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sensor size and image circle pt. 1

Sorry for the hiatus I've been on. I promise, the blog isn't forgotten. I've started writing a book on macro photography. Wish me luck with smooth writing, easy editing, and success in distribution. More on this project as it manifests itself. In the meantime, I'm going to share part of the book through this blog and it's going to take the form of the next few posts. So for a while, we are going to talk about sensor size, image circle size, and optics. Here we go.

Remember that there are essentially three sensor sizes used in interchangeable lens cameras. They are full frame, APS-C, and four thirds. Some folks might be very engaged in the industry and say, “Wait, you forgot about the one inch sensor Nikon 1 and medium format cameras.” No, not really. At the time of writing there is no macro lens for the Nikon 1 system and the number of people who primarily use a medium format camera is a (sadly) minuscule number. So I’ll just deal with the main three.

Each of these sensors is of a different size and I just gave them in descending order. To really explore the way that lens manufacturers think about and design lenses, and the way you should think about them as you explore options, I’d like to construct an analogy. Let’s imagine a projector on a table and a projection screen.These sit across from each other and you’re about to set up a presentation. Good so far?

Now, you set up the table and turn on the projector and the light hits the screen and it fits perfectly edge to edge. Great. The light coming from the projector is the right size for the screen. The size of the circle of light as it hits the screen has a name in the world of photography, it’s called the image circle. And right now the image circle from the lens is the size of the projector screen, which means your projector and screen are perfectly calibrated. There are certain sensor sizes and there are lenses with corresponding image circles. As there are full frame sensors, there are full frame lenses and so forth.

The next day you show up to give a presentation and when you turn on the projector the light not only fills the screen, but bleeds significantly over the sides. It hits the wall and ceiling and the floor. What could you do? Well, the first thought that you might have is to use the zoom function on the projector to make it fit. However, you’re told that these projectors cannot zoom. Indeed, in the world of optics any individual lens is made to create a single image circle size and this is designated by telling you what sensor size the lens matches. It cannot be changed.

Okay, next thing you’d like to do is move either the projector or the screen. However, the screen is fixed to the back wall. It can’t move. Then you learn that this projector, indeed all projectors they have at this particular institution, must be used at a set distance. And this distance is dictated by the projector itself. In the world of optics this distance is called flange distance. So there's no adjusting the projector or its distance, just as flange distance and image circle are fixed attributes of lenses.

A lens must be used at a particular distance from the sensor. For the time being all projectors have the same flange distance, even if they have different image circle sizes. In reality there are different lens and camera systems with their own flange distances, but we’ll ignore this fact for the time being.

These factors feel limiting, but we’ll find a way around them. First, let’s examine the nature of the problem. The image is perfectly in focus, but what we actually see on the screen is a cropped in version of the whole presentation. Example sits just below.

First thing we’ll notice is that we are getting a cropped view of our presentation. This happens with optics and is called the crop factor. A lens that gives one angle of view on an APS-C camera will give a different view on a full frame camera. In essence the larger the sensor that we work with, the wider the angle of view any lens gives on it. Mathematically, we say that the crop factor on an APS-C camera is about 1.5X that of full frame. So a 50mm lens on a full frame gives a field of view on an APS-C camera what about a 75mm lens does on that first full frame sensor.

But take this projector analogy a little further. The projector only puts out a certain amount of detail. This means the detail hitting the screen is a smaller percentage of what the projector is ideally capable of. When you put a full frame lens on an APS-C camera you crop into the optical resolution. In fact, you’re only using about 45% of the glass in the lens. It’s a good thing that full frame lenses tend to be sharper than the average lens, because you’re using less of it. When you take that same lens and put it on a full frame camera it instantly gets sharper. This is because you use 100% of the glass to record detail.

Next week we'll take this projector analogy a step further and look into what happens if the screen is larger than the projection. Later in the series we'll take a look at mathematically analyzing how much resolving power is lost when an image circle is larger than the sensor size. In the meantime, here's a flower picture.

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