Rethinking focal length conventions

Started 7 months ago | Discussions thread
J A C S
J A C S Forum Pro • Posts: 19,312
Re: 6%
1

Phil A Martin wrote:

J A C S wrote:

Phil A Martin wrote:

J A C S wrote:

Phil A Martin wrote:

jrtrent wrote:

tko wrote:

You're thinking you need a 40 mm instead of 35 mm? Of what importance is this? How did you get this conclusion? How would a 85/80 = 6% difference in FL rock your world? I don't get this at all. It would seem like you think your numbers are better since they are "more rounded off," and you want lens manufacturers to come up with new products where the focal lengths are "prettier."

You may be right about the difference between 80 and 85 mm, but apparently there's a noticeable difference around 40mm:

"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."

And that's from Ken Rockwell himself!

He's wrong, it's 43mm is actually the perfect focal length for full frame. It corresponds to the sensor diagonal of 43.27mm.

You are both wrong. There is no such thing as a perfect FL. It is a superstition.

If you want to play word games, then fine. Enjoy yourself.

I only used the phrase "perfect" because Rockwell used it and I was replying to his quoted comment and not that I thought the 43mm focal length to be perfect focal length under all situations and circumstances.

Referring to KR is not a particularly strong argument...

I was not referring to KR, I was responding to his quoted comment.

Maybe I should have been clearer and said that 43mm is the ideal focal length for a standard lens that renders natural perspective on a 35mm or Full Frame camera?

What is a natural perspective

Natural perspective is when near and far objects bare the same relationship in the image, as they do with the naked eye.

This is happening with every lens, every FL, as long as you are shooting from the same position.

and why is the diagonal magical? As weak as the KR argument is, this one is weaker.

It's not a question of magic, it's just that if you take the continuum of potential focal lengths, for a given sensor the focal length that matches the diagonal, is the median focal length and the transition point between wide angle and telephoto for that sensor.

Now you have to define wide angle and telephoto in mm. Next, you have to explain why the "median" happens to be the diagonal of the sensor.

For a full frame sensor, this will be approximately 43mm and on Pentax/Nikon/Fuji APSc sensor approximately 28mm.

Because...?

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