This page on Stroboscopic flash will take you throught the process of calculating the right exposure for your subject.
First, the easy part: Shutter speed.
For stroboscopic flash, the shutter speed needs to be long enough to let all the intended pops of the flash exposure the sensor.
There is a simple formula to calculate this minimum speed:
Minimum Shutter speed = Number of flashes divided by Firing frequency(Hz).
4 flashes at 20 Hz? = 1/5 sec
5 flashes at 5 Hz? = 1 sec
10 flashes at 100 Hz = 1/10 sec
The ideal stroboscopic conditions are total darkness as no ambient light will mix with your flash bursts. In such conditions, the shutter speed can be as long as you want, with the minimum stated by the formula.
What you still need is to calculate the correct flash exposure. That is a bit tricky. The important factors in flash photography are: f/stop, flash power and flash-to-subject distance. When you use stroboscopic flash, you’re splitting your flash power in fractions, that are delivered with each burst of the flash.
This means: 1 burst at full power = 2 bursts @ 1/2 power = 4 bursts @ 1/4 power = 8 burst @ 1/8 power = ….
Yet another factor to consider is the movement of your subject. Here it starts to get very tricky. For a subject with overlap (think about a rotating wheel) the right power setting for the strobo flash will be the exposure needed for the subject divided by the number of flash bursts. For subjects without overlap, like somebody running, the exposure needed for each burst is the full exposure for the subject. (That’s because subjects with overlap build exposure with each burst, like adding a layer of light “paint” with each stroboscopic burst)
Now, to determine the exposure for your subject (without a flash meter):
FIRST: Set your flash in Manual mode (not stroboscopic yet). Set the desired f/stop for your subject (depending on the DoF that you like to achieve) and determine the flash power that you need. You set this directly on the speedlite (1,1/2,1/4,1/8,…,1/128 fractions of the full flash power)
– set your flash in MULTI mode (stroboscopic)
THIRD [for subjects without overlap or fixed parts]
– Think about the number of stroboscopic bursts you want
– set the power setting you determined in the FIRST step and use it as power for the stroboscopic mode
– set the number of bursts you want in the number of flashes
– set the exposure time following the formula you know already
THIRD [for subjects with overlap of fixed parts]
– Think about the number of stroboscopic bursts you want (better if it’s a power of 2!!–> makes your life easier! 1=2^0, 2=2^1, 4=2^2, 8=2^3, 16=2^4, 32=2^5, 64=2^6, 128=2^7
– Now, reduce the flash exposure determined in the FIRST step by the number of stops you get from the previous sequence (16 strobe bursts = 4 stops).
Example: Let’s say that on the FIRST step your exposure was lens=f/8, flash=1/32. You want 16 flash bursts. Then set the flash 1/32–>1/64 (1 stop) –> 1/128 (2 stops). f/8–> f/11 (1 stop) –> f/22 (2 stops)
note that 1/128 is the minimum flash power, that’s why we ‘jump’ on the f/stop to get the additional 2 stops. Your desired stroboscopic exposure is: lens = f/22, flash = power 1/128, bursts 16, Hz (your choice, within the boundaries), shutter speed = 16/#Hz
!!! This results in the fixed object achieving a correct exposure but the moving element in the frame will be only partially exposed, resulting in ghostly shapes!!!
– Fire away!