50mm "Normal" Lens... Ok, but "Normal" on FF or APS-C Sensor?

Started Nov 10, 2012 | Discussions thread
jrtrent
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Re: 50mm "Normal" Lens... Ok, but "Normal" on FF or APS-C Sensor?
In reply to Detail Man, Nov 11, 2012

Detail Man wrote:

It has been my impression that the term "normal" is intended to relate to the ratio of focal length divided by diagonal dimension of film/sensor being close to unity value (1.0), and is otherwise not intimately related to particular characteristics of human visual, or techno-social, perception ...

Well, our impressions are usually based on what we have learned or experienced.  The books and articles I've read over the decades, and my limited personal experience with using normal, wideangle, and telephoto lenses, has me in agreement with Tamron's view that a normal lens renders pictures in a way similar to normal human vision.  After some brief checking, similar statements can be found at Olympus, Toyo, Canon, and Zeiss.  As noted previously, the actual focal length of a "normal" lens will change based on the format size in use, but the relationship of a normal lens to normal human vision remains:

Olympus - This 25mm lens has the perspective of a standard lens with an angle of view closest to the human visual field, so what you see with your eyes is what you see in the images you capture.

Toyo - Normal lenses - For a standard lens, a 150mm lens is the most versatile for 4x5 camera work. It gives a natural perspective, and is useful for any subject from landscape and portraiture to still life.

Canon - A so-called normal lens roughly approximates the perspective, though not the area of, a scene seen by one human eye. By convention a normal lens on a 35mm film camera (and thus a full-frame EOS digital SLR) has a focal length of 50mm or so. A lens with a focal length of 35mm is considered “normal” for a cropped-frame EF-S camera.

Zeiss - The focal length of the Planar T* 1,4/50 is equal to the perspective of the human eye.

Barrie and Lee make the point that what seems normal or correct in terms of perspective will also depend on the viewing distance relative to print size.  That topic also came up in a recent thread related to depth of field.  One article I found by Keith Cooper used the example of 20 X 30 inch display prints taken with 24 and 200mm lenses.  For the wideangle lens, a viewing distance of about 20 inches would be needed for correct perspective, while the picture taken with the 200mm lens would need a viewing distance of about 14 feet.

Of course, people don't normally view images this way; they tend to look at a picture from a distance they find comfortable based on the size of the picture, not the focal length it was taken with (if, that is, they had any way of knowing what lens was used, or how cropping may have affected the "correct" viewing distance).  If a picture is small, they step closer; if it's large, they step back.  Cooper's expectation is that most people will view an image at a distance of about 1 to 1.5 times the diagonal of the picture.

I haven't seen the full study, but there's an interesting abstract at Journal of Vision on depth compression and expansion in photographs.

http://www.journalofvision.org/content/11/11/65.abstract

"Photographs taken with long focal length lenses appear to be compressed in depth and those taken with short focal lengths appear expanded. A common rule in photography is to use a 50-mm lens to create a natural-looking image (i.e., neither compressed nor expanded in depth). We hypothesized that this rule is a byproduct of people's viewing habits and an inability to take viewing distance into account."

What they found is that not only do people have "an inability to take distance into account when interpreting 3D contents of the photograph," the preferred viewing distance is mostly determined by print size, thus they stand too far from pictures taken with wideangle lenses and too close to pictures taken with telephoto lenses.  The study found that images taken with lenses between 50 and 70mm were least susceptible to perceived compression/expansion effects.  While supporting what Barrie and Lee have said, it should also be no surprise that the various 50, 55, and 58 mm lenses marketed over the years as "normal," and even the 60mm equivalent standard lens on the Pentax K-01, do indeed render images in a way that people perceive as normal or natural.

Interestingly, if you multiply the lens focal length by the print magnification for 35mm, that 50-70mm focal length range works out to "correct" viewing distances of about 1.15 to 1.62 times the diagonal of the print, not far off from Cooper's expected viewing distances.

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