Rethinking focal length conventions

Started 8 months ago | Discussions thread
Phil A Martin
Phil A Martin Veteran Member • Posts: 6,699
Re: 6%

jrtrent wrote:

Phil A Martin wrote:

Pixel Pooper wrote:

Phil A Martin wrote:

Pixel Pooper wrote:

For natural perspective the relationship between viewing distance and image size should be the same as the relationship between focal length and sensor size. If the focal length is equal to the sensor diagonal, the viewing distance is equal to the image diagonal and if the focal length is half the sensor diagonal, the viewing distance is half the image diagonal. This, as you said earlier, is part of the ABC of photography and is explained here in The Manual of Photography:

Then what's the disagreement? A 43mm lens on full frame gives natural perspective at the viewing distance described. A 28mm or a 135mm will not give natural perspective at the same viewing distance.

My point is that a 43mm lens does not match the perspective of the human eye, it matches the perspective of an image viewed from a distance equal to the image diagonal. If you view your images from closer it might be 28mm that matches the perspective, and from a bit farther it might be 65mm. There is nothing special about 43mm except that it matches what was once considered "standard" viewing conditions.

So basically you are saying that the conventions of decades are wrong and that there is no such thing as a standard lens.

Andy Westlake, a former DPR employee, wrote, "At the simplest level, a standard lens is defined as one which produces images with a natural-looking perspective (the word 'normal' is often used synonymously in this context). This concept is perhaps best illustrated with regard to what it's not, that is to lenses which clearly don't meet this criterion. A telephoto lens, for example, renders distant objects larger in the frame, and has the effect of compressing the apparent spatial relationship between objects. A wideangle does precisely the opposite; in squeezing more content into the image, objects appear smaller and more distant. It's in the happy medium between these two extremes that the standard lens lies; the apparent sizes and spatial relationships between image elements appear natural, and much as they did in real life."

Tamron wrote, "Generally speaking, a focal length range that provides a similar perspective to the human eye is considered to be somewhere between 40-60mm."

Sigma, in justifying their 41mm equivalent lens on the DP2 as a standard lens, wrote, "A lens with a focal length of 40mm to 60mm on a 35mm film camera is known as a “standard lens” because it delivers natural perspective, close to what the human eye perceives."

I've had "standard" lenses of 41 (equiv.), 45, 50, 51 (equiv.), 52.5 (equiv.), 55, and 58mm. Possibly because of my eyes or viewing distance habits, the 50-58mm range has always appeared more natural to me than the 41 and 45mm lenses, which I find a touch too wide.

Well exactly, I agree and as I've tried to explain above but others are adamant that I and photographic convention, are wrong. But you and I aren't wrong, we are right.

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