The Color Purple

Laynie · Feb 28, 2011

I took some photos at a wedding some time back (I wasn't the photographer, just a guest) where the main color was purple. All the purple dresses came out looking mostly blue. At the time, I thought it was my simple point-and-shoot. Then I started noticing that other friends' cameras did the same thing to varying degrees.

I'll be photographing my brother's wedding in a few weeks, so I've been preparing for that. It occurred to me today that their colors are purple and gold/yellow. I know there are technical reasons why purple is difficult. What I don't know is how to capture the colors in spite of these complications.

Can anyone give me some suggestions on how to approach this for the best results? I'm hoping to learn to do at least some of this with my camera rather than relying on software after the fact.

clanthar · Feb 28, 2011

Film.

Convince them to change colors.

In lieu of that use a camera that captures RAW files. However, this may only help temporarily. At some point you will probably want to convert the RAW files to RGB photos. To do this you must specify a color space for those RGB files, sRGB or Adobe RGB mostly likely. If the color you photographed doesn't exist within the color space then --- well it doesn't exist. Purple is in that problem range.

My dentist grows orchids and has a nikon DSLR. When she found out what I do for a living she asked me why the colors of her orchids (purple) don't capture correctly with her expensive nikon. In digital photography every color must have a number and there are only so many numbers. That's what a color space is; a mapping of colors to numbers. The color spaces we use at this time don't include all the colors we can see and use in the world. Look at this graph of the Adobe 1998 color space. The black triangle represents the mapped colors. Colors outside the triangle don't have numbers and so don't exist.

Notice that purple is in shaky territory. The colors you photograph must be assigned numbers from the existing set of numbers. If you photograph a color ouside the set it must be coerced into the set.

Joe

Lambo77 · Feb 28, 2011

Wow, I just learned something. Never knew that purple was difficult to photograph, and excellent explanation! So when you said shooting RAW is a way to do it, how exactly does that help?

Laynie · Mar 1, 2011

Thanks for the chart and everything Joe. It makes sense.

I was wondering that too, Lambo. I know why RAW images are good, but I'm not sure if there's some specific reason it's better for purple. Unless it's because it gives you more to work with later (?)

Derrel · Mar 1, 2011

clanthar's comments about purples are good to read. I've had pretty serious difficulties with various purple flowers in years past; there was a time when various Nikon d-slr bodies seemed to render purples, lavenders, and violets in unflattering or unexpected ways. One thing that I noticed was that the RAW conversion software could/would make an absolutely monumental difference. Back in the early 2000's, I found that Bibble Labs' "Bibble" RAW converter would crete pretty acceptable purples and lavender colors, but that the then-current Adobe ACR software was not nearly as good at demosaicing Nikon RAW files and getting the purple colors correct, or should I say "pleasing". If there is difficulty in rendering purple hues, I would suggest that the easiest and best way to get past that is to explore different software that CAN do the job properly.

Garbz · Mar 1, 2011

clanthar said:
Notice that purple is in shaky territory.

Purple is actually not an issue at all. Purple the colour is a mixture of pure red and pure blues and is trivial to represent by a camera and in the sRGB spectrum. It lies in the CIE co-ords at around 0.25, 0.1.

The issue here is one of violet. Many things that look purple actually produce a mixture of the colours red, blue and violet. It is this latter one that causes problems for cameras as violet falls outside of the camera sensor's three primary colours (displayed on the CIE as the UV numbers 380-460). Normally if you're trying to record a pure colour that doesn't fit in the sRGB portion it will typically be shifted into the sRGB triangle WITHOUT a shift in hue. I.e. purple should still look purple, but not as strikingly purple as it did. If I take a photo of a colour chart with a purple square it comes up purple. If I take a photo of a cheap black light it comes up nearly perfectly blue. It is this violet component which will cause a shift in hue of the colour you're recording.

There is no real technical solution to this, only workarounds after the fact. If you're lucky enough that the wedding is all one shade of purple that you find is off you can in photoshop shift just that very narrow range of hues over slightly towards the red. This will solve your immediate issue.

Laynie · Mar 1, 2011

It crossed my mind that it would be best if all the bridesmaids' dresses etc were the same shade of purple. I don't know yet. I'm hoping!

clanthar · Mar 1, 2011

Lambo77 said:
Wow, I just learned something. Never knew that purple was difficult to photograph, and excellent explanation! So when you said shooting RAW is a way to do it, how exactly does that help?

RAW capture helps because it allows the photographer to hand process the coercion. If you shoot a camera JPEG and the real color is outside the color space then the camera image processor will coerce the real colors into the color space, often losing detail and posterizing the result. By hand processing the RAW file you can do a better job. You will however have to give up the real color if in fact it was outside the color space. You can't have a color that's not in your working color space.

Joe

Garbz · Mar 2, 2011

For RAW you only give up colour space during export, RAW by it's nature has no colour space until the RAW conversion is done by the processor. Though this only applies for colours which have a hue that can be represented within the colour filter on the sensor. As I mentioned violet falls beyond the blue filter on the camera sensor and will be picked up as blue despite the colour being best approximated as a purple.

Lambo77 · Mar 2, 2011

Great thread. Thanx for the explanation!

Breaux · Mar 2, 2011

A picture is worth a thousand words - and so is a graph! I learned more about color space from this little discussion than from anything I've read so far. Thanks TPFers!