My First DSLR - "Is 'Dunning-Kruger Effect' a Photoshop plugin?"

The Something Awful Forums > The Finer Arts > Creative Convention > The Dorkroom > My First DSLR - "Is 'Dunning-Kruger Effect' a Photoshop plugin?"

pseudorandom: Jun 16, 2010; Yam Slacker

ExecuDork posted:

I don't know but I suspect that way back in the early days of 35mm film, the 1950's and 1960's as many professionals switched from large- and medium-format cameras to 35mm SLRs, lens designers made decisions that resulted in a bag of primes consisting of 28/35/50/85/105mm (with some tweaks, like 55mm). Those decisions might have been based on ease of setting up a production line at a factory for creating and assembling parts that go into making a 28/2.8 or a 50/1.7. Factors that would have been important to those decisions, like factory down-time while re-tooling, are completely irrelevant today but nobody has broken away from old habits - why 28/2.8 and not 25/2.5? Or 75/1.7 instead of 85/1.7? The one exception I know of is the move from 105mm to 100mm for long macro lenses, showing there's not some major optical or mechanical barrier to that kind of change.
The field of view at 28mm on a 35mm SLR is "wide"; smaller focal lengths are "very wide" or "ultra wide", but how different is 25mm, really? It's noticeably wider, yes. So why did the old lens designers not produce lenses in 25/35/45/55mm ? Why 70-210 (or 70-200, or 80-200) for everybody's favourite tele-zoom, and not, say, 100-250 from the beginning?

I am far from an expert, and am still trying to fully grasp the significance of the actual focal length measurement, but this made me curious because I assume there is some actual thought behind this stuff.

After digging through wikipedia, it appears it is based on math.

https://en.wikipedia.org/wiki/Focal_length#In_photography posted:

Camera lens focal lengths are usually specified in millimetres (mm), but some older lenses are marked in centimetres (cm) or inches.

Focal length (f) and field of view (FOV) of a lens are inversely proportional. For a standard rectilinear lens, FOV = 2 arctan x/2f, where x is the diagonal of the film.

...

The focal length of a lens determines the magnification at which it images distant objects. It is equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question. For rectilinear lenses (that is, with no image distortion), the imaging of distant objects is well modelled as a pinhole camera model.[4] This model leads to the simple geometric model that photographers use for computing the angle of view of a camera; in this case, the angle of view depends only on the ratio of focal length to film size. In general, the angle of view depends also on the distortion.[5]

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;[6] this angle of view is about 53 degrees diagonally. For full-frame 35 mm-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 35 mm-format cameras), while a lens significantly longer than normal may be referred to as a telephoto lens (typically 85 mm and more, for 35 mm-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.

Due to the popularity of the 35 mm standard, camera�lens combinations are often described in terms of their 35 mm-equivalent focal length, that is, the focal length of a lens that would have the same angle of view, or field of view, if used on a full-frame 35 mm camera. Use of a 35 mm-equivalent focal length is particularly common with digital cameras, which often use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to achieve a given angle of view, by a factor known as the crop factor.

https://en.wikipedia.org/wiki/Normal_lens#The_image_circle posted:

A normal lens typically has an angle of view that is close to one radian (~57.296˚) of the optical system's image circle.[citation needed] For 135 format (24 x 36 mm), with an escribed image circle diameter equal to the diagonal of the frame (43.266 mm), the focal length that has an angle of one radian of the escribed circle is 39.6 mm; the focal length that has an angle of one radian of the horizontally-bound inscribed image circle, is 33 mm; the focal length that has an angle of one radian of the vertically-bound inscribed circle, is 22 mm. This correlates with the popularity of 35 and 24 mm lenses, and the existence of 40 mm lenses, albeit the latter in a more restrained offer. A 50 mm lens has a vertical-bound inscribed circle angle of view of ~0.5 radians. A 70 mm focal length (typically only available in zoom lenses) has a horizontally-bound inscribed circle angle of view of ~0.5 radians. An 85 mm lens has an escribed (frame diagonal) circle angle of view of ~0.5 radians. Effectively, the 24, 35 and 40 mm trio have a 1:2 relation to the 50, 70 and 85 trio of focal lengths. "Normal" lenses, those that cover one radian in at least one of their inscribed or escribed image circles, belong to the first group, with 35 and 40 mm lenses closer to one radian than 50mm lenses.

These Wikipedia articles seem a bit like academese, at least to me right now, so I think I sort of get what they're saying, but I don't really feel like taking the time right now to really parse the whole thing.

Either way, it's kind of cool. I guess the TL;DR is the common lens focal lengths result in a field of view very close to 0.5 radians or 1 radian (across the vertical, horizontal, or diagonal axis depending on the focal length) on 35mm film/sensors.

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The Something Awful Forums > The Finer Arts > Creative Convention > The Dorkroom > My First DSLR - "Is 'Dunning-Kruger Effect' a Photoshop plugin?"