[Sidefx-houdini-list] aperture focal length relations

Gene Dreitser keyframe at rogers.com
Thu Jun 21 14:41:57 EDT 2007

Heya Mario,

As far as I understand it, 6x6,  6x7, and 4x5 are ratios that represent exposed film area (long before pixel ever come into play)... which means that they should be expressed at 6:6 (a.k.a. 1:1), 6:7, and 4:5 respectively (not sure how the 'x' came into the equation, or when, but it's a common convention in the photography world).

As the site suggests, all those formats use the same type of film stock (in this case, 120), but expose different size negatives onto the same strip of film.

from the site:

6x6 (56x56 mm)
 4.5x6 (41.5x56 mm)
6x7 (55x70 mm)

As to width:height or height:width -- well... ever seen a photographer crank his camera sideways? :)  That tends to be a creative decision in the present context.

With motion picture cameras, as I understand it, this subject becomes even more confusing, because of custom film gates (Uli Gate anyone), anamorphic lenses, scanner offsets, etc.

----- Original Message ----
From: Mario Marengo <mario at axyzfx.com>

Hey man. Thanks!
So the only unclear thing there (to me) is what exactly do you mean by '6x6' 
and '6x7' in the 120 case. You mention ratio, so do you mean 6:6 and 6:7? And 
is that width:height or height:width? And are we talking actual distortion or 
mere 'pixel aspect' here?

@Adrian: Jim found a similar calculator in the Canon site here:
where they make a distinction between '35mm Still' and '35mm Movie' (with 
significantly different width/length values for each!)... just FYI, because 
the site you pointed to uses only the values for '35mm Still', so watch out.

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