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Sep 182012


There is a lot of information out there on the web and some of it can be very confusing and hard to absorb so I decided to do a series of posts that talk quickly and in non-technical jargon so that it may be easier to understand. First topic is Aperture.

What is Aperture?

Aperture is the size of the hole inside the lens. It is also called a F-Stop. Photographers use these terms interchangeably, but they mean the exact same thing. They allow light to pass through the lens and onto your sensor or film. 

So why is Aperture important and why should I care?

Aperture controls Depth of Field. Depth of Field is the area of acceptable focus in an image. Have you seen images where only a small part of the image is in focus and the rest is blurry. Or have you seen an image where everything is sharp. Aperture controls all this. You should care about this because it is one of the creative tools that photographers use to guide the viewer through the image. This is a part of  your creative toolbox as a photographer. 

So what do all those numbers mean?

Apertures are expressed in F numbers like pounds are expressed in lbs. There are whole F numbers and fractional F numbers. The most common whole F numbers are in this order F2.0, F2.8, F4.0, F5.6, F8.0, F11, F16, F22, F32. They represent the amount of light that the opening is letting in, onto the camera sensor or film. On one side is F2.0 which is the largest opening and lets in the most light, and on the other side there is F32 which lets in the least amount of light and is the smallest opening. When you go down the scale from F32 to F2.0, each step down from one whole F number is called a “stop”. For example if you go from F32 to F22 that is considered 1-stop. Each stop means double the amount of light that hits the sensor or film. Now going the opposite way on the scale, if you go from F2.0 to F2.8 you will be letting in half the amount of light. With most digital cameras of the recent past they have fractional F numbers such as F3.5, F4.5, F6.3, etc. These F numbers go in between the whole F numbers. They represent either 1/2 stops or 1/3 stops. Theory remains the same, smaller number, “more light”; bigger number “less light”.

What is an easy way to remember what aperture does?

As photographers, one of the basic things we need to remember is Aperture and what it’s function is. One way that I came up with that is easy to remember is this: “The smaller the F number the less things in focus, the bigger the F number the more things in focus.” If you remember this saying you will know approximately which F Stop to use for what look you want. In most practical shooting situations you will use this mindset for a good portion of your shooting needs. Example, the first question you ask yourself when shooting a close up of a flower would be; “do I want the background in focus blurry”. Now, if you remembered the saying, “The smaller the F number the less things in focus, the bigger the F number the more things in focus”, you can pick a F stop that will give you the desired look. 

Anything Tips for the intermediate photographer?

All lenses have a “Sweet Spot” aperture. This sweet spot, is the aperture that gives the sharpest possible image for that particular lens and is different from “relative sharpness”. You must do a lens test to find the sweet spot for your particular lens and it may differ for zoom lenses at the different focal lengths, and “Prime” lenses which are single focal length lenses, such as a 50mm F1.8.  What is the difference between “absolute sharpness” and “relative sharpness”? Well, the rule of Depth of Field is: “The smaller the F number the less things in focus, the bigger the F number the more things in focus”. This is true for “relative sharpness”, but is not always true for “absolute sharpness”. “Relative sharpness” is a measure of sharpness compared to other areas in the image. Such as the sharpness in the middle of the image is the same as the sharpness in the corners of the image. “Absolute sharpness” is a measure of total sharpness as compared to the lens itself or in comparison to other lenses. Here is an example: I want to shoot a picture of my family with the mountains in the background. Based on the rule, to get my family in focus and the mountains in focus, I should use the biggest number I have available on my lens, such as F32. In theory, my family and the mountains should all be perfectly sharp, and this is true, but only in “relative sharpness”. If I took the same image and I knew, through testing, that my lenses “Sweet Spot” was F11 I could make the same image and  have the “absolute sharpness” of the image be better than the “relative sharpness” of the image shot at F22. The reason for this is a phenomenon called “diffraction”. “Diffraction” is when the opening of the lens (Aperture) is so small that the light that is transferred to the sensor or film is Defracted or Scattered, creating slight blurriness. So the image shot at F22 was the victim of “Diffraction” and when you compare the images side by side, you will notice that the image shot at F11 is sharper to your eye than the image shot at F22.

Anything Tips for the advanced photographer?

Did you know that different lenses have different optical formulas that affect the “character” of the lens. For example, there are some lenses that are formulated to be softer in the corner than in the center. It is built into their optical formula. Other lenses are formulated to be shot at “wide open” (smaller F number) apertures, and don’t perform well when “stopped down” (shot at large F numbers). As your skills improve as a photographer, you should invest time and effort in finding and testing lenses that give you a particular look that you like, this is part of your creative choices as the image maker. Sure, if you are very good technically, you can shoot with any lens and make it work, but picking that specific lens that has just a little bit extra, may make your life a little bit easier. I use this example, if you are a mechanic and you need a 15mm wrench, sure any wrench will probably work, but why do many mechanics choose to use SnapOn tools or Craftsman? Maybe its the grip the wrench has that’s tailored to the mechanics hand, or that the tolerances are tighter for that wrench so it works easier, or that it’s forged instead of cast so it’s stronger and you don’t have to worry about breaking it. Lenses are the tools for the photographer that when you get to a higher level in your craft you should also be mindful of the tools that you are using for your jobs. Pick the right tool for the job. Keep this in mind also, it takes time and effort to find what’s right for you! What’s right for you doesn’t always mean the best and most expensive. Actually, many times it’s the exact opposite! So be wary of sales people, blogs, or advice that seems to steer you in spending more of your hard-earned cash. Nothing replaces testing it for yourself!

There is a lot more to know!! Please use the comments or contact to ask me any questions you may have! I’m here to help!

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