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Aperture, f-numbers, and depth of field

Aperture and exposure

The aperture in a lens – also known as the “diaphragm” or “iris” – is an ingenious piece of mechanical engineering that provides a variable-size opening in the optical path that can be used to control the amount of light that passes through the lens. Aperture and shutter speed are the two primary means of controlling exposure: for a given shutter speed, dimmer lighting will require a larger aperture to allow more light to reach the image sensor plane, while brighter light will require a smaller aperture to achieve optimum exposure. Alternatively, you could keep the same aperture setting and change the shutter speed to achieve similar results. But the size of the opening provided by the aperture also determines how “collimated” the light passing through the lens is, and this directly affects depth of field, so you’ll need to be in control of both aperture and shutter speed to create images that look the way you want them to.

F-number math

The f-number is the focal length of the lens divided by the effective diameter of the aperture. So in the case of the 35mm F1.4 G lens, when the aperture is set to its maximum of F1.4, the effective diameter of the aperture will be 35 ÷ 1.4 = 25 mm. Note that as the focal length of the lens changes, the diameter of the aperture at a given f-number will change too. For example, an aperture of F1.4 in a 300 mm telephoto lens would require an effective aperture diameter of 300 ÷ 1.4 ≈ 214 mm. That would end up being a huge, bulky, and very expensive lens, which is why you don’t see too many long telephoto lenses with very large maximum apertures. There’s really no need for the photographer to know what the actual aperture diameter is, but it’s helpful to understand the principle.


“F-numbers” or “f-stops”

All lenses have a maximum and minimum aperture, expressed as “f-numbers,” but it is the maximum aperture that is most commonly quoted in lens specifications. Take the Sony 35mm F1.4 G, for example. This is a 35 mm F1.4 lens: 35 mm is the focal length (we’ll get to that later), and F1.4 is the maximum aperture. But what exactly does “F1.4” mean? See the “F-number math” box for some technical details, but for a practical understanding it’s enough to know that smaller f-numbers correspond to larger apertures, and that F1.4 is about the largest maximum aperture you’re likely to encounter on general-purpose lenses. Lenses with a maximum aperture of F1.4, F2, or F2.8 are generally considered to be “fast” or “bright.”

The standard f-numbers you’ll use with camera lenses are, from larger to smaller apertures: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and sometimes 32 (for you mathematicians those are all powers of the square root of 2). Those are the full stops, but you’ll also see fractional stops that correspond to a half or a third of the full stops. Increasing the size of the aperture by one full stop doubles the amount of light that is allowed to pass through the lens. Decreasing the size of the aperture by one stop halves the amount of light reaching the sensor.

Aperture and depth of field

“Depth of field” refers to the range of distances from the camera within which photographed objects will appear acceptably sharp.
In extreme examples of narrow depth of field, the in-focus depth might be just a few millimeters. At the opposite extreme, some landscape photographs show very deep depth of field with everything in sharp focus from just in front of the camera to many kilometers away. Controlling depth of field is one of the most useful techniques you have for creative photography.
Basically, larger apertures produce narrower depth of field, so if you want to shoot a portrait with a nicely defocused background you’ll want to open up the aperture wide. But other factors come into play. Lenses of longer focal lengths are generally capable of producing narrower depth of field (partly because, as we learned above, an F1.4 aperture in an 85 mm lens, for example, is a lot larger than an F1.4 aperture in a wide-angle 24 mm lens), and the distance between objects in the scene being photographed will have an effect on the perceived depth of field as well.

Three keys to effective defocusing

There’s actually more to shooting images with beautifully defocused backgrounds than simply choosing a bright lens and opening the aperture up all the way. That’s the first “key,” but sometimes a large aperture alone won’t produce the desired results. The second key is the distance between your subject and the background. If the background is very close to your subject it might fall within the depth of field, or be so close that the amount of defocusing isn’t sufficient. Whenever possible, keep plenty of distance between your subject and the background you want to defocus. The third key is the focal length of the lens you use. As mentioned above, it’s easier to get a narrow depth of field with longer focal lengths, so take advantage of that characteristic as well. Many photographers find that focal lengths between about 75 mm and 100 mm are ideal for shooting portraits with nicely blurred backgrounds.


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