What’s this all about? This article is about setting remote flash exposure correctly using a seldom utilised method: distance priority. It works when you can’t use a hand-held flash meter and the flash is off-camera. It’s also my preferred method for ‘metering’ a single flash with or without an umbrella.
First, the equipment
To use a flash without metering (i.e. TTL, or a hand held flash meter) or chimping (taking a photo, reviewing and adjusting manually), we need to be able to measure distances and calculate flash power. All adjustable flashes come with a table of distance vs power for a correct exposure, either in the manual (boo) or digitally stored in the flash computer itself as a lookup table (hooray). In the later instance, this is called the distance priority mode – dialing in the measured distance, along with ISO and f/stop, automatically sets the flash power for the correct exposure. Magic!
Most people measure distance mentally, and quite well. But I completely suck at this. In fact, I recently had several conversations with some work collegues about this. I asked them to guess the distance of certain walls in our work place. Most were close, some were uncannily accurate, but some, like me, had not even an inkling. So this is why I bought a laser distance meter, and have never looked back. Simply point at the subject and I know exactly how far they are from the flash.
So here’s the workflow:
1: Visualise your photo, subject and flash placement, and set everything up as you wish. It can’t be neglected to mention that this part is the actual art of photography, and therefore the most important bit. The rest is just technical stuff which usually detracts from the ‘art’ if you like.
2: Visualise and set the desired ISO, apeture and shutter speed. This is normal stuff, shutter affect ambient, f/stop affects the depth of field (at this stage it actually doesn’t affect flash power as that hasn’t been set yet) and ISO affects the exposure range as always. Note all the settings and we now switch out of ‘art mode’ to ‘lighting engineer mode’.
a quick note: If you’re using a shoot thru umbrella, and who wouldn’t, take all measurements from the flash head as if the umbrella didn’t exist.A good umbrella will only loose 1 stop of light as it passes through it. Take note of this, though it’s a good idea to test yours. Set the flash to distance priority mode, or manual flash power and
3: Measure the flash to subject distance. I use a laser distance meter as I’m hopeless at guessing and tape measures are too time consuming.
4: Dial in the distance, ISO and f/stop chosen in step 2.
5: Set the compensation for the flash power. Here is where you add your personal flash power compensation to that which you need to compensate for your umbrella. So I usually like a -0.5 compensation to darken the flash a little, and I add the 1 compensation required due to the loss through the umbrella, resulting in a 0.5 compensation.
Shoot and enjoy the fruits of correctly exposed flash! Hooray!
The benefits of this system are many, and there are a few drawbacks. Firstly, the smarts in the ditance priority mode of my flashes show not only the dialed in distance, but also the flash range of power of the particular unit. This changes based on the f/stop, ISO and zoom of the head, and is expressed as a dark line on the LCD with the min and max distance displayed. You can see this in the left diagram with the flash ‘zone’ being between 0.9 mtrs and 1.1 mtrs. Remember this will change according to the other settings I’ve dialed in, but it is extremely useful to know that my desired distance (expressed as the big 1.0 mtrs in the left example) is within the range. If I was below the range, the image would be over exposed as the lowest power the flash is capable of firing would still be too high. Conversely, if I was beyond the 1.1 mtr distance, the flash would underexpose no matter high much positive compensation I dialed in, as the flash would simply not have the power. So as you can see, a lot is happening automatically, freeing me up to not focus on the technical too much, as it’s all just calculated for me.
A drawback is the need to choose f/stop and ISO before setting up the flash. This means if you’re changing setting at the camera, you’ll need to walk to the flash again to reset. There’s no way of getting past this unless you have an assistant, use iTTL remote flash units (very expensive) or TTL cables (very limiting in distance). One thing to remember is that most situaltions dictate certain ranges of f/stop. For example, I mostly shoot in f/2 for portraits in umbrella photography, so this problem is not so pronounced. And don’t forget, shutter speed (within the sync speed range) does not affect the flash exposure, only ambient. This gives you further latitude in selecting settings without affecting the flash exposure.
Lastly, and this brings me onto the promise of my next post, is the issue of using this rig for multiple flashes. I’ve yet to work out a ‘rule of thumb’ for adding a second or third flash. Stay tuned!