Beyond Auto: Aperture – Part 1

GoingBeyondAuto.com - Aperture-Part 1

Alright, so you read about Shooting Modes in my last post, if not, go check it out.  One of the modes I mentioned as a great first step to try was Aperture Priority.  I think you are wondering just what exactly is Aperture and why do you care?  Aperture plays a very important role in the outcome of your photos.  Let me first explain a little about what Aperture is and then tell I can show you how understanding it will greatly improve your photos.

What is Aperture?

To keep things simple, Aperture is the size of the opening in your lens that allows light to pass through to the sensor.  Remember that the sensor is what actually captures the image digitally.  This used to be the amount of light to pass through and hit the film to “expose” the film to the light to make the image.

Inside your lens are a series of blades, known as the diaphragm that block light from passing through the lens.  In the middle of the diaphragm is an opening (the aperture) where the light is allowed to pass through.  The larger the opening, the more light that passes through.  Generally, the more blades that make up the diaphragm, the more expensive your lens is.  The number of blades determine the shape of the aperture, so fewer blades result in more of a hexagonal look, where more blades translate into more of a circular shape.  This is a visual preference because out of focus objects will appear based on the shape of your aperture, more on this later.

In older lenses, the diaphragm was controlled by an Aperture Ring that you turn to adjust the size of the aperture opening.  In modern lenses, like you have, this is all controlled electronically by your camera.  When you are in Auto, your camera measures the light and chooses what Aperture and Shutter Speed combination to use to get a properly exposed image.  However, when you go into Manual or Semi-Automatic modes, you are able to control the Aperture for your exposure.

What is f-stop?

Now that you know that Aperture is the opening in your lens that allows light to pass through, what is f-stop and what does it have to do with aperture?

All lenses are not created equal.  They come in different focal lengths, and with different apertures.  Some lenses, generally the more expensive ones, come with “wide-open” apertures of f/1.4.  The lens that came with your camera may only open as wide as f/3.5.  Before I lose you, let me explain what these numbers mean.

Aperture is all about math, and you don’t need to know it, but I will explain it anyway so you have a general idea.  I will show you the math for Canon’s 50mm f/1.8 lens. The real physical size of the aperture opening if you were to open up that lens and measure it is approximately 27.8mm.  If you take the focal length of that lens (50mm) and divide by the Aperture size (27.8mm) you get 1.79 or approximately 1.8.  However, on Canon’s 85mm f/1.8 lens, the aperture is more like 47.3mm but still gives you f/1.8.  Since both of these lenses allow the same amount light to pass through, they have the same aperture, but the size of the opening is completely different…that is where f-stop comes in.

To make life easy and to keep things consistent, a standardized f-stop numbering scale was created.  So I could use the 50mm lens or the 85mm lens and know that I was shooting at the same aperture even though the sizes are completely different.  You see, f-stop refers to the size of the aperture but in the ratio of the physical aperture size and the lens’ focal length.  This means that (focal length)/(aperture size) = f-stop.  The standard scale of f-stop was determined by taking the square root of 2 and multiples of 2.  For example √2 = 1.4 and would result in a wide open aperture, and √512 = 22 and would result in an almost closed aperture.

An aperture is indicated by an f over the f-stop number.  You may also see it as 1:1.4 showing that it is in fact a ratio.

Here is a chart showing all the standard f-stops.
GoingBeyondAuto.com - Aperture Table

Each “stop” down the list translates to half as much light passing though the lens as the previous stop.  For example, f/1.4 lets in 2x more light than f/2.0.  When you have your lens “wide open,” you are shooting at the maximum aperture for your lens, which may be f/1.4.  This means the aperture is open as far as it can go and is also referred to a fast lens.  If you want to close, or stop down, your aperture, you will head toward f/22 which is called a slow lens as it lets in the least amount of light.  Below is a visual representation of what it looks like to be at each f-stop.

GoingBeyondAuto.com Aperture Chart

I know what you are thinking right about now.  First, that is way too much information, and you are right.  As I said, there was math involved, but you don’t really need to know it.  What you need to know is what the maximum aperture of your lens is and how to open and close it.  But you are also thinking, my lens is f/3.5 and that is not on the chart!  You’re right because that is a standard list of f-stops listed by full stops.  Remember that aperture ring on the old cameras I mentioned?  It would move based on this chart one full stop at a time.  However, with modern electronic apertures, we have finer control over the precise opening of the aperture.  This means we don’t have to go from f/2.8 to f/4, we can take 1/3rd stops instead which gives us 2.8, 3.2, 3.5 and 4.0.  I won’t include all of the 1/3rd stops on this post, but now you know why your camera seems to have a lot more options than the chart above.

Keeping it Simple

I hope I didn’t lose you in that long explanation of aperture.  I want you to understand where it came from, what it is, and how to use it.  Please don’t get overwhelmed by the math or the verbiage.  Just know what your lens can handle and that each full stop down translates to half the amount of light passing through the lens.  The smaller the f-stop number, the more light comes in.

In Aperture – Part 2 we will talk about what this actually means for your photos.  Why do you need different apertures?  How does aperture affect depth of field? How does aperture affect shutter speed? All this and more coming soon in Aperture – Part 2.

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