How can I make sure a dark background comes out pure black with a smartphone?

Question

I recently used a Samsung GT-I9506 to take photos of coins. The idea was to make use of "black" paper as a clean uniform background.

I one of the photos the colour of the paper appears completely black and can not be distinguished from true black in a editing program (while in the other photos it is clear that the paper is not completely black).

I have not been able to reproduce the result.

Is there a setting that allows the phone to detect a black background an adjust the colours accordingly?

Answer 1

One thing that hasn't been mentioned is that you need to achieve the highest separation between foreground and background lighting, so if you can find a way to lift the coin above the surface, you can increase the light on the coin and get a relatively darker background.

Complete the job with exposure compensation and post processing.

Answer 2

More exposure makes black look gray, and Less exposure makes gray look more black. But in practice, for your coins on a jet black background, your best investment would be a yard of black dress velvet from the fabric store. It will photograph extremely black (whereas any black paper will reflect more light).

Answer 3

"Color" would be if you had something white coming out with tinge of a color.

I assume that you mean that the black came out as gray instead of black, much lighter than you expected. To correct for that, you need to lower the exposure, that is dial "exposure compensation" below 0.

This effect is because how "auto exposure" function works. It tries to get even exposure on whole image, it has no way of telling what you intended to be pitch black (as in your case) or 100% white (eg when photographing a winter landscape). If you want the photo to come out predominantly dark or light, then you need to tell that to the camera by using exposure compensation.

Another option would be to use center-weighted or spot metering, when you tell the camera to look at the center and ignore the rest, but I don't know if it's included in the basic camera app. You might need some advanced photo app to do this. You can also move so close that the coin would take entire screen - then the amount of black left around would be too insignificant to lower the amount of light camera sees, but then you may run into troubles with close focus.

Answer 4

Use a camera app that allows you to manual control. The automatic exposure is going to try to make the scene a standard, neutral brightness — which means it will adjust so your dark background is gray. That's normally pretty good, but not what you want in this case. You could use the +/- exposure compensation some apps provide, but that's still unpredictable. So, you need an app which will let you set ISO and shutter speed to fixed values (aperture is usually always fixed on smartphones). Pick values that look right, and there you go. It won't hurt to set manual white balance, as well.

I use the creatively-named Manual Camera, but there are many others.

You may still want to do a little post-processing to bring the black all the way down, getting rid of any texture of the paper or left-over artifacts. This will be particularly helpful if there are dark (but not black) details on your subject that you don't want to completely lose. Generally, you'd use the Curves tool to do this — see What is a "Slanted S" in a curves layer? for some examples.

Many of these apps also support saving in RAW, which will give you a little advantage in post-processing, although not strictly necessary.

Answer 5

I do not think so. Normally the exposure is set to detect the average light and translate that to middle gray.

But if your camera has some kind of histogram, or a built in app that can edit your photos you potentially could:

• adjust the histogram

• increase the contrast

• reduce the exposition

Check your user manual and try to find any of this 3 features.