Aperture, what is it and what does it do for us?

The definition of aperture simply means an opening. In photography aperture refers to the iris or diaphragm of the lens which is usually located at or near the lens mount. The important thing here is that we can control how wide the opening is. But why do we want to control it? What does it do for us?

To answer both questions read on ...

There are two sides to controlling the aperture, one is technical and one is creative. Many people shy away when the word creative is used, but stay with it and you'll see what I mean.

The technical reason to control the aperture is that it controls the amount of light hitting the sensor or film. We need to do this to ensure we get a correct exposure, but it is only one of three controls available to us to manage or control exposure, the other two are shutter speed and ISO. Here is a more detailed explanation of Shutter Speed - Does it Matter? if you would like to also read up on that. Explanation on ISO is coming soon.

The creative reasons to control the aperture are many and include such things as how much of the image is in acceptable focus and how the background looks, the latter is often referred to as Bokeh and is best described as how the lens renders the out of focus parts of the image.

The technical aspects should become second nature requiring little thought, the creative aspects should always be a conscious decision by the photographer.

Apertures are expressed as an f number, this is simply a ratio of the diameter of the opening to the focal length of the lens. These f-numbers are also referred to as stops, they got this name because on older lenses there is an aperture ring that you manually turn and it has fixed "stops" for each opening size. Most modern lenses no longer have this ring, but the term has stuck. Each stop is equivalent to one exposure value (EV). For a full explanation of f-numbers see here http://en.wikipedia.org/wiki/F-number

For our purposes we need to understand that the smaller the number the wider the opening is. Aperture stops in full steps look something like this F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32

At each step the opening is exactly half the diameter of the previous step.

So what effect does changing the aperture have?

To answer this question we have to introduce the concept of acceptable sharpness and depth of field (DoF). This simply means the amount of the image that is, or appears to be, in focus. It is best demonstrated by a series of images. These were all taken with the camera mounted on a tripod, from the same position with the same lighting conditions and the same lens. The only things that changed were the aperture and correspondingly the shutter speed. The images are all cropped in exactly the same way. Apologies for the exciting subject too :evilgrin: You can click through to see bigger versions and the EXIF should be intact.

Technical info: Camera Nikon D610, Lens Nikkor AF-S 85mm f1.8G, Continual studio lighting used.

85mm f2
https://farm8.staticflickr.com/7315/...46657d4a_c.jpg85mm_DoF_Aperture_Demo_D610-7338 by SimplyDiscusPhoto, on Flickr

85mm f2.8
https://farm8.staticflickr.com/7319/...6d9eb233_c.jpg85mm_DoF_Aperture_Demo_D610-7339 by SimplyDiscusPhoto, on Flickr

85mm f4
https://farm8.staticflickr.com/7435/...cb545707_c.jpg85mm_DoF_Aperture_Demo_D610-7341 by SimplyDiscusPhoto, on Flickr

85mm f5.6
https://farm8.staticflickr.com/7334/...7fa20895_c.jpg85mm_DoF_Aperture_Demo_D610-7342 by SimplyDiscusPhoto, on Flickr

85mm f8
https://farm8.staticflickr.com/7302/...b5625f0c_c.jpg85mm_DoF_Aperture_Demo_D610-7343 by SimplyDiscusPhoto, on Flickr

85mm f11
https://farm9.staticflickr.com/8658/...56af7475_c.jpg85mm_DoF_Aperture_Demo_D610-7344 by SimplyDiscusPhoto, on Flickr

85mm f16
https://farm8.staticflickr.com/7301/...aac8d25f_c.jpg85mm_DoF_Aperture_Demo_D610-7345 by SimplyDiscusPhoto, on Flickr

In all the above images the focus is on the second battery from the left. They are arranged at 3" distance and on a 45 degree diagonal (approximately). As can be seen from the above images as we stop down the lens, i.e. increase the f-number and reduce the opening, more of the batteries become in focus. In the first image it is all but impossible to make out the large writing on the last battery, by the time we get to the later images it gets easier to read, and by f16 it is all but in good focus.

One of the creative reasons why we would do this is shown in the following image, the background has just melted away and focus fades away from Kerry's right eye, giving this a very gentle, thoughtful feel.
https://farm9.staticflickr.com/8568/...c8d18106_c.jpgKerry_Titchfield by SimplyDiscusPhoto, on Flickr

It also allows us to separate the subject from the background and achieve an almost 3D effect as shown by this image of an Altum angel.
https://farm8.staticflickr.com/7445/...bf7bbc90_c.jpgAltum by SimplyDiscusPhoto, on Flickr

If you have any questions about this please feel free to ask and we'll do our best to answer them.