then go back to your wider shot on some lenses the focus point will have shifted slightly. This is a disaster from a DSLR video point of view. With a broadcast lens this aberration can be easily fixed with a backfocus chart, this is not the case with a stills lens. You can of course avoid checking focus in this manner by using the digital zoom on your camera, but – and it is a big but - not if you are already rolling. If you are shooting with a sound recordist and you synch up your camera to their external recorder, once you are rolling that is it, you can’t cut, and you can’t use your digital zoom.
My advice would be, if you can afford the more expensive 70-200 lens, get it, and if not buy the cheaper 70-300 IS, but be aware of its limitations, you can still get good results with this lens, theglass is still good quality and you will still get great images, but be aware of its problems. In between these options there are still other choices: you can get a 70-200 that still has image stabilisation and it still has a constant maximum aperture through the range, but it is 4f not 2.8f. This is a good option as it is almost half the price of the 70-200L 2.8IS.
So, in answer to the original question, as to whether to buy primes or zoom lenses, the answer is, you may need both. If you generally shoot in situations where there is enough time to frequently change lenses, primes are great. Generally speaking, primes are built with less glass meaning they allow more light in, they tend to be better in lower light and have a smaller depth of field when wide open. Even if you work on projects that are relatively quick moving, getting a few establishing shots on a 50mm at 1.4 will give you some stunning small depth of field shots, and really add some value to your work, but is likely to be used less often that a standard zoom. I tend to put my standard zoom on the camera and leave it there for 70 percent or more for the shoot, so this is where I don't mind investing some money.