Understanding EOS Flash photography-simplified

Started Oct 15, 2003 | Discussions thread
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mchabbi Regular Member • Posts: 319
Understanding EOS Flash photography-simplified

I have been amazed by the confusion of people about flash photography with the Canon's implementation of E-TTL

So I decided to clear some air here.

Since this is a 300D forum, let us use 300D and an E-TTL flash as an example

First of all 300D is capable of using both E-TTL and Auto Flashes. Let us consider E-TTL only for now.

300D, when using an E-TTL flash uses the "selected focus point" as the point to read the light and meter the flash. This is important.
FEL is an exception, FEL uses the center focus point only.

Assume you are using the left most focus point.

The flash tries to expose the subject under the focus point properly ignoring the rest of the frame. So if you point your foucs point to a white shirt and the rest of the frame is dark, then the whole frame looks underexposed and the white shirt is properly exposed. That is because the flash emits light just enough to make the white shirt look fine in turn underexposing other parts of the frame(remember flash is the only source of light here or your shutter speed is high enough to lose the ambient light). If you point the focus to a black shirt, then the whole frame looks overexposed.

This happens mainly becuse the shutter speeds decide how much ambient light hits the sensor. Shutter speeds are not slow enough to register the ambient lighting and the user is relying on flash to illimunate the entire frame.

So the best results can be obtained if you know what brightness the subject under focus point is.


Use the FEL - Point the "center focus point" to a subject that is not either too white or too black. And press FEL, the flash meters from that subject now recompose and shoot.

If you used somebody's face as the center point and did an FEL then you expose their face properly even if your focus and framing is somewhere else.

There are 2 situations when the flash is used
Case 1) When the flash is the main source of light

In which case you shutter speeds are in the 1/60 to 1/200 range(mostly P and other modes excpet Av,M). Most of the people complaining are using these shutter speeds and dont understand what is happening when the flash illuminates the subject depending on the brightness of the subject and ambient light is not captured enough in the frame.

Case 2) When the flash is used along with other sources of light like sunlight outdoors(1/60th to 1/200) or indoor lights(30" to 1/60th)

There are very few people who use this technique for the fear of camera shake. This mode(usually Av) gives the best flash/ambient balance in a pic.

Will post some detailed shots after a while illustrating all the points.

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Mahendra Chabbi

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