Some enlarging questions

Discussion in 'Enlarging' started by NDP_2010, Sep 11, 2010.

  1. NDP_2010

    NDP_2010 Member

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    I am new to the world of film printing and have done some black and whites.

    I am curious with a few questions.

    What is the advanatage of stopping down an enlarging lens? Is there any advanatage in having a fast enlarging lens?

    I also have a few questions about exposure time.

    I noticed that i left a few pictures enlarging for too long and the contrast was quite low and the picture was very dark. I tried a shorter time and the picture had much more contrast.

    If i enlarge for a short period of time, does this mean highlight details will not be captured? If i have a picture where the forground is quite dark, can i ever effectively recapture these details while keeping the picture relatively contrasty or is it impossible as the picture was not taken correctly.

    Thanks.
     
  2. Marek Warunkiewicz

    Marek Warunkiewicz Member

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    Hi, here are some answers...

    ***What is the advantage of stopping down an enlarging lens?***

    Better sharpness from edge to edge. Most lenses have a sweet spot of sharpness/acutance etc around 2 or 3 stops down from the most wide aperture.

    ***Is there any advantage in having a fast enlarging lens?***

    Yes there is. basically it concerns the point above and printing speed. When you have a 2.8 lens and stop down two stops you get 5.6. Assume your time to get a good print is 12 seconds at 5.6. If you had a 5.6 lens and you stopped it down to 11, your time would be 48 seconds. It's all about the exposure time.

    The ***dark contrast print*** question has more to do with your negative and contrast filter. To really answer that question we would need to know the paper used, time(s) of exposure of the different prints, developer and time in developer, the contrast filter used (if any) and ideally a scan of the negative prints.

    You can capture a wide range of tones in a negative, it's all about the exposure and development (Zone system).

    Marek
     
  3. RalphLambrecht

    RalphLambrecht Subscriber

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    I see another advantage in using a 'fast' enlarging lens. Even if you enlarge at the 2-3 stops down from wide open, a wide-open lens will make it easier to accurately focus the image on the base board before you stop it down and enlarge to get the most out of the lens.
     
  4. RalphLambrecht

    RalphLambrecht Subscriber

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    It's hard to answer this one without knowing what your printing method exactly is, but I can offer two articles, which may help you:

    http://www.waybeyondmonochrome.com/WBM2/TOC_files/TimingExposureEd2.pdf

    http://www.waybeyondmonochrome.com/WBM2/TOC_files/BasicPrintingEd2.pdf
     
  5. Ian C

    Ian C Member

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    In answer to your first question, there are several reasons to use a smaller aperture on an enlarging lens.

    1. Lenses can be slightly sharper closed 1 or 2 stops. Thus an f/2.8 lens might be slightly sharper closed down to f/4 or f/5.6 (any difference is likely to be small).

    2. If you want to give some parts of the print more or less than the basic overall exposure—called printing in and dodging—it’s useful to use a smaller aperture to give you sufficient time to perform these operations.

    3. Composition and gross focusing of the projection is most easily done with the aperture fully open for the brightest, most easily seen image. But when you fine tune the focus with a grain focuser the image may be uncomfortably bright for your darkroom-adjusted vision. By closing down to the working aperture you’ll find grain focusing the dimmer image more comfortable to your eye and the grain is more easily resolved with the lens stopped down 1 or more stops.

    4. In some cases, if you have no glass negative carrier and the negative isn’t as flat as you’d like, closing the lens may increase depth of focus at the negative sufficiently to give a slightly sharper print.

    5. Though not a problem with modern lenses, older lenses sometimes had a defect called spherical aberration. Such a lens might be focused wide open, but would change focus as the aperture was closed down to make the exposure. That’s why darkroom workers were often cautioned to refine the focus after stopping down to the working aperture to compensate for focus shift due to possible spherical aberration in the lens.

    The advantages of a fast lens are:

    1. Brighter, more easily seen image for faster composition and gross focusing.

    2. Potentially faster printing time. For example, an f/2.8 lens closed 2 stops at f/5.6 will print in half the time of an f/4 lens closed 2 stops at f/8.

    For the second question, longer exposures don’t generally result in reduced contrast. I suggest checking carefully for light leaks prior to the lens. Some enlargers spill light from around the negative carrier or elsewhere striking the wall near the enlarger and bouncing back to the paper. This is exacerbated by the greater reflectance of a white or light colored wall close to the enlarger.

