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  1. #11
    Steve Smith's Avatar
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    Quote Originally Posted by jeroldharter View Post
    I understand that with LEDs, dimming them is more complicated than with other light sources.
    Dimming LEDs is fairly easy. The most efficient way is to drive them with a rectangular wave of varying duty cycle e.g. ranging from 95% on, 5% off to 5% on, 95% off with 50-50 in the middle. You could set up a single control which increased the brightness of the green at the same time as reducing the brightness of the blue (and vice versa).

    A fixed, one off adjustment to the relative levels of the two colours should give you a single control which can be marked off in equivalent grades.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  2. #12
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    Quote Originally Posted by Steve Smith View Post
    Dimming LEDs is fairly easy...
    Steve.
    I suppose it is like large format black and white photography. It is easy if you know what you are doing. I have no background in electricity. I dabbled with the idea of building an LED light source. But with no background I think I figured out which LEDs to buy but was stymied by what to do next. There is no LED Depot.
    Jerold Harter MD

  3. #13
    Steve Smith's Avatar
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    Quote Originally Posted by jeroldharter View Post
    I dabbled with the idea of building an LED light source. But with no background I think I figured out which LEDs to buy but was stymied by what to do next. There is no LED Depot.
    I suppose it's the same as everything. It's easy if you know how but not if you don't.

    The other method of varying blue/green ratio is just to have two separate exposures one after the other. e.g. 20 seconds of green followed by 30 seconds of blue may put you into grade 3 territory whereas 45 seconds of green and only five of blue may equate to grade 1 (very simplistic, obviously). Similar to the way some people use two exposures using grade 0 and grade 5 filters

    I think the LED light levels are going to be lower than comparative incandescent light sources so exposure times should be longer and more manageable.

    I have an on/off (mainly off) project ongoing to build a 5x4" LED lightsource enlarger so I have a bit of interest in this. Unfortunately I have about three thousand and seven other things on the go as well!

    If you want to give this another go, I would be happy to give advice regarding the electronic control (or anything else I may claim knowledge to!).


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  4. #14
    RH Designs's Avatar
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    The electronics of controlling LEDs, whether you're using a split grade exposure or the single grade equivalent, isn't difficult. As I see it the major difficulties are much more to do with the mechanical design - how to get an even enough light distribution and how to interface the light source to the wide variety of different enlargers that exist in the market at an economical cost.

    My experiments have shown that achieving a high enough contrast from the blue LED is also an issue - the "bluest" LEDs I can find (455nm peak) still only managed about a grade 4 contrast equivalent, though his can be improved at the expense of exposure time by adding some magenta (or presumably blue) filtration.

    But going back to the OP, Geary is quite correct that for split grade printing, there is no need to control the intensity of the light sources. If the hard light is brighter than the soft light, the required hard exposure time for a given contrast is reduced. It's as simple as that. Using a mix of green and blue light for one or both of the exposures just reduces the maximum range of contrast adjustment available from the system.

    Regarding the optimum negative contrast for split grade printing - well, the object of variable contrast paper is to fit the paper contrast to that of the negative and not the other way around, whatever method of paper contrast control is in use. You only need to ensure that the negative contrast falls within the range of paper contrast available. I should add that this analysis omits any considerations of the subtle effects that the interaction of the negative and paper characteristic curves have on the "look" of the resulting print - these are much harder to characterise - but if the negative contrast lies within the available paper contrast range then both highlights and shadows can be rendered on the paper. And this can be achieved by either split exposures or a single exposure through the appropriate filtration.
    Regards,
    Richard.

    RH Designs - My Photography

  5. #15

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    Quote Originally Posted by RH Designs View Post
    The electronics of controlling LEDs, whether you're using a split grade exposure or the single grade equivalent, isn't difficult. As I see it the major difficulties are much more to do with the mechanical design - how to get an even enough light distribution and how to interface the light source to the wide variety of different enlargers that exist in the market at an economical cost.

    My experiments have shown that achieving a high enough contrast from the blue LED is also an issue - the "bluest" LEDs I can find (455nm peak) still only managed about a grade 4 contrast equivalent, though his can be improved at the expense of exposure time by adding some magenta (or presumably blue) filtration.

    But going back to the OP, Geary is quite correct that for split grade printing, there is no need to control the intensity of the light sources. If the hard light is brighter than the soft light, the required hard exposure time for a given contrast is reduced. It's as simple as that. Using a mix of green and blue light for one or both of the exposures just reduces the maximum range of contrast adjustment available from the system.

