In the previous post, I presented a method of creating multiple image composites by photographing a number of frames and then combining them in Photoshop for the desired effect. A while back I was interested in capturing motion and passage of time in one frame and produced a collection called “Time Compressions”. They used the slow shutter speed while either the subject, or the camera, or both moved during the long exposure. You can tell whether a photograph used a long shutter speed since the subject gets smeared, creating patterns of shapes and lines depending on lighting and the subject. Here are some examples of long exposure along with some movement.
In these examples, the image or part of it “drags” across the sensor and creates the results based on tonal values. One may argue that these have multiple images in each frame, that is a bit of a stretch. The result I am trying to achieve should show distinctly captured instances of the subject appearing several times in the frame. In order for that to happen, the image needs to form multiple times on the sensor so that it would record those instances. One way of achieving that is to open and close the shutter multiple times to record multiple instances of the subject, which what André Gallant showed using his Nikon camera. We know that not every camera, Canon digital cameras included, can do that. But, what if I was able to turn on and off the light that hits the subject during a long exposure? That would certainly create multiple, and cumulative instances of the same subject on the same frame. I think by this time you are getting the idea. Since I cannot turn on and off the sun, I am going to use a light source that can be controlled in that fashion: Canon Speedlites will offer that capability very nicely. Here is a preview of what we are trying to achieve.
As you can see in the image on the left, the flowers appear several times in the frame. Exposure data is 0.5 sec at f-8. During that half second exposure, the flash that was mounted on the hot shoe of the camera, a Canon G9, fired multiple times to register the number of images you see here.
What determines the number of images captured, superimposed on each other are the settings on the flash and the duration of the shutter speed. Stroboscopic flash is typically used to capture the swing of a golfer or a tennis player, or the movements of a dancer, and the like. Depending on how many times the flash fires during the long exposure, a new image is photographed with every burst of light that hits the subject, which is moving during the exposure. In this technique I am outlining, there may be a small subject movement, like a breeze rocking the flower head back and forth, the actual source of the movement is the camera movement. Here is the process:
- Choose a low light situation, late afternoon, early evening where the ambient light would not make things harder (it is possible to do this at other times too with a bit more planning)
- You will need a Canon Speedlite that has stroboscopic capability. Since they have different ways of setting the parameters, I will stick to the general explanation using a Canon 580 EX, which you can apply to your flash
- After setting up the flash and the camera (see below), focus on the subject, practice the movement path you will follow
- A little outside where you want the capture to photograph, start moving the camera at the practiced speed and release the shutter and continue moving until the flash stops firing
- Review your image and repeat as necessary. WARNING: MAKE SURE TO REST YOUR FLASH BETWEEN FIRINGS, this is a high-stress operation for the flash. Read the manual.
The flash unit needs to be set to multi flash mode. On a Canon 580 EX, this is achieved by pressing the “Mode” button until “MULTI” appears on the display. There are three variable after this point that control the appearance and the exposure of the final photograph.
- The flash power: Since the unit needs to fire in very rapid succession, it will not be able to do so at full power. Keeping the distances reasonably close, you will need to set the power to 1/4 to 1/128. This will control the amount of light that will fall on the subject. Press the center button on the control dial until the power flashes and then rotate the dial to the desired power level. Remember, multiple flashes will accumulate on the same frame and the total exposure will be determined by the sum of the flash firings. So, 1/64 or 1/128 is not necessarily too little light.
- The number of flashes to fire: This setting will determine the number of distinct images that the sensor will capture at the end of the process. Let us say that we want 10 flashes. So, press on the center button again until the second group of dashes or numbers start blinking and rotate the dial to the number of flashes you would like, in our case it will read 10.
- Herz (Hz), is the frequency of something occurring in one second. If you want the flash to fire at the rate of 5 flashes per second, you will set the last parameter Hz to 5. Press on the center button until the third block of numbers start blinking. On the 580 EX, Hz can go as high as 199. On others this may be different. Remember, this is not the number of flashes that will strike, but how fast they will fire. Let us say we want it to fire at the rate of 5 Hz, the dial will read 5.
The flash is set, now you need to set the exposure on the camera. Set the camera to the manual exposure and determine the f-stop you would like to use, say f-8. You may need to refer to your flash manual for guide numbers to calculate the proper f-stop to use. Now you need to calculate a shutter speed that will stay open long enough for the flash to fire 10 bursts at the rate of 5 per second. Can you deduce the fastest shutter speed you need, five per second, and I need 10 flashes? It needs to be set at 2 seconds or longer for this to work; flash count divided by Hz. If you set the shutter to a longer time, say 4 seconds, then the ambient light may also add some information in the form of streaked or smeared image.
All this may sound complicated, but once you have your Canon Speedlite in front of you, it will make more sense. Now, you can capture multiple images on one frame using Canon gear. Experimentation is the key for you to find the results that please you. Vary the power, number of flashes, or Hz for different effects; vary the speed at which you move the camera; change the f-stop; have fun. Remember the warning to rest your flash unit during these experiments, they really need to rest. The same process may very well work with Nikon flashes, read the manual.
Here are some photographs that have been captured with a flash, firing anywhere from one to many bursts during the single exposure to kick-start your experiments.