Page Contents
The Choices
- Flow phenomenon: Water boiling? Faucet dripping? Why does it look like that?
- Visualization technique: Add dye? See light distorted by air/water surface?
- Lighting: Continuous? Strobe? Sheet?
- Photography
A: Framing and Composition
B: Cameras
C: Lenses
D: Exposure
E: Resolution - Post processing: Creating the final output. Editing: at least cropping the image and setting contrast.
Still on lenses. Here we’ll see what ‘out of focus’ means, and the relationship between aperture and depth of field.
Aperture
The second major specification for a lens is the maximum aperture, which is basically the hole in the lens that light passes through. Camera lenses have a variable sized hole, created by a clever set of overlapping leaflets, as shown in Figure 1. The larger the maximum aperture, the more expensive the lens will be, since more precision shaped glass will be needed.
Now here’s the confusing part. Aperture is defined as the focal length f, divided by the diameter D, e.g. f/D . Both quantities are lengths, so the f number, or f/, is just a ratio without units. Although it is written f/ it is pronounced “eff stop”, for reasons we’ll save for the next page. For a given focal length, the larger the diameter is, the smaller the f/ is. So the largest aperture a lens can have will be at the smallest f/. This is the value used to specify a lens. Generally f/ ranges from f/1.4 to f/22 or so, but some large format or antique cameras can get up to f/64. Yes, that means a really tiny hole.
The aperture has two major effects. It can be used to reduce the amount of light passing through the lens, but we’ll delay discussion of that to the next section, on exposure. The other thing it can do is control the depth of field (DOF) in the image. To understand DOF we have to see what happens when an image is out of focus.
Out of Focus: Circles of Confusion
For an image to be in focus, the lens location needs to be adjusted so the sensor plane is in the focus plane. But what if the sensor is a little too close to the lens, or a little too far? Figure 2 illustrates this: all the light rays coming from one point on the object that are captured by the lens form a cone, converge in the focus plane and continue on to form another cone. When the sensor plane cuts the cone, it sees a ‘circle of confusion’ instead of a point on the object .
Depth of Field (DOF) and Aperture
Figure 2 shows an exaggerated case. If we dial it back and say the sensor plane is close to the focus plane, the size of the circle of confusion will be small, possibly even less than one pixel of the sensor. Even if it covers a few pixels, it may be too small to be noticed when viewing the whole image. If the object moves a little bit forward or back, the focus plane will also move a little, resulting in small circles of confusion. If the circles stay small enough to escape notice, the movement of the object is considered to be within the ‘depth of field’, the region out in front of the camera where everything will be reasonably in focus . Also note that DOF is not symmetric around the object in focus; due to the nonlinearity of the lens focus equation, you’ll get more DOF behind the object than in front.
Now, let’s place another object in front of the first object, as shown in Figure 3. It is outside the DOF, so it generates a large circle of confusion, and will appear blurry in the image. Here’s where the aperture comes in. If we use the aperture to reduce the diameter of the light cones between the lens and the sensor plane then the circle of confusion for Object 2 will be smaller as well, effectively expanding the DOF (Figure 4).
If you want everything in your image to be sharp, use a large f/ which gives a small aperture. The great landscape photographer, Ansel Adams, was famous for his incredibly sharp and detailed images such as Figure 5. He used a very large format camera, with film plates 4×5 or 8×10 inches, and the smallest possible apertures. In 1932 he co-founded Group f/64, an association of West Coast photographers dedicated to presenting the world as accurately as possible, in contrast to using photography like a painting . So why not use as small an aperture as possible, all the time? Aperture has a big impact on exposure (our next topic) and on resolution (also coming up).
When photographing a surface at an angle, sometimes a large f/ is insufficient to ensure that the whole subject is in focus. In most cameras the sensor plane is perpendicular to the lens axis, so the object plane is too, and only things on or near that plane are in focus. If your subject is sloping away from you instead of being perpendicular to the lens, wouldn’t it be nice if the object plane matched that? View cameras, like the one Ansel Adams used, can do that by allowing the lens barrel to be tilted as shown in Figure 6. This results in the object plane being inclined even further, according to the Scheimpflug principle , as shown in Figure 7. This can be achieved in a DSLR by a special adapter like a Lensbaby .
At the other end of the aperture range, a small f/ can be effective for bringing attention to your subject while letting the background be blurred, as demonstrated in Figure 6. In fact, the aesthetic quality of blur in an image has a name, bokeh . Bokeh depends on details of lens and iris design, with some lenses having better bokeh than others. A word of caution: be thoughtful about using a shallow DOF. Blurry foreground objects can often be distracting, while blurry backgrounds are more easily accepted.
If you are shooting with a phone camera, however, you won’t have to make this choice, because most phones don’t have a variable aperture at all . Because the lenses are so short, you are likely to get excellent DOF unless you are focused on a very close object. There are software modes in camera apps or in postprocessing that can give you an artificially blurred background if you want.
Hands on! Do this now!
What is the range of apertures on all your cameras and lenses? What happens to that range when you zoom? What happens to your DOF? Can you preview the DOF in the viewfinder? Do you like your bokeh? Make some test shots and examine them closely; do they match what you expected from your viewfinder?
