hp5 135 and hp5 120 roll film - different base density?

[hellomikmik]

Hello,

beginner's question:
is it possible that hp5 135 film would have different base density* that hp5 120 roll?
or is it same film just cut in a different way in the factory?

Thanks for your help.

*I'm not sure about the expression but I mean the shade of developed unexposed frame (or shade of space between frames).

 

[Leigh Youdale]

It is possible - other manufacturers do it. The film base might be of different specification, especially thickness, and then it may well depend on the choice of antihalation layers etc.
Best source here would be Roger Hicks, but the Ilford site might offer the opportunity to ask the question directly.

 

[sevo]

120 has a different base material (except for the odd few cases of master rolls intended for other uses packed to 120 or 135).

135 usually is stronger so that the perforation does not tear and it has a dye in the base (needed to block out light pipe effects that can fog the film while loading). 120 must be thin, usually has a gelatin back coat for better flatness, and can be cast on a highly transparent base as none of the film edge or rear is ever exposed to light.

 

[ChrisN]

Very close, actually. I've just compared the two with a micrometer: 135 is 0.135mm in thickness; 120 is 0.130mm.

 

[sevo]

Very close, actually. I've just compared the two with a micrometer: 135 is 0.135mm in thickness; 120 is 0.130mm.

Such a 20% difference is quite noticeable. It is 0.125mm base for 135 vs 0.110mm for 120, if we believe Ilford (http://www.ilfordphoto.com/Webfiles/20106281054152313.pdf). If we assume the 0.01mm constant difference between your and Ilford's figures to be the actual coating and the backing to be in symmetry with the coating, the actual acetate base in 120 would be 0.100mm strong, by the way.

Sevo

 

[hellomikmik]

Thank you for the answers.