Why doesn't narrowing the aperture have the same effect as lowering the flash power?

Question

I have a small doubt here. I'm no strobist and just got my first flash (Canon 430ex II Speedlight). While I was playing around, I found that narrowing down the aperture DOES NOT have the same effect that lowering down the power of the flash has. I'm using reflective umbrella here to bounce the flash back.

Logically, narrowing down the aperture should lower the flash power reaching the sensor and hence darkening the image. But it doesn't have the same effect that lowering down the power does. And yes, I've tried it in both TTL and Manual modes.

Answer

Narrowing the aperture will reduce all light coming in to the sensor, including light coming from the flash itself and ambient light.

Reducing the flash power changes the ratio of flash light to ambient light. True, it will reduce the overall amount of light, but the reduction is not as evenly distributed as when you reduce the aperture (which reduces everything).

Taken to the extreme: if your flash power was very low and your ambient extremely high (such that the ambient dominated the exposure), lowering your flash further still would have no effect. Think, for example, of a low-power flash on a very sunny day. You'd have to look closely to notice any reduction in flash power, though aperture will have a very significant effect.

So, if ambient light is a significant factor in your exposure, then you will indeed get different results by changing one or the other.

Now, if you were using a high shutter speed to completely eliminate the effect of ambient light, then there might be finer details to discuss here. Perhaps you could post a photo of your shot and describe the details of your exposure & lighting setup?

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