Red Fringe In Snow Photos – Exposure Examined

[Boatman]

A week ago I took a series of snowscape photographs and reported seeing an unusual red fringing or border along the side of the image. The histogram on my camera showed the graph to be very bunched up as the light was flat and there was a limited range of brightness and colors.

Keeping with the recommendation to keep the curve “high and to the right”, I had increased my manual exposure adjustment to +0.3EV or +0.7EV on all of the images. This kept the histogram in the upper one third of the graph but did not cause it to move past the end of the exposure range.

After posting my photos and asking about the cause of the red fringing, forum member Black Pearl suggested shooting a series of photos of a plain white card to see if I could recreate the problem. I did this. It does recreate the problem and it raises a couple of questions that, if explained, may be very useful in figuring out the correct exposure for low contrast photos, like a snowscape on an overcast day.

To test this I took a large piece of whiteboard and set it by the slider in my living room where it received ample light from outside. The weather today is very similar to the weather a week ago when I took the original photographs.

The camera showed that it wanted to take the picture at 125th second, f6.3. ISO was set to 64. I set the camera on manual control and took a series of pictures at f3.5, f4.5, f5.6, f7.1 and f9.0. In the following series of images, you will see that there is significant red fringing in the two over exposed shots and a hint of fringing in the f5.6 shot. (Yes, I should have shot one at 6.3) I think the lesson here is that in snowscape situations you need to avoid overexposure or even slightly underexpose although this will create other issues with other items in the image, which are presumably much darker than the snow.

Now, this is where I’m getting really confused. The board I photographed was white. All the shots were taken in RAW mode and opened into Photoshop without making any adjustments to the image. From there I reduced the image size to 72dpi but made no adjustments in Photoshop to them. As you can see, they are all gray, not white!

When I opened these images in Picasa, which I use as my file manager, they all appeared pretty much white, but Picasa does adjust the RAW image before displaying it. Somehow Photoshop Camera RAW sees them differently, and this I don’t understand. In order to get the images to be white you need to bring the brightness slider from +50 to +150. I noted where the image histogram (a spike) intersected the diagonal line in the Photoshop curves adjustment. Since the input and output were nearly exactly the same in all cases, I’ve only listed one number. From the most under exposed to the most overexposed, the range is: 77, 119, 140, 170, 212.

The Picasa histogram does not provide any numbers but the curve placement in all of the RAW images is in the upper third of the graph. The histograms for the jpg conversions in Picasa are to the left for the underexposed images and to the right for the over exposed images. This is more in line with what I would expect.

I guess this means that Picasa is adjusting the photos and displaying the histogram for the adjusted image, not the RAW image, but I’m not sure. I am pretty certain that overexposing a snowscape is a bad idea and will result in the red fringe. I’m also pretty certain that I’m missing something in Camera RAW and need to go back and study how it opens a RAW image and what it does to it.

I also think that testing on a white board isn’t correct, though it may simulate a snowscape and therefore be useful. Using an old trick from my black and white film days, I took a reading on my hand in the same light as the white board – two full stops darker. A couple of photographs using the reading from my hand that included my hand, the white board and some of the surrounding room looked very normal and displayed a histogram that reached across the full width of the graph . But over exposing two stops in a snowscape puts me back into a situation where I would be getting a red fringe… so much to learn.

[Geoff F]

Quite a few issues here, Homer, which will probably require someone more experienced in explanations. But to start the process here are a few simple ideas:

Read a few tutorials on White Balance to try to understand the basic principles.

Did you say which camera and the photo modes which you used to take these test shots? If so I appear to have overlooked that information. What were the exact settings? Shutter speeds, etc, and was that a constant ISO? ISO 64 sounds a bit low but some of the 'point & shoot' cameras have different defaults to the Dslr range.

A camera, on basic settings, will underexpose pure white to prevent overexposure (somewhere around 15% grey is average) so that is why your white card has become grey.

If you used that shot to create a Custom White Balance you would return the image to white (but run some risk of over exposure if you aren't careful) other standard WB basic settings will produce a variety of shades.

Some editing software does create an estimated average starting point for Raw conversions. I don't know about Picasa but Photo Plus gives me a totally different default setting from Raw Therapee.

Metering from the palm of your hand is certainly and old trick which can still be effective as a last option, providing you have average white skin. As an alternative for snowy conditions, I have successfully created a Custom White Balance by photographing snow in light shade and using that to set a Custom Balance. Any not quite pure white area can be used to create a very rough setting but, of course, if you want perfect results you should use one of the correctly calibrated cards.

With most Raw converters you should be able to reset the White Balance values to suit each individual image. Very, very roughly, a lower number (say 5000) will produce a bluer result while a higher number (say 6000) will produce more red.

Once you have created a correct White Balance for the scene you can start to experiment with any colour fringing problems.

Very basic snow settings, without creating a Custom Balance, are to over expose slightly to keep the snow pure white, but if you over do this you may start to lose fine details, which requires less exposure until you obtain a suitable setting.

Shooting with the histogram 'well to the right' has been recently discussed on another post; but basically this is correct (providing you don't overexpose) for a bright photo. However no two scenes are exactly the same and some shots require a more 'moody' histogram with maximum values around the midtone values.

But this is just something of a quick, but rather rambling, explanation; I'm sure somebody else can do better!

This CinC Tutorial may explain things more coherently.
https://www.cambridgeincolour.com/tu...te-balance.htm

[revi]

First about the whites appearing gray:
White balance settings have nothing to do with that.

What is important is, that your camera doesn't know where on the grayscale your card is.

In other words, it has NO way to know if the card is black, white or gray. All it sees is a certain amount of light coming through the lens. So, the manufacturer decided that the light would always be considered as coming from a middle gray (13 or 18% reflectivity). So, using the meter's exposure, your photo will show a middle gray card...

Your exposures went from about -2 to +1 relative to the metered value (or 0.25x to 2x the metered light), your white card would reflect about 4x as much (2 stops), so to get a white image would require an exposure correction of +1 to +4 EV. I don't use photoshop, so the values you give don't mean anything to me ...


As for white balance: auto white balance should work reasonably well here, and the pictures you show have no colour cast.

For snow, you have a white balance problem on sunny days. Snow receiving direct sunlight will require a WB setting around 4000-5000 K. Snow in shadow will need something more like 7000 K, as it receives its light from the (blue) sky... So that gives us a nice mixed lighting problem

Remco

P.S. To help see what happens with the wite card, think of how a white cube lighted from the side looks: lighted side is very bright, opposite side looks dark gray or even black, others in between, yet all sides are 'white'...

[Boatman]

Geoff and Revi:

Thank you for your responses. I think that what is happening is that the camera's light meter is being fooled by the monotone of the flat light of snow or the white of the cardboard. As you suggest, it has nothing to work with in terms of setting the white balance, though that can be fixed easily, post shoot. Perphaps the best solution is to go with what the camera wants for basic exposure and bracket the shot. That would cover the bases pretty well. Clearly over exposing is a bad idea and is resulting in the red fringe I was getting.

By the way, the camera was a Minolta A2.

[Colin Southern]

Hi Homer,

I've written a little about metering in the past ... you might find these threads interesting ...

Custom Settings for 1D Mark III

What is the best method for learning how to expose photos correctly?