Velvia

[Lounge Lizard]

I was thinking about colour spaces and relating this to Velvia - simplistically we say that Velvia has an extended green sensitivity. Is this simply saying that Velvia has a more extended colour space, particularly in the greens, than other film or am I trying to make this too complicated by comparing digital colour with the spectral response of film?

[Fen]

am I trying to make this too complicated by comparing digital colour with the spectral response of film?

Yes!

Go out and take some photos

[Lounge Lizard]

Sorry, but I've gone big on colour management and it's a lot to get straight in one's head.

[Fen]

Your post scared me at first...

I thought it was about Volvos!

[Lounge Lizard]

Your post scared me at first...

I thought it was about Volvos!

I'm not sure if anybody has been knocked off their bike by a roll of Velvia.

[Benchmark]

I would have expected the spectral response of a digital sensor to be more linear than film, although I guess that depends on the filters used, and whether processing algorithms are used to produce certain effects..

I tend to think of colour space rather like an elastic band. You can stretch it to include an almost unlimited number of colours, but as you do so, the information about each colour in the gamut gets thinner and thinner.

So for example, RGB (as used by photographic papers) has a very limited colour gamut, but there can be a huge amount of information about each colour (which we see as shadow and highlight detail).

OTOH, modern inkjet printers like my Epson R800 use eight inks to provide an incredible colour gamut, with bright, intense colours, but they do not represent the subtleties of those colours as well as RGB.

I could be completely wrong on this, but that is how I see it.

[Benchmark]

Your post scared me at first...

I thought it was about Volvos!

I'm not sure if anybody has been knocked off their bike by a roll of Velvia.

Airport security staff seem to take it very seriously.

[Lounge Lizard]

OTOH, modern inkjet printers like my Epson R800 use eight inks to provide an incredible colour gamut, with bright, intense colours, but they do not represent the subtleties of those colours as well as RGB.

I could be completely wrong on this, but that is how I see it.

I can't say I follow that one Nigel. Your printer may have a gamut wider than sRGB but it tries to accommodate the colour space of your image as best it can by using the printer ICC profile and an associated rendering intent or by some clever proprietary stuff in the print driver.

If you use AdobeRGB or ProPhotoRGB then your image colour space may be wider than the gamut of the printer (in some but maybe not all parts of the spectrum) but, again, it's down to whatever colour-managment you use to compress that colour space into what's available.

OTOH, if you are talking about the gradation in a given colour space then sRGB, due to its smaller gamut, gives much smoother transitions and moving up to AdobeRGB or ProPhotoRGB really needs 16-bit data to retain a degree of smoothness.

[Per]

My observations on Velvia are that it artificially emphasises some wavelengths (principally greens). This 'punch' suits some subjects, especially in dull light. However, as with filters, if you can see the effect then it is overdone.

I consider other film stock (e.g. Reala) to show more complexity and variety in the reproduction of Greens and thereby appear more truthful overall.

As always, it depends what effect you are after and if you consider emphasis to be necessary...

[Benchista]

Hmmm - my experience is actually that what Velvia does is to differentiate between greens more than any other film, and that's why I liked it - take a picture of a mixed wood in summer on any other film and all you get is an amorphous mass of green. With Velvia, you can distinguish much MORE variety between numerous different greens - something that Kodachrome is particularly bad at, IMHO.

[Lounge Lizard]

...which bears out my thinking in that Velvia has an extended gamut for green while other films result in clipping and produce the nearest green it can resulting of compression of the green tones and thus a lack of separation between them.

[El_Sid]

I was thinking about colour spaces and relating this to Velvia - simplistically we say that Velvia has an extended green sensitivity. Is this simply saying that Velvia has a more extended colour space, particularly in the greens, than other film or am I trying to make this too complicated by comparing digital colour with the spectral response of film?

Any system which involves colour, sensitivity and resolution has a definable colour space. Most photographers have only become aware of colour space since the advent of digital imaging and by that I mean image scanning as well as digital cameras themselves.

The response and and sensitivity values of a film, like Velvia, can be plotted in the same manner as any digitally derived colour space to produce a characteristic plot. I have no doubt that Fuji have a copy of this very colour space as part of their design specification for the film, it's just that in the past most 'togs were not aware of it's existence.

With the advent of digital imaging systems we have become aware that these systems need to conform to standardised colour spaces if there is to be any hope of reliable intercommunication while a film such as Velvia is essentially a stand alone system - as long as it's processed correctly the resultant slide should correspond to it's specified colour space and appear correctly when viewed as a transparency.

Issues start when you attempt to transfer the transparency to another medium such as print or digital which has a different colour space. It is at this point that the issue of non-congruity between spaces rears it's ugly head and clipping and out of gamut errors can occur. IIRC Velvia is a notoriously difficult difficult film to scan and I am aware that Cibachromes - while being about the best print interpretation of slides possible - still tend to lack the real vibrancy. This occurs because the colour spaces of the Cibachrome system and digital scanners are not quite the same as the film being printed or scanned The end result means there are likely be areas where the film space extends beyond the space of the receiving medium - at this point out of gamut clipping occurs and we lose the colours beyond the boundary causing the image to look weaker and less vibrant. Just to make matters worse we also need to remember that the subtractive colour spaces of prints (based on CMYK type systems) are slightly inferior all round than those of RGB systems - their spectral intensity at any given shade tends to be weaker than the RGB derived equivalent and falls off more rapidly towards the limits. As a result prints are less vibrant and saturated than we would really like.

As long as one is considering Velvia for projection it's fair enough to say that it has extended green sensitivity. Once you start to print or digitise it then how extended that sensitivity is and how the colour space of the film compares to the receiving medium becomes more of a (complex) issue...