Why Are My Purple Flowers Blue?

Started Jun 6, 2008 | Discussions
twofurrycats Contributing Member • Posts: 571
Why Are My Purple Flowers Blue?

I'm using the H9 and tried various settings, all with 200 ISO and these dark purple irises look blue. What am I doing wrong?

1/1000s; f/2.7:

1/25s; f/6.3:

Sue

jrdu Forum Pro • Posts: 21,371
I have the same problem with my R1

It's a really hard colour to get right. The red goes out of the right side of the histogram and the blue goes out of the left side. There's no way to get all the colour in.
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OP twofurrycats Contributing Member • Posts: 571
Re: I have the same problem with my R1

Thanks. My reds seem ok, it's just the purples.

Sue

binary visions Contributing Member • Posts: 552
Purple & digital sensors (long post)

Purple is one of the hardest colors to reproduce.

There are a lot of reasons for this.

First of all, I have yet to find a white balance system that handles well when pointed at a purple object - virtually all of them tend towards producing a solid blue, rather than purple. The only way around this is to white balance with a calibrated item like a proper gray card or an Expo disc.

Second of all, purple is a very hard color to capture in nature. Purple doesn't actually exist as a wavelength - violet does, but it's not the same color. Purple only exists as a combination of blue and red. Just from life experience, you should recognize that purples tend to change dramatically under different lighting - while a red is a red is a red regardless of the light, a purple may change to blue with a simple shift in light intensity or color.

Lastly, there are the technical details of the Bayer sensor. I'm a little muddy on this since it's been a year or two since I read a paper on it. Let me start here:

The light receptacles on a Bayer sensor (yours) only sense one color. Each light receptacle has a filter over it, and is sensitive to only one wavelength - red, green or blue. When light strikes the filter, the receptacle gets a charge, if that charge is very strong, the camera says, "that's a pure color" and reproduces it as such. If the charge is weak, the camera gathers the data around that receptacle and looks for other clues - if the green and the red sensors both got a little, it might be reproduced as orange. So that's how a Bayer sensor works (very simplified, of course), by looking at a bunch of light receptacles and figuring out how much of what wavelengths are there, then saying it's a certain color.

This runs into at least a couple problems with purple. Part of it is due to the Bayer pattern having twice as many green filters as red or blue - pure reds and blues render fine, and the green channel is reproduced faithfully, but there is simply more green data available than red or blue data, so combining red and blues doesn't always work as well, simply because there's much more green data surrounding the reds and blues, and purple has no part of the green spectrum. Having so much green data is actually a very good way to make sure colors are reproduced similarly to what we see, but it does lead to a limitation of purples.

It's compounded by the fact that violet is actually a shorter wavelength than blue and the camera cannot record violet properly. It records it as a blue. The blue filters are sensitive to the violet light, but it's still sensing it on the blue filter - there is no violet filter, and there is no combination of red blue and green that produces violet.

There's another facet about how the wavelengths of red and blue fit together (being opposing colors on the color wheel) but I'm not so familiar with the effects of that on the sensor.

Hopefully that's all somewhat accurate since I'm working on memory here, and someone can correct me about anything that's wrong.

Phew, that turned into something of a book. I hope at least one person muddled through it and found it useful.

Stephan
--
dev/null

OP twofurrycats Contributing Member • Posts: 571
Re: Purple & digital sensors (long post)

Thanks, Stephen. I did read all the way through it and found it very interesting. So, I guess the only workaround might be to fiddle with the white balance? Or, stop taking pictures of purple flowers with my H9! Maybe I'll try a different camera and see what happens.

Sue

jrdu Forum Pro • Posts: 21,371
Well the reason that your purples look blue

is that the red is clipping.
So to get the red back you have to underexpose, but then you lose the blue.

It's a tricky colour on any digital camera with the possible exception of the Minolta A2. I don't know why, but it just seemed to do well with purple.
--
John Dunn

Portraits: http://www.fototime.com/ftweb/bin/ft.dll/pictures?userid= {8B9B811D-AD1C-4A7D-923E-A4D0930BB5EE}

binary visions Contributing Member • Posts: 552
Yep.

The best way to take pictures of anything purple is to be extremely careful about your white balance, and to nail your exposure since purple is so sensitive to changes in light levels. A guy on another forum posted some purple flowers he was having trouble with - the photo was over exposed by about 2/3 stop, and the color was just gone.

It's not that it's impossible to take pictures of purple things, it just requires a little more care and sometimes some post processing.

Stephan
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dev/null

binary visions Contributing Member • Posts: 552
That's it!

I said in my post that I couldn't remember the reason why the red/blue interaction caused such problems.

You've got it: underexposing to control the reds causes the blue to drop off. Overexposing to control blues causes the red to blow out.

Bingo. Thanks.

Stephan
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dev/null

mozarkid Regular Member • Posts: 380
Re: Why Are My Purple Flowers Blue?

You've got some good advice so far, I'll just add to it:

Do a web search for "CCD sensor sensitivity"

Unfortunately, you'll find that CCD's have reduced sensitivity in the blue area of the spectrum and down-right poor sensitivity in the upper blue to near UV area. This makes the higher frequency blues challenging, thus purples as well.

What can you do?

First, DON'T use a UV filter - this just cuts more sensitivity at the upper end of the blue spectrum.

Second, use a white card and the "Onepush Set" white balance option. Non-glaring surfaces such as a clean, bright white cotton fabric or a brite white matte photo paper work well.

