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Thread: Lighting -- What am I doing wrong?

  1. #11

    Join Date
    Nov 1998
    Posts
    339

    Lighting -- What am I doing wrong?

    Don't forget the bellows factor!

    Also, you could be getting some short-exposure reciprocity failure; the flash duration may be 1/1000 or shorter.

    I know your initial reaction is to buy a powerful flash, but in working with l arge format and tabletop subjects unless you spend a huge amount of money on a h igh-power pack you'll still be working with multiple pops, just not so many of t hem. By high-power I'm thinking of 2400 w/s or more.

  2. #12

    Join Date
    Jan 2001
    Posts
    522

    Lighting -- What am I doing wrong?

    Jennifer, as david says it should be 8 pops at f32...FWIW, we do most of our studio shooting at work in terms of mulitple pops...we just shut off the modeling lights, and do the multiple pops in the dark with the lens on T...sometimes we don't even close the lens, just change holders, wait for things to settle out & then continue...and we will shoot on location this way as well, if we have to either by "beating" the available light with the strobes, or by covering the lens during recycling...you can never have enough power when you shoot 4x5....we use 2 2400 ws speedotron packs (blackline), and an 800....multiple pops are a convenient way of bracketing as well...but, you do need to take bellows factor in effect, as well as the subject matter....for instance, glass bottles etc...will be hard without a modeling light...not a problem for exposure really, but more of a general lighting problem. I have a Dynalight 1000 ws pack myself, and while this is great for small & medium format, when I use it for tabletop 4x5 work, I get into many multi-pops...like a minmum of 8 at least, for a basic shot...so, I agree that you might need at least 2400 ws minimum if you do this alot....we also don't use any fancy meters, just basic Sekonic models, all you need is to get the basic exposure, you can figure the multiple pops out from that...but having a flat disc diffuser, and knowing how to use the meter is important, as well as using Polaroid....

  3. #13

    Lighting -- What am I doing wrong?

    I really appreciate the feedback from everyone, this has been great! I calculated the bellows extension factor, which came to 1.1 since I have 22cm of extension and a 210mm lens (22 squared divided by 21 squared).

    I ran a new set of tests. First I set up my strobe so my light meter in single-flash mode registered an aperture of f11. I took an exposure -- looked pretty good. I skipped f16 and took another at f22 with 5 pops (it takes 2 to get to f16 plus one additional pop for every four pops, per Ellis). Again, it looked pretty good. I took one last exposure at f32, using a total of 10 pops. Again it looked pretty darn good. To accomodate the bellows factor I actually used approx. f20 and f29 instead of f22 and f32.

    I know I haven't yet accounted for reciprocity failure, but at least the subject matter is identifiable, which is a far cry from where I was last night.

    And, I'm still looking forward to getting a set of studio strobes for faster recycling times and more sophisticated lighting setups.

    Thanks again to all.

  4. #14

    Join Date
    Jul 1999
    Posts
    84

    Lighting -- What am I doing wrong?

    Since you are shooting polaroid and are in an experimenting mood, how about using multiple pops to fill-in the shadows? You already got a picture with correct exposure (and ugly shadows) using a single pop. Well simply add another pop from a different direction to fill-in the shadows. This might give you a more interesting look too, sort of simulating a multiple strobe setup. Good luck! (And how about posting your photo here once you done ;-)

  5. #15

    Join Date
    Oct 2000
    Location
    Dorset, UK
    Posts
    50

    Lighting -- What am I doing wrong?

    Are you sure about that bellows extension? Unless your 210mm lens is a telephoto then 220mm of extension should get you into focus at around 4.5m or 15ft. That doesn't sound like a table-top shot of a wine glass and bottle to me!

    As others have said, this definitely sounds like a metering and/or reciprocity problem. You seem to be combining flash and ambient light and depending on the relative intensities that would very likely explain the strange scaling. I would definitely try to maximise the light from the flash by bouncing it off a reflector much closer to the subject.

    If you can shut out all ambient light then the multiple flash thing should work easily enough, but I would just meter one flash and recalculate the number required for the f-stop in use using the doubling sequence. Even then, I'm still not sure how well Polacolor Pro 100 handles a large number of low intensity flashes (not something I've tried with it).

  6. #16

    Lighting -- What am I doing wrong?

    Huw, quick question for you. How did you determine that I should get into focus around 15 feet? When I did the bellows factor calculation I did think that that seemed quite short, but I just went back and looked at the camera again, and it is definitely set at 22cm.

    While the image isn't as sharp as I would like (and another thing to work on), the focus is fairly good. Could the fact that I'm using some tilts change how far my bellows needs to be extended?

  7. #17

    Join Date
    Oct 2000
    Location
    Dorset, UK
    Posts
    50

    Lighting -- What am I doing wrong?

    Jennifer, the standard optical formula relating focal length(f), subject distance(u), and bellows extension(v) is 1/f = 1/u + 1/v [put another way, that is u = fv/(v-f)]. That is for a 'thin' lens, but it works fairly accurately for typical LF lenses, which have multiple elements and groups. You measure the distance from the film plane to the optical centre of the lens, or where the aperture is. It doesn't work for telephoto designs, but I imagine your 210mm is not one of those. If it is a telphoto that could account for the apparently short extension. In that case I would calculate the exposure compensation by considering the magnification at the film plane - if the magnification is m, then multiply the f number by m+1 to get the new effective f number.

    If, as I assumed, it isn't a telephoto then I am mystified. The tilts shouldn't make too much difference, although the level of illumination will vary very slightly across the film plane. Simply measuring the distance between the film plane and the centre of the lens should still give a result that fits the formula fairly accurately.

    I hope that helps, but I've a feeling that the mystery is only deepening - I can certainly see why you were beating your head against the wall!

  8. #18

    Join Date
    Jan 2001
    Posts
    522

    Lighting -- What am I doing wrong?

    I don't know if this is going to matter or not, but I've never had much luck with accurate proofing on color polaroid materials...I've found that the b&w films were much better in terms of speed matching...so unless polacolor itself is the final product, you may have better luck with type 54 or 55....type 55 is nice because you can check focus on the neg easily, and the short range of the print matches up to transp. film well...but like I said, forget this if the polacolor print is what you want.

  9. #19

    Lighting -- What am I doing wrong?

    The end goal is actually a transparency. I'm using the polaroids for doing things like checking shadows, verifying approximate exposure, checking composition, and checking focus. I've read to expect the colors to be off slightly and the exposure won't match quite right either (I can't remember which way the exposure will be off and I don't have my notes in front of me), but it is a good tool for getting a good overall sense of how the final photo will turn out. Is this a fair statement? Since I think sharp focus will ultimately be my real challenge in large format should I be using a different type of Polaroid or just bite the bullet and start shooting and exposing more transparency film?

  10. #20

    Join Date
    Mar 1998
    Posts
    1,972

    Lighting -- What am I doing wrong?

    In general I find the Polacolor Type 59 to be about 1/2 stop faster than any ISO 100 color transparency film. And Polacolor is lousy foir exposures that require reciprocity time corrections.

    try this too but it might be a moot point since you are not synced to the shutter but assuming you have a pretty solid camera sitting on a pretty solid tripod & head you might try doing . Instead of leaving the shutter open for long periods, do a series of short exposures, the # in the series obviously linked to the number of pops you need. Since you are not sync'd , try doing a series of 1 sec exposures during which you fire the flash by hand. TYes you'll have to recock the shutter between exposures but assuming you don't kick the tripod this should work and you'll also cut out any low level ambient light from being recorded.

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