Normal lens for DX format cameras
What is a "Normal" lens? It is the lens which produces a view similar to an "eye view" of a given scene, ie "1:1"…
Not exactly. A normal lens is a lens that has the same angle of view at capture as the viewing angle of the resulting image at display. This results in the perspective of the photograph being "right" for the viewing distance.
Since the lens makers, and often the photographers, don't know at what viewing distance the final image will be viewed this is often ignored and only lenses that are "normal" at the "standard" viewing distance are called normal lens.
The so called 'normal' lens for ANY GIVEN FORMAT is roughly equiv to the film's (or sensor's) DIAGONAL. …
Exactly - if the viewing distance is the standard viewing distance, which normally is the diagonal of the photograph.
If the lens has a wider angle of view, then the viewer has to be closer to the photograph to get the perspective he would expect (this would be the typical cinema viewing situation).
The 50mm as holy grail standard got entrenched, IMHO, from the early days when not many choices of lenses were available, and most cams, including Cartier-Bresson's Leica, CAME with a 50mm lens. So...
It probably has more to do with the flange distance of most cameras and with the look of the images.
Lenses that have a longer focal length than the flange distance are easier to build - if the focal length doesn't get to long. Easier to build results in lower priced lenses.
A slight telephoto lens creates images that look more intimate - if they are viewed at a standard distance they look like the viewer is closer to the subject as it would normally be the case at that viewing distance.
… So while I've always thought the '35mm equiv' thing was more confusing than helpful, I can now, when there are so many different digital sensor sizes, a common-base reference and if called "Standard Equivalence" or the like, could be helpful.
Why not just call a normal lens a normal lens, a wide angle lens a wide angle lens and a telephoto lens ... There is no need to make any references to 35 mm film*. If someone wants to be more precise he could talk about the angle of view of the lens.
So now I have to take my 16mm-70mm lens on the DX 5100, set to 28mm and compare images to those on FX taken with an FX 50mm lens... Do you think they will be close to each other, lens quality aside?
No. The 28 mm on DX will have a different perspective or a different framing compared to the 50mm on FX. The 28 mm is very close to normal on DX and the 50 mm is slightly telephoto on FX.
*) I guess you know that 35 mm equivalent refers to the name of the film format and not the focal length of the normal lens on DX.