OTD Forum Pro • Posts: 18,701
Is that why I'm blue?.....nt
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Marion
casual-photos@q.com

OTD Forum Pro • Posts: 18,701
+/- 0.7EV and bracket might catch it then?.....nt
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Marion
casual-photos@q.com

OTD Forum Pro • Posts: 18,701
Another reason to NOT use a UV filter as a lens protector..nt
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Marion
casual-photos@q.com

tc333 Veteran Member • Posts: 3,967
Re: Why Are My Purple Flowers Blue?

Hi Sue

I have no experience doing flower pics but thought i might try a purple flower in our garden quickly just before it started to rain....you gave me the inspiration

I set it to vivid, cloudy WB and bracket for exposure. I cropped a lil and this is the end result. Pretty much close to the real colour. I guess you just have to try a variation of things. I think using bracket on WB would be a good choice too.

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Tony

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Panasonic Lumix DC-GX9
binary visions Contributing Member • Posts: 552
That would probably work.

On the other hand, I generally don't bracket unless I'm trying to shoot HDR or the situation is so absurdly tricky/one time only that I have no other choice.

I didn't pull up the histograms for the original post but the exposure looked okay, it was just the white balance that was shot. John suggested that there were some blown channels but unfortunately he also indicated they were blown at both ends so there's no real recovery there, just a proper WB and hope for the best.

Stephan
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dev/null

MTT Veteran Member • Posts: 4,954
These pic's makes me appreciate my 717.

Did great on purple. I think better then my D70 and D300. It's a WB thing. If WB to cool then the purple will be to blue. Have you tried presetting WB?

Mark

Sony 717,

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Hutchman Veteran Member • Posts: 8,487
Re: Why Are My Purple Flowers Blue?

I don't do flowers but here is one with the A100 that on my iMac runs the gamet from Red to Purple to Blue.

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John
'Hutchman'
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(unknown member) Forum Pro • Posts: 23,648
All digital camera's can....

make purple look purple or close to it anyway.

one factor that counts "Correct exposure *

as u know i did many flowers with all kind of point and shoots some examples that i could find in a quicky.

i must have some made by the H1 as well ..........the examples

where taken with two other point and shoots Nikon/Canon but believe me u can do the same with any other camera.

some shots including exif to give u a idea.

Camera: Nikon Coolpix P80
Exposure: 0.017 sec (10/601)
Aperture: f/3.5
Focal Length: 11.9 mm
ISO Speed: 143
Exposure Bias: -7/10 EV

Camera: Nikon Coolpix P80
Exposure: 0.008 sec (1/125)
Aperture: f/8
Focal Length: 19.4 mm
ISO Speed: 200
Exposure Bias: 0/10 EV

Camera: Nikon Coolpix P80
Exposure: 0.017 sec (1/60)
Aperture: f/8
Focal Length: 33.3 mm
ISO Speed: 200
Exposure Bias: 0/10 EV

Camera: Canon PowerShot A650 IS
Exposure: 0.077 sec (1/13)
Aperture: f/6.3
Focal Length: 7.4 mm
ISO Speed: 80
Exposure Bias: 0/3 EV

Camera: Canon PowerShot A650 IS
Exposure: 0.01 sec (1/100)
Aperture: f/8
Focal Length: 7.4 mm
ISO Speed: 80
Exposure Bias: 0/3 EV

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OP twofurrycats Contributing Member • Posts: 571
Re: All digital camera's can....

Thanks for the help. I'll go out and try some of them if it ever stops raining!

Sue

Stephen McDonald
Stephen McDonald Forum Pro • Posts: 13,890
High-Frequency Colors

Purple and lavender are at the high end of the frequency spectrum of colors visible by people. Blue is just below them. Many digital cameras and camcorders have trouble in sensing and processing them at those high frequencies and end up reproducing them as lower than they appear to your eyes directly. You can minimize this problem by keeping the exposure set no higher than is barely adequate. That way, the camera doesn't have to deal with so much of the high-frequency intensity. You can experiment with ND filters and may gain some advantage with these colors in bright light. Purples and blends of purples and blues can also get washed-out more than lower-frequency colors by overexposure. There's another factor in this, which is caused by the color frequencies we can't see, up in the ultra-violet range, but which are detected by a camera's sensor. Purple flowers are reflecting a lot of ultra-violet as well as visible purple and this adds to the difficulty for the electronic sensors.

Birds, insects and others can see a whole extra set of color and patterns in the ultra-violet range, that are unknown to us. Flowers present signboards to potential pollenators, advertizing what they have to offer, in these ultra-violet patterns. Some spiders can actually spin large webs that display fraudulent UV patterns, that emulate those of flowers and confuse insects and even some small birds into entering their traps.

Here are two examples of JPEG still images from my CMOS camcorder, an HDR-HC9. The first picture was shot at the same exposure I used successfully for flowers of other colors, such as reds and yellows, that were nearby and had the same facing to the Sun. The second picture was taken with an exposure 2 clicks lower (2/3 of a stop), on a later occasion, but under similar light conditions, after I had figured out the cause of the problem. These Giant Vinca flowers have shades of both purple and blue and something in between, that occur in different parts of the same bloom. The second picture shows their colors just as they appear directly, which is always the best test of color balance. This is a big advantage in taking flower photos in your own yard, as you can easily compare your results on a screen, with the real thing.
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OP twofurrycats Contributing Member • Posts: 571
Re: High-Frequency Colors

Thanks, Stephen. I've been shooting them on a cloudy day as I thought I'd get truer colors. I'll try again tomorrow as the say it will be sunny and warm.

Sue

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