    It’s likely not much of a problem with a large aperture as the total exposure time is so short that the spill light has little effect. But with a small aperture the printing time is much longer. It’s possible that such a long printing time will allow the spill light to “flash” the paper. If so, then the contrast will be reduced. That might explain your observation.
     
  6. Bob Carnie

    Bob Carnie Subscriber

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    I think a lot of expectations of improved sharpness by closing down is highly overrated.

    An APO enlarging lens is designed be sharp at open apetures.
    We in fact focus on the grain , with a grain focuser, do we not.

    Can anyone here say that the grain gets sharper as you close down an Apo lens.
    I for one have never seen this effect when using glass carriers in my darkroom. The grain is sharp, my print is sharp.

    I am only stating what I see every time I focus, if anyone here can explain to me the theory of closing down improving grain sharpness, I am open to here it.
     
  7. Bob Carnie

    Bob Carnie Subscriber

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    I should add , the theory should also be backed up with actual prints showing better sharpness when one is using an aligned enlarger, good optics, image centered to the neg, condensor and bulb. Also I would expect a glass carrier.
     
  8. fotch

    fotch Member

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    I think what it means that if something is not perfectly aligned, or parallel, the slight increase in depth of field will help in edge to edge sharpness. Not everyone has top of the line equipment nor the skills and patience to get the enlarger, lens board, easel, etc., into perfect alignment.
     
  9. Bob Carnie

    Bob Carnie Subscriber

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    To my knowledge we are not talking depth of field , rather depth of focus , which with an Apo enlarging lens there is extremely little or none.

     
  10. fotch

    fotch Member

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    Bob, I will admit this is way over my pay grade, however, in what the OP asked "advantage of stopping down an enlarging lens?", so,how is either different? Both depth of field and depth of focus are strongly dependent on changes in aperture, are they not?

    The APO lens is not the average enlarger lens so I am not sure how that fits in to the answer for the original question.
     
  11. RalphLambrecht

    RalphLambrecht Subscriber

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    As with taking lenses, stopping down an enlarging lens will improve image quality and cut down on lens aberrations. This is especially true and visible in the image the further you get away from the image center.

    I find it easier to use the grain focuser with a wide open enlarger lens, because I can more easily see the 'snap' in and out of focus.

    Depth of field (at the baseboard) and depth of focus (at the negative stage) are in a geometrical relationship with each other (see attached illustration).
     

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  12. ic-racer

    ic-racer Member

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    The optimal aperture for any lens is not some nebulous guess at how many stops down to go. The published MTF curves and light fall off curves tell all.
    The DOF is related to aperture size and magnification and acceptable size of circles of confusion. It is independent of lens design or construction or focal length.
     
  13. RalphLambrecht

    RalphLambrecht Subscriber

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    I've never seen any EL-Nikkor MTF graphs. Does anybody know where to find them?
     
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  15. Bob Carnie

    Bob Carnie Subscriber

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    On a very practical level. Visually looking at prints

    With all good things in place , like I suggested above.

    Focus on grain with paper under grain scope.

    make a print wide open , make a print two stop downs adjust time.

    Do not re focus between.

    Look at the prints.. If someone here can honestly say they say the one print is sharper than the other I will eat my shorts.
     
  16. RalphLambrecht

    RalphLambrecht Subscriber

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    Bob

    You should be able to see at least a difference in edge sharpness from wide-open to a couple of stops down. If you can't, you must have some wonderful lenses. Try the test with a picture of a resolution target and you should see it.

    I don't have a lot of enlarging-lens MTFs, but I have attached one for a Rodenstock Rogonar-S. One can clearly see the performance difference between f/2.8 and f/5.6.

    Regardless of this evidence, I'm not holding you to your promise. I don't think it's healthy.
     

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  17. Bob Carnie

    Bob Carnie Subscriber

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    I shop at Victoria Secrets

     
  18. RalphLambrecht

    RalphLambrecht Subscriber

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    That's different. Go for it then, I like to see you eating your shorts in that case.
     
  19. michael_r

    michael_r Subscriber

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    I might add an additional advantage when stopping the lens down a few stops - uniformity of illumination. This of course all depends on the design and focal length (particularly in relation to negative format) of the lens, but generally light falloff is greatest wide open.

    Regards.
     
