Focal length question

[benstewart]

I have posted here before requesting information on lenses, and now I hav nearly made my choice, except for ONE thing. Bearing in mind that I carry a Digital Rebel XT: Is it true that some manufacturers display the focal length of their lens WITH consideration for film/digital differences. In other words, will this: http://www.tamron.com/lenses/prod/2875mm.asp lens be a real 28-75 on my XT, or is it a 44(?) Same for this one: http://www.ritzcamera.com/product/2...digital-cameras;cislr-lens;cilenses-for-canon This is especially important because I want a wide angle lens and I want one that will work once I get a full-frame sensor - somewhere down the line.

 

[Don Simon]

A 28-75mm lens is a 28-75mm lens, whether it's on a compact camera, dSLR, 35mm, medium or large format. It will remain a 28-75mm, so lens manufacturers will always call it a 28-75mm, although they may also give the comparable focal length for 35mm. This 28-75mm can be considered "equivalent" to 44-120mm on the 35mm format. If you want something "equivalent" to 28-75mm on 35mm film, you will need something like an 18-50mm, like that Sigma you listed.

Similarly if you wanted something more comparable to 18-50mm on 35mm film, neither of those lenses are wide enough; you would probably need something like 12mm on the Rebel to compare to 18mm on the 35mm film format.

 

[fmw]

The focal length of a lens is the focal length regardless of the camera to which it is mounted. A 28mm lens will be a modest wide angle of view on a 35mm camera but will be a normal angle of view on a digital. It will be an extreme wide angle of view on a medium format camera. To get the same angle of view of a 28mm lens on a 35mm camera, you will need a 17 or 18mm focal length on your digital.

Some lenses are made with a smaller image circles specifically for digital sensors. At the wide angle end of things these lenses will normally have focal lengths like 10 or 12mm at the short end of a zoom. That would be a fairly extreme wide angle on a digital. It wouldn't work on a 35mm camera at all because it wouldn't cover the frame. It would vignette by projecting a little circular image on your sensor that is smaller than the sensor.

 

[Digital Matt]

although they may also give the comparable focal length for 35mm. This 28-75mm can be considered "equivalent" to 44-120mm on the 35mm format.

It's equivalent to a 440-120 when it's on an Rebel Xt, not 35mm.

The focal length of the lens is just the distance from the rear element to the film plane. It's always the same no matter what system you put it on, but the field of view that the camera is able to see changes. Since we base our understanding of a lens' field of view based on this abstract measurement, you'll need to know what field of view you are actually seeing in an equivalent manner. Since you don't think of a 50mm lens as a 1.9685 inch lens, it would be stupid to say that on a digital SLR, a 50mm lens gives the field of view of a 3.1496 inch lens. It's always a 50mm lens, but the smaller sensor in the digital rebel xt only sees a portion of the center, which gives it the field of view of an 80mm lens.

Lenses are always named, or rated by the distance from the element to the film plane, so therefore, a 50mm lens is always a 50mm lens.

 

[Don Simon]

It's equivalent to a 440-120 when it's on an Rebel Xt, not 35mm.

I didn't say "when it's on", I said "on". What I meant was that the 28-70mm, when used on the Rebel, is comparable to a 44-120 used on the 35mm format. Obviously I could have phrased it better though. Saying that it's equivalent to a 44-120 when on an Rebel Xt is surely just as confusing, unless you state that the comparison is with the 35mm format. I.e. neither of us really qualified the comparison.

In future maybe we should just post a chart to explain it

 

[Digital Matt]

This might help drive the point home.

http://www.millhouse.nl/digitalcropfactorframe.html

 

[fmw]

Lenses are always named, or rated by the distance from the element to the film plane, so therefore, a 50mm lens is always a 50mm lens.

OK, if we are going to get really picky, this is true for the back focal length of normal, telephoto and short focus wide angle lenses only. The problem is that, with SLR's, you can't use short focus wide angles because they would interfere with the mirror. WA's for SLR's are designed with an effective focal lenth that is quite a bit shorter than the distance from the rear element to the film or sensor plane. They are called retrofocus designs and work something like a projector lens does. The distance from the rear element to the sensor is sigificantly longer than the EFL of the lens. Sorry, I couldn't resist.

 

[Digital Matt]

Hehe, no problem Fred. This is a can of worms that to try and explain simply is really just impossible. I was trying to avoid getting into lens design, since for the most part, it only clouds the issue of focal length comparison between systems.

 

[fmw]

I was just practicing my sea lawyership.