Creative use of DOF

When finished, hopefully you’ll know what is an aperture and why it’s important. So, let’s start:

An aperture is, quite frankly, a hole. By varying the size of this hole, we can increase or reduce the amount of light that reaches the sensor. Say what? Let me put it another way. Let’s talk very briefly, first, about exposure. Exposure is the amount of light + the amount of time that the light is left on the sensor, or film.

A jug of water
In order to have a property exposed picture, you need the right combination of light + time. If you have a container that holds liquid, for example, a 1 gallon container, when you go to the sink, or perhaps to the garden hose, you can control the filling of that container by varying the quantity of water that is coming out of the hose. If you open the spigot to its maximum opening (aperture), then the container will fill very quickly. However, if you let but a trickle of water come out at a time, it might take several hours to fill the container. This is exposure. Time + quantity (intensity) of light. The intensity being controlled by how wide you open the spigot. The time that is needed to fill the container, shutter speed.

Away from water. Back to cameras.
If you were to take one of your lenses, hold it to the light and look through it, you’d see a hole created by diaphragm blades. If your lens has an aperture ring, turning the ring would make the opening larger or smaller, depending on the direction that you turn. The purpose of this hole is to increase or reduce the amount of light coming into the camera. This is your aperture.

What do all of those numbers mean?
Looking at my 50mm lens, I see the following numbers: 1.8, 2.8, 4, 5.6, 8, 11, 16. Looks like some weird math sequence that you’d have to figure out for a college entrance exam, doesn’t it? Well, these numbers are aperture numbers, or f-stops, and each one of them, starting left to right, represents a smaller aperture, that is, it lets in less light (1/2 the amount of the previous number). The bigger the number, the less light it lets in. Think of it as a divisor.

A little math! Piece of Pi !
Uh, oh! No one said that there would be math involved! You can skip this part if you don’t want to know why these numbers are as they are. Let’s just say that it has to do with circles and Pi.

So, if you decided to keep reading, here’s what those numbers mean:
Given the above formula, let’s plug in some values for a 50mm lens:
f = focal length
N = the aperture Number.
Area = 3.14(pi) * ( 50/2 * 4 )^2
Area = 3.14 * ( 50 / 8 ) ^2
Area = 3.14 * (6.25)^2
Area = 3.14 * 39.06
Area = 122.7 square millimeters (area of the circle).

Using the same calculations for the same lens, but different aperture (5.6), gives and area of 62.6 square millimeters, or approximately 1/2 of the surface area, so one half of the light! Isn’t math wonderful?. Try it for the others and you’ll see that the math works. Now you know why each step to a higher number results in half of the light being let through, and each full step to a lower number, twice the light. You are changing the surface area of the hole.

For your math savvy folks, the above formula is the calculation for the area of a circle. pi * r^2. The division in the ( ) is the calculation for the radius, but enough of this math stuff!

So, that’s all very interesting. How can I use this?!
OK, now that we know how to control the the quantity of light coming through, what advantage does it offer us? This allows us to control our depth of field. Depth of field is how much, front to rear, of our picture is in focus. We might not always want everything in focus from the tip of John’s nose to the end of the street. We might want to isolate John and provide a nice, blurred background.

DOF and Bananas (click for larger image)
Depth of field can be altered by many factors: focal length of lens, distance of lens to subject, aperture, distance of subject to background, etc. For the purposes of this article, we’ll stick with depth of field as it pertains to aperture. We’ll also only demonstrate DOF for one lens, a 90mm f/2.8.

Because the birds won’t hold still long enough for me to take 8 shots in a row, I had to use some bananas that I had sitting around. Bananas are very cooperative, as a bunch, generally! ;-)

As you can see, each step down (move to a higher number, smaller aperture) in aperture causes a corresponding increase in depth of field. When I was at f/2.8, the banana was very well isolated from the background and only that point that I focused on, at the stem, was in sharp focus. As I decreased the size of my aperture, more things start to come into focus, which may or may not be what I really wanted.

In this way, DOF can be used not only to control the amount of light coming in, but to also control what appears in acceptably sharp focus. So, now you know!

Aperture Priority / Aperture Value (AV)
This is the mode on a camera where you choose one half of the exposure equation, the aperture, and leave the camera to choose the other, shutter speed. That’s it!

Other things: Fast lenses
OK, so you’ve heard the term, perhaps, “fast lens”. It has nothing to do with how fast you can put it onto your camera! :-) It may have to do with how fast it can drain your bank account, though! Fast lenses can be a lot more expensive than their slower counterparts. They are also heavier because they use larger pieces of glass to let in more light.

A tale of two lenses
For example, a Nikkor 300mm f/4 costs $1,124, while the same type of lens, one stop faster, the Nikkor 300mm f/2.8 VR costs a whopping $4,449! You pay a lot for that extra stop of light. A lot of engineering goes into that lens. Also, the former weighs in at 3.5 pounds (1.6 kg), the latter, 6.3 pounds (2.9 kg). Certainly something to consider when making a purchase.

A fast lens means, generally, those with maximum apertures of 2.8 or more. Remember, the smaller the number, the wider the opening. Think divisor. Fast relates to shutter speed. The wider the lens opening, the faster shutter speed you can use for the same lighting conditions. For example, a 100mm f/2.8 is a faster lens than a 100mm f/3.5, by almost a complete stop (1 stop equals twice or 1/2 as much light, depending on the direction that your heading.). So, it is a stop faster.

Now, hopefully, you know about apertures, how they affect exposure, and why having control of it can be important.

Your favorite photography podcasts or books?
Missing pieces.

7 Comments

LEAVE A COMMENT