Questions about scanning B&W negatives: 2-humped camels and other beasties

[guyburns]

The background to this is that I'm scanning B&W negatives from the 1950s through to the 1970s – 120-format negatives either 6x9 cm or 6x4.5 cm. I'd like to know why some of these negatives have certain characteristics. They're amateur, and quite a number are underexposed because the photographer was not using a light meter.


TWO-HUMPED CAMELS
Quite a number of the negatives have a deep valley in the histogram, what I call the "2-humped camel" look. Why is that? This valley shifts around. Sometimes it's at one end of the histogram, sometimes at the other end, sometimes in the middle. Is it due to the original film, the way it was developed, ageing, heat, the lighting conditions? I'd be surprised if it was the lighting because it happens in a variety of situations. Camels are quite common in these negatives and I'd like to know why.


Black Beetles
Sometimes Black Beetles appear with the 2-humped camels. Usually only in seriously underexposed images, these beetles appear as tiny black spots where there would have been shadows, but there is very little gradation to the surrounding parts of the images, and they look just like tiny black beetles spread over the landscape. In positives otherwise bereft of blacks, why do these little black spots appear?

Three Positives showing the above can be downloaded here. Two have been corrected for maximum contrast (but without loss of levels), and further corrections will be required, while the other is an uncorrected original. Note that the Black Beetles are not as obvious in the uncorrected scans, as they begin life as "Gray Beetles" amongst an almost white image.

Any suggestions most appreciated.

[EinstStein]

I have observed similar phenomena.
Not sure I can answer your questions, and not sure I am all right, but this is what I observed and guessed when "scanning" negatives using digital cameras.

1: The histogram directly out of camera has unaligned RGB histogram. I believe during the negative to positive calibration the RGB value will be weighted and merged.
2: The RGB channel merged algorithm for positive images are something like (0.21 × R) + (0.72 × G) + (0.07 × B). This is certainly not for "scanning" negatives. If you blindly convert the color negatives to B&W negatives or B&W positives, your channel weighting for merging the RBG matters. Depends on the weight, you can see three peaks, 2 peaks (your camel humps), or 1 peak.
3: Typical B&W negative film has a built-in spectrum sensitivity (the chemistry channel weighting). The histogram is not necessary a gaussian curve.


   

[Doug Peterson]

Are you following the industry standard procedure outlined in Digitization Workflows: Transmissive Material or your own ad hoc workflow? If the latter, are you using a linear capture curve, or a default curve that adds contrast?

[EinstStein]

I use Hasselblad CF digital back +  Phocus. Hasselblad Phocus has an easy command to handle the negative films, both color and B&W. This is how it works.

1. Tether the camera to MAC with Phocus, take the picture.
    -- or, instead of tethering, you can shoot the picture first,
        then import the raw files into Phocus
2. In Phocus,
    -- enable the "reproduce" command in the export pull-down
    -- select the export output to "Negative"
        (change the default "standard" to "negative")
        it will show positive mage if the film is negative.
    -- Do white balance ( I use the droplet to pick the grey region, if any)
    -- the image is now nice looking positive. No brainer.
    -- of course, you can do further twea, but it is optional.
3. Export the image to TIFF format (or JPG)
    -- Do not export to DNG format.
    -- DNG or any raw format will lose all the post-processing adjustment.

[Doug Peterson]

For clarity, my question was to the OP who is having issues.

[SergeyT]

"TWO-HUMPED CAMELS"

 There is plenty of "bright" sky in each, so it it will make for the second hump.
 The first one come from the rest.
 I would not worry about histogram looks. It is just a tool that hints about value distribution. For as long as the image looks good to you the shape of histogram should not matter.

"other beasties"

It is not clear how you scan.
The black spots could be a result of clipping in the scanner firmware\hardware and\or errors in colospace conversions.   Try to scan as you see the negative (as a positive) in 16 bit with no tonal adjustments and clipping and do all the conversion and adjustments in your edition tool of choice)
It could be that the black spots are the areas where nothing was recorded on film (clear base) due to insufficient exposure at the time of picture taking. If that the case - nothing can be done about that.
Again before being to concerned about it I would seriously think about the final purpose of digitizing. Is it for exhibition quality prints or just to preserve memories. If the latter - the examples look good to me as they are.


SergeyT

[yaya]

Some good advice from Doug and Sergey,

I'm assuming you are using an RGB scanner/ camera for capture so I'd recommend capturing as much linear data as possible and trying to avoid clipping. For WB I'd use the film base (the area that hasn't been exposed, outside the frame.
Then when doing any adjustments, you have to work inside the crop. That way the histogram will ignore the frame which is typically black.
Work on the R,G,B curves separately to a) maintain balance and b) avoid further clipping.
If done correctly, you may not need to convert to B&W at all.
But do read the guide that Doug has pointed to. It will teach you a very good practice for this type of projects.
BR
Yair

[guyburns]

Thanks for the replies. I should have clarified how I scan in my original post. I am scanning with an Epson V700 flatbed, at 16-bit, 4800 dpi, RGB, with all corrections turned off, linear capture curve, and full contrast range.

Archiving
I archive the negative scan at full resolution RGB in jpg2000 format, between 80-300MB usually.

Working Images
For working purposes I convert to grayscale tiff (sGray), with several non-destructive adjustment layers for cleaning, correcting contrast, and adjusting shadow detail. These negatives are destined for Blu-ray, so I have chosen sGray as a close match to the gamma of the Blu-ray colour space. It's not exact, but close enough for my purposes.

The green channel is typically sharpest (I always check), so when I convert to grayscale I discard the red and blue channels. Usually I down sample to 2400dpi for the working images. Just before being sent to Premiere, where these images will become part of an audio-visual, I generate a third file, a 1080P jpg.

It's a long complicated process. Each image takes about an hour to scan, archive, edit, index, and create a version for Premiere. Then after all that, maybe another 30 minutes per image getting it into, and manipulating it inside Premiere.

It's the valley, not the humps
Another thing I should have made clear: it's the valley I'm interested in finding out about, not the two humps. Why are a large number of levels effectively missing from these negatives? What's the cause? I've rarely, if ever seen a valley in scanned slides.

This is not a question about how I fix the valley, or about scanning. I'd just like to know what causes that valley. Probably the only people who might be able to explain these valleys are those who have been extensively involved in B&W developing and printing. To me, it seems that a chemical explanation is required.