Factor for enlarger head height adjustment?

Discussion in 'Enlarging' started by declark, Dec 11, 2008.

  1. mmerig

    mmerig Member

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    It's easy to fall into a Rube Goldberg approach to simple things, but I was curious about using negative height, so I made some measurements -- Exposure Value versus negative height, or EV versus lens height. Negative height is easier to measure on my enlarger, but it is not ideal as it does not follow the inverse square law (ISL). A quadratic equations fits it nearly perfectly, but it's not as easy to use as the ISL, and probably unique to each enlarger setup. Lens height follows the inverse square law very closely -- the main error in my measurements is EV value. I may try and improve these measurements when I have time, but the error is mostly about 1/6 stop. Of course, the light coming through the lens at the iris could be assumed to be a point source, so there should not be much concern about using the inverse square law, or enlarger head characteristics above the point source.

    Getting a reasonable estimate of a new time for a new paper size (say 8 by 10 to 16 by 20) via the ISL, and using this estimated time on a small paper (like 5 by 7) at the bigger enlargement, is usually faster for me than starting over with a test strip. The narrow strips are usually not wide enough for me to judge the final outcome.
     
  2. mmerig

    mmerig Member

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    [QUOTE="mmerig, post: 1995872, member: 60150" -- Exposure Value versus negative height, or EV versus lens height..[/QUOTE]

    Substitute time, in seconds, for EV.

    I re-measured and got more precise results. I can post results if anyone is interested. The quadratic equation does not fit as well now, but the conclusion is the same:

    Using negative height and an empirical curve to estimate time is not worth the trouble. Stick with lens height and the inverse square law to estimate the new time.

    I would still be interested in how enLARGE works, from a theoretical perspective.
     
  3. pentaxuser

    pentaxuser Subscriber

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    Me too but the more time that passes then the less likely, I suspect, we are to hear from the App inventor. A pity. If he has proof that his method is either better than any other or more worrying for us other users, he has proof that anything other than his App definitely gives wrong results then shouldn't he be beating a path of enlightenment to our doors?

    pentaxuser
     
  4. cornflower2

    cornflower2 Subscriber

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    Hi pentaxuser - You may wish to view my instructional video on YouTube called ‘Save time, photo paper and chems when enlarging - enLARGE Calibration and Enlarging’, which is designed as a ‘video instruction manual’ for people who have just bought the app, which by the way is priced, I think, to pay for itself in saved time and materials, and in better looking prints, in just the first one or two print sessions. The video demonstrates the enLARGE calibration process and then demonstrates the app in use with a so-calibrated enlarger, but I am going to go over the process here with some notes that have not been included in the video.

    Note that you can perform the calibration process without owning the app at all - it’s only when you enter the test data from the calibration process into the app, which can be done at any later time, that the app becomes calibrated to the enlarger. Ralph W. Lambrecht, who has compiled a fantastic body of work on this and related matters, says I have a unique talent to make things complicated, but to my way of thinking enLARGE solves a fundamental problem in enlarging which can otherwise not be solved practically at all. And David Brown says that we should all simply ‘make a new test strip’, which will ‘solve the problem’, but I could not disagree with David more; test strips, when made at actual larger final enlargement sizes, and together with the usual run of imperfect initial enlargements which typically follow them, are in my opinion a major waste of darkroom printing time, materials and processing chems. If you’re making lots of big enlargements (eg. 16x20”, 20x24”), which I was doing when I developed enLARGE, then the wastage is significant. Every traditional darkroom worker I’ve ever known has complained of the same thing: too much paper (and associated time and print chems) wasted in the making of traditional enlarger enlargements.

    I don’t wish to sound obscure here but unfortunately I have an investment to protect. The cost of getting my app onto the Apple AppStore, in $AUD, was significant and so it would be foolhardy for me to disclose its inner workings. However, this is not to stop anybody else from perfecting a similar or different app or method. Prior to developing enLARGE I was simply making large test strips, typically at 16x20” and 20x24” enlargement size, and then using 3, 4 or more whole sheets of paper to get my first good print - by which time my trays of photographic chems were becoming exhausted - all very expensive and wasteful. My enLARGE app is simply designed to bring all the creative work involved in making an enlargement - the decisions that need to be made about exposure, contrast, dodging & burning, overall composition and finished enlargement size - down to a conveniently small, fast, cheap size; you then use enLARGE to compute the exposure time needed to make matching enlargements of other, eg. larger sizes.

