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| p.2 #6 · Do I need a light meter? |
The goal of the exposure exercise is to record the same range of tone and detail you see. The way it is done indoors with two flashes (starting in a dark room) is to add enough fill to record the shadows at whatever f/stop the lens is set at, then overlap the key light over the fill until they are also reproduced correctly with detail in solid white just below clipping.
Back in the days of film the means to that goal was knowing the numerical key:fill ratio needed to fit a full range black/white subject (e.g. bride and groom) to the range of the print or transparency. For color prints that was a 3:1 ratio:
1:1 Even Fill
2:0 Key 2x (1-stop) brighter (incident)
3:1 reflected ratio.
On color film we would meter the fill (meter pointing at light from shadow position) based on the amount needed to record the detail in the black suit on the negative, then set the key light incident meter reading one stop brighter based on knowing that's what was needed to reproduce the white dress correctly. The result was a "normal" looking full range which was correctly exposed everywhere.
With digital the concept is the same (detail everywhere seen by eye) but the results can be judged visually. Try this:
Set your lens to f/8, camera 8 feet from full range subject. Spare the wife, just throw a black suit and white suit over a chair.
Start with flashes in M mode, Group A as fill over the camera and and Slave (key) in Group B off to the side an arm span between flash and subject. Set both at 1/128th power.
First raise fill to 1/64, 1/32, 1/16, 1/8, until you see detail in the folds of the black suit in the playback.
Now raise the power of the overlapping key light the same way until you see the highlighted parts of white shirt start to clip, then back down the power until it doesn't.
Note the group A and group B power settings. The same power settings will give you the same full range with detail everywhere at the same fill / key distances. Today, tomorrow, forever. It's the basic set-up I've used for portraits for over 40 years because it is predictable and consistent.
Next repeat the same "fit the scene to sensor" exercise in ETTL ratio mode.
Lens at F/8
Ratios on, fill over camera in Group A, Slave/key in Group B. Ratio set to A:B = 1:2 (2x more key than fill [incident]). Why 1:2 as the starting ratio? It should be very close to perfect at recording the entire black / white tonal range.
Now just shoot test shots adjusting FEC until highlights in white shirt are just below clipping.
Once the highlights are rendered correctly, check how A:B =1:2 is fitting the shadows to the sensor range:
Do you see detail in the shadows in the black suit? If not, change the ratio to A:B = 1:1. That adds more fill. The metering will adjust the flash output so a FEC adjustment after changing the ratio should not be necessary.
If the black suit is being rendered gray at A:B=1:2 (with FEC set for correct highlights) it indicates the 1:2 ratio is creating too much fill for your sensor range. Change the ratio to 1:3 which will reduce fill. Metering will adjust the flash power to keep the highlights correctly exposed at same FEC setting.
Upon completion of those two simple tests you will know both the manual power ratio and ETTL A:B ratio needed to fit a full range scene to the camera sensor range, without a meter. The difference?
In M mode the ratio and exposure will only be correct at the same distances between key and fill. Good for consistent shot-to-shot exposures but best suited for static shooting situations like formal portraits or still life.
In ETTL once you find the A:B ratio that fits the full range scene to the sensor range as you move around and your shooting and light distance to subject changes the metering sorts out how to keep the ratio and the exposure the same. Will in do it perfectly? No. You need to "chimp" looking at the highlights as a gauge and tweek FEC to keep them under clipping with detail. Err on the side of slight under exposure. You can make underexposed highlights correct in Photoshop, but you can't put detail back into blown one.
Now with that new understanding of your flash gear and camera do you still think you need a flash meter? If so and you buy it the exercises above will equip you with the knowledge to expose a full range in the event you drop and break it, or the battery in it dies (which happens unexpectedly at the worst times).