crazy reds, what's this?

ivar

My reds are crazy. I've noticed it before, but never this bad. This is straight from a CR2 file, no processing, whatsoever. It's zoomed quite a bit to show the red jacket:



What's going on? How do I prevent this? :dunno

DoctorIt

i dont think there is anything you can really do. many sensors get "overloaded" in the red channel. i've seen it on almost all my cameras, my d2h was particularly sensitive to it.

i wish i could provide a better answer, but it certainly has something to do with the wavelength range that can be interpreted by sensors.

jfriend

ivar wrote:

My reds are crazy. I've noticed it before, but never this bad. This is straight from a CR2 file, no processing, whatsoever. It's zoomed quite a bit to show the red jacket:

What's going on? How do I prevent this?

As Erik said this is "just one of those things about digital" that you have to watch out for. Red flowers, red soccer uniforms, red clothing all have risk of overexposure. I suspect it has something to do with the way the exposure sensor measure light and it gets fooled when red is dominant.

Anyway, whenever I have lots of red, I always make it a point to check the RGB histogram on a few test shots and often crank in a little negative exposure compensation to protect the detail in the brightest red parts.

Tanuki

ivar wrote:

My reds are crazy. I've noticed it before, but never this bad. This is straight from a CR2 file, no processing, whatsoever. It's zoomed quite a bit to show the red jacket:What's going on? How do I prevent this?

No processing whatsoever? In that case, all is not lost. Your red channel is completely blown in the posted jpeg, but is probably not blown in the RAW file. If you're using Lightroom or ACR, try reducing the red saturation saturation and brightness. Turn on the "preview clipped highlights" until you see it come back into gamut. Give it a try and if it doesn't work to your liking, there are other things you can try.

Regards,
Mike

ivar

Thanks guys

Things are indeed not lost, Mike. I can fix things in post, but I was just wondering what was going on, and if I could prevent it. Seems like I'll keep fixing it in post

Chrissiebeez_NL

i sometimes shoot at a presentation night with lots of red light and sometimes the images just come out like a duotone red and black. Red does crazy things to a sensor

Tanuki

ivar wrote:

Thanks guys

Things are indeed not lost, Mike. I can fix things in post, but I was just wondering what was going on, and if I could prevent it. Seems like I'll keep fixing it in post

If the reds can be recovered from the RAW file then the sensor was not overwhelmed, but rather the gamut was clipped when the color space of the RAW file being converted into the sRGB or Adobe RGB color space.

If you want to minimize post processing with RAW (or if you shoot in jpeg), you can prevent this (if your camera has a 3 color histogram) by chimping exposure based on the individual channels instead of the luminance. Then, if the scene looks a little underexposed, you can apply some levels to bring it back. But I think you'll prefer your results if you expose properly in the first place and then handle gamut clipping during the raw conversion stage.

Regards,
Mike

Slinky0390

Wow my physics class came in handy.. . We learned that red light has a wavelength of 600 some nanometers, which makes it one of the slowest visible lights. I'm almost positive we learned something along the lines of how red light doesn't diffuse through the atmosphere as efficiently as blue light (other end of the wave spectrum) so colors like red tend to be blown out by things that are sensitive to visible light (our eyes, camera sensors, etc.)

jfriend

Slinky0390 wrote:

Wow my physics class came in handy.. . We learned that red light has a wavelength of 600 some nanometers, which makes it one of the slowest visible lights. I'm almost positive we learned something along the lines of how red light doesn't diffuse through the atmosphere as efficiently as blue light (other end of the wave spectrum) so colors like red tend to be blown out by things that are sensitive to visible light (our eyes, camera sensors, etc.)

I think more likely what's going on is that the exposure sensor is sensitive to all colors. To get a medium exposure, it expects there to be a medium amount of all wavelengths of light that add up to a particular sensor value. If you now have a medium tone and it's only one color like red instead of all colors and that one color you have isn't green (which tends to dominate), then the sensor will see a much smaller signal than it expects of a medium tone. When it sees a smaller signal, it thinks that it needs to add exposure to get to a medium tone. As such, it tells the camera to go to a longer shutter speed or larger aperture to let more light in. When it does that and there's only this one color, it can easily let too much light in and that color channel oversaturates. There's a technical reason why this doesn't happen with green things which I've seen an explanation for, but don't know how to explain myself. I believe it does sometimes happen with bright blue skies (they wash out to a white sky), though you usually aren't metering off bright blue things.

Tanuki

jfriend wrote:

I think more likely what's going on is that the exposure sensor is sensitive to all colors. To get a medium exposure, it expects there to be a medium amount of all wavelengths of light that add up to a particular sensor value. If you now have a medium tone and it's only one color like red instead of all colors and that one color you have isn't green (which tends to dominate), then the sensor will see a much smaller signal than it expects of a medium tone. When it sees a smaller signal, it thinks that it needs to add exposure to get to a medium tone. As such, it tells the camera to go to a longer shutter speed or larger aperture to let more light in. When it does that and there's only this one color, it can easily let too much light in and that color channel oversaturates. There's a technical reason why this doesn't happen with green things which I've seen an explanation for, but don't know how to explain myself. I believe it does sometimes happen with bright blue skies (they wash out to a white sky), though you usually aren't metering off bright blue things.

That's a good point. In my assessment I had assumed that the red jacket was only a small part of the scene and that the rest of the colors in the photo appeared properly exposed. If the red jacket influenced the meter (either because the camera spot metered on the jacket or because the jacket dominated the scene), then it certainly could have caused overexposure. It's hard to know whether the culprit was overexposure or gamut clipping without seeing the whole photo.

Mike

Nikolai

ivar wrote:

My reds are crazy. I've noticed it before, but never this bad. This is straight from a CR2 file, no processing, whatsoever. It's zoomed quite a bit to show the red jacket:

What's going on? How do I prevent this?

Was it Sony? They are (in)famous for oversaturated reds. All mine did that...:-( Once a channel is saturated 100% there is no way back. No other way but account for that and underexpose by a stop or so...

scottcolbath

Interesting thread. I'm guessing my pic here fits the subject.


It was mentioned to me prior, that the pic was oversaturated. No PP on this pic. Just a JPEG conversion.

S.C.

LiquidAir

jfriend wrote:

I think more likely what's going on is that the exposure sensor is sensitive to all colors. To get a medium exposure, it expects there to be a medium amount of all wavelengths of light that add up to a particular sensor value. If you now have a medium tone and it's only one color like red instead of all colors and that one color you have isn't green (which tends to dominate), then the sensor will see a much smaller signal than it expects of a medium tone. When it sees a smaller signal, it thinks that it needs to add exposure to get to a medium tone. As such, it tells the camera to go to a longer shutter speed or larger aperture to let more light in. When it does that and there's only this one color, it can easily let too much light in and that color channel oversaturates. There's a technical reason why this doesn't happen with green things which I've seen an explanation for, but don't know how to explain myself. I believe it does sometimes happen with bright blue skies (they wash out to a white sky), though you usually aren't metering off bright blue things.

Nominal Luminance is 30% red + 60% green + 10% blue. As a result, a luminance light meter is actually twice as sensitive to green as it is to red. What this means is that if the brightest object in the scene is monochromatic red, the camera will overexpose it by a stop.