Some differences between 110-film and 120-film.

After being preoccupied with 135-film (24×36 mm), I wished to revisit other formats briefly. I took the below pictures with a Pentax Auto 110 for 110-film (13×17 mm) with an 18 mm lens and a Rolleiflex 2.8C for 120-film (60×60 mm) with an 80 mm lens. 

The Pentax picture, on the left, is taken using Orca 100 ASA, and the Rolleiflex picture, below, is taken using Fomapan 400. Both films are processed in Rodinal 1+25 for 5:30 minutes.

Considering the small frames of the 110-film, I am surprised by how good the Pentax photo is. I expected it to show much more grain and much less detail. 

But there's more to it than that. The impression of the two images is different, and compared to the Rolleiflex image, the Pentax image appears flatter, partly because the Pentax image object (the row of fishing huts) is more distant than the objects in the Rolleiflex image. 

However, the difference in focal lengths also have effect, but comparisons of focal length need to account for differences in the film format, i.e. the crop factor. 

The crop factor depends on the diagonal of the film format. The 135-film (36 mm × 24 mm) diagonal is about 43.3 mm. It can be calculated using the Pythagorean theorem as width2+height2

The diagonal of the 110-film is about 21.4 mm, which with reference to 135-film gives a crop factor of 43.3/21.4 ≈ 2.0
 
With a focal length of 18 mm and a crop factor of 2, the Pentax lens is equivalent to a 36 mm wide-angle lens for 135-film. 

The Rolleiflex diagonal is about 84.9 mm, which gives a crop factor of 43.3/84.9 ≈ 0.51, which means that the 80 mm lens corresponds to a slightly more normal (41 mm) lens for 135 film.  

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