I'm surprised that I can't find this one being asked before, but maybe it's because everyone else knows the answer. Lenses have focal length measurements in mm signifying the field of view when viewed on a 35mm (FF) camera, and 50mm on these cameras being roughly what the human eye sees. What I don't know, is the mm designation actually a physical measurement of anything? In a 50mm lens what actually measures 50mm? Or is it just a way of comparing magnification between different lenses? I had thought briefly that maybe it was the distance between the film plane and a piece of glass in the lens, but as the total length of my camera and 70-300mm lens never gets as long as 30cm, this can't be the case. Sorry for being so dumb, please enlighten me.
What does the mm measurement of a lens signify?(6 posts) (5 voices)
from wikipedia - "A lens with a focal length about equal to the diagonal size of the film or sensor format is known as a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance of the print diagonal, which therefore yields a normal perspective when viewing the print; this angle of view is about 53 degrees diagonally. For full-frame 35mm-format cameras, the diagonal is 43 mm and a typical "normal" lens has a 50 mm focal length. A lens with a focal length shorter than normal is often referred to as a wide-angle lens (typically 35 mm and less, for 35mm-format cameras), while a lens significantly longer than normal may be referred to as a telephoto lens (typically 85 mm and more, for 35mm-format cameras). Technically long focal length lenses are only "telephoto" if the focal length is longer than the physical length of the lens, but the term is often used to describe any long focal length lens."
I have also wondered about this, and did much reserch.
This is the best answer I have found, spent a little time looking at it, gave up and accepted it for what it is.
hope niko allows this one because it is interesting, even though confusing.
I knew it as simply the distance of a lens from the focused plane in order to create an image focused on infinity. This is regardless of whether it's a single glass or an old school lens design.
I say old school because at one time the rear element of the lens was the focal length distance from the film plane in order to focus on infinity (and in some lenses I believe it still is). So a 50mm optic was 50mm away from the film plane at infinity. As single-lens-reflex cameras became popular, wide angle optics started to become a problem because their short focal lengths meant their rear element would interfere with the mirror mechanism. This is why cameras like the Nikon F had mirror lock up, so you could stick lenses like a 6mm or 8mm fisheye to shoot. The rear elements were literally 6- or 8mm away from the film plane at infinity focus.
As you focus closer the rear element starts moving away from the film plane.
Lens design started changing and manufacturers started designing lenses that had equivalent field or view of shorter focal length optics but the rear element now cleared the mirror mechanism. The same holds true for longer focal length lenses which were design to be more compact for their stated focal length. The rear elements were now closer (and therefore the lens more compact) than their traditional focal length equivalent.
You can still get traditional focal length optics in long focal lengths, typically for refractor telescope optics. You can see how much longer they are in comparison.
Because of these reformulations of traditional optics, the concept of a lens focal length started to get lost in the overall scheme of things. Typically the focal length is used to denote the optic's field of view on the film plane.
If you change the size of the film plane, you need to formulate the optic to cover it. This is not a problem going from FX to DX, but going from 35mm to "2-1/4" 60mm cameras the lens needs to fill the larger film plane. Changing the size of the film plane also changes the field of view of the optic relative to the film plane. So while a 50mm is considered a "normal" lens (field of view similar to the human eye) in an FX camera (24mmx36mm film plane), it's considered a moderate telephoto in a DX camera (typically around 16mmx24mm), and is a wide-angle in a 2-1/4 camera (60mmx45mm, 60mmx60mm, 60mmx70mm typically).
Thanks for your responses guys, from what I understand of your replies, in modern camera/lens combinations at least, the mm designation of a lens doesn't refer to an actual distance internally, and as I thought may be the case, is just a way of describing the field of view of a lens in a universally understood way. Have I understood that right?
The focal length of a lens is actually the distance between the image plane and the principal plane of a lens when you focus at an object at "infinity". If you have a single lens or a very simple lens design the principal plane is the middle of the lens but as pointed out by vidrazor in a modern lens design the principal plane might not have any physical correspondance at the lens (barrel). So the mm designation of a lens still refers to an actual distance internally, but not necessarily to an object inside the lens but a virtual plane (actually the image plane is also a virtual plane as you can't see it - you can only place a sensor/film/screen at it and make the image visible).
If you know the focal length of a lens and the image size (size of your sensor/film/screen) you can also calculate the field of view. Again as vidrazor pointed out the same focal length lens will produce different field of view depending on the imager size.
So what is the difference between a DX 50 mm lens, an FX 50 mm lens and a large format 50 mm lens? As you might know the sweet spot of a lens is usually in the center of the image and the further you go to the outside the less IQ you get. So lens designers need to add additional elements to make the usable area of a lens bigger. And this is the only difference: the usable area of the lenses image plane. If you would attach a large format 50 mm to a DX body you would get the same image (FOV etc) as when you use the DX 50 mm. If you attach a 50 mm DX lens to a large format camera you would also get the same image as when using the large format 50 mm lens, only that stuff like vignetting, distortion etc would be well visible within the image...
Hope this helps
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