What is a Normal lens?

It just happens to be a focal length that represents the world with a comfortable amount of perspective, similar to what you'd be normally processing with your eye + brain.

Sure, you can look deep into the distance with your naked eye and see a telephoto perspective on something, or pay attention to your peripheral vision and notice a wide angle effect on your world, but most of the time you're probably scanning and attending to the world from a roughly normal perspective.

Just happens that a film diagonal focal length roughly matches that, I guess.

BTW reading over the first bit of the thread I guess my first response was a bit repetitive. Sorry for that.

cotdt said:

and what makes the format diagonal so special, so normal?

most 50mm SLR lenses are double gauss designs which are retrofocus.

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But but... your human eyes are ultra wide angle lenses, much wider than 50mm for 35mm.

cotdt said:

But but... your human eyes are ultra wide angle lenses, much wider than 50mm for 35mm.

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Yes, but your attention is focused on a much narrower field of view. Your peripheral vision exists only to attract the attention of your forward looking perception. We have the eyes and perception set up of a carnivorous predator, not a herbivore.

cotdt said:

the retrofocus 35mm Distagon actually outperforms the 35mm Biogon, it's just such an amazing lens. maybe retofocus is not so bad after all.

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Totally possible. Again, the essential difference between retrofocus and non-retrofocus design is how far from the film can you put a lens. If someone manages to make a better lens one way or another, I'm not arguing. However, I've seen a lot of rave reviews about the Biogon, but not much about the Distagon, so I don't know more than that.

The original Barnack design camera used a 50mm lens designed for 18X24 & he had a lens made in the same fl with enough coverage for what we now know as "full frame"
The fl just became a convention & isn't worth worrying about.