Target influenced differences in exposure readings.
This might also be titled, “Why I shoot in M with custom WB.”
Background: It used to be that all reflective meters thought that they were looking at an 18% gray target. So if you pointed it at a white house, a transparency slide would come out looking 18% gray. If you pointed it at a black sculpture, a transparency slide would come out looking 18% gray. Examples here.
So we used incident meters which would read the light falling on the meter, not the light reflected toward it. Note that I said “transparency slide”. That’s ‘cause negative film had more exposure latitude, & the guy in the lab would adjust exposure for your prints, so you probably didn’t notice the difference very much.
I took some pics in Full-Auto-Mode intending to illustrate to a class how exposure reading on different color values will throw off the exposure readings, & was surprised to see that there wasn’t as much difference as I'd expected. The light subjects weren't as dark, & the dark subjects weren't as light as past experience predicted.
Then I realized that there was a constant involved – the highlights. It didn't work the way I expected because the images had highlights & digital exposure meters key off the highlights to some extent, trying to “expose to the right”, just like video will try to set 100 IRE.
1st result: Full-Auto-Mode
So, in the same lighting, I first shot flat, colored, construction paper targets without highlights. I filled the frame with the paper for each shot, so nothing else would influence the exposure.
The top group below shows the resulting colors under manual exposure set with a gray card, & custom WB. The last color swatch, outlined in gray, is black background cloth. Exposure was set at 1/30 sec @ f-8 for all.
The middle group below were Aperture Priority exposure, lens at @ f/8. The final range covered 4+ stops, just what I’d expected from my “days in film”! This shows the value of setting exposures with a gray card in situations where there is no reflected light to assist the meter. This may explain why sometimes you get the right exposure, & sometimes not when you’re still in the same apparent situation. Maybe you walked in, or zoomed in, & eliminated a highlight from the frame?
The last group below was “Green Box”, fully automatic mode including WB. Note that the cam assumes that you're hand holding and did not go below 1/60 @ f/5.6. It did want to use the flash on the last exposure of the black background cloth, but I wouldn’t let it.
Finally, notice that the very last exposure was of a gray card. Full auto WB didn’t do much good there, did it?
xchangx's Auto vs. Custom WB example: https://photography-on-the.net …p?p=16880109&postcount=12
Last, look at Wilt's series of shots here. The WB varied from 2028K thru 5916K, in 5000K light. An 80% error rate!
I recently used this info to take a pic in a dimly lit expo (pics below) where the lights above threw off the histogram, so I just took a pic of a white paper without including anything else, just as you would for a white balance shot, except in this case I bumped the exposure up just short of blowing out the whites.
(For WB, you need about a 2X underexposed pic so the cam has some gray info to work with.)
Let me clarify this. To use white paper to set exposure:
1. Take a reading of the white paper.
2. Increase the exposure by 2X. Actually, from experience, I open up about 8 clicks ( 8 times 1/3 stops = 2-2/3 stops.)
3. Take a test pic which includes the white paper & NO reflective highlights from anywhere else.
4. Chimp, which means to look at the Histogram and see how close you are to the right side. You want to be close, but not touching it. This will give you a good, general "start exposure" for most situations.
5. I would then enter that exposure into my "M" settings, & usually I'll delete the test pic.
At first, you might want to keep it to use for a post-processing WB setting. With Custom WB, I usually just adjust by eye in RSE.
More on that is in Need an exposure crutch? Using your hand as a substitute for a white/gray card exposure. It's the same principal as a incident meter.
Another example That's what I used for my exposure, & the highlight on the top of the tank gave me a reading of about 220 in PS. The lights in the ceiling were completely blown out at 255 in the pic below, so if I'd only relied on the histogram of the overall scene to set the exposure, the result would have been a pic about 2+ stops underexposed. RAW probably would have saved me (again!), but why take the chance!
See the pic below for a comparison. There's another "final" version at:
More on this method is in Post #9, below, or look at: Need an exposure crutch?
Do I ever use AutoWB? Only for tests!
Do I ever use a Preset? Sometimes for "walk around" shots I'll use the Sunlight one. For dawn or evening shots I'll use the Cloudy one as I like the warm colors & a CustomWB will destroy them.
Where do I use CustomWB the most? Always in the studio, or in other color critical applications.
Do I tweak after in RSE? Usually a bit if my eye says to, or a lot if I've screwed up! We used to use filters more often than not to tweak Ektachrome to get the results we wanted. Why not tweak with Digital?
I hope this is a help to some of the beginners out there. Any questions or comments?
There’s an update/continuation of this at:
Gray Card…White Paper. What’s best?
What’s best for exposure, Gray cards, white paper, expensive attachments for the lens?
Curtis has expanded on this in the thread
Comparisons of metering modes - an eye opener!
I recommend that everyone take a look at his results & join the ongoing discussion!
Another informative thread by Curtis that everyone should see:
How NOT to expose to the right