Re: Step wedges

Peter Marshall (petermarshall@cix.compulink.co.uk)
Sun, 24 May 1998 09:33 +0000

In-Reply-To: <f44b1300.35671cb0@aol.com>
> In a message dated 98-05-22 14:31:24 EDT,
> petermarshall@cix.compulink.co.uk
> writes:
>
> << I've done this before and it works more or less for a few steps.
> However the real problem if you haven't go a densitometer is in
> deciding what is density 0.15.
>
> Actually the original density does not even have to be 0.15 as long as
> one
> process the film with the right gradient so that the final wedge has
> approximately 0.15 difference in each step.

But there is no 'original density' as I'm talking about exposing film to
light - there is only one density involved - that of the developed film.
If you don't have a densitometer how do you know what is .15, .3, .45 etc.
Of course it isn't critical to have exact densities, but it would be nice
to know that your 10 step range was roughly the same as other peoples'.

Either you need a densitometer, or you can actually do it accurately
enough for alt-photo purposes by visual comparison with an existing step
wedge. A card with two holes - one for a step on the wedge and one for the
film you are comparing to it - on a light box is all you need.

>
> >> Its actually easier to do a test strip or strips at various times,
> develop, measure the densities, draw a curve of density v time and
> read off the times you need for the 0.15 steps under the same
> conditions. >>
>
> Yes, and I have made many of my own step wedge this way. But if anyone
> is
> trying to do it this way, you must make sure that your processing
> (temperature, agitation, etc.) is fairly repeatible. I use automatic
> processor
> (Jobo).
>
>
> Dave

Absolutely agreed on this. Choosing a film/dev combination that gives a
reasonably long dev time (perhaps 10-15 mins) at room temperature helps. I
use a Jobo on almost all of my film too and tend to forget that others
aren't so lucky!

Peter Marshall

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