Cropping.

Discussion in 'The Darkroom' started by PlasticSpanner, Nov 8, 2005.

  1. PlasticSpanner

    PlasticSpanner TPF Noob!

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    When I raise the enlarger head to create a crop when printing, do I need to adjust the f stop setting at all and what effect will changing it cause?

    Thanks all!:thumbup:
     
  2. terri

    terri Administrator Staff Member Supporting Member

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    You'll more than likely need to adjust your exposure timing, but if you're satisfied with the f-stop you've selected you can let that alone and simply run a new test strip to see how your previous exposure time was affected.

    Mr. Know-it-All will doubtless expand on this answer. :mrgreen:
     
  3. PlasticSpanner

    PlasticSpanner TPF Noob!

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    Found out about the exposure! From a near perfect print to a faint grey image!:x :lol:


    What effect will the f stop have on the image anyway? All I know about that is it affects contrast (or does it?:lol: ) Oh and exposure time!
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Naturally! :lol:
    Terri is right about using the exposure time to control the exposure - but try to avoid having too long an exposure time as paper suffers from reciprocity failure just like film.
    You can actually calculate the exposure time change if you feel inclined (some enlargers have scales printed on the main column to help with this).
    In printing - as in so much else in Photography - the Inverse Square Law comes in to play. If you double the distance of the lens from the paper you reduce the illumination level by a factor of four - which is 2 stops.
    Obviously if you increase the distance by a factor of 1.5 then it is 1 stop.
    To increase exposure by 1 stop you double the time. So 20 seconds becomes 40s.
    To increase by 2 stops: 2x2x20 = 80s.
    If you are mathematically inclined you can work the increase out for most height changes.
    You should still do a test strip but by doing the ISL calculation - even approximately - you will give yourself a starting point and save paper and time.

    Of course the ISL holds true the other way as well. If you move the lens nearer the illumination level increases and you need to reduce the exposure time.

    *Increasing the size of your print will reduce contrast so you may have to go up a grade in paper. If you know that you are going to be printing big you can actually compensate on the neg.


    Happy, Terri?
     
  5. PlasticSpanner

    PlasticSpanner TPF Noob!

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    There's a whole can o worms in that post Graham!:shock: :D


    My enlarger does have a scale on the main column (I thought it was something to do with image size!:lol: )
    How do I use the scale? Is it just a conversion scale?

    ISL I can handle, so if I have a good print at 5 X 7 I can actually calculate the exposure times for larger or smaller prints?

    Are paper grades relative to 1 stop each?

    Lastly! What is an unacceptably long exposure? I did a huge crop which meant the exposure was 8 minutes!
     
  6. terri

    terri Administrator Staff Member Supporting Member

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    You know it's all about making me happy, dontcha Graham? Luckily for you, I'm easy to please. :lol:

    I am actually excited to learn what that damn scale is all about that is on my enlarger column. :thumbup: This is the stuff they don't bother explaining in a 6-week darkroom course. :mrgreen: I'm all for mastering anything that can give me a better starting point than simply viewing my test strips and guessing +/- percentages.

    Thanks! :)
     
  7. PlasticSpanner

    PlasticSpanner TPF Noob!

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    I like the challenge of learning everything I can, regardless of whether there's an easier way or it's pointless!:D
     
  8. alexecho

    alexecho TPF Noob!

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    Eight(!) minute exposure? I usually aim for 20-30 seconds! Maybe it's OK, we'll wait for the expert answer. I would just die of boredom with an exposure as long as yours!
     
  9. terri

    terri Administrator Staff Member Supporting Member

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    Holy crap! What kind of paper were you printing on? What f stop were you using? That seems wayyyy too long.
     
  10. PlasticSpanner

    PlasticSpanner TPF Noob!

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    It was at f11 and the head was approximately 2' above the easel. I was messing with crops like I would in PS to get a face only onto the print. Printing on Kodak Bromesko grade 2 paper.


    It's also a colour head and the bulbs are rated at 14v/35w but it's on a 12v supply so I am going to get some new, standard 12v/35w bulbs.
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

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    My Durst has about 4. I've never managed to find out exactly what their relationship to anything is. All manufacturers are somewhat vague on the subject. But to be honest I've not put that much effort into finding out.
    I do know that the scales are also used for calculating enlargement factors when using the enlarger as a copy stand (a lot of enlargers will allow you to do this).
    The best way that I have found is to do a series of test strips with the enlarger at various heights. Use the most appropriate scale - or use a tape measure.
    Either use a test neg or no neg.
    Stop the lens down to f8 maybe, and set the enlarger head to give the full frame size of your standard print. I do 10x8 but it's up to you.
    Do a test strip as a step wedge, giving increases along the lines of 2-3-4-6-8-..
    You may have to play with the lens aperture or the intervals but what you are after is a good grey scale.
    Mark the position on your column with some indication of what it means. I use x1.
    Move the head up the column half as much again (1.5x).
    Do a step wedge with the same intervals.
    You should find that the 2 sec exposure on the first strip is the same as the 4 on the second. The others should show a doubling too.
    Mark it on the column 1.5x - or something.
    Do the same for 2x the height, 1/2x the height and so on.
    As long as you standardise your processing and take care you should be able to do 1/2 even 1/3rd stop intervals.
    If you normally print a different size then start with that and do the same sequence. You will get a different scale but the principle is the same.
    You can do the same for 6x6 negs, 6x7 and 5x4. Just make sure you mark them clearly so you know the difference.
    In use you choose the scale that corresponds to the enlargement you are doing. If you make the image bigger (or smaller) just read off the exposure mod factor and adjust.
    You don't have to do the test strip - you can just use an accurate tape measure and a calculator. But I did the test strip to make sure there were no problems or inaccuracies in the system.
    It's a bit of a fag and it will take a while but I found it worth it in time and paper savings.
    I standardised my negs too so I could put one in, put the head on a mark, stop the lens to f8 and the exposure time was the same (or near as damn it) every time.
    You can do a similar calibration for paper grade too.
    Get a standard neg and just move the head up (and down) in measured stages doing test strips on the grades to see when it's appropriate to change.

    Hard to say. Paper has quite a low speed and is designed for longish exposures but I think 8 mins would be getting there. Emulsions only obey Bunsen-Roscoe within a small window. It's just that reciprocity failure effects are not immediately obvious.
     
  12. PlasticSpanner

    PlasticSpanner TPF Noob!

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    Thanks Hertz!:thumbup:


    I'm going into the DR now to try all that & see what it relates to on the scale.

    I'll get the new bulbs tomorrow & see what kind of difference they make to the exposure times.

    Thank God for MS Excel!:lol:
     

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