    Regarding the optimum negative contrast for split grade printing - well, the object of variable contrast paper is to fit the paper contrast to that of the negative and not the other way around, whatever method of paper contrast control is in use. You only need to ensure that the negative contrast falls within the range of paper contrast available. I should add that this analysis omits any considerations of the subtle effects that the interaction of the negative and paper characteristic curves have on the "look" of the resulting print - these are much harder to characterise - but if the negative contrast lies within the available paper contrast range then both highlights and shadows can be rendered on the paper. And this can be achieved by either split exposures or a single exposure through the appropriate filtration.
    Well thats all well and good for a straight print. But when you want to print part of the image hard in relation to the rest of the print, then having a negative which fits the middle of the paper contrast, allows printing the chosen areas at upto 3 grades higher contrast. There are people who like to do this instead of having a standard fomulaic look. Its called creative printing. And I know some stuffy people will tell you its not a fine print but not everyone wants to print according to their rules. And rules were made to be broken. i.e. just because VC paper exists doesn't mean you don't have to worry about creating a negative of good contrast leaving you room to play with.

  6. #16
    RH Designs's Avatar
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    Quote Originally Posted by rob champagne View Post
    Well thats all well and good for a straight print. But when you want to print part of the image hard in relation to the rest of the print, then having a negative which fits the middle of the paper contrast, allows printing the chosen areas at upto 3 grades higher contrast. There are people who like to do this instead of having a standard fomulaic look. Its called creative printing.
    I'm well aware of creative printing, and that's why I said that the analysis ignored the more subtle effects. It's always wise to get negative contrast to around the middle of the paper contrast range to give you some margin for creative dodging/burning etc. However, my point was that if the negative contrast lies within the available range of paper contrast available, then it can be printed, whatever method of adjusting print contrast you use. So there is no need to adjust the negative specifically with split grade printing in mind, that's all. Whether the final result is an aesthetically pleasing print or not relies on the skill and vision of the printer. Whether you use G0 and G5 or some other intermediate grades for split printing, you can still achieve the same result with appropriate dodging and/or burning.
    Regards,
    Richard.

    RH Designs - My Photography

  7. #17
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    Steve,
    I think that this is core to the discussion. Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

    The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?

    Cheers,
    Geary

    Quote Originally Posted by Steve Smith View Post
    Dimming LEDs is fairly easy. The most efficient way is to drive them with a rectangular wave of varying duty cycle e.g. ranging from 95% on, 5% off to 5% on, 95% off with 50-50 in the middle. You could set up a single control which increased the brightness of the green at the same time as reducing the brightness of the blue (and vice versa).

    A fixed, one off adjustment to the relative levels of the two colours should give you a single control which can be marked off in equivalent grades.


    Steve.
    But your flag decal won't get you into Heaven any more. They're already overcrowded from your dirty little war.
    Now Jesus don't like killin' no matter what the reason's for, and your flag decal won't get you into Heaven any more. – John Prine

  8. #18
    Will S's Avatar
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    In the workshop I did with Les McClean I'm pretty sure we always used 0 and 5, and never anything else. Time was varied during dodge/burn to achieve local contrast changes where needed.

    Will
    "I am an anarchist." - HCB
    "I wanna be anarchist." - JR

  9. #19
    galyons's Avatar
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    Rob,
    IMO, this is contrary to the concept of split grade printing. There are no grades in split grade printing, just the relationship of maximum hard to soft. That is the maximum hard and soft produced by your light source. (Of course this assumes that you have relative range). There is no need to calibrate to an "intermediate grade". Quite frankly there is no calibration.

    I find split grade printing combined with the StopClock Pro f-stop timer artistically intuitive. I adjust the hard and soft exposures to get the overall contrast desired and the dodge/burn for localized contrast. To what contrast grade these judgments calibrate is not even in my thought process. That is the core of split grade printing!

    Cheers,
    Geary

    Quote Originally Posted by rob champagne View Post
    Just using G0 and G5 won't necessarily do it unless the negative is of perfect contrast for the required print.

    That obviously creates a major problem in designing an LED light source for split grade printing because the mix of green and blue for any intermediate grade has to be calibrated to the specific paper and developer combination. That appears to be what the the heiland split grade head does although it is not an LED source, but it has to know how the paper will respond to any combination of Y+M or G+B to be able to work out intermediate values for starting point or burning and dodging.
    But your flag decal won't get you into Heaven any more. They're already overcrowded from your dirty little war.
    Now Jesus don't like killin' no matter what the reason's for, and your flag decal won't get you into Heaven any more. – John Prine

  10. #20

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    Quote Originally Posted by galyons View Post
    Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

    The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?
    AFAIK, aside from reciprocity issues, the two are equivalent, no matter the light source. After all, it ultimately just comes down to the number of photons that strike the paper. Certainly I frequently dim my light source and increase exposure time (or vice-versa) to get the exposure time into a range I want for dodging/burning or just to not be sitting at the enlarger twiddling my thumbs for minutes at a time.

    If there's some difference in the results (contrast shifts, maybe?) from dimming vs. reducing the exposure time, I'd like to hear what these effects are. I've never heard of nor observed such effects, though. (I've also never looked for them in a systematic way.)

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