Rethinking focal length conventions

Started 7 months ago | Discussions thread
Phil A Martin
Phil A Martin Veteran Member • Posts: 6,429
Re: 6%

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.

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

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

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.

Obviously other formats will have standard lenses of different focal length. APSc 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 exagerate perspective? (It is part of the basic ABC of photography afterall) 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.

If you understand and agree with this basic photographic theory and there's no reason you shouldn't. 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. 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. Assuming the sensor to be 24x36mm, as they are claimed to be.

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

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