[pure-silver] Re: what speed do you rate HP5+ at?

----- Original Message ----- 
From: "Shannon Stoney" <sstoney@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, February 24, 2005 5:53 PM
Subject: [pure-silver] Re: what speed do you rate HP5+ at?


> >Yes, there is a difference.
>>
>>
>>
>>
>>Regards
>>
>>
>>
>>Ralph W. Lambrecht
>
>
> I am thinking that the roll film is designed so that under 
> exposure
> is almost impossible, in order to be idiot-proof.  In 
> fact, they say
> that you can shoot it at ISOs up to 3200!  So, when you 
> shoot it at
> 400, of course the shadows are going to be placed a little 
> high.
>
> --shannon
>
>>
>>
    I ernestly advise you to get a book on elementary 
sensitometry. Even the Kodak film booklet, _Kodak 
Professional Black and White Films_ publicaton F-4, if its 
still published, is a good starting place. For a more 
complete lesson have your library find _Photographic 
Materials and Processes_ Strobel, Compton, Current, Zakia 
The Focal Press. The first edition is better than the second 
but either will do. There are other good books but they have 
become hard to find.
    Film speed does have a meaning and one can not assign a 
speed arbitarily. The ISO method of measuring speed of B&W 
negative film for still cameras (there are other standards 
for other kinds of films). The ISO speed is measured under 
specified conditions. These specify essentially a contrast 
value. If the contrast is changed by changing the degree of 
development the effective speed changes. The current version 
of the ISO standard does not specify a developer but 
requries that the developer used to measure the speed be 
specified with the results.
    The ISO speed has virtually no safety factor. It is 
intended to yeild the minimum exposure conssitent with good 
shadow detail. Most films have about two stops of latitude 
for underexposure but underexposing will result in a 
progressive loss of shadow detail. On the other side most 
films have many stops of latitude for overexposure.
    "Pushing" film really does not increase its speed. 
Rather it increses contrast. Since the low exposure region, 
called the "toe" has lower contrast than the rest of the 
characteristic, increasing development will make the image 
there more printable. However, it makes the contrast of the 
more fully exposed part of the image very high contrast. 
Some films are more tollerant of pushing because of the 
shape of the curve. There are a few films, T-Max P-3200 for 
example, which have a curve shape especially suitable for 
pushing. This film measures about ISO-800 but can be pushed 
to very high speeds, even to EI-25,000 under certain 
conditions.
   For best tonal rendition film should be exposed normally 
or given some overexposure. Because the ISO speed has no 
safety factor some overexposure, perhaps a stop, will often 
result in better tone rendition but this must be determined 
by experiment.
   Ilford has stated somewhere that it does not follow the 
ISO standard. I think the reason is that the contrast of the 
ISO standard is higher than is commonly used. Ilford 
specifies speed for a contrast value intermediate between 
that suitable for diffusion sources and condenser sources, 
so the speed is perhaps a bit lower than would be the case 
for the strict ISO test. This is really of little 
significance in normal use.
   In order for exposure indexes to have any meaning one 
must be sure than shutter speeds and stop scales are 
accurate. Ultimately, the speed index is just a value to 
plug into a light meter and some judgement based on an 
understanding of what the value means must be applied.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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