Finding a Densitometer

Discussion in 'Film and Processing' started by jim_gardner|4, Mar 1, 2021.

  1. I have decided it is (long past) time I did some tests with a Densitometer. Looking on the web, I see they are from a bit under £1000 up to £6000. Does anyone know of a type / sales outlet that makes something at a more reasonable price point or, is there anyone in the UK that would be prepared to lend/rent one out?
     
  2. I would suggest that you make friends with a local photofinisher (one hour photo shop). They have densitometers which they should be using daily to test their processes. In UK, many supplied by Noritsu, find their address and email and inquire about price of their densitometer.
     
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  3. Look for something like an Xrite 810 on eBay.
     
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  4. Since posting I have looked in some of my books to see if there is another way of testing. I found in AA "The Negative" that; Value 1 should have a density of 0.10, Value V a density of 0.70 etc. This is clearly a difference of 0.60 He then goes on to say that a spot meter can be used and that each 1/3 stop exposure interval on the meter will correspond to a density increment of 0.10 From that I would have thought 1 stop would then equal a density increase of 0.30. How then is it that he states the value difference between V1 and V5 is a density difference of 0.60, which he also says above in , blue is 2 stops, when we have learnt it is a difference of 4 stops?
    Confused? I certainly am.
     
  5. The difference in the scene of 4 stops results in roughly 2 stops of density on the negative.
     
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  6. AJG

    AJG

    I used the 1/3 stop change on my Soligor spot meter with a good light box to determine Zone 1 when I was calibrating my film speed/developing times and it worked fine. While I firmly believe that understanding the process is useful and important, sometimes the best thing to do is to follow directions from someone who clearly knows what they are doing.
     
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  7. I have a densitometer that I got from Goodwill, but haven't got working
    (or tried very hard). I think it needs a new mirror which reflects the light
    though the hole, and then the negative. Well, it cost $10, so I decided to buy
    it and figure out later what to do with it.

    I did learn some about how it works, though. The current from a photomultiplier
    tube is proportional to light intensity and exponential in applied voltage.
    So, there is a circuit to run it at constant current, so voltage is logarithm
    of the intensity!

    Anyway, you might look at the used market. Much darkroom (or not)
    photography equipment is being sold off, and you might find one for
    a low price.
     
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  8. If you look at the characteristic curves you will see this depends on the development but even at highest contrast development the slope is still less than 1.
     
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  9. C41 negatives, including XP2, usually have gamma close to 0.5.

    Black and white might be 0.6 or 0.7, though push processing can
    get close to or over 1.0.

    Well, the data sheets have both the characteristic curve, which usually are
    curves without the long straight section that you hope for.

    Then a contrast index, which is the slope of a line between two specific
    points on the curve. They show both of them.
     
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  10. Density is a measurement of the blackening.
    We measure the blackening of film which is a result of exposure and subsequent developing, using a unit called “density”. The root of this value is a delta of 2X. In other worlds, a change in exposure of 1 f-stop is equal to a doubling of halfling of density. By tradition the density unit is logarithmic. A 2X change in density equals 2^0.3 (2 elevated to the 0.3 power). In ordinary math notation this is 0.30 units of density. It goes like this: 1/6 f-stop = 0.05 units of density – 1/3 f-stop = 0.10 – ½ f-stop = 0.15 – 2/3 f-stop = 0.20 – 1 f-stop = .30 – 1 1/3 f-stop = 0.40 – 1 ½ f-stop = 0.45 – 1 2/3 f-stop = 0.50 – 2 f-stop = 0.60 – 3-f-stop = 0.90 – 4 f-stop = 1.20.

    This method dates back to the turn of the 20th century. Photo scientist used log notation because before the calculator, they used the slide rule. By the way, a gray card with 18% reflectivity reads 0.75. This is zone V, the calibration point for exposure.
     
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  11. But if you can get something like the Xrite 810 would be nice.
     
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  12. The reason the 810 is nice is because it does both reflection and transmission. I think it also gives color values. I remember those huge old Macbeths where you needed two separate units for T/R.
     
