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 1

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?

Answer 2

What was your ISO setting? If your camera is set on Auto ISO, narrowing the aperture will cause the camera to compensate with ISO and resulting exposure will not change, whereas changing flash power/compensation will not affect the ISO.

Answer 3

It depends on mode. Here are the basics:

In manual mode, narrowing the aperture will absolutely reduce the amount of light hitting the sensor as long as you are at sync speed or below. Naturally, the lower the speed, the more the ambient light factors into the equation, which clouds the issue a bit.

In TTL mode, narrowing the aperture will cause the flash to power up in an attempt to get a correct exposure. You can exceed the capability of the bounced flash -- 430EX flashes aren't the most powerful kids on the block. Anyhow, the more the flash powers up, the longer the duration and the more the sync speed matters. Again, ambient light will cloud the issue.

The best way to test it is this: Set up a simple target, relatively close to the camera. A still life or test card will do. Put the flash on camera and try the experiment of varying the aperture in a dim room to remove ambient light from the equation.

Next, add the bounce into the mix. Try it on manual and then auto. You should get the results you expect.

Finally try on-camera and bounce with some ambient light mixed in.

With these results and the camera in the same position looking at the same subject, you should be able to see what is causing your unusual results. But be aware, stopping the lens down is not exactly the same as powering down the flash -- just mostly the same. Flash vs. ambient is the largest differentiating factor; "smarts" built into the camera and flash are another factor.

It would be great to hear what your results are.