Aperture – 31+ Days To Better Photography

Yesterday’s Topic: Shutter Speed

Part 2 of the Exposure Triangle is Aperture.

Aperture affects Depth Of Field. Do you hate math still?  You might hate it more.  Check this out.

Yeah. That’s Depth Of Field (from Wikipedia). There’s also the Circle Of Confusion. And all that is likely to make your head spin, so let’s break it down to more simple terms.

Getting back to yesterday’s diagram, the aperture on a camera is actually located in the lens. It is behind all the elements (the pieces of glass or plastic in the lens that do the focusing of light) and closest to the camera. Before light leaves the lens, it passes through the aperture. Most apertures are made up of a number of blades which effectively make a smaller and smaller hole for the light to pass through. Why? Because it will increase Depth Of Field as it does so. It looks, more or less, like this:

From left to right, the aperture is open, closing, closed as far as it will go. Now, it doesn’t actually block the field of view as it might seem, it’s blocking light that isn’t coming into focus at a given plane.  Let me try a diagram.

Light comes from a source, let’s say the sun, hits the bunny and bounces into the camera lens. In this over simple diagram, one lens is then moved front to back to focus. Focus means the light from the bunny hits the plane of the sensor evenly (front to back), in a two dimensional way. In this diagram, the aperture is open all the way. This focus is only good for the bunny, for how far away the bunny is from the camera.  The Bunny Plane (r) if you will. If the bunny or the camera moves, this happens.

Because the focal plane is not on the sensor and is way behind behind the camera, the bunny will be blurred.  Bad for you and the bunny. It’s a fact that you can only focus on one plane at a given distance with your lens. Maybe that bunny is 15 feet away. In the top diagram the focus was set to 15 feet.  In the bottom one, maybe it was set to 20 feet and thus, the focal plane was off of the sensor.  IMPORTANT NOTE: ALL of the bunny light is still hitting the sensor.  It is just hitting it while being focused on a plane past the sensor. If the light (in these diagrams) is not hitting the plane of the sensor, it is blurred.

“Well then,” you may be asking, “how the heck do I get things in focus that are different distances away from the camera?”

Aperture! First let me show you what it looks like having different objects at different distances as the light comes into the camera.

The camera lens can only focus on one distance or plane. It picks the bunny. Who wouldn’t? In this case then, the tree and the Yeti, being further back and having the aperture open all the way, do not have their light focused on the correct plane of the sensor and come out blurred. Now let’s use the aperture and close it a bit. I’m going to remove the bunny for a moment, don’t freak out, the Yeti doesn’t get him.

Here is what the light coming from the tree looks like. Same as above. Closing up the aperture…

With the aperture partially closed, as represented by the increased darkness around the edge of the aperture, that ‘extra’ light is cut off and only the light that will come to a high focus is let in. From this point forward, the tree will continue to be in focus. This can be a hard concept to illustrate in two dimensions.

When you close down the aperture, you cut out the extra light from an object that was not going to hit the sensor in sharp focus and are attempting to only let in the light that will be in focus.

Aperture settings are often called f-stops. Remember when I mentioned stops yesterday? These function the same way, letting in half as much light as the f-stop goes from one setting to another and doubling the amount of light hitting the sensor when changed in the opposite direction. In the case of the aperture, the sequence looks like f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45 and so on.  The f-stop number is a function of the lens focal length and the size of the hole the aperture makes (which is the same concept as how your pupil works, by the way).  The smaller the pupil size, the less light. Because we are working with fractions again, and the equation looks like it does at right (thanks Wikipedia), the higher the number, the less light.

The higher the f-stop number, the less light and the greater the depth of field. Depth Of Field simply means the field of view that can be in focus.

If we focus on the bunny and use f/1.4 for instance, the depth of field might only be a foot. Meaning from the point you are focused, maybe the bunny’s nose, the only objects that will be in focus will be .5 feet in front of that point and .5 feet behind that point = 1 foot total.  Now change the aperture to the next stop, f/2 (which, as it is another stop, lets in half as much light) and you might have increased the Depth Of Field to 2 feet. Now 1 foot in front of the bunny’s nose and one foot in back of its nose is in focus. Can you see how more and more things will come into focus as the f-stop is increased?

Here’s another piece of the puzzle:

The person at top is to represent portrait work, where a wide open aperture and its shallow depth of field is often desired to help remove the person from the background. The mountain scene at the bottom represents a time when you might want many things, front to back, to be in focus and thus a smaller aperture is desired.

While changing to a higher f-stop number does give a greater Depth Of Field, it lets in less light. What does that do to the Exposure Triangle? It means either Shutter Speed needs to change to compensate for a good exposure, or ISO does. Knowing what you do of Shutter Speed and how it works with stops, can you now see how if you change the Aperture by one stop darker, you need to change the Shutter Speed by one stop more light?  (We’ll get to ISO tomorrow, for now we’ll keep it simple with Shutter Speed and Aperture.)

What does this look like in the real world?  Here’s a series shot of things I found while picking up random objects off the floor in my house.  Each object is about 6″ away from the one in front of it.  Focus is locked on the golf ball in all shots. Click on a picture for a larger image.

Aperture f/5.6 Shutter Speed 1/160

Aperture f/9.0 Shutter Speed 1/40

Aperture f/14 Shutter Speed 1/15

Aperture f/36 Shutter Speed 1/3

Can you see how you more and more of the objects (and dirt on my floor) come into focus as the aperture gets smaller and smaller (remembering that as the aperture itself gets smaller, the f/ number gets bigger)? Do you also notice what happens to the shutter speed? It gets slower and slower to let in more light to compensate for the aperture.