    I think there are two main questions of interest here: first, how does enLARGE ‘know’ about your particular enlarger, and second, how do you use enLARGE in practice to actually make enlargements? Firstly you should know that enLARGE is simply a ‘calculator-style’ app; it doesn’t physically measure or control anything directly but simply receives numerical data from you, the user, in order to compute certain results, exactly like a simple dedicated math calculator.

    First, it is necessary to ‘inform’ enLARGE about your enlarger - this is the calibration process. enLARGE is essentially interested in your enlarger’s relative light output at different magnifications. In an ideal simpler world, if our enlargers behaved according to the Inverse Square Law - which states that as the projected image size is doubled then its brightness decreases to one-quarter - then a change in exposure could be computed purely on the basis of a change in enlargement size, and instead of using enLARGE we would use a simpler calc app which would use the Inverse Square Law and simply say, “Okay, the print size has doubled from 5x4” to 8x10” so therefore the exposure time needs to be quadrupled from 10 secs to 40 secs.”

    But as I have explained, our enlargers do not behave according to the ISL, and in fact behave differently to each other. I have two 6x9cm condenser enlargers - a Leitz Focomat IIc and an Agfa Varioscop 60, both of which have their own 60 mm enlarging lens. If I make an 8x10” print with the Agfa at 10 seconds, and then make a 16x20” matching print, my new exposure time for the second print is 34.3 seconds; whilst if I do the same thing with the Focomat, it’s 37 seconds. Neither of them is 40 seconds, and they’re both different by about 10%, which is enough to show up as a difference in a side-by-side print comparison. Why are they different? It’s because the enlargers are physically different: they have differently sized and shaped lamphouses and interior components, different condenser lenses, and the actual focal lengths of their two ‘60 mm’ enlarging lenses may also be slightly different. When enLARGE takes a ‘snapshot’ of an enlarger’s ‘unique exposure curve’, as I call it, during the calibration process, it reads all of these differences collectively and holistically in one single measurement, as will be shown. You can’t dissect these different influences on the enlarger’s exposure performance and there's no point in doing so. My point is that unless this unique difference is recognized, you can’t begin to compute an accurate new exposure time, for any particular enlarger-and-lens combination, for any change in print magnification.

    So how does enLARGE get to ‘know’ your enlarger’s unique exposure curve? To function in its role as a Predictive Exposure Computer it simply needs to know the nominal focal length of the enlarging lens and the enlarger’s relative light output at any two different print magnifications, eg. at a small print magnification and at a large one. Instead of using a light meter to measure the light output at these two different magnifications it uses photographic print paper, such that when we make any two consecutive differently-sized but identical-looking enlargements from the same negative we acknowledge that the print paper, in each case, has effectively received the same amount of light. Incidentally, the particular choice of print paper does not influence the accuracy of an enLARGE calibration or its subsequent accuracy in actual enlarging, thus once a calibration has been made you can use it with all different types of print papers and materials with equal success, there is no need for separate calibrations for different materials.

    Let me step you through the enLARGE calibration process and you will see how it works:

    The enLARGE Calibration Process

    First we will make a low-magnification calibration print. Imagine setting up your enlarger (let’s say with a 50mm lens and a 24x36mm negative) to make the smallest enlargement that you would ever typically want to make with it, let's say it’s an enlargement of about 2X or 2.5X (the exact magnification isn’t critical), thus a print of approximately 6x9cm. And let’s suppose that we make this enlargement and it looks just right using an exposure time of 5 seconds with the 50mm enlarging lens set to f8. Having now made this enlargement, we now enter this data from it into the ‘Create Enlarger Profile’ screen of the enLARGE app:

    • We type in the negative-to-print distance (typically measured using a tape measure) used to make the print - in this case, let’s say it’s 250mm;
    • We type in the print’s exposure time, in this case, 5 secs, and
    • We type in the enlarging lens’s nominal (ie. as-written-on-the-lens-barrel) focal length, in this case, 50mm.
    enLARGE now knows that our enlarger is producing a ‘perfect print’ when the negative-to-print distance is 250 mm and the exposure time is 5 secs, using a lens of focal length 50mm.

