Why does black look purple?

Simple question - Why does black on pictures look dark purple?
This (d)effect haunts me and makes me crazy. We make car tests, and often the car seats are black. But on pictures they appear as violet. I can't tune it down, without really photoshoping it.

Plus, i just found out, that it's not just on car seats. I went to the zoo, and that pitch black llama also turned purple. Why? And is there a way to correct it in camera?

I used different cameras, this happens with all of them.

A) you're overexposing your blacks, which isn't helping.
b) try a polarizer to reduce the reflected light off the surface.
c)try a uv filter

as a wedding photographer I'm fully aware of the problem...(.Leica had a nightmare with thier digital rangefinder) we want our black tuxes to look black!

turns out that cameras with good IR sensitivity really have problems with blacks that absorb most of the visible spectrum but can reflect back alot of the not so visible spectrum.

some photoshop work can help. a grey card can help you in post by getting your exposure right and a reference for correcting any color problems.
ps Mattmatic uses the expo disc which is great, and I've discovered that for setting your white balance, the translucent bodycap that comes with the dslr's does a great impersonation of the expo disc, though not as accurate for exposures, not too far off either.

What cameras do you use? What software? And most back isn't actually black anyway...

Pentax K100D, Nikon D50, Canon 350D, maybe something else too, don't remember.

Software - used to use Photoshop. Now turning to Lightroom. But the thing is, i want to avoid this problem "in camera", so i don't have to use software later.

"And most back isn't actually black anyway..." - well, yes, but how to make it look black then?

Had a quick look -
Nearly all down to White Balance.

If you use AWB (Auto White Balance) the camera is making certain assumptions about the type of image you are photographing - and all-black doesn't usually come into it

The white balance affects the "skew" of the colours. Get the WB wrong and everything gets tinged. (That's one of the benefits of shooting RAW - you can always adjust the WB at the PC).

Another part of it is the black point. If the black point is too high then black objects will look more grey, and if the WB is off as well everything goes seriously pear-shaped!

If you want good colour, you should use a manual white balance (even sampling off a sheet of A4 paper is better than nothing).
The little QPcard is very helpful too - not expensive and easy to carry around.
There's also the Expodisc (which I love), but that's expensive too.

A cheaper Expodisc is the Lee filter caps (three for £3 last time I checked). They're good enough to get a rough colour balance from.

HTH!
Matt

Well, that is good to know. Although AWB on the K10D has gotten almost all my shots just right. Not really taken any indoors for lack of a fast lens so can't really test that out.

Btw, is this sort of the same?

It will do half of the Expodisc

The Expodisc was designed as an exposure tool first. The white balance followed. You can use it as an incident light meter because the lens arrangement (like bobbles of plastic) on the front will collect the light and spread it evenly across the frame. It works very well, and because the light sources are averaged as well it gives a good WB.

The "Expodisc replacement" you linked to just does the WB. (The Lee filter caps will do the same thing.)

You will get a better WB off a QPcard, compared to the "replacement" or Lee caps

Matt

That is allmost sort of the same
The surface of the Expodisc is covered in small pyramid shapes which help to diffuse the light more.

although I've never tried it myself I have been told that the lid off a tube of Pringles makes a pretty good white balance doo-hicky

But pringles doesn't have a translucent cap here anymore as far as I know.
They have an orange cap for one flavour, that will make your whitebalance do nice things...

well it would certainly give your photos an individual look

Pringles lost their appeal for me when they ceased to be molded out of pure MSG and therefore now taste like the cardboard they are

I've used a white yogurt carton as an incident reader attachment on many film cameras in the past and it worked really well, I suppose it could also be used as a WB tool but have not tried it myself. There are no end of things that could be used such as the tops from aerosols etc. Must be worth a go I would have thought.
Harry