[Leica] Crop factor ramblings

[ken at iisaka.com (Ken Iisaka)]

With Mark's remarks about crop factors and "full-frame" sensors, I was
inspired to ponder the true meaning of the "crop factor."

One of the things that is often not discussed is that it is dependent on
the aspect ratio. Different formats have different aspect ratios, and it
has a meaningful effect on the "crop factor."

If you are comparing sensors of the same aspect ratio, it's simple: just
compare the diagonal measurements, or one of the edges.

When you are comparing two different aspect ratios, and possibly a third
aspect ratio for the final print.

A 35mm "full-frame" is nominally 24x36mm, though it is not uncommon for the
outermost 0.5mm or so to be masked, leaving us with a 23x35mm frame.

However, if I want to make a 8x10 print, I would only be using 23x28.75mm
of the frame.

Thus, if I am using a FourThirds or 13x17.3mm, the valid comparison would
be between the short edge of the frame, i.e., 23mm vs 13mm, or 1.77.

So, my Panasonic 20mm lens really behaves like a 35.4mm lens on a "full
frame" 35mm film.

If I want to produce 5x7 prints, however, the factor is different again.

>From the 35mm film, I'd be using 23x32.2mm, while I'd be using
17.3mmx12.38mm of the four-thirds sensor. Here, the "crop factor" is 1.86,
closer to the "official" crop-factor of 2.0.

Anyway, I have always felt that the Panny 20mm is too wide for my taste,
and I expect to be happier with the new Olympus 25mm lens, which is really
equivalent to a 43mm lens on a 35mm film.

-- 
Ken Iisaka
first name at last name dot org or com