spot meter as a REFLECTION densitometer on the cheap

Discussion in 'B&W: Film, Paper, Chemistry' started by kwmullet, Nov 27, 2004.

  1. kwmullet

    kwmullet Subscriber

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    There's a good handful of descriptions at various places on the net as to how to use a spot meter as a substitute for a transmission densitometer in measuring negatives. I've looked all over the place and haven't yet found any descriptions of a good method for using a spot meter as a cheap REFLECTION densitometer for measuring prints. I tried measuring contact prints of a step wedge, but as the angle of measurement changed as I measured different spots in the same step, the EV value changed.

    Has anyone come up with a good method for doing reflective densitometry with a spot meter?

    scratching my head...

    -KwM-
     
  2. Bob F.

    Bob F. Member

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    Not tried it, but making a foamcore or thick card (or plywood if you want something more permanent) holder for the meter so it is always at the same angle should do it. Keep it on the holder with a couple of elastic bands. As long as the light source is constant, with the holder keeping the meter at the same angle, it should work. The BTZS book shows a similar, but more substantial idea.

    Cheers, Bob.
     
  3. pelerin

    pelerin Member

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    I took a class 10+ years ago that was based on Davis's BTZS principles. All the sensitometry work was performed using a Pentax analog spot meter. The meter was mounted in a hand fabricated plastic cradle. The cradle also held a closeup filter so that step wedge sections were in focus when you read them. A work lamp provided the illumination and the collar that held the lens of the meter had a rotating section that either opened or blocked a port that allowed for reflected metering. You then converted EV to log D by referencing a chart. I'm pretty sure that all this was modeled after the directions in the workbook that came with the first edition of BTZS.
    Some options:
    1)find a copy of the BTZS workbook which I'm pretty sure is out of print, (maybe try on the BTZS board)
    2)failing that I'm pretty sure that the doohicky that I'm describing still exists and I could probably get you a photo
    3)buy a used xrite reflection desitometer from a print shop that is going out of business or going digital. This was my option. The densitometer was very cheap, less of a pain to use and required no construction labor. YMMV.
    Good luck.
     
  4. Donald Miller

    Donald Miller Member

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    I don't think that a spot meter will give the degree of precision that I would want to determine reflection density. For instance, the Pentax digital that I have will only get as near as 1/3 stop. If I missed the reflection density by 1/3 stop at the low value and missed it by 1/3 stop at the highlight end then this would translate to .20 in exposure scale variance under the best of circumstances.

    To take this a step further, I think that this best case scenario is unrealistically ambitious.
     
  5. doughowk

    doughowk Subscriber

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    Phil Davis Beyond the Zone System (my copy is 4th edition) has an Appendix section on building a reflection/transmission densitometer with a Pentax spot meter. The working drawings are quite detailed.
     
  6. gainer

    gainer Subscriber

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    Spot meters have been known to suffer from flare, which will tend to "smear" the measurements. This is more likely the cause of variation of readins along a step of constant reflectivity than the angle, especially if the step is on a diffusely reflecting material. Some years ago there was an article in Photo Techniques that showed that flare, but I can't remember when, or who wrote it.
     
  7. argentic

    argentic Member

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    No it's not.
    The latest edition of Beyond The Zone System can still be ordered at www.focalpress.com. "Beyond the Zone System (4th Edition), Phil Davis, £37.99, ISBN 0-240-80343-4".
     
  8. JHannon

    JHannon Member

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    Our own Dr Bob wrote a very good article on using a spotmeter to measure density.

    Here is the link: Spotmeter Article

    I was getting usable results using a spotmeter with a light table. I used a series of adapters on the meter to get it to fit on a reverse mounted Pentax 80mm lens. I added a rear lens protector then drilled a hole in the center of the lens protector. The reverse mounted lens was to get it to focus closely.

    I hope it helps,
    Regards,
    John
     
  9. Deckled Edge

    Deckled Edge Member

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    How about using a different piece of hardware for densitometry--a scanner? I have not done anything like formal testing, but I use SilverFast software when scanning prints. If you use the straight line algorithm when scanning, with no manipulation, you should get an accurate representation of your print. SilverFast SE 6.0.2 has a densitometer. I have played with this, and it seems useful. Does anyone have experience using a scanner to formally analyze your dried-down prints?
     
  10. kwmullet

    kwmullet Subscriber

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    John,

    Great article recommendation. I'll certainly go through it in more detail when measuring negs, but in this particular instance, I'm looking for a way to do REFLECTION densitometry of a print, not TRANSMISSION densitometry of a negative.

    -KwM-
     
  11. gainer

    gainer Subscriber

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    I have done this with limited success. The response of the scanner is not logarithmic if you read the values as they show on screen. It is possibly, but not essentially, that it is a linear scale. You can test that by scanning a reflection step wedge and plotting the values on semilog paper or by plotting the logarithms of the values on linear paper. If you are lucky the result will be a straight line. You will see that the readings are integers, so the resolution of density will get poorer toward the shadow end of the step wedge.

