The focal length of a lens tells you how much of the scene can be captured from a specific position. It's much easier to explain this with some still images, so here you go.
This is the same scene shot with different focal lengths from the same position. As you can see, the image gets wider with a smaller focal length. Extreme wide angle lenses are called fisheye lenses. It's possible to shoot extreme perspectives with this type of lenses, but they will also give you a lot of distortion (as seen in skateboard videos), which you have to be aware of. So you'll definitely need a wide angle lens (18mm or less) for shooting big objects (e.g. buildings) from a position that is close to the object. Lenses with focal lengths around 50-85mm are great for shooting portraits of people because you don't have to be too close to the person. Lenses with focal lengths above 85mm are called tele lenses and are used to capture things that are far away from the shooting location. If you own a camera with and APS sensor like the Canon 550D be aware of it's crop factor (1.6). A 50mm EF lens on a 550D or 7D will give you an image like a 80mm lens on a 5DMKII (
Prime lenses do often have better aperture (f-stop) values than zoom lenses which makes them much better when it comes to shooting video under low light conditions. Larger (wider) apertures mean the the lenses have bigger openings for the light to fall into them, which makes them faster because they don't need as much time as lenses with smaller apertures to capture the same shot. Fast lenses do have smaller f-stop numbers like f/1.2 f/1.4 or f/1.8 (the aperture gets smaller (more narrow) and the lens gets slower with increasing f-numbers). Here's a video to demonstrate the effect of the aperture to the image under low light conditions.
As you can see in the video above, a fast lens is much more capable for shooting video than a slower lens. But you have to be aware of the fact that your image will get a very shallow depth of field (only a thin area of your shot is in focus/sharp) with a fast lens that is wide open (big aperture). Check the following pictures to see the effect of the f-stop values to the depth of field of a still image.
Smaller f-stops will result in a very shallow depth of field. This means that the background and foreground of your shot will look very blurred. A lot of people like this effect and in fact shots with a shallow depth of field look very cinematic. If you don't want your video to have a shallow depth of fiel when shooting in low light, you'll have to choose a higher f-number and turn up the ISO. As you can see in the video below, the aperture stays the same and the image gets brighter (but also much noisier) with higher ISO values.
But there's one important thing you have to concider when shooting videos with a fast lens in bright sunlight: because the lens is so fast you will have to set a very quick exposure time for your videos if you don't want them to get overexposed (to bright). But short exposure times will result in a clipped video which is terrible (exposure time should be twice the frame rate to result in a smooth video - I set it to 1/50s in most cases). To avoid this problem you can close you lens (narrow aperture) which will result in a deep depth of field (everything in focus) or attach a ND filter to brighten down the image and make longer exposure times possible. I reccomend the last option because it makes it possible to shoot with a wide open lens in bright sunlight without changing the exposure time of your video. I will show you the effect of ND filters on videos in a seperate post with my new Fader ND.
Do you need image stabilization (IS) in a lens? For handheld shots without a steadycam or rig - yes. If you shoot from a tripod - no. This depends on what you're shooting and how much you want to spend. An IS lens is definitely better (you can turn IS off when you don't need it) but also much more expensive. If you're on a very tight budget and you don't need IS you can keep your eyes open for old M42 lenses and adapt them to your DSLR. I will post a video with a M42 vs. Canon EF comparison soon. What you definitely don't need for shooting videos is a lens with ultra fast focusing (e.g. USM) because you will have to focus manually (autofocus and video? No.)!
So this is what I recommend to put on your shopping list if you want to shoot videos with your DSLR:
- fast prime lenses (f/1.8 or faster!) and some ND filters or a ND fader.
- next thing should be a wide angle lens or a zoom which starts at a short focal length (18mm or shorter).
- what you need when filming animals or other objects that are far away: a tele lens.
- the most important thing: go for primes if you have the money to buy more lenses because they are faster and sharper than zoom lenses.
If you're shooting skateboarding / bmx / ... videos and don't want to spend too much:
- go for a fisheye lens or a zoom lens which starts at a short focal length.
- if this is to expensive screw a cheap fisheye lens to your kit lens (will give you some distortion but who cares).
If you want to have only one lens for shooting video and photos with:
- I feel sorry, but that won't be possible. I would buy a zoom lens like the Sigma 18-200 and a cheap
- fast prime lens like the Canon EF 50mm 1.8 for low light videos.
What do you think? Which cheap lenses are the best for shooting videos with? If you have any tips write a comment below.