Aperture

Aperture

One of the basic parameters in photography is aperture, an ideal example of something that trades off different advantages and disadvantages. It is controlled, usually, via a diaphragm of 7 or more blades, the more the better as a rounded diaphragm opening means less jagged looking highlights and better bokeh. It can also be controlled by Waterhouse Stops, where brass inserts have various sizes of hole in their centre so the appropriate aperture can be slotted in. These of course can be perfectly round, but the system is more clunky and the purview of older large format film cameras.

The main effects of aperture are on depth of field and image quality, and the two are mutually exclusive. In theory, the lens wide open would give the highest resolution and as we stopped down this would gradually be reduced by diffraction. However, lenses are not perfect so in reality most lenses will reach a peak around two stops down. So an f/2 lens might peak around f/4. Older lenses can behave differently, as do cheap zooms, and the lower quality lenses may not peak until f/16. However, the f/4 quoted may not offer enough depth of field, so we stop down to say f/8 or f/11 to make sure everything we want in focus is in focus.

This is further complicated by format, so crop sensor cameras offer more depth of field at a given aperture than full frame ones. This could be an advantage or it could be a drawback, depending on your point of view. I think we'll keep it simple and not get dragged into that here, but stick to the generalities. Of course, small apertures also mean longer shutter speeds, so another consideration becomes camera shake and then subject movement. But for today let's see some images where aperture was considered the most important parameter.

Widest aperture possible, f/6.3 here but at 300mm, to concentrate on the eyes



Small aperture, f/16, to maximise depth of field, but when close up even that may not be enough



f/8 and be there! Good advice for general outdoor shooting



Widest aperture, f/1.2, subdues the background



F/8 and 20mm ultra-wide lens results in plenty of depth of field