Getting a clean black background when focus stacking.
Very often I encounter a beautiful image with black background but with a slight set back — the background is not as “black” as the author (and I) intended to be. And again often, you can see lots of photoshop efforts to make it black, such as spotty blobs of different shades of black, traces of black brushes, etc, but these efforts fail too frequently to wonder how to get a good clean black background!
I have to admit that when I first started trying to take some photographs with black background, I had trouble with it. But after some practices and understanding of light, achieving such result is not as difficult anymore, though still limited by space. Here are some thoughts and I hope it can help you.
First of all, not all black background material are equal. Many photographers trying to get black background would put a piece of black cloth behind their subjects, thinking this would give them black background. But putting a black cloth behind the subject as background is actually only half the story. Why? Because not all black material yield good result. Some material are reflective even if it might appear black (think black plastic bag). When light hits these material, some lights are reflected into camera lens. This is particularly true when flashes or strobes are used because we do not see how the scene is when flashes or strobes are fired.
So how do we pick the right material? Well, we can only pick the right one comparatively, ie, we can pick one that is “blacker” if we have a choice. Sure we can line up different materials and take a picture of them and pick the one that is darker. Here is an example where a white wall is covered by a black cloth, which in turn is covered by a professional black background cloth:
This image is obtained by setting camera to spot meter mode and is exposed using the middle cloth. As can be seen, the “Pro Black Background” is the obvious choice.
As can be seen in above picture, the “Pro Black Background” is the obvious choice here. But how do we quantitatively analyze this? Using pixel values does not seem to produce accurate results — there seems to be some “haze”, this is caused by some additive value to the “black”, so you can not simply take a ratio of pixel values.
Then, how do we analyze this? The answer is reflective metering. If you have a light meter like Sekonic L308S, you can use its reflective meter mode. With light meter, you can point the lens (exposed by removing the dome) at each part of the example and get a reading. Then you can compare the exposure reading to get accurate ratio.
Light meter in reflective metering mode (the dome is removed)
However, if you do not have a light meter, your camera can be used. Here is procedure to do so:
- Set your camera metering mode to center spot. Different cameras have different settings but most modern camera support this metering mode
- Since we are measuring dark cloth, set ISO as high as possible, say ISO 6400
- Set lens aperture to wide open so we can get reasonable reading, in this case f/1.8.
- Set camera to Aperture priority mode (Speed priority mode works, too).
- Aim the center of view finder to the white wall, middle black cloth and the professional black cloth (whatever you are comparing) and take a reading.
Following above procedure, the reading I got were 1/4000, 1/250, 1/25. So the exposure ratio is, in terms of stop, 1:4:7.32, meaning that the white wall is 4 stops brighter than the middle one and 7.32 stops brighter than the professional cloth. The professional cloth is 3.3 stops darker than the middle black cloth.
What does this mean? It means the professional black cloth can yield a much “blacker” background — any reflective imperfection on the cloth will be reduced by 3.3 stops, thus reducing the chance of messy background.
Here is a comparison
In this example, the top image was shot with “regular” black cloth and the bottom one was shot with “professional” black background cloth.
You can see, the background in top one is uneven as the back drop is not very flat, thus any reflection will cause uneven background. The bottom image, however, show nice clean background because the professional background is 3.3 stops darker than the regular one, thus the uneven surface does not show
Second of all, to get clean black background, keep the background as far away as possible. Why? This is where the inverse square law of light applies — the further away from light source, the less intense the light is. So if you double the distance between light source and background, you can reduce the exposure by 2 stops, when the scene is properly exposed, your background might be exposed by 2 stops less, making it much darker and again reduces the chance of getting messy background.
What does this mean? It means, even if you do not have the best professional black background, just by moving the background further can greatly improve “clean black background” result.
Though it still shows some messy background, the one on the right is much better than the one on the left because the black paper as background was moved further away from scene, reducing exposure on background by at least two stops.
Third of all, if we are limited by space, move the lights as close to the scene/subject as possible! Say what? This is a bit mind boggling, wouldn’t that increase the exposure on the background, too? What I am saying is that, if you do not have a large studio with a lot of space, try to keep lights as close to the scene as possible and keep the background as further as possible constrained by space. The logic behind this is the same, to increase the exposure ratio between the scene/subject and background. By moving the light closer to subject/scene, it increases the light intensity on the scene/subject, but, because of inverse square law of light, the increase of light intensity on the background is much less even though over all light intensity increased. Because of the differential between increase of light intensity on the subject/scene and background by moving the lights closer, we have increased the exposure ratio between subject/scene and background. Here is a example
From the higher contrast of the table cloth and longer shadow on the left, you can see the light is closer and the background is cleaner. Overall exposure for the right one is reduced (but still looks a bit more) as the light is moved closer and so is the background but at steeper rate (light fall off) due to inverse square law of light
- Use better black background cloth or paper
- Keep background as far away as possible
- If limited by space, keep lights as close to the subject/scene as possible
- Keep background as flat as possible
These rules apply to all situations in photography, be it portrait photography, product photography, macro photography, water drop photography, etc.