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The Exposure Triangle - Aperture
Exposure Triangle

Aperture Settings What is Aperture
Aperture is one part of the exposure triangle. It is the small hole in the lens that light passes through on its way to the light sensitive sensor. The size of this hole is adjustable. Adjusting the aperture size controls the volume of light coming into the camera through the lens. The larger the aperture, the more volume of light enters into the lens. The smaller the aperture, the less volume of light enters the lens. The aperture on a camera controls light like the pupil in a human eye. It does so by closing up to restrict light, and opening up to let it through.

Aperture settings are measured in f-stops. Each stop is double or half the light of the previous stop. The Aperture is located in the lens, not the camera body which means the aperture range will vary depending on the lens. Older cameras had apertures with values that double or halve with each change: f2.8, f4, f5.6, etc. Each change was one complete stop making calculating in your head easy. Today's modern cameras have more settings in between. Usually one third stops. This gives us many more f-stops to choose from but if you want to increase or decrease the aperture by one stop you'll have to move the dial 3 clicks. Once again, because of the many options, having the camera do our calculations makes it easier but it is important to understand what is happening even if you don't ever want to do the math in your head. It is important to note that the larger the number, the smaller the opening.

Aperture Priority Mode Aperture is the first part of the exposure triangle the camera adjusts to obtain a correct exposure when more light is needed. It can be controlled manually in Aperture Priority mode or full Manual mode. When it comes to image sharpness, aperture is the first thing that should be considered for various reasons that will be explained later.

F-stops are a calculated as a ratio between the aperture of the lens and the focal length setting: f-stop = focal length / aperture (n=f/d). so if you have a 50mm lens and the aperture diameter is 4.54mm the f-stop is f11. Because it is a ratio (or fraction) the larger the number, the smaller the aperture and the smaller the number, the larger the aperture.

Lens Speed Lens Speed
It is important to understand that the range of f-stops available are limited to the lens. Every lens has a different range of f-stops or aperture settings. Most lenses are capable of going down to a very small f-stop from f22 to f36. It's the large apertures that are important. The larger the aperture can go, the more light can be captured. Lenses with very large apertures up to f1.2 are considered to be fast lenses. These fast lenses come at a price. Each lens has it's largest f-stop stamped somewhere on the lens, usually right in the front around the glass but could be stamped on the outer case. This number, denoted as a ratio, might look something like 1:3.5. Zoom lenses may have a range such as 1:3.5-5.0. This is because some zoom lenses' maximum aperture will be different if the lens is zoomed in or out.

Depth of Field
The most common reason to adjust the aperture setting is to control depth of field. Depth of field refers to the sharp part of the photo in front and behind the focal point. In other words, how much of the photo that is in focus. The smaller the aperture, the deeper the depth of field. A good way to remember this is to say the larger the f-stop number, the larger (deeper) the depth of field and the smaller the f-stop number, the smaller (shallower) the depth of field.


Sweet spot
Using small apertures (large f-stop numbers) is a great way to keep much of your image in focus. Using large apertures (small f-stop numbers) is a great way to achieve that artistic blur in your photos but using f-stops close to either end of the f-stop range comes at a cost. That cost being sharpness. Yes, using small apertures will get a greater focus field but the over-all image will become soft. There are two phenomenon that cause this: Diffraction for small apertures and Spherical Aberrations for larger apertures. The sharpest images that can be obtained by your lens is usually about 2 stops down from its largest aperture (smallest f-stop number). This is known as the lens' sweet spot.

As light passes through the iris in a lens some strange things happen. The light bends. This is known as diffraction. If you shine a light on an object in a perfectly dark room it will cast a shadow. If light travels in a straight line, then why isn't the edge of the shadow sharp? Why is it fuzzy? That's because light bends around objects that it hits. When light travels through the iris of a camera it hits the edges of the aperture and bends. The smaller the hole, the more the light bends. This has two unique side effects. The first is that the image will be slightly fuzzy. It loses its over-all sharpness. It also creates a star burst effect around lights. If you can live with a slightly softer image, the star burst effect can be very appealing in your photo.

Below are examples of three different aperture settings. The diagram shows light entering your lens, passing through the aperture and hitting the light sensitive CCD. The image below the diagram shows the star burst effect due to diffraction. The image below that is a blow up of a sign on one of the buildings showing the sharpness of the image.


Spherical aberrations Spherical Aberrations
For the sharpest image, all light must be focused to a very fine point. The finer this point, the sharper the image. But how fine can this point be? That depends on the quality of the lens. The problem is that the rate light bends as it passes through a lens at different angles is not a constant. The greater the angle between the light ray and the lens surface at the entry point, the greater the change of angle as it passes through the glass. This means that the rays of light that pass through the lens at its outer edge bend more than the light that enters in through the center. Yes, we expect this to happen but look at the diagram to see what I mean. The light rays towards the outside edges are bending too much so that they are not all meeting at one fine point. This makes the sharp tip of the cone of light dull. It's like drawing with a dull pencil. No matter how hard you try, you can't draw sharp lines. This is known as spherical aberration. No matter how much you try, you cannot focus down to the smallest point at the tip of the cone.

There are ways lens manufacturers can design lenses that will minimize spherical aberrations but this brings the cost of the lens up. If your lens is not creating sharp images, it could mean that it is suffering from severe spherical aberrations. Sharpness is not the only thing to be concerned about. There are also resolution concerns.

Coming Soon:
Depth of field preview button
Lens resolution

For more information about aperture read the "Shaping Bokeh" and "Circle of Confusion" tutorials.