Table of Contents
How to Determine the Correct Exposure for Flash Photography Based on the Exposure Index of the Flash
Determination of Exposure Index
Both the flash bulb and the electronic flash have a specified sensitivity index. The exposure index, GN24 or GN32, etc. are printed on the product manuals of all kinds of small and medium-sized electronic flashes.
The GN exposure index is determined by the sensitivity of the photosensitive film (usually with ASAI00 as the standard), the exposure distance of the flash and the F value of the aperture. For example, some electronic flashes are marked with GN32, which means that using ASA100 photosensitive film, in the case of aperture Fl, shooting the scene at a distance of 32 meters can get accurate exposure. Conversely, when taking pictures with an aperture of F32, the subject should be placed 1 meter away.
Calculation of Exposure Index
The formula for calculating exposure index is as follows:
Exposure Index = Aperture × Flash Distance
Here we can regard the exposure index as a fixed value. When the aperture is enlarged or reduced, the photographic distance (that is, the flash distance) will be adjusted accordingly. Conversely, when the shooting distance changes, the aperture must also be adjusted accordingly.
In this way, the following calculation formula can be obtained:
Aperture = Exposure Index ÷ Distance
Distance = Exposure Index ÷ Aperture
For example, if the flash guide number is GN24 and the known shooting distance is 3 meters, the aperture=24÷3=F8 can be calculated according to the formula.
However, it must be noted that the index of some products is calculated in feet, so it should be converted into feet as the calculation standard to avoid errors.
The f-factor table is usually engraved on the back of the lampshade of the electronic flash or on the lamp handle. According to the distance between the flash and the subject, it is easy to find out the f-factor that should be used. This kind of photosensitive table is generally formulated according to the photosensitive film of ASAI00. If you switch to a faster or slower photosensitive film, you must narrow or open the aperture accordingly.
How to Eliminate the dark Shadows Appearing Behind Flash Photography Portraits
Shooting portraits indoors, if you shoot from the front, a black shadow will appear on the wall behind the subject, etc. In fact, this shadow is the projection of the subject himself. The closer the person is to the background, the more pronounced the shadow will be. This kind of dark shadow is thick and deep, very stiff, often destroys the harmony of the picture, and is generally not welcomed by people.
The easiest way to get rid of shadows is to keep the characters away from the background. Make the shadow cast to the ground, preventing the shadow from clinging to the back of the character. If the environment does not allow this, try to choose a dark background so that the shadow cannot be seen.
You can also use split lights to illuminate the background from the side to eliminate shadows. In daytime photography, the position of the characters can be arranged in front of the open doors and windows, and the shadows will not appear. However, the outdoor brightness must be estimated at the same time when exposing.
How to Do Multiple Flash Photography
Many photography enthusiasts face interesting subjects, such as tunnels, caves or very long walls, etc., and want to shoot, but the light is too dark. Or a long wall, shooting at night, the effect is peculiar, but it is not easy to solve the lighting problem. With a flash, it is impossible to light up such a large area. As a result, he is often at the mercy of the Yuan policy, and has no choice but to look at the ocean and sigh.
Use the flash correctly
So is there no way? Of course not! How can a flash light evenly illuminate a large area? That is to use the flash for multiple flash shooting. That is, use the flash as a “brush” and use it to paint on the subject.
Implementing this lighting scheme does not require special techniques, nor does it require special fixtures and equipment. Rather, it requires careful design. In order to make all parts of the scene illuminated evenly, the distance from the lamp to the object should be roughly equal for each flash, and the number of flashes and the position of the lamp should also be planned in advance, and there should be no blanks in the photo that are not illuminated by the light.
Use the shutter correctly
Since the flash is moving, and possibly quite a distance from the camera, synchronizing with the camera via the pulse line is impossible. At this point we have to take a rather slow shutter. This means that you need to open the camera’s B door or T door multiple times. And it also means that you have to use a tripod.
If your camera doesn’t have a multiple exposure mechanism, you can’t use the B or T repeatedly. You can only keep the B or T open from beginning to end. Between each flash, you can block the head lens with something like card paper, lens caps, or the like.
Correct exposure
The range of flash illumination is proportional to the distance from the flash to the subject and inversely proportional to the exposure.
The calculation of exposure is quite simple. Just remove the guide number of the flash from the distance from the lamp to the object as usual. Of course, there is another difference from the indoor flash. There is no doping on the four walls, and it is in complete darkness. Therefore, the actual exposure should be opened with a larger aperture. And there are also several flashes at the same position.
This is often the case with adding color chips to the lights. When it comes to overtones, it produces beautiful, colorful images that are much prettier than photos without overtones. As mentioned above, just add a little more exposure. Of course, the arrangement of the color blocks should be suitable for the creative intention and should not be self-defeating.
(To Be Continued)