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I want to be able to blend ambient light with fill in flash, using off camera flash in manual mode. I don't understand how to translate the information from the light meter into settings to input into the flash light. Is there a formula i can use as a rule of thumb?

My equipment:

  • Canon 550D
  • Flash speed light 4030ex
  • Seconic flashmate L308S
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It's all in stops (EV). Remember that a stop is a halving or doubling of the light. So, the manual power settings on a flash are a full stop scale (1, 1/2, 1/4, 1/8, etc.). Look at the aperture setting you want to use in the camera. Look at the reading the meter gives you. Whatever the difference is between those two settings? That's how much of an adjustment you need to make on the flash. For example, you want to be at f/4, but the meter reading is f/8. Then you want to bring the flash power down by two stops (e.g., going from 1/4 power to 1/16 power).

Neil van Nierkerk teaches a good trick, if you're not good at juggling f-numbers in your head. You can just use the camera itself to tell you how big an adjustment you need to make by adjusting the aperture from where you want it to be to what the meter's reading out, and counting the clicks. Then put the setting back where you want it on the camera, and adjust the flash's power output by the same number of "clicks" (just make sure the meter, camera, and flash are all set to use 1/3-stop adjustments).

See also: What does f-stop mean?

  • Hi and thank you answering my question. You make it sound easy, I will try this out and see how i get on. – Andy Mar 31 '15 at 10:22
  • Hi, Andy here. I have tried to use Neil's trick (counting click) But my canon flash wont let me set it to 1/3-stop adjustments. I have read the manual over and over again but their doesn't seem to be a setting to do this. So i'm guessing I either need a more sophisticated flash light or I need to learn how to use the metered number and calculate this for my flash light. Sorry If I am sounding really daft, but i am finding this really difficult to understand. – Andy Apr 1 '15 at 9:46
  • Say i want to be at f8 . My flash light is set on 1.1 full power. I take a test shot and the meter reading is giving me f 14, am i correct in thinking i would dial down the flash light from 1.1 to 1.16? – Andy Apr 1 '15 at 9:58
  • @Andy No. That's two stops too many. :) Let's round up since you can't do 1/3 stops, and say the meter is giving you f/16 (f/14 is f/16 -1/3EV). f/8 to f/16 is two stops. So, at full power, you'd want to come down two stops, or halve the number twice. So 1-> 1/2 -> 1/4. Maybe print out the f-number 1/3 stop scale and use that as a reference? f-numbers are on a square-root-of-2 scale (i.e., the values don't double, their squares double), so most of us just memorize the numbers. :) – inkista Apr 1 '15 at 18:25
  • @Andy One more thing--there's a custom function on the L308S to display only full stops. It's on page 20 of the manual. – inkista Apr 1 '15 at 18:37

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