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  1. #1
    Gay Larson's Avatar
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    need help with my math

    I usually use the same film all the time to avoid the problem I have now. I was out on a field trip and someone traded me a roll of 125 iso film for my roll of 400 iso Ilford HP5+ just for grins. I changed my ISO on my Pentax MF camera and shot the 125 iso then later put in a roll of Ilford Hp5+ to do a portrait of my brand new granddaughter. I realized what I had done after the roll was completed. So here is the question. I'm thinking I need to pull the processing one an a half stops but don't know how to figure the time. Normally I process in Ilfotec Hc 1-31 at 20 C for 6.5 minutes. How do I figure out how many minutes to develop the film shot at the wrong ISO? Sorry if this is too basic a question but I need to know and where else would I go for the answer but my learned friends on APUG?
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  2. #2
    Photo Engineer's Avatar
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    Gay, thats about 1.5 stops for a good B&W film. You should have no problem with it even in a 'normal' process. In fact, you might be better off with a normal process as the pull will reduce contrast.

    PE

  3. #3
    Gay Larson's Avatar
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    but suppose I want to know how to figure out the time, how do you do that?
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  4. #4

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    Gay,

    For that amount of overexposure you may not need to compensate with reduced development time unless you want to reduce contrast in the negative. The effect of this amount of over exposure would be to move the exposure off the toe of the film characteristic curve and it probably will not pile high light densities up on the shoulder of the curve.

    This would not have been too great an over-exposure for a high contrast scene and in that instance you would reduce development.

    I would make my development determination on the inherent contrast in the exposure. If it was high contrast, I would reduce development by one minute (10-15 %)...if of normal contrast I would develop as normal. The negatives will have greater density but not any greater density range.
    Art is a step from what is obvious and well-known toward what is arcane and concealed.

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  5. #5
    Alex Hawley's Avatar
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    Quote Originally Posted by Gay Larson View Post
    but suppose I want to know how to figure out the time, how do you do that?
    No math is necessary Gay, beyond what you have already done, determining how many stops from N you exposed. Because each type of film is fairly unique, as is its response to a given developer, there's really no way of doing it mathematically.

    Here's one way of doing it. Go to the Massive Development Chart (http://www.digitaltruth.com/devchart.html) and look up HP 5+ in Ilfotec HC 1:31. The slowest ISO listed is 400 and the time is 6.5 minutes. Knowing that you overexposed by 1.5 stops, and knowing HP5+ has lots of latitude, developing for 5.5 minutes would not be a bad idea as our esteemed colleagues above have suggested. If you want to hold back contrast a bit, go with 5 minutes. Increase contrast, go with 6 or 7 minutes.

    No math involved, just intuition based upon experience.
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  6. #6
    Gay Larson's Avatar
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    There is a lot of misinformation out there because the owner or the shop I take my b&W film to be developed on occasion when I am in a hurry (not often) told me there was some kind of formula to determine the time for one stop difference. Perhaps she preferred I didn't figure it out so I would bring it in or she didn't know. They have a really good printer but I didn't talk to him.
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  7. #7
    BradS's Avatar
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    Why not refer to the manufacturer's web site? There, you will find the data sheet for HP5+. It contains all manner of information about this film. May I suggest going to http://www.ilfordphoto.com/ and poking around a bit? Follow the "products" link on the right side...

  8. #8
    BradS's Avatar
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    Here is a link to the Ilford Hp5+ product page. If you hover over the "fact sheets" tab and select "HP5+ fact sheet" from the pop up menu, you should get the data sheet.

  9. #9

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    Quote Originally Posted by Gay Larson View Post
    They have a really good printer but I didn't talk to him.
    Talk to the printer who can probably give you better advice than the owner.

    If you cut back on development to reduce density then contrast will suffer. Remember development determines contrast while exposure determines density. Remember many photographers deliberately over expose by a stop to get better shadow detail. You are still within the latitude of the film. Stop worrying.

  10. #10
    Alex Hawley's Avatar
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    Quote Originally Posted by Gay Larson View Post
    There is a lot of misinformation out there because the owner or the shop I take my b&W film to be developed on occasion when I am in a hurry (not often) told me there was some kind of formula to determine the time for one stop difference. Perhaps she preferred I didn't figure it out so I would bring it in or she didn't know. They have a really good printer but I didn't talk to him.
    That formula is most likely for an automated processing machine where only one type of developer is used for everything and there is lots of development test data availalable for each type of film. This is what we engineers call an empirical formula based upon empirical results, which means lots and lots of development testing. Quite doable, and necessary for someone producing processing machines, but it would be a huge PITA doing it at home.
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