Canon 90 EX for beginner as a Master unit?

Started Jan 4, 2013 | Discussions thread
TangoMan
Senior MemberPosts: 1,168
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Here's how to control flashes manually.
In reply to DoctorPDA, Jan 5, 2013
This is my first time buying an off camera flash, and have only used DSLR for the past 6 months. I don't know if I have enough knowledge yet for manual flash, but I bought a few books to learn about it. I also have to figure out whether a soft box or umbrella is better for my use (portrait).

It's really quite simple.

The flash to subject distance determines how much light reaches the subject. (The camera distance is entirely irrelevant.)

The light falls off according to the square of the distance between the flash and the subject. For example:

  • 1 meter = x amount of light
  • 2 meters = 1/4 x amount of light

(but if your light source is very large, the fall off is much less important)

That's only good to know as a general rule and to understand what's going on, but in practice, you put a light somewhere, set a power level and take a test shot. (the "1/4" setting is handy because it lets your light recycle quite fast. 1/8 is even faster but the light output is weaker).

From the test shot, you can do three things.

Too bright?

  • Reduce ISO setting
  • Reduce aperture size (use a higher aperture number)
  • or Reduce the flash power.

You could also increase the flash to subject distance but it's really not practical.

Too dark?

  • Increase ISO setting
  • Increase aperture size (use a lower aperture number)
  • or Increase the flash power.

Moving the flash closer could also be a good solution and alter favorably the lighting of the scene.

Note that nowhere was it mentionned that you should change the speed setting. Changing the speed will not affect the flash, because the flash duration is so short (1/500 to 1/6000) that is can't be interupted by the camera shutter. The camera shutter will however affect the ambiant (continuous) light and you should use it to balance the ratio of ambient to flash lights.

Remember : Manual mode is you friend. Change the aperture setting to control the flash, change the speed setting to control the ambient light. It's that easy! (don't exceed the x-sync speed of you camera or you will get a black bar in your pictures. That critical speed is usually 1/125 to 1/250)

Does that help?

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