Estimating print tonalities with Photoshop

[Katharine Thayer]

I need to illustrate some points about print tonality in gum, and to do so I'd like to be able to express the tonal range (DMax and Dmin) of any print in a way that I can compare prints to each other, without a reflection densitometer, which I don't have and won't be acquiring.

When I was thinking about this issue several months ago, I was reading the values from the scanned file in Photoshop, and someone told me, as I recall, that you can't do this in Photoshop because Photoshop spreads the tones for any given image across the whole spectrum from black to white, so a particular value in the 0-255 histogram, or in the %brightness in HSB, or %K, wouldn't reflect the same absolute tonal value in one image as it would in another. So given that advice I abandoned that project on the spot, thinking I'd never be able to compare tonal values across prints without a densitometer.

But now I'm back to thinking about the same question again, and now that advice, as I'm remembering it, doesn't make sense to me. Yes, if you set the black and white points to the darkest and lightest values in the image, or let the scanner do that, then it would be true that the tonal values in the photoshop file would be arbitrary, would not reflect the actual tonal values in the print, and wouldn't be comparable across images. But I wouldn't do that with a scanned image; I shut off automatic settings in the scanner and let the minimum and maximum tones fall where they will on the histogram, and as far as I can see, the histogram then reflects the actual tonal range of the print.

I've been spending rather an inordinate amount of time today with a Stouffer 12-step reflection step tablet and the old two-holes-punched-in-cards-routine, to visually judge the tonal values in the print itself, and then compare those numbers to the histogram and the brightness values of the scanned print. There is a very good match between the "% reflected" values of the darkest and lightest areas in the print as taken from the step tablet and its accompanying spec sheet, and the brightness value of the HSB scale for the same areas in Photoshop, and the 0-255 values, if calculated as a percentage. So it seems to me that the Photoshop numbers do accurately reflect the actual tonal values in the print, that those values are meaningful either as % reflected (brightness) or as approximate density values from the Stouffer table, and it's a whole heck of a lot easier to let the computer do it than trying to judge the values visually. Is there something wrong with my logic; am I missing something just real obvious? If so, please tell.... thanks.
Katharine

 

[Joe Lipka]

The percentages recorded by the eye dropper tool are the same as the dot area measured by a reflection densitometer. When you match those two (within a percentage point or two) you have achieved sensitometric nirvana.

When measuring prints, you need to start thinking like a pressman, not a photographer.

 

[Katharine Thayer]

Sorry, Joe, that didn't help; I can't translate it into terms that are useful to my understanding.

My question was as follows: as far as I can tell from looking at a couple of print samples, the % reflection estimated visually from a Stouffer step tablet (which can be easily converted into log density values) is the same for a given area on a print, as is the HSB Brightness value, as is the 0-255 brightness value (if expressed as a percentage) of the same area on the same print, after being scanned and converted to a Photoshop file; they are all approximately the same value, give or take a percentage point. So is there anything particularly wrong with using one of these values from photoshop as a reasonable approximation for % reflection, or with translating them into log density just as one would % reflection? That was my question; I can't decide if your answer was an answer to my question, or not, since your statement "the percentages recorded by the eye dropper tool are the same as the dot area measured by a reflection densitometer" doesn't mean anything to me.

My understanding of the reflection densitometer is that its main use is to simply measure the amount of light shined directly on a surface that is reflected back at an angle from that surface; the resulting number is the % reflection, yes? That number is then translated by a simple formula into optical density. It doesn't seem to me I would need to "think like a pressman" to understand % reflection and optical density, but no doubt I'm still missing something.

Thanks for the answer, even if I wasn't able to make sense of it.

 

[Loris Medici]

Katharine,

As long as you scan the same target first and lock the exposure, all the subsequent scans (color balance set to white point - without clipping) will be consistent in reflecting the actual tonal values of the prints. The key points are:

- Locking the exposure by first scanning the same target every time. (A Kodak Gray Card for instance...)
- Setting the color balance to white point, no clipping.

You have to read the manual / help files of your scanning software in order to learn how to make these settings... I use Vuescan and it gives me the necessary tools to apply these settings to my scans.

Hope this helps,
Loris.