Whew!  I know that is a lot to take in and it is one of the harder topics for people to grasp when they are learning about a camera.  Aperture affects Depth Of Field.

And with all that reading, it’s time to put it into practice!!  Here is an assignment for you to try at home to help reinforce the idea of Aperture and Depth Of Field:

Tomorrow we dive into the last part of the Exposure Triangle: ISO.

And the answer to yesterday’s question is: 4 stop. 1/60-1/125= 1 stop.  125-1/250= 1 stop.  1/250-1/500= 1 stop.  1/500-/1000= 1 stop.

31+ Days To Better Photography is a series written by professional photographer Peter West Carey on The Carey Adventures.Com. The series is designed to unravel the mysteries of photography so you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.

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18 thoughts on “Aperture – 31+ Days To Better Photography

  1. I think I get it. Holy cow, (or bunnies) years of trying and I think that makes sense. I love landscapes and I am trying to think through the shot instead of just taking a picture. It is difficult to stop and remember the whys. This helps.

  2. If I am the child, then these daily lessons are the reassuring parent, holding my hand as I cross the dizzying streets of photography. Thanks for making sense of this stuff, Peter! Although I’m beginning to wonder why you didn’t pursue your obvious passion for graphic design :) Jokes aside, they really do help – I’m excited for the subplot of the Yeti and the Bunny to unfold.

  3. I think I get it. Now how long will it stick in my brain? (New DSL is on order, but hasn’t arrived yet!)

  4. Thank you. I HATE math, and am more than a little confused everytime you mention a number. But, I think I might, just maybe, get the bottom line. I think some practice exercises would help out all the dummies (like me).

  5. Okay, so here on my little point and shoot I can adjust the aperture range from 2.8 to 5.8. So, from pretty wide open to slightly less wide open. I ought to be able to resolve staggered objects into focus so long as they are within a very short distance of each other. Right? Because although a distant object would need a very narrow aperture (a high f-stop) in order to cut off those wide light rays that are not converging on the sensor (which is what causes blurring in improper focusing), I only need a small difference in aperture to cut off the wide light rays coming off an object only slightly behind the one I have in focus.

    For adjusting exposure so I get the same amount of light in the shot whether I’m trying to get one object, or staggered objects, in the picture, I need to slow down my shutter speed (leave it open longer), because while the aperture closing cuts off wide, un-focused light rays, cutting off those light rays also makes the exposure darker if not compensated. When cutting off blur-causing light rays, compensate with longer exposure time, i.e. slower shutter speed (or higher ISO, but that’s on a different post :} ). However, with my little point-and-shoot my shutter speed seems to be adjusted automatically based on an input I give the camera about how to meter the light in the frame (whole frame, center object, central point). I can’t manually change the shutter speed, so I have to trust that the camera will slow the shutter speed enough to light the exposure well enough, despite the more narrow aperture.

    Further problem with the point-and-shoot: In order to change the aperture, I have to engage the “zoom” feature. So I’m not only closing the aperture, cutting off improperly angled light from distant objects, I’m making the subjects take up more of the frame than they were before; but I’m not at the same time bringing them into focus. I can manually adjust the focus distance to compensate for things a little, so if I want a higher f-stop, and to keep the same subjects in focus despite the zooming, I have to back up a little. If I could manually adjust the aperture I wouldn’t have to back up when I lose the focus on my subject as I zoom in to close the aperture.

    Did I understand aperture, shutter speed, and focus properly there? Do you know of other ways to help a point-and-shoot compensate for the zooming-aperture linking? Or is focus actually tied up with the zooming feature in a way I haven’t understood yet? Is there a zooming equivalent on SLR’s?

    • Sean,
      You understand it well, especially the second paragraph. Most P&S don’t allow for much aperture control and yours is the first time I’ve heard of linking the zoom, which will force a higher aperture number just by nature of the action and smaller lens type.
      On a DSLR, when you zoom in, the aperture will change too on all except the highest quality (read: $$$) lenses. This is also when a standard 400mm 2.8 will cost much more than a zoom that goes to 400, because it can offer a wider aperture and thus work better in lower light (think of the lenses you see on the sidelines at football games).

  6. That is such a good explanation of Aperture. I have been struggling to get my head around it for a few months now. I must say that I am going to print and laminate the picture with the scale on it for a quick reference while out in the real world taking photos. I always think I understand sitting at the computer but when I go to take the pictures I forget half of what I have read so the card will be a handy tool to have with me.

    Keep up the good work and thats so much for sharing your knowledge!!!

  7. I am a pretty technical person. I knew the fact that smaller aperture higher DOF and vice versa. Never understood why? This article made it crystal clear. Peter, great job done man!

    I would like to provide a little bit of feedback to improve the clarity of the content.

    1. Please consider labeling your images such as Fig. 1, Fig. 2. So you and others can refer to images when discussing the topic.

    2. The first picture where you are showing the bunny, tree and yeti. At first, I was thinking the tree and yeti is simply reflecting out of focus light rays on the sensor. Only later it became clear that all out of focus objects regardless of distance actually reflect some in-focus rays on the sensor along with mostly out of focus rays. When aperture is closed down it cuts down the out of focus rays. What a wonderful concept!

    3. I was also for a moment thought that when aperture is closed down, light actually bounces off of the boundaries of the aperture to get the in focus rays. Which is not the case.

  8. You Are PHOTOMAN!!!
    Thanks for the simplified Aperture explanation, will be doing the exercise today, and reviewing my results.
    ciao4niao

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