Michael Leuty wrote:
> On 23/04/07, Dr Adrian Midgley (In the office)
> <[log in to unmask]> wrote:
>> being equivalent to an 80mm lens on a full frame 35 mm (film) camera
> 
> I am loathe to dispute any statement by Our Ade, but I am
> told that this is not really the case. The position of the
> sensor means that your field of view is more restricted than
> it would be with a 35mm film camera *but* the optics of the
> lens remain the same, so that you will get the same (slight)
> amount of foreshortening with a 50mm lens on a digital camera
> as with a film camera.

We're talking here about "Equivalent Focal Length". Because 35mm SLR users
are very familiar with the way lenses of a given focal length behave,
Digital SLR users often use this simplification. 

It comes down to the fact that for "normal" pictures, a focal length that
approximates to the length of the diagonal of the film is appropriate. So
most 35mm SLRs have a focal length of 45 or 50 mm.

Digital cameras in general have smaller sensors than 35mm film, so the focal
lengths of the standard lenses are correspondingly shorter. A 300 mm
telephoto lens on my Nikon D100 behaves like a 400-450 mm lens on a 35mm
camera, because the image being collected by the sensor is smaller - so you
don't have to be so close to an object to fill the frame with it.

The big-nose effect you get when getting a full-frame of someone's face
using a 50mm lens on a 35mm SLR arises because the nose is significantly
closer. You'll get the same distortion with a 30mm lens on a D100. The
solution is to stand further back, so that the nose is not much closer than
the rest of the face. If you want to fill the frame, you'd use a "portrait"
lens on a 35mm camera, with a focal length of around 100mm. The D100 will do
this with a 70mm lens.

Foreshortening effects aren't to do with the optics of the lenses, but to do
with your angle of view. A 50mm lens on a 35mm SLR takes a view which
approximates to the way you see it with the naked eye. This makes the
resulting picture look natural. If you take a very small part of this
picture and enlarge it, you get the same effect as you would using a
telephoto lens, and because you perceive the resulting image as being of an
object closer than it really was, you misjudge the distances of the objects
beyond it.

I've been very pleased with my D100, having been familiar with Nikon film
cameras over the last 30 years. As with all hi-tech equipment, you can get
something much better for much less if you wait, but I hope mine's going to
serve me well for another 5-10 years.

Michael