Rethinking focal length conventions

Started 7 months ago | Discussions thread
Phil A Martin
Phil A Martin Veteran Member • Posts: 6,307
Re: 6%

FrancoD wrote:

This is your problem :

" 43mm is actually the perfect focal length for full frame. It corresponds to the sensor diagonal of 43.27mm."

just because someone came up with that idea it does not mean that it is correct.

It probably is for some but not for others.

What I do mean by that is that human field of view is not a fix number.

Find 10 charts showing what our field of view is and you will get close enough to 10 different set of numbers.

Here is one

just for fun this is another one

there is some agreement that the main field of vision is around 60 degree but your pet lens , the 43mm Pentax has a field of view of 53 degrees , so yes somewhat around the middle of the various charts but again it isn't an exact number and that is why other manufacturers have not bothered making one.

If you'd actually followed the thread of the conversation, you'd have realised that my comment about perfection was in response to the Norman Rockwell quote:-

"40mm is the "Perfect Normal" focal length for full-frame. Unlike 50mm lenses which often are too long or 35mm lenses which are often too wide, 40mm is always just right."

Because clearly no lens is perfect for all occasions, just that a 43mm lens would be the "perfect" focal length for a standard lens on 35mm/FF and not 40mm as Rockwell claims.

As for your diagrams, yes very interesting but it has little to do with standard lenses, which are lenses that render perspective more closely to the experience of the average human eye and not the angle of view seen by them.

Incidentally, that's why with 35mm/FF, many people favour lenses in the 35mm range, where the angle of view is closer to the human eye and whilst the perspective shift is there, it isn't as exaggerated and as visible as it is with lenses of 28mm and wider.

As for other manufacturers, Fujifilm make a 27mm lens which approximates to a 40.5 mm lens in FF. Both Sigma and Sony make a 30mm lenses for Sony e mount which is the equivalent of a 45mm on FF.

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