    Now let’s make a visually matching high-magnification calibration print, which thus effectively uses the same amount of light at the print paper but at a bigger magnification. To do this we will raise the enlarger’s head to make the largest print that we would typically want to make with it using its current configuration of negative format and lens. Once again the exact size isn’t critical; let’s say it’s a print of about 13X, thus about 13x20” at the print easel. We now refocus the lens for this new print but we don’t touch, handle or change the lens iris setting, thus we leave it set at f8 (or at whatever f-stop setting was used to make the earlier low-magnification print). enLARGE works by forecasting a change in exposure time (not lens aperture) so it expects the lens iris to be left unchanged for the making of all prints in any one particular set.

    The task now is to make this big enlargement so that its tones exactly match those of the smaller first print. We’ll need to experiment with different exposure times to find the exposure that achieves this. (If you’re really going to do this then there’s a smarter, cheaper, faster way, and that’s to print both the small and large calibration prints with no negative in the carrier, so that you just print a pair of medium-grey matching tones; then, when you’re printing the high magnification test print, you just print a matching tiny patch-print of the center of the projected field. This method is recommended, but make sure you focus the lens, when making both prints, using a dummy negative first). Let’s say that an exposure time of 137 seconds does it. We now enter this new additional data - from this new matching high-magnification calibration print - into the same enLARGE ‘Create Enlarger Profile’ work screen:

    • We measure and enter the negative-to-print distance used - in this case, let’s say it’s 969 mm, and
    • We enter the print’s exposure time, in this case, 137 secs.

    enLARGE now has the negative-to-print distances and exposure times needed to produce both matching test prints. Specifically, it knows that when using a 50 mm lens and the negative-to-print distance is increased from 250 mm to 969 mm, then the exposure must be increased from 5 secs to 137 secs. To complete the calibration we add a brief descriptive text label to it which describes the enlarger setup, eg. ‘Durst M800, Opticon85 Condenser, 50mmRodagon’.

    That’s it! That’s all enLARGE needs to know about our enlarger. It now has a picture, so to speak, of its unique exposure curve, being the ‘exposure performance’ of the enlarger, as exemplified by just two positions, being negative-to-print distances of 250 and 969 mm.

    Now, if we change any physical or optical aspects of the enlarger - eg. we change its lamp to one of a different size or position, or we change its condenser lens or its enlarging lens - then this information would become inaccurate and void. In practice, when using enLARGE, we make a separate calibration for each different commonly used enlarger setup and then select the appropriate calibration, from enLARGE’s ‘enLARGEr Profile’ library, to match the equipment in use, thus making it serve as the active program. enLARGE stores an unlimited number of such calibrations, each called an ‘enLARGEr Profile’, so that you can conveniently use it with any number of different enlarger setups.

    Now that enLARGE ‘knows’ the exposure performance of our enlarger, it uses this knowledge to compute and thus forecast new exposure times required for all magnifications made with it - both within the tested negative-to-print distance range and also beyond it - although I recommend that the tested range of negative-to-print distances encompasses the distances to be used for actual enlarging.

    Having made our calibration, we can now load it from the user library into the main enLARGE workscreen and use it to make one or more matched pictorial enlargements with the so-calibrated enlarger, as follows:

    The enLARGE Enlarging Process

    Initially we don’t use enLARGE at all, but simply begin by setting our enlarger to make a small enlargement. The actual size isn’t critical so long as its negative-to-print distance preferably falls within the calibration-tested range. We’ll focus the lens sharply for the print and then stop the lens down to an aperture - eg. f8 - which is not so small that it prevents the sharp examination of film grain through our focus-finder when we later need to refocus the lens for larger projections with the lens in its already-stopped-down setting.

    We now make test strips and preliminary first prints - adjusting the print’s exposure and contrast as needed - to make a print which looks perfect, the way we want it to look. In fact we do all the things we would otherwise normally do at a much bigger final enlargement size, but we now do it using much smaller bits of paper and not much processing chemistry. One of the advantages of using enLARGE is that it keeps your processing chemistry fresh and strong for the making of final larger perfect enlargements - rather than wasting it on large-scale testing - and another advantage is that you can print and view the print as a complete full-frame print rather than have to judge everything on the basis of just a larger portional test strip. At this smaller complete size you can make better decisions about a final desired enlargement size, its composition and cropping, and you’ll also notice if the image isn’t such a good one after all, in which case you can abandon it early and move onto something else. At this small print magnification, exposure times will be short (good) but may be too short to actually test any dodging or burning effects (bad), so these may simply need to be estimated for application later in the larger enlargements, as will be shown.

