Beginner What's the difference between ISO 1600 and ISO 400 pushed to 1600???

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Tom
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Just getting my head around the world of B&W film and developing at home etc. I know that pushing film makes it grainier and contrastier but ISO 1600 will be grainier than ISO 400 anyway. So would it just be a bit contrastier?

I'm trying out pushing HP5 to 1600 (it's really bright and sunny at the moment so I could have chosen a better time to experiment!).
 
There are two aspects here.

1) the exposure. You are underexposing by two stops. If you used a film with 1600 ISO sensitivity you would not be underexposing. This affects the relationship between the tones. By underexposing by two stops, you are putting your shadows on the heel of the log intensity/image density curve causing tonal compression in the shadows. The highlights should still be fine.

2) development. If you extend the development time you increase contrast. You do not actually need to extend development time so long as you are happy to print from a thin negative.

So, the end difference will be blocked shadows and high contrast with the pushed film compared to the more sensitive film.
 
Thanks John,

Also the explanation of why you get more contrasty images is interesting...
 
Few years ago I preferred Ektachrome 200 pushed rather than natively higher iso rating.
 
Just coming back to this as I’ve just pushed some kentmere 400 and also shot a roll of Kentmere 100 at box speed.

So pushing affects the contrast - does it make for more grain too? Is that just a product of the higher contrast? I.e. you’re bringing our the grain by increasing contrast? Or does it actually create more grain? (Don’t really know what grain is so an explanation of that would be a bonus!)

Cheers,
Tom
 
Film is made up of silver halide crystals suspended in gelatine. Exposure converts a molecule or two (or more) into metallic silver, which shows up as black. Development enhances the effect and converts even more, working out from the original silver atom. The end result is a series of silver grains in the emulsion, and what we see and call grain is the effect of these clumping. All this being an approximation to the truth...

Grain size (for black and white films) is increased by extra exposure and development. Some developers can reduce grain at the expense of film speed, others the reverse.

You can still see grain in a low contrast subject, like a clear sky, so the contrast per se isn't a factor.
 
If you take a look at the data sheets produced for films, you should see graphically the effect of extra development. At least, Ilford provide it.

At the risk of invoking what many seem to consider higher maths :p, film speed is proportional to the grain volume, but as seen on a negative what you're aware of is the cross sectional area. As area depends on the square of the radius, and volume the cube, a 400 ISO film will have double the grain size of a 50 ISO one.
 
Some developers can reduce grain at the expense of film speed, others the reverse.
Hi Stephen,

Thanks for your reply. Can you explain your statement above please? If a developer reduces grain what’s the relationship between film speed? And what’s a good developer for reducing grain?? I hear Rodinal is quite a grainy one...
 
OK, in simple terms: grain reducing developers usually work by a solvent action - they dissolve away some of the silver, which by definition reduces the grain size. But by the same token, the net effect is that the effect of the original exposure is reduced, which means a loss of film speed.

From memory, if you go to most of the usual suspects to buy developers, they divide them according to their characteristics, so you can see which developers do what.

Given that I'm a large format photographer (so grain doesn't matter) photographing mainly stationary subjects (so film speed doesn't matter) I don't worry overmuch about these things for my own work.

If you want to go further into developer characteristics, read "The Film Developing Cookbook" by Anchel and Troop (check name spelling); and if you have access to a decent library, you might be able to find "Modern Photographic Processing" by Grant Haist where vol 1 has some very informative photomicrographs of developed film.

I use Rodinal which is an acutance developer (think sharpening in Photoshop/scanning terms) which increases grain (think adds artefacts in Photoshop/scanning); I find it fine on roll film and large format, but wouldn't like to try it on 35mm.
 
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a 400 ISO film will have double the grain size of a 50 ISO one.

I should have added "all other things being equal". Tabular grain films change the rules slightly.
 
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