Aperture Basics - Part 2
- Large f-stop numbers (i.e. f/22) represent narrow apertures
- Has a large DOF (depth of field)
- Everything is in focus
- Used for landscape images or when you want the whole image in focus
- Small f-stop numbers (i.e. f/2) represent wide apertures
- Has a small DOF
- Only the subject is in focus and the background is blurred
- Used for portrait or images where you want the background separated from subject
- As you increase or decrease the aperture, the amount of light that reaches the camera / image sensor changes.
What is the impact of changing the aperture?
Now for the tricky part. If this is your first time reading about aperture, read the following sentences 3 or 4 times. Personally, it took me some time to fully grasp what is written below.
As you increase the aperture, say from f/4 to f/8, the hole that the shutter makes gets narrower. Thus, as you decrease the aperture, say from f/8 to f/4, the hole gets wider.
Intuitively, this seems backwards. If you are increasing the number, then you should also be increasing the size of the hole. However, this is not what is happening and once the numbers and what they represent are explained it becomes a little easier to grasp.
So I've increased the aperture by one stop. How does this affect the light getting into the camera?
From the above diagram, one can see that the hole gets continually narrower as we increase the aperture. This also means that the amount of light that gets to the image sensor continually decreases. The more technical way to say this is that as the aperture is increased by one stop, the size of the opining is halved which means the amount of light let through will also decrease by half. The opposite is true when decreasing the aperture. What this means is that if the aperture is increased, or made narrower, you will also need to extend the shutter speed to let more light reach the image sensor in order to get a properly exposed image.
If you feel confident that you understand aperture, skip this section. For everyone else, lets give this a try.
One thing that helped me when I was just starting to get used to this idea was to relate this idea to something I've used/dealt with before. Personally, I would relate aperture to a hose with water coming out.
Let's pretend you have a hose and you are trying to fill a bucket with a certain amount of water. There are two ways to fill the bucket with water. You can either use a small garden house and leave it on for a long period of time or you can use a large fire house and turn it on for a very short time. In the end, the same amount of water will reach the bucket.
Now lets try the same sentence with inserts relating each story item to its designated camera item.
Let's pretend you have a hose (the size of hose is related to the size of the aperture) and you are trying to fill a bucket (the bucket is the image sensor on the camera) with a certain amount of water (the water will be the light particles you are trying to capture to make an image). There are two ways to fill the bucket with water. You can either use a small garden house and leave it on for a long period of time (use a narrow aperture with a long shutter speed) or you can use a large fire house and leave it on for a very short time (use a wide aperture with a quick shutter speed). In the end, the same amount of water will reach the bucket (the image will be exposed the same amount).
Translation: For a properly exposed image, you can either make the aperture wide and have a quick shutter speed or have a narrow aperture and have a slow shutter speed. Now for those of you that are a little more advanced, I realize that this doesn't take ISO into consideration but this is meant to explain the inverse relationship between aperture and shutter speed to the beginner crowd.
Depth of Field (DOF)
The depth of field (commonly refereed to as DOF) is the amount of the scene that will be in focus when you take your photograph. If you have a large DOF, most of the photograph will be in focus regardless of distance. This means that something that is 5 feet away will be just as "in focus" as something 100 feet away. This is especially useful when taking landscape photographs or images with people where the background is equally as important. To that effect most landscape photographs are taken at a narrow aperture, between f/8 around f/16. Only in extreme circumstance would you use a narrower aperture as some funky things start happening due to limitations of your lens. (A brief explanation of these side effects can be found here.)
This means that I can have several apertures that give me a properly exposed image. Why would I pick one aperture over another?
Personally, I consider aperture to be the most important setting on my camera and is generally the first thing I select when composing a photograph. The greatest strength of aperture is that it allows the photographer to add dimension by separating the subject from the background or alter the exposure to make the photograph brighter or darker. As with everything in photography these two effects have names: Depth of Field and Exposure. (There are several other very technical aspects that change with aperture and are covered here.)
For a small DOF, only part of the image will be in focus, normally the object closest to the lens. The rest of the image will be fuzzy or blurry. (As with all things in photography, this effect has a name: Bokeh). This is especially useful when taking portrait photographs as it commands all the viewers attention to be on your subject. All distraction that the background might of provided is gone and all that is left is for the viewer to pay attention to the subject. Most portrait photographs use a wide aperture, between f/1.2 and f/2. If your lens doesn’t go down that far, you can still get decent separation from the object and its background with an aperture around f/4, if the background is far enough away. The closer the background is to the subject, the harder it will be to get good separation.
When you set your camera to a certain f-stop number, what you are doing is setting the shutter opening to a fraction of the lens opening. Therefore, setting the camera to the aperture of f/10, will set the shutter to be 1/10th of the total lens opening and setting the aperture to f/2 will set the shutter to be 1/2 of the total opening. Since the f-stop number represents a number in the denominator (bottom number) of a fraction, the smaller apertures (f/2) denotes a larger portion of the shutter being open as compared to the larger apertures (f/10). (i.e 1/2 is larger then 1/10). Thus, smaller aperture openings are given larger f-stop numbers and larger aperture openings are given smaller f-stop numbers.
If you are a visual learner, as I am, the visual representation of aperture (below) might help you understand what happens when the aperture is increased or decreased. (These are not to scale and are only intended to help you understand the above concept.)
Below you can compare the difference between large and small DOF. The same image was taken with two different apertures: f/1.2 and f/9. You can see that the image with the small DOF (taken at f/1.2) has a very blurry background, so much so that you can't even make out what is in the background. You can see the background gradually getting blurry on the lawn chair as it gets farther away. On the other hand, everything in the image with the large DOF (taken at f/9) is in focus. You can see the subject that is close to the lens just as well as the subjects in the background.
Exposure is the amount of light that gets to the image sensor. As the aperture is changed in size, the amount of light that reaches the camera sensor also changes. Remember earlier, when increasing the aperture by one stop also changed the amount of light that reached the camera sensor (assuming everything else is kept the same). This relationship is the driving force behind all photography decisions and as such it is important to know and understand because not every type of photograph will allow you to chose from a wide variety of apertures. For example, in low light situations, the camera should be set to the aperture that will let in the most light; something like f/4. However, even with your aperture set at its widest setting, you still might not get a properly exposed image. What's next? This is where the shutter speed comes into play. Once you truly understand this relationship, the possibilities are endless which is why taking photographs is an art form not a science experiment. No matter how creative you get, at the end of the day it all boils down to the fact that you need a certain amount of light to hit the image sensor in your camera (and your camera doesn't care how it gets there).
Shutter Speed: 200
Shutter Speed: 200
Shutter Speed: 200
Shutter Speed: 200
Photos: The above photos show what happens when only the aperture is changed (all other settings were kept the same). The photo on the left has a very wide aperture. As you go right, the aperture gets narrower and the images gets darker.
What Aperture is right for me?
Hopefully, now you see why the answer to this question is not an easy one for the simple reason that every situation is different. You not only have to take into consideration the current situation, but also what you want your photograph to look like in the end. Do you want a small or large DOF? How much light is available? Can you select the aperture you want with the available light? Hopefully the above discussion gave you some insight on why certain apertures will be selected over others.