Why do we use focal length to discuss lenses?

[AutumnJazz]

Wouldn't it make much more sense if we simply used field of view? Focal length is so arbitrary...For example, you can't directly compare the focal lengthes of 135 and 4x5. What is normal on 135 (50mm) would be insanely wide for 4x5 (90mm is a normal lens, correct?).

Is it just a tradtion that has been rooted into photography in the beginning? One of those things that just stuck.

Jazz

 

[David A. Goldfarb]

As long as you know the focal length and the format, then you know basically what the field of view is. With large format equipment, we can change formats fairly easily by changing the camera back or by putting the same lenses on different cameras, as long as the lens covers the format. Also in large format we usually distinguish between the angle of view and the angle of coverage (and within that between the angle of good definition and the angle of illumination), so specifying the focal length and format gives some more information.

There's no reason also to say something like "I used a moderately wide lens for this image," when the precise focal length and format aren't relevant to your audience.

 

[Ian Grant]

As the same lens can often be used on a variety of formats it's focal length that's important.

Ian

 

[Ole]

As the same lens can often be used on a variety of formats it's focal length that's important. ...

Yes and no.

Stating which focal length was used does not help unless you also specify the recording format. I can use the same 210mm lens on any film size from 645 to 12x16", with widely different angle of view as a result. It's long on MF formats, longish on 4x5", normal on 5x7", wide on 8x10", and very wide on 12x16".

Maybe just specifying "long, longish, normalish, wideish, wide, very wide, or ridiculously wide" would be precise enough?

 

[cowanw]

I think 150mm is closer to normal for 4X5.
Regards
Bill

 

[keithwms]

Field of view matters when you compose; focal length matters more when you consider depth of field and when you meter. It'd be roundabout to discuss depth of field in terms of field of view; likewise, since the aperture f/# ratio depends very directly on the focal length.... it's much more helpful to know focal length than field of view. Everybody has a few focal lengths in their heads that they equate with ultrawide, normal, etc., and their opinion depends on their format.

Of course, there are big differences in how these issues are discussed in smaller versus larger formats. LFers are generally more interested in image circle, which is something that 35mm shooters seldom fret.

 

[Nick Zentena]

Most people aren't talking about multiple formats. If people are discussing 35mm cameras then the focal lengths provide all the info.

The only time it's a problem is when people assume formats.

 

[Lee L]

Focal length is a fixed property of the lens. Angle of view is arbitrary in the sense that it's a combination of focal length and film format. If I told you I had a lens with a 45 degree diagonal field of view you wouldn't have a clue what I was talking about unless I also told you the film format. If I tell you what the focal length of the lens is, you could figure out angle of view for your format. As you become used to a given format, it's easy to grasp a general angle of view given the focal length.

Lee

 

[Photo Engineer]

You have to remember that a lens also has to be designed to cover the image area at the focal plane without vignetting. If you use a 50mm Digital lens on a 35mm Analog camera, you may suffer severe vignetting, and the same is true of an APS lens on a 35mm camera. A 50mm lens for a 4x5 camera is wide angle, but it also has a uniform density projection area at the 4x5 focal plane.

This is something often forgotten and not mentioned above.

PE

 

[David A. Goldfarb]

Consider the alternative, with APS and all the smaller than full frame digital formats, where lenses are often described in terms of the "35mm equivalent." On the one hand, that helps ordinary consumers figure out about how wide or how long the lens is in terms familiar to them, but it doesn't really help them understand the broader principles of photography too well, and when they want to learn a bit more and sign up for photo.net or one of the other discussion boards--maybe even APUG--there are a lot of misconceptions about depth of field and its relation to format or sensor size or how it is measured, or what "telephoto" means in terms of lens design, because they don't know what the focal length and format of their camera really is or what it means.

 

[Photo Engineer]

Well, Lee has used the technically correct term, "diagonal". This is critical in lens work and lens design. Imagine a 50mm huge telephoto designed for 8mm work. The rear end of that lens would be tiny indeed and the diagonal for an 8mm frame would be tiny. Also consider that due to this concentration of image, the f stop for a 50mm lens for 8mm would be wider than a comparable 50mm lens for 35mm. This explains the maximum aperature of a 4x5 lens being what it is. The concentration of light is a key factor in the light gathering ability of a lens.

I think there is no single answer. We have to consider all of the details in choosing a lens or describing it.

PE

 

[DWThomas]

Just my background I suppose, but I like numbers! An eighty millimeter lens is always an 80 millimeter lens. Presumably I know, or quickly learn, my specific gear to take it from there.

DaveT