    The numbers I get from any of several programs including the Photoshop LE range from 0 to 256, which is only 8 bit precision. This is sufficient at the upper end but pretty poor in the shadows.
     
  12. kwmullet

    kwmullet Subscriber

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    Yep --- I've read a few accounts of folks using a scanner to do what I want to do. I'd just rather not have to start a project on a computer every time I want to read a print density.

    Although I also want to measure dynamic range on finished prints, the specific thing that got me started on this is that I'm trying to get into a mode where I replenish my developer tray rather than dump it each day -- especially when I have several days in a row where I print. The thought of dumping developer that's still got a lot of capacity left, said developer being one of the major costs of a printing session, was starting to be a disincentive to doing a short print session.

    I switched from a powder-based (Dektol) to a liquid concentrate (Ilford MG IV) developer, then filled up a 4x5 sheet film box with pre-exposed contact prints of a 21-step test wedge. I keep the first of these "test strips" developed in that chemistry as a benchmark, then at the end of each session, I cover the developer and fixer by setting a sheet of freezer paper on top, keeping most of the air out.

    At the beginning of each session, I test the fixer with Hypo check and do a film clearing test, and I process one of my canned test strips. As the developer begins to get exhausted, the set of dissernable steps starts shifting to the left. At that point, I'll add 10-20 ml of and run another strip to get the scale back to the benchmark. (I use big 16x20 cement mixing tubs instead of trays, and use about 3L of solution in the developer and fix tray, and 4L of solution in my cheap-as-dirt single-use citric acid stop tray).

    Right now, I'm just visually comparing the strips, but I'd like to get to the point where I can get the gamma from each one, plot trends and have a real controllable process. Yeah, I'm trying to reproduce what I used to do with roll film processors when I was in the Navy.

    -KwM-
     
  13. Deckled Edge

    Deckled Edge Member

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    I see what you are saying--256 is 2 to the 8th. Therefore, the difference between Zone 8 and Zone 9 is 128, but the difference between Zone 1 and Zone 2 is only 2. On the other hand, this gives a reproducible, recordable number, while the analog meters have many more variables (and, while they are set up logarithmetically, they suffer most inaccuracies at the near and far ends of the scale).
    Question: Did you buy or make your reflectance step wedge? We are all familiar with the Kodak transmission wedges. I think mine is half-toned and would be of limited usefulness, even scanned as a transparency, in this setting.
     
  14. lee

    lee Member

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    maybe kevin you might like me to come up and bring my densitometer? I have a little portable job that if you are not using a staining developer will make your life easier. This is only for film as it is a transmission only unit it is an X-Rite.



    lee\c
     
    Last edited by a moderator: Nov 30, 2004
  15. kwmullet

    kwmullet Subscriber

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    Hey Lee,

    Wonderful offer & I might take you up on it when I get around to doing curves on film. Right now, though, I'm focused on trying to find the best way to read my step wedge contact prints.

    Thanks very much,

    -KwM-
     
  16. Donald Miller

    Donald Miller Member

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    If you want reflection density readings, I will read them for you. IM me off post if you are interested.
     
  17. kwmullet

    kwmullet Subscriber

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    Thanks Donald, but my project right now for reflection densitometry is to read test strip prints I make before each printing session as a guide for how much I should replenish my developer tray.

    So far, the most workable thing I've heard is the suggestion to scan it. Once I pull my test strip print from the fix and give it a quick hot wash, I can dry it in 40 seconds in my microwave and scan it on my scanner. I was resisting the idea of pulling a computer into my process for anything but music and reading APUG during dead time, but it's beginning to sound like scanning is my best option. I realize the accuracy won't be all that great, but for the purposes of doing process QC, the scans need only be consistent with each other, not with any kind of real-world benchmark.

    (side issue -- how does one actually measure drydown? Surely, people aren't putting wet prints on their densitometer. Is it strictly by eyeball?)

    Besides, I'd end up doing my control charts in excel anyway.

    Later on, I'll get a real densitometer to do real sensitometry. After several visits to eBay, I'm drawing the conclusion that two cheap/old single-purpose (one transmission and one reflection) densitometers would probably be cheaper than one dual-mode densitometer.

    cheers,

    -KwM-
     
  18. Kirk Keyes

    Kirk Keyes Member

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    KwM - that article has all the info in it you need to do this. But instead of backlighting a negative to get transmission values, front-light your control strip prints and then read them. This will give you the data you need to calculate reflection densities.

    And for what you are trying to do, you don;t need to worry about a fancy bracket to hold the meter. Just using your hand-held meter will get you there. A close-up filter may be nice, to get the focus right, but if your print areas are large enough, that may not be necessary either. As to flare, you can block out extraneous light from your subject and meter to minimize that too.

    Kirk - www.keyesphoto.com
     
  19. argentic

    argentic Member

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    I bought my Gretag reflection densitometer for $ 40,- on a photo fair. And with a little patience you can find one that cheap on Ebay. I wonder now why I put up with my BTZS modified spotmeter-densitometer for so long.