Understanding Aperture In Photography

In Simple Terms – “Aperture is ‘the size of the opening in the lens when a picture is taken.”

The main function of a camera lens is to collect light. The aperture of a lens is the diameter of the lens opening and is usually controlled by an iris. The larger the diameter of the aperture, the more light reaches the film / image sensor.

Aperture is expressed as F-stop, e.g. F2.8 or f/2.8. The smaller the F-stop number (or f/value), the larger the lens opening (aperture).

[Note: Many camera user manuals today will refer to the aperture in terms of “aperture value” instead of f/value. I’m not sure when this trend started but don’t get confused between “aperture” and “aperture value.” Aperture value” is simply another way of saying f/value.]

Lenses with larger apertures are faster because, for a given ISO speed, the shutter speed can be made faster for the same exposure. A smaller aperture means that objects can be in focus over a wider range of distance (depth of field).

Portrait and indoor (sports and theater also) photography often requires lenses with large maximum apertures in order to be capable of faster shutter speeds (and narrower depth of fields) in order to combat the low light problems with no camera shake.
The narrow depth of field in a portrait, as well as in macro photography, helps isolate the subject from the background.

Maximum Aperture Typical Lens Types
f/1.0 Fastest Available Prime Lenses
(for Consumer Use)
f/1.4 Fast Prime Lenses
f/2.0 Fast Prime Lenses
f/2.8 Fastest Zoom Lenses
(for Constant Aperture)
f/4.0 Light Weight Zoom Lenses
f/5.6 Extreme Telephoto Primes

In More General Terms

As you know from general experience, the bigger a hole, the more can go through it. Think about turning on a tap (water faucet): open it a little and the flow is only a trickle, open it up and more water flows through.

It’s the same with lens aperture: the larger the aperture, the more light gets through to the sensor. Obviously this affects the exposure of your image.

Now, giving the film or sensor the proper exposure is like filling a cup of water: if the water flow is slow (from a small aperture), it takes longer to fill (the exposure time is longer).

And obviously, if the flow is faster (we turn the tap on to make the aperture larger), it takes less time to fill the cup (exposure time is shorter).

Lens Focal Length
(35mm equiv)
Lens Type Use in Photography
Less than 21 mm Extreme Wide Angle Architecture
21-35 mm Wide Angle Landscape
35-70mm Normal Street & Documentary
70-135 mm Medium Telephoto Portraiture
135-300 mm Telephoto Sports, Bird and Wildlife

The Science Behind

Suppose we have a 50mm focal length lens. If we have a big size hole – a big aperture, it might measure 25mm. So 50 divided by 25 gives us 2: the f/number is 2, which we write as f/2.

If the aperture is smaller, say, 3mm in diameter, 50 divided by 3 gives us about 16: the f/number reads f/16. As the hole is smaller, less light gets through. So f/16 is said to be a small aperture or small f/number.

That’s why you could get confused if you read about an aperture of 16 being smaller than 2: that does not make sense and is, in fact, wrong. A photographic aperture is written as ‘f/number’: it means the focal length divided by the aperture diameter. So f/16 is indeed smaller than f/2. (Microscopists talk about numerical aperture, but that’s a different thing.)

Wait, What are STOPS ?

Here’s another source of confusion. The word ’stops’ is used in two senses. One goes back to the days when lens aperture was changed by dropping in a metal plate with a hole cut in it. You changed aperture by taking out one plate and dropping in another one with a different sized hole. These were called stops (actually Waterhouse stops, after the inventor). From that we get the term ’stopping down’.

Now, these stops were arranged so that each smaller hole halved the exposure (and conversely, each larger hold doubled exposure). From that we get the term ‘f/stops’. From this you still hear photographers talk about ‘one stop’ meaning a halving or doubling of exposure. Goes all the way back to late nineteenth century!

Carting sets of metal plates with holes in them is a bore, not to mention really slow to use and before long the aperture diaphragm was invented. This was a set of leaves which were pivoted on the rim so that they fanned across the gap – the more they overlapped, the smaller the central hole. And that’s what we still use now.

The Cheat Sheet

This is for all those, who face trouble, reading all those cryptic aperture values on lenses. This is what it really means-

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