By Andrea Ghilardelli
What if I told you that you could light up any indoor scene with just a pocket flashlight in order to get a beautiful picture of it? Yes, that is right! No need of powerful lamps or flashes, but just a tiny faint flashlight. No matter how dark the interior is (even completely dark) and not even its dimensions. Keep on reading if you want to discover how all this is possible.
All the theory we need to know, underlying this smart feasibility, is related to light and image sensors. No matter which kind of sensor, be it an electronic one (CCD) or a traditional film, all sensors register in some way all incoming photons (light). It does not matter the exact time when the photon hits the sensor: each incident photon just adds up to all the previous photons. Moreover, photons coming from different parts of the scene hit different parts of the sensors.
Therefore, if we want to take a picture of a dark room, we have basically three options. The first trivial one is to light up the room with lamps or flashes so that the light is enough as in any standard photograph. The second option we have is to make a very long exposure photo with the aid of a tripod. No matter how dim is the lighting, with an arbitrarily long amount of time we will eventually collect as many photons as needed. Our third unusual option, the one we are going to exploit in this article, is to take several pictures of the room and in each of the picture we light up only a small portion of it. After that, we will have to add all the pictures in a unique final image with a photo editing software. The final image will be the sum of all the single short pictures.
There are two fundamental concepts here. The first is that we can light up a portion of the image at a time and then, after summing all the images, we have the entire scene completely illuminated. The second is that we can integrate incoming photons indifferently in a unique long exposure or equivalently in a series of single short exposures and then summing all them up. Let me say that, in digital still cameras, the maximum exposure time is always quite short, while in film cameras it can be arbitrarily long. Hence, with digital cameras, summing up several images is the only way to take a photograph in very low light conditions. Rephrasing what we have said thus far, the method proposed in this article performs both a temporal and a spatial integration.
Each short picture we take needs only a small quantity of light; that is why a simple pocket flashlight is usually enough. The fainter the flashlight (or whatever you would like to use) the more the photos needed for summing up. The brighter the scene you want to appear, the more photographs you will have to take. Typically, you will have to take a few pictures with the same portion of the image brightened in order to achieve a reasonable result with a simple pocket flashlight. As an order of magnitude, I could achieve a good result with a one-diode flashlight by summing up about 30 pictures of a living room, 15 seconds each at 50 ISO.
Let me now give you some tips and tricks in order to achieve better results. For starters, as we are stacking many images one over the other, a tripod is essential, or the alignment of all the pictures will be problematic. The images we are going to sum up will need to be aligned at pixel level. Also, to avoid micro movements, always use the self timer of your camera to let the oscillations dampen after pressing the shutter release button.
It is not easy determining how long to point the flashlight, in which direction and from which vantage point. It depends on the effect you want to achieve, too. If you are looking for a uniform lighting, keep in mind that you will need more time lighting far objects than near ones. Hence, take more pictures lighting up distant objects. Another important trick is not insisting with the flashlight in a fixed position, because the final image could contain brighter areas as if illuminated by a spotlight (unless this is exactly what you want). Rather, keep moving the flashlight around a pretty large area. Note that the speed you move the light at is not influential.
By changing the position of the flashlight, you will change the direction the light is coming from. So, if you just stay behind your camera, the effect will be similar to using a camera flash. But you can hold the flashlight anywhere, thus providing the better light direction your image requires. Note that this might prove an extremely creative benefit. Moreover, you do not need to take all the pictures holding the flashlight from the same position. Instead, you can take a subset of the whole set of pictures while holding the flashlight from any arbitrary point, thus creating the effect of multiple lights! For instance, if you take some pictures while holding the flashlight from the left of your camera and then another set of photographs while holding it from the right, you simulate the presence of two light beams. This could be useful to lighten up distant or dark objects, as well. Move closer to distant object with your flashlight to lighten them more effectively. Note that in some pictures you could even hold the flashlight within the scene frame. In that case, however, a post processing will be required to delete the part of the image containing the flashlight or unwanted results could arise. So, usually you could take only a portion of the whole set of photograph holding the flashlight within the scene.
I hope this article spurred you to experiment this fascinating and creative technique. So, now grab your camera and start shooting!
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