On 2/10/2015 2:22 AM, Peter Klein wrote:
We were treated to quite a show during our anniversary dinner.
<https://www.flickr.com/photos/24844563@N04/16304267069/>
I'd like some advice on a technical problem with this picture, and in fact with all of the pictures I took of the city
from atop the Needle. Red car tail lights turn white. It isn't overexposure
Well, yes it is.
, because it happens even on shots that I deliberately underexposed drastically, where all the RGB values are under 255.
Are you sure of this, at the pixel level? If I throw a lasso around the area of taillights on the screen sample, I get a
decided stack of 255 values in all three channels at the top of the histogram of just that small area. That's classic
clipping, and will indeed turn red to white.
There is similar clipping of the blue channel of the blue lights I checked, but no red and only a very little green
channel clipping, so they are probably reasonably accurate color.
The difference between the red and the blue highlights is the nature of the lights themselves. Most taillights are still
incandescent bulbs behind red tinted plastic. Although mostly red looking, they are wide spectrum, with a lot of green
and blue. So when clipped a lot, all three channels clip, and 255 for all three channels makes a white pixel. The blue
lights, OTOH, are most probably 'neon' or LED, putting out narrow spectrum, almost pure, blue light. So when they clip,
it's only the blue channel and no harm done.
There are also clipped highlights in the relatively white office lights, streetlights, etc. but you don't notice because
they look fine.
Chuck is exactly correct. Very small areas of blown highlights are just too small in number to show up in an overall
histogram. I often select a small area or Select Highlights (a PS menu item) to check the state of clipping.
In shots with any decent city detail, the red channel does hit 255, but the green and blue are much lower. Lowering
the exposure in post, or using highlight recovery has no effect.
This doesn't happen with larger areas of red light where the pixels have the same values as the little tail lights.
But taill lights turn white, as do other points of colored light like the blue and green Seahawks colors that still
adorn some construction cranes. The key seems to be that the light sources are just a few pixels in diameter.
Indeed, that is the key, as Chuck and I have pointed out. Those little areas
are simply overexposed.
Maddeningly, on the onscreen preview, the tail lights appear red. But when I "develop" the JPG, they turn white.They
also turn white if I blow up the onscreen preview to 50% size or larger.
Size, sampling to screen resolution and your eyes at work. They are always
blown.
The camera is an Olympus E-M5 with 45/1.8 lens, and the RAW developer is Capture One v. 7.1.2. Here's a screen clip of
the whole picture, followed by a section with and a string of car rears blown up to 400% so you can see what's going
on with the pixels.
<http://gallery.leica-users.org/v/pklein/temp/SpaceNeedleMoonriseScrPreview.JPG.html>
I like the moon exposure better in this one.
<http://gallery.leica-users.org/v/pklein/temp/SpaceNeedleMoonriseCars400pct.JPG.html>
Advice, anyone? Is this just an inevitable result of the Bayer array, or is
there a way to fix it?
Nothing to do with a Bayer array. As Chuck outlined, there's no way to fix it for this sort of scene. At least no simple
way.
What I would try is even greater underexposure of the whole scene, using a lower ISO, to keep noise down as far as
possible. Bracket, and use the highest exposure where the lights stay red at 100%. Then in post, raise the midtones, and
use NR to control noise there. Separately whack the shadows with a combination of masking them out of the increased
brightness used on the mids and really heavy NR.
I've used this strategy successfully on other subjects with similar problems. It is time consuming and I can't imagine
doing it in an editor that doesn't have layers and layer masks.
As Chuck suggests, the other solution, without reshooting, is to replace the offending pixels with correct colors. You
may do it by retouching, one by one. Or, in PS, select them by color and area and use replace or Match Color. Either way
is likely to be tedious.
Red Moose
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What if the Hokey Pokey *IS* what it's all about?
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