Suppose that you are out in the woods with your Canon EOS 5D, a full-frame camera and a 50mm normal lens, and you want to take a picture of the tip of a pine needle.
First you want to take a picture of the moon. That's pretty far away, so you feel comfortable setting the lens focusing helical to "infinity". The "nodal point" of the optics will now be 50 millimeters from the plane of the sensor. [Note: exposure for the moon should be roughly f/11 and 1/ISO-setting.]
The effort of setting up your tripod is so great that you become tired and fall asleep. When you wake up in the morning, there is a bear standing 10 feet away. You refocus your 50mm lens to get a picture of the grizzly. As you turn the helical from "infinity" to "10 feet", notice that the optics are racked out away from the sensor. The nodal point is a bit farther than 50 millimeters from the sensor plane. The lens is casting an image circle somewhat larger than the 24x36mm sensor. Some of the light gathered by the lens is therefore being lost but it isn't significant.
After snapping that photo of the bear, you notice that his fangs are glistening. These aren't going to appear very large in your last shot, so you move up until you are about 1.5 feet from the bear. That's about as close as the lens helical will let you focus. The nodal point is now pretty far from the lens. Extra light is spilling off to the edges of the frame , but still not far enough to require an exposure correction. The bear's face is 1.5 feet high. You've oriented the camera vertically so that the face fills the 36mm dimension. 36mm is about 1.5 inches. So that means you are working at "1:12". The subject is 12 times the size of the subject's image on the sensor.
You're losing some light, but also you notice that you don't have too much depth of field. A 50mm lens focussed down to a foot from the subject only has a depth of field of 1/16th of an inch at f/4. No problem. You haul out a big electronic flash and stop down to f/11. Now your depth of field is a whopping ... 1/2 inch.
Looking down, you become fascinated by some pattern's in the bear's claws. Each one is about 1.5 inches long. You'd like to fill the sensor's long dimension (36mm) with a claw, which means that the subject and its image will be the same size. You want to work at "1:1". But the folks at the lens factory skimped on the helical. You can't rack your optics out far enough to focus at 1:1. It looks like that pine needle tip photo is completely out of the question.
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