How to use a densitometer?

[carioca]

I have finally bought a densitometer for evaluating and linearizing my digital negs for carbon printing. It reads out the typical density values, but what would be the best approach for translating them into % (or 0-255 )values for the correction/linearization curves?
Thanks for any advice.

Sidney

 

[Joe Lipka]

Is it a reflection or transmission densitometer? To calibrate my digital negatives I have a photoshop file with fifty different patches. I make a digital negative of this patch and then make a print. I measure the print densities and then change the adjustment curve until the screen reading and dry print match.

I have an X-Rite 400 reflection densitometer. I have found that the % dot area scale of the densitometer is the same as the percentage gray scale in photoshop. The goal is to get the percentage gray scale in photoshop to match the reading on the densitometer within a couple of percentage points.

My web site has a section which covers this in a really general way.

 

[carioca]

Dmax values / OHP / QTR

Hi Joe,
thanks for your reply.
I'm using a transmission densitometer x-rite 361T.
From what I've read so far, using all inks of a printer would result in smoothest tonal gradations and minimum grain.
I had been using PDN for a while, but my 'ideal' blocking color (green+blue) showed too much grain for my taste.
My goal therefore would be to use all inks (composite black) for 0-100% of the greyscale, that's why I'm tempting the QTR approach which seems to have a good curve control.
Obviously, the different inks do not reach up to Dmax (my ink Dmax needed to obtain paper white) except K and Y.
I see a lot of testing and math coming my way.....

When I print my 2115 Stouffer stepwedge, I get steps 1-20 printed perfectly, replicating this with digital negs is a big challenge!

Sidney

PS: I have checked the DOT option on my densitometer, but probably being transmissive, shows unusable values since there is no dot pattern on my test negs.

 

[Neil Poulsen]

I have an X-Rite 400 reflection densitometer. I have found that the % dot area scale of the densitometer is the same as the percentage gray scale in photoshop. The goal is to get the percentage gray scale in photoshop to match the reading on the densitometer within a couple of percentage points.

Please check my logic.

My densitometer reads in standard logarithmic density units. In his book, The Print, A.A. defined these units as LOG10(1/R), where "R" is the proportion of light reflected off the surface. (In this case, the printed paper.)

If "Rw" is the proportion of light reflected off the (white) paper and Rb is the proportion of light reflected off of maximum black, I'm thinking that "100%x(R-Rb)/(Rw-Rb)" would correspond to the "% dot area" that Joe mentions. This number goes from 0% to 100% as the printed surface of the paper goes from maximum black to maximum white.

Does this seem reasonable?

If so, then it's algebraically a simple matter to calculate this number, given density units D of a patch, Dw, and Db.

 

[Joe Lipka]

neil and sidney - Some of our more technically gifted should be involved in this discussion. I don't believe digital negatives fit well in the world of the transmission densitometers because digital negatives require a "blocking color" to make up for the lack of density provided by the ink pigment itself. The density of the digital negative is composed of both a color component and ink. Adding the complicating factor that pigments and inks behave differently in transmitting UV light using a transmission densitometer is a real problem.

Using a transmission densitometer for a digital negative prepared from a image setter that would output graphic arts film would be a reasonable expectation. But using a reflection densitometer for a digital negative (for me, at least) would be a very difficult thing.

My choice for the reflection densitometer came from the very simple thought that it makes a direct comparison between the initial image and the print. It answers the question "Can I make a final print that has the same tonal separation as the image on the computer monitor?" If it doesn't then the only thing I change is the shape of the curve and that is the only thing I change until the two match.

One other thought on avoiding the use of the transmission densitometer for digital negatives. I would assume that you have a transmission densitometer because of its' association with the Zone system. Many photographers (me included) lived and died with the zone system negative densities for our negatives. The digital negatives are different in that they are less flexible than a film negative. Remember you make a digital negative to match a specific set of printing parameters. We fix the sensitizing solution, the exposure time and developing and optimize the negative to meet those print characteristics. With the Zone system we do the opposite. Aim for a specific negative density and then fiddle with the paper, developer, dilutions, time, temperature and anything else we can think of.

Hope this helps explain some of why we do what we do. If Sandy or Michael chime in with some real solid technical help, we should be able to answer your questions specifically.

 

[Neil Poulsen]

Reflection Densitometer

Hi Joe, I was thinking in terms of a reflection densitometer. It sounds like you get a percentage reading, whereas I get standard density readings on my reflection densitometer. So, I was seeking input on a way to transform my readings into something that corresponds to the percent that we see on our computer screen.

I think there's an interesting contrast between the what we do with digital negatives and what we do with the zone system. With the former, we calibrate the system. With the latter, we calibrate ourselves by internalizing a feeling for what a Zone III and a Zone VII (or VIII) looks like.