Why the color differance?

[mjs1973]

The set up for these two shots was identical. The lighting, the back ground, camera, and lens didn't change. The only thing that I changed was the exposure. The longer the exposure, the redder the background became. I was using a solid black piece of cardboard for a backdrop. The only post processing done was a crop and some USM. So why would a longer exposure turn the backdrop red?

[another view]

Clarify a couple of things - you're only using incandescent light, no flash - right? And you compensated your aperture for the shutter speed?

If both of these are the case (which is how it looks from the images) then I'm stumped. It looks like just a little incandescent light spilling on the background, but why it would show up on one when the exposure (aperture + shutter speed = EV) is the same. Any more details you can share?

With slide film - especially Velvia - you will have a little reciprocity failure, which in the real world means that longer shutter speeds need a little extra exposure (even longer speed) and the colors will shift a little too. The greens are slightly different on my monitor but very close (not enough to worry about unless you're shooting clothing IMO). Check Fuji's website for the details on when you need to make an adjustment for this (again, if it's critical).

[mjs1973]

AV, your assumptions are correct. This was shot with a small lamp as my main light source, no flash. I had a tissue over the lamp as a make shift diffuser. The overhead lights in the room were on also. The EV was the same, just used different aperture & shutter speed to get it. This was nothing important, just tesing my macro lens. This was shot in digital if that helps any.

My only guess would be that with a longer exposure, the back drop reflected more light from the rest of the room, but it's only a guess. Part of the walls and the carpet in the room are a dark maroon color.

[Chunk]

How long were the exposures and what were the aperture and ISO settings?
Could this be noise? Try shooting the same exposure with the lens cap on.
As you can see, I don't have a clue. The ratio of room light to lamp light should be the same, no matter how long the exposure.

BTw, nice macro, I like the reddish background.

[another view]

My only guess would be that with a longer exposure, the back drop reflected more light from the rest of the room, but it's only a guess.

That's what I first thought of, but since you're using available light (no flash) and therefore changing your aperture to compensate for the longer shutter speed (which we can see because the subject has nearly the same exposure), the exposure for the background is the same. BTW, forget all that stuff about reciprocity failure - it doesn't apply to digital.

I thought this because that's how I adjust the background exposure for flash-on-camera stuff like wedding receptions. Using the max sync speed of 1/250 at f8 will look like a cave, but 1/40 at f5.6 (as long as you can hold it steady and people aren't moving!) will allow some exposure of the background so the shot looks better - but the subject is lit by flash so that exposure is the same. Different situation but for some reason the results are similar. Still stumped...

Digital noise looks something like film grain - but the look of it can vary from camera to camera. Some has more color to it than others. I don't see any real amount of noise in the background, but a dark smooth area like that would be the first place you'd see it. Chunk's tip of taking a shot of the same shutter speed with the lens cap will show you how much noise the camera actually produces (use same ISO of course, but aperture really doesn't matter here).

[Chunk]

Digital noise looks something like film grain - but the look of it can vary from camera to camera. Some has more color to it than others. I don't see any real amount of noise in the background, but a dark smooth area like that would be the first place you'd see it. Chunk's tip of taking a shot of the same shutter speed with the lens cap will show you how much noise the camera actually produces (use same ISO of course, but aperture really doesn't matter here).

Aperture shouldn't matter but have it the same anyway. When you are trying to solve a problem and don't understand what's happening, try to vary only one thing at a time and reset to the original setup before trying another change.

Mike, are you shooting raw images? If not there are white balance, saturation, contrast, and maybe some other settings that need to be considered.

[Sebastian]

Were you shooting film or digital?

Digital: White balance was off and as the exposure went up the color shift became more and more apparent. The background was underexposed in the first one and that's why it went black. Had the white balance been right, and the background true black, it would have ended up grey, but since the white balance was off, it shifted to red.

If you were shooting film all the same applies, except that you would either have to color correct in post, use tungsten film or filter the light to match it to sunlight.

[Sebastian]

Let's take a closer look at your first image.

Right off the bat the shot looks way too warm, but that might not be aparent unless your monitor is at least closely calibrated, so there is another way to check. When you first look at your levels immediately you can see the something's not right, the highlights should be white but the blue and green are nowhere near them, whereas the red component dominates:



Just moving the midtones over to open them up shows the true color of the background, which is anything but black.



Get a grey card and white balance properly.

[another view]

Good point - Auto White Balance can change between exposures even if what you're shooting didn't (much). Zoom in or re-compose a little and AWB can and probably will change. Same with auto exposure. Custom WB is usually best (or RAW) but not always needed. Here, you can see the difference though.

Can you tell us the aperture, shutter speed and white balance for the two shots?

[Sebastian]

Good point - Auto White Balance can change between exposures even if what you're shooting didn't (much). Zoom in or re-compose a little and AWB can and probably will change. Same with auto exposure. Custom WB is usually best (or RAW) but not always needed. Here, you can see the difference though.

Can you tell us the aperture, shutter speed and white balance for the two shots?

Steve,

I'm positive this has nothing to do with AWB, I'm sure the WB was the same for both shots. Like I pointed out, the original shot with the "black" background had a red background, it was just really, really dark and it LOOKED black. The exposure did not shift colors, it only magnified the fact that the light was too warm for the chosen white balance.

[mjs1973]

Thanks for all of the replies! Both shots were taken with the same custome WB. It was an old WB that I had set up a while back and just haven't gotten around to changing it.

Here are the shot specks: Info in red is for red BG, black info for black BG.