  20. Bob Carnie

    Bob Carnie Subscriber

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    I should add that for each format I am using a longer lens.
    ie

    35 - 80mm
    6x6 ,6x7 - 90mm
    6x9 - 105mm
    4x5 -150 and 180mm

    I am still convinced that sharp grain at wide open will not be sharper grain at 2 stops down.

    There are a lot of factors that contribute to edge defects, but if you do all the good things your edges are sharp wide open. At least in my darkroom
     
  21. RalphLambrecht

    RalphLambrecht Subscriber

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    Bob

    I already provided evidence to the contrary. All lenses suffer from aberrations to some extend. Enlarging lenses not excluded. Some of these aberration effects are reduced when the lens is stopped down.

    Maybe your lenses suffer from a focus shift when stopping them down, offsetting the effect?
     
  22. michael_r

    michael_r Subscriber

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    I was simply pointing to light falloff as something that can usually be reduced substantially by stopping down, assuming there is any falloff to begin with. If you're using an 80mm lens for 35mm, falloff is less of an issue than if you were using a shorter focal length. But I raised falloff because it is a different lens property than was being discussed, and an additional reason someone might want to consider stopping down. It is not related to resolution/sharpness. Although regarding sharpness I will back Ralph up on this. Unless you are using a completely corrected, truly diffraction-limited lens, sharpness toward the edges and corners will improve when the lens is stopped down to a degree. It might not be visible in small enlargements, but it is a fact. There are several common abberations which can be reduced by stopping down. Unless you have a perfect lens, there is always an aperture somewhere in the middle of the lens's range that gives you the best balance between the reduction of various abberations, and the increasing effects of diffraction. Even today's most corrected Rodenstock and Schneider enlarging lenses benefit from stopping down 1-2 stops.

    On the other hand if you're pleased with the results you get wide open and everything is working for you, stick to it I guess.

     
  23. Ian Grant

    Ian Grant Subscriber

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    For many years I'd have agreed with Ralph, and that's how the lenses I used behaved in the 60's and early 70's.

    However at some point I had to use a Rokkor lens wide open for some 20x16 prints and was amazed that the grain was sharp from the edges/corners through to the center. My Schneider Componon's are the same.

    There may be a slight difference but unless you're printing Technical Pan (or similar) it's difficult to spot. Modern enlarger lenses have excellent flat field optimisation so should perform well wide open.

    Ian
     
  24. Bob Carnie

    Bob Carnie Subscriber

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    It should be pointed out that I am only referring to grain sharpness within the print. I do understand falloff , abberations at the edge,of large prints can be problematic.

    I too close down the lens, mainly for optimum printing times,

    but if we are talking about focusing on a nose of a subject, being grain sharp then printing wide open, then closing down without refocusing and adjusting time, and then telling me the nose is sharper of a subject on the second print , I will start making the stew of my shorts.

    Its really simple make two prints,
     
  25. ic-racer

    ic-racer Member

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    Yes this holds true by the MTF charts. When looking at the MTF charts, only go out to 50% on the X axis. Same with the light falloff chart; only go out to 50%. Then it is evident that larger apertures will be OK in this case because you are only using the center of the image circle. But, as I indicated earlier you really need to look at the MTF cart of the lens in question rather than making generalizations. For example, the Rogonar-S MTF chart posted earlier shows different characteristics than the Schneider lenses I use.

    Also, a little clarification and explanation of MTF charts. We know from physics that the lens will have the least diffraction wide open. And it can be that the 'Line Pair Per Millimeter' readings on the center of the field will be greatest when wide open. However, the contrast of those line pairs will be suffering. This is one of the reasons for going with the MTF chart as it takes into consideration the contrast. So, that is why we see the center of the Rogonar-S lens produce a better MTF when stopped down but that characteristic should not be applied to all enlarging lenses. (See below).
     
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  26. ic-racer

    ic-racer Member

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    Just to show that you shouldn't generalize comments about enlarging lenses, here is the MTF of a lens I use. It shows the following:
    1) Wicked sharp in the center even when wide open f4 (probably better coated than the Rogonar S and smaller lens elements)
    2) Sharper at the corners at F8 than F5.6 BUT...
    3) If you only use the center of the lens (ie use it for a smaller format negative) it will be better overall at the wider aperture (what Bob is saying).

    [​IMG]
    The 3 graphs across are F4, F5.6 and F8 in that order.