We have covered in great depth the doubling/halving relationship and how it works with f/stops and shutter speeds to control exposure. You now understand that, for a given illumination on a subject and a given ISO, there are many combinations of shutter speeds and f/stops that give the same amount of light on the film or sensor. This is key point since the shutter speeds and f/stops you choose have implications in how your final photograph will look in ways other than just the amount of light on the film. For example, as you stop down the aperture, you get more depth of field.
If you don’t have a tripod, there are limits to how slow your shutter speed can be before your body movements blur the photo, so there are some constraints. But the point remains, all these combinations yield the same amount of light on the film and an identical picture in terms of brightness. What does vary is the ability of the camera to stop action and the depth of field, or how much is in focus in front of and behind the subject. That is the topic I want to discuss next -- depth of field.
Depth of Field
Depth of field is the amount of subject matter in front of and behind your focus plane that appears to be in sharp focus. When we say "that appears to be in focus" or "is acceptably sharp", understand that this is a continuum, it's not like objects in the depth of field range are razor sharp and then suddenly the sharpness abruptly becomes fuzzy. Things get gradually less sharp until they are perceived as being "out of focus". It is very much like a normal or Gaussian curve.
Basically, these three things affect depth of field:
1. The f/stop
The smaller the f/stop (the larger the number, that is, the smaller the diameter of the aperture), the more depth of field you get. At f/2 (small number, big aperture), you will have comparatively narrow depth of field, with little in focus on either side of your focus point. At f/16 (big number, small aperture), you will have comparatively more depth of field, with more subject matter in focus on either side of your focus point.
I say "comparatively" for a reason. As I’ve already discussed, from a brightness point of view, measuring the amount of light hitting the film or sensor, f/2 is f/2 regardless of the lens. This isn't the case with depth of field. The amount of depth of field at f/2 will also depend on ...
2. The focal length of the lens
The shorter your focal length, the more depth of field you will have. A 25mm lens will have more depth of field than a 50mm, and a 50mm will have more than a 100mm. With really short lenses, like 4 mm, you will have immense depth of field.
That is how simple cameras work. They have a small lens with a fixed focus, but the depth of field is such that everything from one foot to one mile is in focus. That is how the camera in the iPhone works. It has no focus adjustment, but it has a very small lens. With long lenses (telephoto), like a 400mm, you will have a very small depth of field. (Note that you usually have a relatively high f/stop on long lenses, so the depth of field limiting is not so severe, but it is there.)
3. The distance to the subject
The closer you are to your subject, the less depth of field there will be. The further away you are, the more depth of field you will have. Depth of field is actually a percentage of the distance around the focus point. So, at one foot it may be one inch, but at ten feet it is ten inches and at 100 feet it is 100 inches or a little over 8 feet.
Narrow depth of field can be obvious in some photos. Portrait photography is a pretty standard use of limited depth of field. Portraits are typically taken with the subject close to the camera and an f/stop of f/1.4 - 2. That way the subject is in sharp focus, while the background is out of focus yielding a "soft" effect and forcing the eye to go to the person. The point of a portrait is the person being photographed, and not the background. It is also possible to focus on a distant object, and have the foreground be softly out of focus. Very artistic shots can be composed using these techniques.
That second item, the focal length of your lens, has an interesting aspect to it. On one hand, it's pretty straightforward in that the depth of field of a 50mm lens at, say, f/8, is the same regardless of what camera the lens is on.
The wrinkle is that the 50mm lens can be wide angle, normal or telephoto depending on the camera body to which it is attached and format (size) of the film or sensor of that camera. In the days of 35mm film, a 50mm lens was considered "normal." Less than 50mm was called "wide angle" and greater than 50 mm was considered telephoto.
This sounds like some theoretical detail, not significant to we simple photographers, but in fact has some important implications in this digital age. This is because in the film days, just about everyone used 35mm film and lenses in the same focal length range. Now, with digital cameras, sensor sizes vary from tiny to full-frame 35mm sized and even beyond.
With modern digital cameras, there is no standardization on sensor size, and each size comes with its own requirements for focal lengths and even lens design. In general, now that digital has taken over, sensors are smaller, the lenses are shorter, and the depth of field is greater. In fact, recent technological changes have led to even smaller sensors with yet higher resolution. If you want everything in focus all the time, this is great. If you'd like to be able to make some things out of focus ever once in a while, this can be a problem.
Why would a photographer want something not in focus? As I explained earlier, it is actually a great artistic effect, and you may not want everything in focus all the time. It's a way to force the observer to the important parts of the photo by making other parts out of focus. It can be part of the composition of the photograph. We're really moving from the science to the art of photography here.
Sadly, if all you have is one of those lovely little digital cameras or the camera in your cell phone, you will find it easy to get lots of depth of field and surprisingly difficult to get limited depth of field. This has to do with the relationship of film formats and sensor sizes to the focal lengths of lenses used. That is one reason to spend more for cameras which allow direct adjustment of exposure controls, larger sensors, and -- even -- removable lenses. If you do own a camera with interchangeable lenses, this will be a guide to which lenses you should have in your collection and for what reason.
This large depth of field of the simple pocket cameras can be annoying because limited depth of field, to make your subject stand out against the foreground and background, is a pleasing way to isolate and emphasize your subject.
In fact, a lot of filmmakers are quite interested in this fact as DSLRs are being used to do high-definition video for this very reason; they get to use longer lenses and get less depth of field and, in addition, the ability to do real wide angle work without spending a fortune on wide angle lenses. This is called the "film look". A big part of this is the limited depth of field resulting from longer lenses and smaller sensors. Video photography is benefiting from this without resorting to expensive equipment.
In the next installment, I’ll go into this more. Remember, my goal is to explain everything there is to know about focus and exposure. So watch for the next installment and we’ll dig into sensor sizes and how they relate to focal length, lens classifications, lens selection, f/stops, and other esoterica. Plus, and you're going to really love this, we get to do more math. Not only are we going to square numbers, but we're going to take square roots. Why I can hardly wait for it.