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  13. I think my favorite named law that I learned about in college is Beer's law.
    (How can it not be fun if it is named after beer!)

    That is, that transmittance is exponential in absorbance,
    and absorbance is proportional to path length, and to
    concentration of attenuating material.
    (It seems that it was discovered in wine, and not beer.)

    If one filter lets through 10% of the light, two such filters will
    let through 1% and three 0.1%. So using a quantity that is
    logarithmic in transmittance makes sense.

    By the way, SPF on sunscreen is not exponential. The actual
    amount of UV absorbing chemical is log(SPF value). Think about
    that when buying it. Usually, though, pricing is close enough to
    log(SPF), as maybe it should be.
     
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  14. There was a lot of discussion, some years ago, about using scanners as densitometers. There are too many caveats to go into here, but it's certainly feasible. Search for 'scanner as densitometer' or something similar.
    You're obviously looking in the wrong place.

    Small handheld pencil-type densitometers were used for analysing medical X-rays and personal radiation-monitor badges. There should be piles of those things lying around unused these days.

    Search for a Sakura PDA-81, or a Wogan Radix. If you're lucky the seller won't have a clue what they've got.

    I'm not sure what you hope to gain by using a densitometer anyway. Just expose some film at box speed on an 'average' subject, and develop it according to the recommended time and temperature. You should end up with a roll of negatives that have no more than a Dmax of 2.1, with the printable highlights hovering around 1.8D above base+fog.

    If what you're getting doesn't look like these reference negs, then it's time to alter your exposure or development regimen.

    Obsessing about densities and 'calibration' is just a waste of time. The guys (or gals) at Kodak, Fuji and Ilford have done all that tedious stuff for you. As long as you don't go off piste and try to reinvent the wheel.... which incidentally isn't a very useful mode of transport off piste!

    P.S. I just did a quick search.
    There's one here for 43 quid.
     
    Last edited: Mar 2, 2021
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  15. Real life brightness values don't have a 1:1 relationship with the density of the developed film.
    A typical negative has a 'gamma' of 0.55 to 0.65. So you have to multiply the exposure stop differences by about 0.6 (and then by 0.3) to get log density differences.

    So 4 stops exposure difference will give 4 x 0.18 (0.6 x 0.3) = 0.72D difference in density - approximately. It's approximate because the slope of film gamma varies depending on where you measure it. That's what H&D curves tell you.

    If you're not familiar with film curves showing density against log exposure (H&D curves), then a densitometer will be useless to you until you learn about them.
     
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  16. I purchased my Kodak model 2 A.K.A. X-Rite 811 with calibration scale on eBay back when photographers were offloading them due to the Digital revolution. I think I paid less than $200 for mine. If you know how to use one fine, other wise you might have to download the X-rite 811 manual and spend a weekend reading it. It is not easy reading...
     
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  17. You're not wrong there. I read the instructions online. Phew!
     
  18. And that's the problem. Without a Densitometer I cant tell what the Dmax or densities are. Will keep looking on the web though, thank you.
     
  19. I have had a lot of help with this, particularly from Alan Marcus who was kind enough to PM me. I think without a densitometer and the knowledge of its usage, I cant do much more than stick with what I am doing.
     
  20. Here are two simulated density wedges. One in sRGB 'space', and the other in Adobe RGB.
    Density squares.jpg
    If you place your negative over a full-white area of your screen you should be able to judge which density square most closely matches the Dmax or highlights.

    If any area of your negative is denser than any square shown above, then you need to drastically cut your development time. OTOH, if it doesn't have any density that closely approaches those squares, then probably more development is called for.

    A well exposed and properly developed negative should have a base+fog (minimum density) not much more than 0.2D, and then the darkest printable highlights should be a match for the 2.0 or 2.1D squares at most.

    Basically, if you can barely see a domestic light bulb through the darkest parts of your negative, it's too dense. And if you can easily read a newspaper through it, it's too thin.
     
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