Good explanations above. I'll give my own spin on it:
Originally posted by Farmer_Terry If I now take a 50mm lens from my old 35mm camera and put it on my K-x it behaves like a 75mm lens would have done on my 35mm film camera.
A 50mm on 135/HF (half-frame) or APS-C does NOT behave just like a 75mm on 135/FF. The smaller frame crops the FOV to that extent, and DOF is different, and perspective will change because we'll shoot from a different distance to get the same framing of a subject. We can say that looking through the viewfinder, a 50/2 lens on APS-C would be
similar to a 75/2.8 lens on 135/FF.
Quote: First question – is this because the sensor on my K-x is smaller than 35mm film?
A smaller frame sees less of a projected image than does a larger frame. Try this: Cut a picture from a magazine. Draw a 60x45mm rectangle on it. Draw a 36x24mm rectangle inside that. Draw a 24x18mm rectangle inside that. Those are the frame sizes of MF, FF and HF cameras. The picture remains the same, but each frame crops a different amount. The amount of cropping, compared to 135/FF, is the crap.factor. (I prefer the German term
format-faktor.
Quote: Second question - this would suggest that a “normal” DA type lens for my K-x is still 50mm. Is this the case?
Final question – does this mean that 50mm is “normal” for all DSLRs (Pentax, Nikon, Canon etc.) as it was for all 35mm film cameras?
As mentioned, an optically 'normal' focal length is the diagonal of the frame, which for 135/FF is 43mm, which is shorter than the depth of the Pentax mirrorbox (45.46mm). Rangefinders often had lenses in the 35-40-45mm range because their rear elements could sit close to the film plane without resorting to exotic designs. SLR lenses had to accommodate the swinging mirror, so kit lenses were often 58mm, then 55mm, then 50mm, which were easily designed for speed. We see many more f/1.2s in those focal lengths than elsewhere.
We've discussed elsewhere the varied meanings of 'normal', derived from the Latin
norma or carpenter's square. Normal can be: at right angles; within so many standard deviations of a mean; the diagonal of a frame; what is acceptable; what is familiar. 50-55-58mm were 'normal' due to familiarity, not optics; 45mm is closest to the FOV of one human eye, about 50 degrees. (Our binocular vision is 160 degrees, about 4mm on 135/FF.) But with an Olympus XA, 35mm was 'normal' (familiar), and so was 58mm for a Zenit. That's a pretty wide range of normality!
Optical normality varies with frame size (film or digital). For nominal APS-C it's 30.1mm. For my K20D it's 28.1mm. For Canon APS-H sensors it's 34.4mm. I don't know the precise size of a Kr sensor; check your manual. You can figure that a lens in the 28-29-30-31mm range will probably be 'normal'. But if you want to replicate the look of the old 135/FF kit lenses, then 35-37mm are on target. My Mir-1 37/2.8 would be closely FOV+DOF-equivalent to 55/4.
So if you want undistorted 'normal' images from your Kr, use a 28mm lens (also called "short normal"). Or a 31Ltd if you can afford it! For a view similar to your old 50mm on 135/FF, use a 35mm lens (also called "long normal"). Just remember the DOF and perspective differences. Better yet, just learn what each lens does on a specific camera. When I was shooting 135/HF and 135/FF and 6x6/MF and 9x12/LF on the same day, I didn't think about crap.factors, just about which lenses did what I wanted with each format.
Hope this helps!