Rethinking focal length conventions

Started 7 months ago | Discussions thread
Phil A Martin
Phil A Martin Veteran Member • Posts: 6,341
Re: 6%

Gerry Winterbourne wrote:

Phil A Martin wrote:

Gerry Winterbourne wrote:

Phil A Martin wrote:

Gerry Winterbourne 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.

There is no single "perfect" focal length. Human eyes vary in size; if "perfection" means matching human vision then it varies depending on the individual. 43mm was picked out of the range of visually "normal" focal lengths because it corresponds to the diagonal of 135 format - an arbitrary choice that doesn't confer perfection.

It is the focal length of the "true" standard lens, where perspective is seen as the human eye sees it. This has been an acknowledged truth throughout the history of photography and is easily demonstrated in practice.

There is no such thing as "the" human eye.

There is, I can't have speak for you but I have two.

That is not "the" human eye; each is "a" human eye.

As I said (but you ignored) human eyes vary.

Which is irrelevant because I am clearly speaking about the average human eye.

It isn't irrelevant at all. You said it was perfect -

As explained elsewhere in this thread and something you should have realised had you followed it closely, I used the term "perfect" in response to the Ken Rockwell quote, where he claimed that the "40mm is the 'perfect normal focal length for full-frame" and that is all. It was not intended to insinuate that the 43mm focal length of full frame is the perfect lens for all occasions and circumstances.

but it can only be perfect for a individual whose eyes are exactly the average (in which case 43mm wouldn't be perfect - 43.27mm would.

The history of photography started 100 years before a format with diagonal of 43mm came along:

When it is clear I'm talking about 35mm format/full frame but I'm sure you realised that.

Why would it be clear from the term "throughout the history of photography" that you really meant only the second half of that history? Why don't you say what you mean instead of setting guessing games?

Obviously other formats will have standard lenses of different focal length. APS-C for example, the standard lens will be around 28mm.

show me a convincing demonstration that it is true for every human eye.

As explained above, this comment is irrelevant.

I assume that you would agree that wide angle lenses exaggerate perspective?

No, I won't. I'll agree that they exaggerate the perception of perspective.

Again pedantry. You know exactly what I meant yet choose disagreement for the sake of it.

(It is part of the basic ABC of photography after all) and as the focal length shortens, the angle of view increases and with it the exaggeration of perspective. Likewise with telephoto lenses, increasing the focal length narrows the angle of view and increasingly foreshortens perspective.

All of this depends on how the picture is viewed. If you can view the picture so that the angle it subtends at the eye is the same as the angle subtended by the lens on the sensor there is no appearance of distortion.

If you understand and agree with this basic photographic theory and there's no reason you shouldn't.

Well, I understand and agree with the basic theory; what I don't agree with is how you explain it.

Then you should agree that if we go through all the potential focal lengths for a given format, we will reach a point of cross over where the focal length neither exaggerates nor compresses the apparent perspective and that is what we call the standard lens. Which for 35mm/FF, will be in the 40-50mm range.

Well, other people extend that range but, yes, I'm happy with that. But that isn't demonstrating that 43mm specifically is the "perfect" focal length - which is what you started by claiming.

Because in the continuum of focal lengths, the focal length that marks the transition from wide to telephoto, corresponds the the length of the diagonal of the sensor. In this instance 43mm or thereabouts. Obviously in custom and practice, the 50mm is usually cited as the standard lens but many fixed lens 35mm cameras of the 1950s to the 1980s, often had a lens in the 40-45mm range.

The Pentax 43mm f/1.9 ltd being a good example of such a lens, because the focal length is almost identical to the negative/sensor diagonal of 43.27mm.

Why does matching the diagonal matter? What is special about it?

See above.

Assuming the sensor to be 24x36mm, as they are claimed to be.

Actually, they "claim" 35.9 x 24.0mm.

And this sums you up perfectly, pointless pedantry in the extreme.

That's it, I've had enough of your pedantic point scoring. Good night.

-- hide signature --

Gerry
________________________________________________________________________
I'm happy for anyone to edit any of my photos and display the results
_________________________________________________________________________
First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
gerry.winterbourne@ntlworld.com

Post (hide subjects) Posted by
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
tko
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow