Technically, you're correct. Once all DSLRs are "full frame" cameras, the discussion will be academic. I think most folks are aware that it's the difference between the benchmark 35mm frame size and digital sensor size that produces the 1.5X effect. On a practical level, though, once you scan the 35mm negative and upload the digital image into Photoshop, the only really apparent differences are in the aspect ration, file size and (arguably) sharpness of the digital image (assuming both are, say, TIFF files). So, while we may talk about the 1.5X increase in focal length, we handle it in the field just as we would with a 35mm camera: If the lens that's on is wrong for the scene, we change it.....

The focal length is a fix number given by the design of your lens (plus the distance from rear element to sensor IIRC). When you put a 50mm lens on a digital camera, you only crop out a part of your image due to a decreased angle of view. The maginification stays the same (as the focal length didn't change).

That focal length multiplier is marketing hogwash in my eyes.
Lenses should not be sold via their focal length, but via the field of view. That would make them easier to compare if you use several sensor/film sizes.

M arketing wants you to think that you are getting spmething for nothing. the lens still captures the same area regardless of the viewfinder or sensor/film plane. The viefinder is merely masked off.
Even in a full frame camera, the area is masked off as well; lenses are ROUND, apertures are round, but pictures are SQUARE or RECTANGULAR. The same principal applies.

If you make a 4 x 6 print from the full frame of a 35mm film and a full frame 4 x 6 print from a D70 or similar with a "focal length factor" around 1.5x, the image on the digital print will be larger without question. By the same reasoning, a six inch lens on a 4 x 5 functions as a "normal" focal length lens but put it on a 35mm and it is a 3x telephoto.

You are right that the focal length does not change (and therefore neither does the effective f stop), but with equal enlargement to the same final size, the lens on the smaller digital sensor will have an effective longer focal length and larger image.

Looking at this another way: If you make "contact prints" of a 35mm image and of a digital image (same subject, lens and setup), it's easy to see that the size of the subject in both prints is identical. The digital print will appear to be the 35mm image, cropped somewhat. If you enlarge both images to 4x6, yes, the digital image will be larger, but the 1.5X "telephoto" effect is really a 1.5x magnification effect in Photoshop. It's not occurring in the camera at all. As someone else here pointed out, the so-called telephoto effect is mostly marketing hype invented to add a little "color saturation" to the advantages of interchangeable lenses. Kind of like a car dealer saying you'll get better mileage if you buy the 17-inch wheel option for your new 4WD pickup. Yeah, right....

>>If you enlarge both images to 4x6, yes, the digital image will be larger, but the 1.5X "telephoto" effect is really a 1.5x magnification effect in Photoshop.<<

Bill,

This is a misperception of the reality that is involved. Using an APS digital SLR is the same as using a frame in a film 35mm SLR that reduces the imaging area by 1/3.

As I have said before the parallel to using an APS digital SLR as far as the relationship between lens focal length and image area size is replicated by using a 35mm film back on a Hasselblad. The normal 80mm Hasselblad lens then functions identically to using an 80mm lens on a 35mm SLR.

It IS NOT something Photoshop is doing that is involved in the 1.5x lens focal length factor that is a factor using an APS digital SLR. The only thing different is the fact an APS sensor area is about 2/3rd the area size of a 24x36mm standard 35mm film frame.

What the manufacturers of APS digital SLR's are really telling you when they refer to a 1.5x lens factor is this: if you have both a 35mmm film Canon EOS camera and a Digital Rebel and just one lens, a 50-200 zoom, and you put that lens on the film camera and set the focal at 150mm; to get the same subject framing at the same distance from the subject with the Digital Rebel, you will need to set the lens at 100mm.

I don't think that supports your claim that it is marketing hype.

It should be quite obvious when you consider the new "digital" wide angle zooms offered by some of the camera manufacturers. On an APS digital SLR like the Canon Rebel, their new 10-22mm wide angle zoom is close to comparable in providing the same angle of view range as a 17-35mm zoom on a 35mm film SLR .

nice clarification David!