For Iliah Borg - D100 Characteristic Curves Test

Started Apr 18, 2004 | Discussions thread
tony field Forum Pro • Posts: 13,802
H&D curve of film vs digital

fotogenetic wrote:

Would you be able to compare the tonality of the D100 with that of
Velvia or Provia using the Excel spreadsheet I created?

Your curves match what I did a year or so ago on the D100 and negative film material. The only problem with relation to film is to match the high density end of the curves with what you have in digital. However, they can be directly related with a simple linear equation on the straight line portion of the H&D curve. My basic proceedure was to map Zone 5 on film to the out-of-camera light meter reading of a grey card.

I first used a flat bed scanner to measure the density of the films. This proved to be less than excellent.

Subsequently, I went to the local university graphic art department that has a transmission densitomiter and measured the density of the film zone exposures (also try the Fine Arts department if they have a photography course - or maybe a very good pro printing shop). I then scaled the image density numbers so zone 5 was mapped to the same density as a digital camera's light meter reading (not quite prefect since meters are set for 12-15% reflectivity and not 18% that Zone 5 should be). Zone zero was pegged at the same digital density as digital noise (almost RGB 0,0,0 but I arbitrarily tried 0,0,0 and 1,1,1 as well).

After plotting (on paper) the curves, the net effect was that negative film had about 4 more stops of highlight information than the d100. The film curve was the classic H&D S-curve. The digital curve was essentiall a straight line with a slight compression in the highlights. (Humm, guess the electron wells can froth and foam

From this test, I concluded that the primary reason for the "beauty" of film images is that the contrast compression in the highlights and shadows allow a much finer tonal gradation than digital. Digital does not have the fine modulation in the shadows or highlights. The worst case is to photograph the sun in the picture and see the concentric rings of reddish tonality on digital that is represented as smooth tonality on film.

Like you, my custom curves have slight compression in the highlights to mimic film's preservation of highlight. I had not considered adjusting the curves midpoint as you did.

I basically concluded that I could directly relate the densitometry readings with digtal by using a simple factor - but I don't have those notes since I moved.

To define the exact shape of the digital vs film curve at hight and low intensities, I photographed a white 16x20 card that was lit with 45 degrees (relative to the card plane) incident light and the light was about 2 inches away from the leading edge of the card. This gave a continuous gray scale because of inverse square law. I chose zone 2 and zone 11 exposures using my Minolta one degree spot meter for the leading edge (bright side) of the images and and attempted to relate the digital and film captures. This did little more than confirm that digital was essential a "linear" log density function and that film had the classic S-curve as expected. This "proved" that digital cannot hold shadow or highlight texture. I vaguely recollect that I should have chosen zone 3 for the dark end test point to ensure that I would be on the linear portion of the film H&D curve.

Any digital experimentation with a custom full S-curve (both toe and heel comrpessions) proved useless - probably due to the lack of dynamic range of the digital camera. The only meaninful gain was in highlight compression.


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