Understanding Exposure : Aperture 101
The series of Understanding Exposure trucks on with this installment: Aperture. As an appetizer let’s take a quiz. I have three questions, answers below (no cheating), you get them right… you pass the quiz and read no more. You get so much as one wrong, you go back and read this entry. Deal? Deal. Let’s go!
What is the f-stop number when stopping down two full stops from f/5.6?
a. f/2.8
b. f/8
c. f/11
d. f/7.2The near and far points from the fixation point encapsulate an area called “The ___________________ .
a. Depth of Focus”
b. Circle of Confusion”
c. Depth of Field”
d. Fixation Point Distance”The creamy silky background blur behind your subject is pronounced _________.
a. Bo-Kuh
b. Bo-Kah
c. Bo-Keh
d. Boke…like Coke
Let’s get down to the lesson shall we….
Stops , F-Stops , Stopping Up, and Stopping Down
To begin with, the “f” in “f/8” stands for “focal length”. If you replace this f in the fraction with the value of the focal length of the lens you’re using, you will get the diameter of your aperture. Let’s say, you’re now using an 80mm lens. If the f-stop you want to choose is f/8, you will get the fraction 80/8. 80 divided by 8 is 10. So the opening in your lens is exactly 10 millimeters across. If we had a 50mm lens and an f-stop of f/1.0, our aperture would be 50 millimeters across. In other words, its diameter would be equal in length to the focal length of the lens.
Here, though, it’s important to distinguish between the diameter of the aperture (measured in millimeters) and its area (measured in square millimeters). If we choose f/1.4 on that same 50mm lens, the diameter of the aperture will get smaller by a factor of 1.4 but its area will be reduced by a factor of 2. This might sound complicated but you don’t really need to think about it. What you should know, however, is that by changing the aperture by one stop we either reduce or increase the amount of light by a factor of two. So, if we stop DOWN the lens from f/4 to f/5.6, we allow twice as little light. If we stop UP the aperture from f/5.6 to f/4, we double the amount of light.
The range of f-stops a lens supports is f/1 – f/32, consisting of the following f-stops:
f/1,f/1.4,f/1.8, f/2.0, f/2.8, f/4, f/5.6, f/8. f/11, f/16, f/22,f/32
Depth of Field and Aperture
In short, depth of field (DOF) is the distance between the near point and the far point of the fixation point were objects in your image are acceptably sharp. It’s measured in feet or meters.
The depth of field can be shallow (narrow) or large. When it’s shallow, just a small part of your photograph appears sharp. When it’s large, a great area of your image is in focus.
How does aperture affect depth of field? The chosen aperture value (or f-stop number, as it’s called) directly affects the depth of field in your image. The lower your f-stop number, the shallower the depth of field. At f/1.8 for example, only a part of your object of interest will appear sharp. The rest of the photograph will be blurred.
Bo-Bok… Background Blur
I threw this in the lesson because there seems to be a misunderstanding on the proper way to pronounce the name used for background blur. The name itself originates from Japan and the syllables, consonants, and vowels offers some confusion hence why people say Bo-Kay or Bo-Kah when Bokeh is actually pronounced Boh-Keh. Good Bokeh is soft silky background when using a shallow depth of field on a wide aperture (f/2.8 on up).
ANSWERS: Confucius say; in multiple choice always choose “C”. 1.C 2.C 3.C
In Closing
I want to thank you for swinging by the week. Today, tomorrow, and Saturday are going to be a little hectic as I’m going on vacation. I’ll say now that I make no guarantee of a blog this next week, but I’ll do my best to get one out. Flickr Friday happen regardless as I have some shoots planned for Project 116*3. At any rate, see you all tomorrow for Flickr Friday. Cheers!
