Aperture selection and sharpness

By pilliwinks

Techniques

15 May 2008 10:15

Aperture and shutter speed are related; for a given level of light and ISO, you have a series of aperture/shutter speed combinations that will give the same exposure. Which combination you choose will depend on whether you need to use a particular shutter speed to freeze (or blur) the subject (or camera) movement, or if you need a particular depth of field.

Most landscape photographers favour a large depth of field, which implies a small aperture. Which is why tripods are usually used, since that means that camera movement need not be taken into account. But you can still have a problem with subject movement. The idea of blurring water is a common one, but use of a longer shutter speed will also blur any moving branches, leaves or grass. This may or may not be desirable.

However, the point of maximising depth of field is only to ensure that all the objects that you want to be sharp are sharp; once that end is achieved, there's no point in stopping down any further to gain more depth of field. And stopping down more than you need can be counterproductive to overall sharpness. This may be skipped; but shows why stopping down reduces sharpness, rather than increasing it.

Depth of field "works" because the eye can be fooled into seeing a circle as a point, provided only that the circle is small enough. How small is "small enough" depends on how closely you are looking at the print; and because it's the size on the print that matters, the degree of enlargement comes into the equation. But I'm not going into that here, because on stopping down an effect other than depth of field comes into play.

If your lens were perfect, and there was no optical effect called diffraction, then, at the plane of focus, a point in the subject would create a point in the image. As you move further away from the plane of focus, the light rays spread out, and the point becomes a circle of steadily increasing diameter. The limit at which the eye will see the resulting circle as a circle defines the limits of the depth of field. Think of the light being focused as a cone, and the size of the base being the aperture. As the base diameter drops, the angle of the sides becomes shallower, and you can cut a section ever further from the apex and have a small cross section. Which is why small aperture = great depth of field.

The circle formed is called the "circle of confusion", and once you've decided what value is appropriate for your print, you can calculate the depth of field (or look it up in tables!). Unfortunately, there is an optical effect called diffraction, which is caused by the nature of light itself. This effect means that not even a perfect lens can reproduce a point as a point, because light itself will not focus to a point, but to a circle (called the Airy disk, after its discoverer). And the more you stop down, the larger this minimum point becomes.

Clearly, there is a cross-over point at which the size of the Airy disk equals, and then exceeds, the size of the circle of confusion. When this point is reached, the lens is said to be diffraction limited, and you can increase the sharpness of objects at the limits of the depth of field only by reducing the resolution (sharpness) of objects within the depth of field.

It is possible to calculate the maximum resolution you can achieve when diffraction is taken into account, and as a reasonable starting point, you can say that the maximum resolution in line pairs per mm is 1000/(f number). The actual size of the Airy disk depends on the wavelength (= colour) of the light, so this figure is open to argument; some would use 1500/(f number) instead. The difference in the numbers comes down to the contrast between the lines; the actual resolution will be greater than the 1000/(f number) gives, but at a lower contrast in the fine details.

And it has been known for years that, if a certain resolution is reached, you get a greater impression of sharpness if you increase the contrast rather than the resolution. The resolution will actually be greater than the formula gives, but it won't look like it... The size of the circles are an absolute size, independent of the camera format. At f/8, the maximum resolution you can get will be 1000/8 = 120 lppm (approx). If you need to be able to resolve 6 lppm in the print to create the illusion of sharpness (a figure often used, and like all others, can be debated), then clearly you can only enlarge by a maximum of 120/6 = 20 times.

This gives a print 20x30 inches from 35mm (or full frame sensor); 15x20 from most DLSR sensors, and considerably smaller from the very small sensors used in most compact cameras. Drop the aperture to f/16, and the print sizes halve, if you still want to have the same degree of sharpness. All this assumes that you have that impossible object, the perfect lens; and that your sensor or film is capable of infinite resolution. In the real world, things are worse... It is because of the effect of diffraction that the smallest aperture the makers allow you to use is dependent on the film/sensor size. It's not a coincidence that digital compacts don't stop down very much; or that f/22 is the smallest aperture available on nearly all 35mm lenses, and f/32 on medium format ones. And that to go to f/45, f/64 or smaller needs a large format camera. It's all to do with the degree of enlargement needed to make a reasonable size print.

The conclusion is that once you have stopped down enough to overcome the problems most lenses have when used wide open, and gained enough depth of field, stopping down further is a bad thing. There are times when absolute resolution isn't the most important thing, and the necessity for a long shutter speed will take priority. The "bad thing" is "bad in terms of resolution", and clearly there will be times when you will ignore this. If you want to look at the effect of diffraction applied to the real world of digital cameras, take a look here: www.cambridgeincolour.com/tutorials/diffraction-photography.htm