Length instead of FOV?

rbiemer · 06-12-2008

Memphis started a thread ( http://www.rangefinderforum.com/foru...ad.php?t=60093 ) that got me wondering, again, why lenses are referred to by length rather than by field of view?
I know I can't change the current way of describing lenses but it seems to me that the FOV is a better way to differentiate between lenses.
I pick different lenses not because one is a 50mm lens or another is a 21mm lens but because one lens covers 45° or another one covers 86°.
If I post a photo and say "I took this with a 50mm" but don't specify the format, the pic could be wide angle or "normal" or telephoto. But If I post a photo and say I took this with a 45° lens, then it is clear (I think ) that it was a "normal" lens regardless of film size.
Any thoughts folks?
Rob

laptoprob · 06-12-2008

True, very true. Guess the 24x36mm format is almost standardized as a reference. And then remember Barnack was the one who started using 35mm for still photography by building the small screwmounts. Is that vision?

Rob-F · 06-15-2008

Yes, it would have made sense to identify lenses that way. It would tell you more about what you really need to know when selecting a lens. Thanks for this original thought!

In fact, why not specify lenses by radian measure? One radian equals about 57.5 degrees. That's about the horizontal angle of a 35mm lens on 135 format. Many consider the 35mm to be the "normal" lens. By that standard, the normal lens is a one-radian lens. A 25mm is a 1.4 radian lens; 90mm equates to .39r, and so on. It could be another way of relating to the field of view of the lens.

David William White · 06-15-2008

Some sympathy here, because that usually is what we want to know, but the focal length is the design parameter of the lens and can be stamped on the barrel. The field of view is limited by the size of the film behind the lens, and is therefore dependent upon what camera you've mounted it on. For instance, I've a lens that can cover an 8x10 (and would be wider-than-normal) but I use it in a 4x5 and it obviously then has a narrower field of view and becomes a normal lens. So lens focal length AND film size are required to calculate the field of view.

D.

oftheherd · 06-15-2008

It might indeed have made more sense, but we have become used to it. And for the most part, at least at whatever point in time it becomes importanto to know, we know what is normal for a given negative size. We also know longer and shorter and what they cover.

Imagine if we all knew and identified our lenses by angle of view, and some one in a forum put forth the revolutionary idea of identifying them by focal length ...

Rob-F · 06-20-2008

Quote:

Originally Posted by David William White

The field of view is limited by the size of the film behind the lens, and is therefore dependent upon what camera you've mounted it on. For instance, I've a lens that can cover an 8x10 (and would be wider-than-normal) but I use it in a 4x5 and it obviously then has a narrower field of view and becomes a normal lens. So lens focal length AND film size are required to calculate the field of view.

D.

But I don't think you'll be fitting that lens to a 35mm or MF camera! The proposal continues to make sense for lenses meant for a specific format, such as Leica or Nikon (or Canon, etc.) lenses for 35mm cameras; Zeiss lenses for Hasselblad or Rollei (etc.).

rbiemer · 06-22-2008

So, as I was looking out my window at the dim rainy morning I realized that FOV instead of length wasn't specific enough--need the length to figure f/stops.
Between that and knowing that lenses once were made independent of film size, I can see why the length got to be the major reference point.
Rob

oscroft · 06-22-2008

Quote:

So, as I was looking out my window at the dim rainy morning I realized that FOV instead of length wasn't specific enough--need the length to figure f/stops

Yes indeed. In addition to the FOV being dependent on the frame size as well as the focal length, if you think about how f/stops work (that any aperture that is defined as the focal length divided by a fixed number will transmit the same light regardless of the focal length), I think it becomes clear that focal length is the only practical way to defines lenses.

rxmd · 06-22-2008

The focal length is a parameter of the lens, the FOV is a parameter of the lens and the arbitrary choice of medium. Since we want to describe the lens itself, the focal length is really the only option.

rbiemer · 06-22-2008

Well stated, concise rebuttal to my half formed idea!
Rob

Quote:

Originally Posted by rxmd

The focal length is a parameter of the lens, the FOV is a parameter of the lens and the arbitrary choice of medium. Since we want to describe the lens itself, the focal length is really the only option.

And thanks to the rest of you gentlemen for your contributions as well!
R

payasam · 06-22-2008

Goes to show that what might today seem an arbitrary decision would, when it was taken, have been based on sound reasoning.

In practical terms, it's no problem: a photographer with a bit of experience reaches for the correct focal length depending on the field of view wanted.