Rangefinder camera pedant
jlw is offline
Join Date: Aug 2004
First thing you need to understand is that diopter has nothing to do with finder magnification.
The finder magnification refers to the perceived size of the image seen through the finder, and is something that's designed into the finder optics.
The finder's diopter "power" only has to do with making the view through the finder easier for your eye to focus. You can change diopter settings freely with basically no effect on the finder's overall magnification. (Okay, an extreme minus-diopter eyepiece may make the finder image appear slightly smaller, but that's more because of how the eyepiece interacts with your eye.)
Here's a lot more than you probably want to know about this, but I'm feeling especially pedantic today:
-- A diopter number measures the refracting power of a simple lens. The larger the diopter number, the more the lens bends light. Positive numbers refer to convex (positive) lenses that cause the light rays to converge; negative numbers refer to concave (negative) lenses that cause the rays to diverge.
Combinations of convex and concave lenses can be used to design complex optics such as camera lenses, but in the case of a finder "diopter" lens the important application is making the finder image more comfortable for your eye to view. In this respect it works just like corrective eyeglasses lenses, which also are defined in diopters.
It's usually assumed that your eye is most comfortable when focused at infinity, where the ciliary bodies that focus it are relaxed. When you view something close, the ciliary bodies tighten up and make the eye's lens more convex. Over long periods of time this tires the eye, and also the eye's lens gradually hardens over time so that it becomes more difficult to focus. (This eye fault is called "farsightedness," because you can see things far away more easily than you can see things up close; another term for the same thing is presbyopia.)
So, adding a convex (positive diopter) lens in front of the eye takes over some of the ciliary bodies' work, allowing its lens to be focused nearer infinity while still viewing something close. This is exactly what positive-diopter reading glasses do, and it's also what magnifying loupes do. They don't actually increase the image magnification; they just let you get your eye closer to the object without losing focus.
In terms of finder design, we'd say that the positive diopter lens increases the apparent viewing distance of the finder image, meaning that the eye "thinks" it's farther away than it really is.
Because of the variations in individual humans' optical equipment, many people have a problem that's the opposite of farsightedness; it's called "nearsightedness," or myopia. In this fault, the eye's lens is too convex, or the eyeball itself is too long; this keeps the eye's lens from focusing at infinity. We correct this by adding a minus-diopter lens in front of the eye; this diverges the incoming rays slightly, so that when the eye re-converges them, they come to the correct focus.
In finder-design terms, a minus-diopter lens moves the apparent viewing distance closer.
Why would manufacturers design finder eyepieces with slight minus diopters as standard, if the eye is more comfortable at an apparent viewing distance of infinity? The basic reason is that the eyepiece is only one part of the total finder system.
You can design a perfectly good viewfinder with no optics at all; you may have seen an old press camera with an "open-frame" finder, consisting of a wire frame on the front standard to define the field of view, and a peep sight on the camera back to let you align your eye accurately behind the wire frame. This type of finder is reasonably accurate and very bright; its main drawback is that it's very difficult to build it into a small camera such as a 35mm camera.
The first step to get around this was to get rid of the wire frame and substitute a rectangular negative (minus-diopter) lens at the front. The negative lens reduces the field of view to match what the camera lens sees; you can view this image by placing your eye several inches behind the viewing lens, again using a peep sight to line it up properly. The Ermanox, the classic available-light camera used by Dr. Erich Salomon, had exactly this type of viewfinder.
Again, the main drawback of this type of finder was size; you have to put your eye several inches behind the viewfinder lens to get it to focus properly, and this meant the camera body had to be several inches thick. An obvious improvement on this was to position the eye closer to the front finder lens. The unaided eye would have difficulty focusing on the finder image at this short distance, so the designer adds a convex (positive diopter) lens to help it focus more comfortably.
That's the beginning of the modern camera viewfinder, but subsequent improvements have added lots of other things to its optical path: a rangefinder spot, bright frame lines, and now exposure readouts. These are all packed into tiny spaces inside the finder mechanism, and require their own optical elements to get all of them to appear at roughly the same apparent viewing distance so the eye can take them all in at once.
Most of these optical elements are positive, so it's often easier for the manufacturer to add a slight minus diopter to the camera eyepiece to even everything out. Again, this doesn't have anything to do with finder magnification; its only effect is on the apparent viewing distance of the finder image.
All you really need to know is this: Diopter powers are additive, so if you know the total diopter power of your camera's finder system and your eyeglass prescription's diopter, you can add them to figure out what diopter correction you need to replace your glasses. In practice, it can be difficult to find out how much of the finder system's total diopter power is in the eyepiece lens and how much is in the rest of its optics, so usually you have to resort to a bit of trial and error to determine what eyepiece will give you the best overall correction.
(Also, in addition to plain old diopter error, most people's eyes have a certain amount of astigmatism -- this is caused by the eye's lens being thicker in one direction than the other, and requires a cylindric correction in your eyeglasses lens. Standard camera diopter correction lenses can't include this cylindric correction -- so if you've got a bad case of astigmatism, even the correct diopter correction lens for your camera won't make the view as clear as what you see through your eyeglasses.)
Note that I'm not an optician -- all this eye stuff is just info that I've pieced together over the years as a result of having crummy vision but wanting to use cameras and other optical stuff. If in doubt as to what's best for you, explain your situation to your eye doc and get his/her advice.
But don't worry about diopter correction's effect on finder magnification, because basically there isn't any. Just remember that magnification determines the finder image's size and diopter determines its apparent viewing distance.
"Never trust a graph without error bars."
Last edited by jlw : 12-11-2006 at 07:38.