Ever wondered how film photographers took portraits with perfect lighting? The technique works just as well with Digital cameras if you're willing to learn.
Every flash has a few unique characteristics:
Intensity or Power
Bulb Shape, or Modifier Shape
Lens Design, Zoom settings
Portability & Power Supply
I may come back an write something about all of those topics eventually, but for today we're going to focus on intensity, and how to calculate an alignment with your camera settings using math - instead of peeking at the back of your camera over and over!
I'm going to make some assumptions here;
You understand Flash-Sync-Speed
We're not working with High-Speed-Sync or Hyper-sync
You know how Aperture and ISO affect Brightness, and what a "stop" is.
You know how to work your camera, trigger, and flash's controls
Let's get started! The first step will be determining just how powerful your flash is. This should be the most complicated part of the process. If you have a light meter, or can borrow one, that is the best option.
- Alternatively, you can load your images into photoshop and use the color checker or eyedropper tool to match an image of a gray-card to middle-gray (r:102, g:102, b:102).
As a last resort, if you can't get a light-meter, and you don't have Photoshop or a similar editing software... You can eyeball how you like your images to look this one time... it's not ideal, but it's better than nothing.
To determine your flash's power with a graycard, I recommend setting your strobe up on a lightstand, and measuring with your arms (your wingspan) to your subject - the graycard. You will take an image at ISO100, Your camera's max sync-speed (usually between 1/160 and 1/320) at f/8. Flash set to 1/1, keep the ambient light at a minimum (Just enough to enable autofocus), and take an exposure. Hopefully, the graycard will be overexposed at these settings. Next you are going to lower your flash power, one click at a time, taking an exposure at each setting. Depending on your flash, this may be 1/3, 1/2 stop or 1 full stop for each click - take note of yours.
Next, you will load all the images into photoshop and test the color of each image, find the one that is closest to 102,102,102 on the RGB scale... This image is the properly exposed one! Woohoo! Now you will count how many images in it is - for example let's say it was the 8th image. Now remember how many stops each click adjusted your flash? I'll use 1/3 stop for my example... We started at 1/1 in image #1, so image #2 would be 1/2+2/3, #3 would be 1/2+1/3, and #4 is taken at 1/2. Continue the pattern as far as necessary. #8 was taken with a flash power of 1/8+2/3 (some displays may show 1/4-1/3). Now you have your base exposure number!
To determine the above without Photoshop, a gray card, or light meter... you can do the above instructions with a friend's face, and stop when you feel that you have a good exposure (again, this is NOT the ideal method)
And lastly, with a light meter! Simply place your light meter the distance of your wingspan away from the flash and take a reading, reduce the flash power until you get a reading of f/8 on your meter... this will be your base exposure number!
No matter what method you used from above, we need to make those numbers useful, so you will want to make a chart. On the left side column, list possible camera apertures. I like to use whole stops to keep the chart minimal. On the top (or bottom) row, list possible Camera ISO settings. Again, I prefer full stops. To fill in the cells of the chart, we will start with your "Base exposure number" that we calculated above. So place your calculated number into the cell that is below ISO100, and to the right of F/8.
You may remember I got 1/4-1/3, and that's a lot to type for each cell, so I would simply remember to add a tiny bit of distance to my measurements when placing the flash! My wingspan is 5'9", So I'll go with 6'.
Now you can work your way across the chart and fill in the cells, most flash units don't go below 1/128, so I'll leave those smaller fractions blank, if your flash goes lower, go ahead and fill them in.
Now you have your very own personal flash guide that is custom for your flash! How do we use it? This is the fun part! First, you will want to get an ambient light reading of the scene you will be photographing. I'll make up some numbers. Let's say that it's evening, and the subject is gently backlit. We decide that f/5.6 will look good, and we're using an 85mm lens, so we want at least 1/120 shutterspeed. The meter is saying we're very underexposed, so we raise our ISO from 100 to 400. That puts the light meter about 1/2 of a stop underexposed - pretty close to balanced. Finally we will look at the chart and see - my aperture on the left is already set to f/5.6, and we will travel over to the ISO400 column. The chart tells us that our flash should be set too... 1/32 power, we dial that number into the flash and take the picture. Perfection.
Bonus tip! It's fairly safe to assume that most modifiers like shoot through umbrellas and soft boxes with single layers of diffusion material will cut your exposure by 1 stop. Knowing that makes it super simple to adjust, I just do it in my head, but you can always look 1 row higher on the chart for a reminder.
2nd Bonus tip! You can buy rectangular stickers that run through standard printers, slap that on the back of your flash or on your flash-transmitter, and you have a perfect reference sheet exactly where you need it all the time.