Photography & Digital Arts

Photography Tips

Photography Tips – The Aperture Priority Setting: Understanding The Av Setting, What F Numbers Mean & Depth of Field

First of all, the first two questions are really asking very similar things. The Av setting is one of the most confusing issues for photographers wanting to move beyond the Automatic settings on their camera. Av literally means Aperture Value, and is the aperture priority setting.

Sometimes called the “f number”, aperture or aperture size, it is the conventional way of expressing how wide open the lens is. Understanding how to use this gives a photographer much greater creative control.

Understanding How The Camera Calculates Exposure Is Key To Understanding Aperture Priority

The correct exposure for any given image is determined by:

  1. the size of hole the light passes through (the aperture size) on the way to the sensor
  2. the amount of time the light has to get through the hole (the shutter speed)

(It is of course also affected by the ISO value, sometimes called ISO speed – a measurement of how quickly the sensor reacts to the incoming light. This is usually determined independently of aperture size and shutter speed).

Aperture Value (Av) Means Size of Hole In The Lens – And That Affects Depth Of Field

Choosing the Av setting tells the camera that the photographer wants to take control of the aperture size, (the size of hole the light has to pass through on the way to the sensor) and the camera should use this when working out the right shutter speed to make the exposure correct. Taking control of the aperture size gives the photographer control over something called depth of field. This is a topic in itself, but simply put greater depth of field means more of the image is in focus, and less depth of field means less of the image is in focus (usually allowing the background to be intentionally blurred out).

Understanding Av Values – The Numbers Are Reversed!

Confusingly, the larger the f number, the smaller the hole.

There is a simple reason for this, in that the number is actually a fraction – so f4.0 means that the lens is ¼ as wide as the maximum possible diameter, and f22 means that the lens is 1/22 as wide as the maximum possible diameter. 1/22 is obviously a lot smaller than ¼, hence the number that appears to be larger actually giving a smaller aperture (or hole for the light to pass through).

In practice a simple rule of thumb is to remember that it’s “the wrong way round”:

  • small f number = large hole and
  • large f number = small hole

A typical range of f numbers from smallest to largest ranges from

f45 … f22 … f11… f8 … f4 … f1.4

with f45 and f22 giving greatest depth of field (most area in sharp focus) and f4 and f 1.4 the least depth of field (least area in sharp focus).

Controlling Aperture Value Gives The Photographer Greater Creative Control

Why would the photographer want to do control the size of hole? Quite simply for creative control. Left to its own devices, the camera will normally try and get as much as possible in focus in good light, or as fast a shutter speed as possible to prevent camera shake in low light. This isn’t always going to give the desired result the photographer was looking for – or indeed the best result. Taking control of this setting allows the photographer to choose the result they want, whether it is as much as possible in pin -sharp focus, or a background intentionally blurred out.

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