Matt Sep 13, 2011
In this fourth lesson, we are (finally!) going to start discussing the meat of photography technique, with a very important parameter: focal length.
As we saw in lesson 2, focal length is what determines how “zoomed in” you are, also often called angle of view. Focal length is an actual length, expressed in millimeters (it corresponds to the distance between the optical center of the lens and the film plane, though you need not worry about that). The lower this number, the less zoomed in you are. We speak of a wide angle, since you can view much on the sides: you have a wide view. Conversely, if the number is high, the angle will be narrow and you will only see a small portion of what is in front of you: you are zoomed in, this is what we call a telephoto.
Though we will see later that it is not exactly true, as an approximation, you can zoom with your feet: walking 10 meters closer to your subject or adding 5mm to your focal length will result in the same image (these are random numbers, by the way). The choice of a focal length is the very first step in composing a photograph, and probably the most important, as it determines framing. All the other choices (exposure, depth of field, etc) are dependent on your framing having been decided on.
So far, so good. But things become a little bit more complicated when you start looking at the actual numbers. An 18mm lens on a medium format camera will produce a very different angle of view than the same focal length on a compact camera. A modern compact like the Canon S90 has focal lengths between 6.0 and 22.5mm, yet the same values on a lens for a FX DSLR like the Canon 5D would be unbearably wide and totally unusable.
The culprit is what we call the crop factor. The focal length is a physical property of a lens, but the resulting angle of view, which is what we are really interested in, depends on another factor: sensor size. The bigger the sensor, the wider the angle of view for the same focal length. In order to convert angles of view between different formats, we use the crop factor, which is a ratio between the standard 35mm film area and the actual sensor size. For instance, Nikon DX cameras have a smaller sensor than their FX counterparts, which results in a 1.5x crop factor. This means that a 28mm lens on a DX camera will have the same angle of view as a 28*1.5=42mm lens on FX. This explains why, when DX cameras started appearing, the focal ranges of most lenses changed accordingly: the 18-200mm DX lens counterpart is the (just announced) 28-300mm FX lens, etc.
Of course, this works in the other direction too: if your sensor is bigger than 35mm film, then you will need longer focal lengths to obtain similar angles of view: on 4×5 large format cameras, 150mm is considered normal, whereas it would be firmly in the telephoto domain on a DSLR.
Because it can all be a bit confusing, especially with lenses that can be used on several different formats, it is common to give a “35mm equivalent” focal length: the focal length which on a 35mm/FX camera would give the same angle of view.
Concretely, you just need to be careful when discussing actual focal lengths: remember that the final angle of view (which is probably what you are discussing) depends on the crop factor, and that everyone may be using different ones.
Remember how a bit earlier, I said you could zoom with your feet? Well, it’s not quite true. The reason is that perspective will change. One effect of using a long focal length is that it will compress perspective, making everything appear to be on the same plane. Wide angle, on the other hand, will exagerate depth, sometimes to extreme lengths. This is why landscape photographers like to use ultra-wide lenses.
Compare for instance this image, shot at 16mm (with a 1.5x crop factor):
to this one, at 155mm:
Notice how in the second one, the moon seems to very close to the mountain, while in the first one, the climber appears very far away from the ground (he wasn’t more than 8-10m up)? This is an effect of focal length, and a very important creative tool at your disposal.
Sometimes, it will be worth getting closer to your subject and using a shorter focal length, if you want to create depth and emphasize perspective. Sometimes, you will have to walk backward and use a longer lens, if you want to compress perspective. You can sometimes see this effect in movies, usually when someone is feeling sick or about to pass out, and the relative position of objects seems to change but the framing remains the same (see this youtube extract from the Goodfellas). This is achieved by moving forward while zooming out at the exact same speed.
Now that you know more about focal length, let’s take a look at the different ranges usually found in lenses, and what their uses tend to be. Of course, there are many, many exceptions, but this is the “normal” use they were designed for. All focal lengths are given for 35mm sensor size (crop factor 1).
The assignment today is about getting a bit more familiar with focal lengths. You will need a camera and a zoom lens (or a series of prime lenses). Go somewhere where you can walk freely. Bonus points if there is a mildly interesting subject. Start by staying immobile and take a picture of the same subject at 5mm increments for the entire range of your lens (compact cameras users, just use the smallest zoom increments you can achieve). Now, remember the framing of your most zoomed in image, walk toward the subject and try to take the same image with the widest focal you have.
Back on your computer, compare the last two images. Do they match exactly? What are the differences? Take the series of immobile pictures, reduce the size of the most zoomed in image and overlay it on top of the widest one. Does it match exactly?
If you are not tired yet, try taking a wide angle image which emphasizes perspective and a tele image which makes use of perspective compression.