In the good old days,there were only a few sizes of film, and most
people had 35mm. This made comparing lenses easy. 35mm meant wide
angle, and 135mm was a tele lens. Today, where image sensors vary
in size with almost every manufacturer, 35mm doesn't mean wide
angle any more. We have to “multiply” to find a 35mm “equivalent”,
or we use the ridiculous “3 times zoom” or “10 time zoom” terms,
which only mean something if you know what the starting focal
length is, and can work out if it's wide angel, or super wide
angle. So the confusion goes on.
Thirty years ago, you had to make do with double-digit numbers of film formats (off the top of my head: 8x10", 4x5", 20x30 cm, 9x12 cm, 69, 68, 67, 6-square, 645, 135, half-135, 110, cine 16 mm, cine 8 mm), and nobody thought about making "focal length equivalents" of any sort.
So, lets all get into the 21st century and start quoting focal
lengths relative to image sensor size, so we all know what everyone
else is talking about.
Uh-huh. And it's a perfect cover-up for manufacturers to obscure the true size of the sensor even further.
Here is the science bit. A “normal lens” is one that makes the
picture look “normal” to us humans in terms of perspective. This
is approximately equivalent to the diagonal measurement of the
focal plane, i.e., the image sensor or film. 35mm negatives have a
diagonal of 42 mm, which means the normal lens for a 35mm camera is
a 42mm lens. “Ah” I here you say, “but its 50mm!”. Actually, I
say, it's not. A 50mm lens might be standard, but it's not normal.
50mm lenses are used on 35mm cameras because that is what the first
Leica camera used, and the idea stuck.
Actually, you know, it's almost correct, except that it's not. With focusing plane to flange distances between 42 and 44 mm, it's extremely difficult to make non-retrofocus lenses shorter than 50 mm. That's why all 45 or 43 mm designs were pancake lenses, based on the Zeiss Tessar, which makes for a wonderfully sharp optical formulation, but also makes it woefully difficult to make fast lenses. Double inverted Gauss makes fast lenses very efficient and simple to make (hence, f/1.7 and f/1.8 lenses can cost below $100).
So, for an image sensor on the Nikon Coolpix P5000 measuring 7.18
mm x 5.32 mm, the diagonal is just under 9mm. So, the normal lens
would be a 9mm lens. Anything smaller, is wide angle, anything
larger is telephoto. The equivalent to a 84mm lens in 35mm
photography, for a Nikon Coolpix P5000 would be an 18mm lens (or
zoom to 18mm), just right for portraits. Inother words, both the
84mm and the latter 18mm lens can be classed as a D2 lens, meaning,
they are both 2 times the diagonal of the image sensor or film.
Similarly, a wide angle 28mm lens in 35mm cameras would be the same
as a 10mm lens in a Nikon Coolpix P5000, and both could be called a
D 0.56 lens, meaning the focal length of both lenses is 0.56 times
the focal length, and will produce pictures of the same
magnification, and therefore the same perspective.
Uh-huh. So an 11 mm lens (on 2/3") and a 93 mm lens (on 68 medium format) would both be D1.
If you agree, talk about this all over the world, and get everyone
talking sense on this issue. If you don't, please come up with
another mathematically correct and easy to administer method.
I disagree. For one thing, it'd make it easier for manufacturers to further obscure sensor size differences.
Your method is NOT easy. How do I calculate aperture in your system? If I get no absolute number, just two relative ones (that's as ridiculous as it can get), I'd have to know the sensor diagonal to know what DOF to expect. I can't imagine doing the maths in the field.
You want a mathematically correct way? One that's sound? We're already using it. You've got a quoted absolute focal length, plus relative aperture.