    Having got our first small print looking perfect we now measure its negative-to-print distance (eg 275mm) and enter it into the main enLARGE workscreen together with its main exposure time (eg. 12.8 secs). We can also enter our required dodge and burn percentages (eg. 10, -15, 25, -30) into as many as four separate fields. Now we’re ready to raise the enlarger head to any new size (as mentioned, preferably within the calibration-tested negative-to-print distance range) for the making of, say, a much bigger print (although it could be a smaller print, as enLARGE computes and forecasts in either direction).

    We raise the enlarger head to the new desired height, lock it and then fine-focus the lens in its already-stopped-down position. Then we measure the new negative-to-print distance and enter it into the same main enLARGE work screen. Then we tap on a button that says ‘New Exposure’ and the workscreen instantly displays the required new exposure time, such that, with identical print processing, the new print will visually match the previous smaller print exactly. enLARGE also displays the required relevant dodge and burn times for the new print, computed as percentages of the main exposure time and displayed as both percentages and seconds-of-time, and it also displays a secondary (user-customisable) ‘dry-down’ main exposure time. In a supplementary screen enLARGE will also compute split-exposures, in which the main ‘wet’ and ‘dry’ exposure times are broken down into a sequence of as many as 9 shorter equal exposures, with default and user-programmable correction for their reciprocity-failure effect which otherwise produces a print which is slightly too light. There is some discussion here in photrio about whether or not a print changes its contrast with a change in magnification; enLARGE will show you that it doesn’t.

    And that’s enLARGE. Essentially, it lets you do all your creative print testing - which is important but always ends up in the bin - at a small, cheap, convenient size, and it gives you superior control over the look of your larger enlargements by letting you see them as large ‘thumbnails’ first. enLARGE is also great for making sets of differently sized matching prints, which can - to further save processing time - be exposed and then processed together in batches.

    I hope this info has helped to explain. I am not aware of any other app or method that does what enLARGE does. Mind you, there’s nothing stopping you from carrying out the calibration process as described above and simply noting the factorial change in exposure that occurs at different negative-to-print distances, and then using these when printing at the same distances. However, this method limits you to a limited number of set print sizes, whilst enLARGE works through a continuous and unlimited range of magnification from 1X upwards.

    Sorry to be so long in answering your query! Let me know if you have any other queries. There are other interesting facets to enLARGE - things you will discover for yourself if you ever use it - but I try not to confuse people with them. If you’re an Apple iPhone, iPod or iPad user then let me know and I’ll let you download a copy of enLARGE at a discounted price. Best Regards - Andrew Wittner, Melbourne, Australia
     
  5. pentaxuser

    pentaxuser Subscriber

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    Thanks for the detailed reply, cornflower2. I'll need to take some time to study it properly and hopefully others who contributed to the thread will be interested in it also.

    pentaxuser
     
  6. mmerig

    mmerig Member

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    Per enLARGE:

    Two bivariate measurement points (negative-to-print distance and exposure for two print sizes) will likely not be enough to calculate a "unique exposure curve" for an enlarger, unless every enlarger has a function that can be described with only two parameters. Either the "curve" is actually linear (straight) in the original units of distance and time, or one or both of these variables can be log-transformed (or some other transformation) and the line is straight in that space. Using a zero time at a zero print size provides another point, but this would be superfluous for a linear function in non-log space, and zeros do not work in log space (there is no log of zero or negative numbers, unless you like negative infinity).

    Also, using print results to determine an exposure curve can be confounded by reciprocity issues of the paper if the two print sizes are very different, and the resulting times are quite far apart. If that is the case, then a new calibration could be needed when switching paper types. Better to use a meter to get the times, and handle the reciprocity separately if necessary.
     
  7. cornflower2

    cornflower2 Subscriber

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    Fair enough! Each to his own!
     
  8. RalphLambrecht

    RalphLambrecht Subscriber

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    if this is still an issue for you, send me an email to rwlambrec@gmail.com and I will send you a free pdf document with all the relevant answers. Ralph W. Lambrecht
    all the best.
     
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