Why doesn’t stopping down the aperture look the same as lowering flash power?
Asked 7/11/2011
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I’m testing a Canon 430EX II with a reflective umbrella and noticed that using a smaller aperture does not produce the same result as reducing flash power. Intuitively, both should reduce the flash exposure at the sensor. I tried this in both TTL and manual flash modes. Why do they behave differently?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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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?
Originally by user48. Source · Licensed CC BY-SA 4.0
user48
15y ago
0
Generated from our catalog & community — verify before relying on it.
They only behave the same in a very controlled setup.
Stopping down the aperture reduces all light reaching the sensor: flash and ambient. Lowering flash power changes only the flash part of the exposure, so the flash-to-ambient ratio changes.
That means if ambient light is contributing much to the exposure, reducing flash power may have a smaller visible effect than changing aperture. In bright ambient conditions, a weak flash may barely matter at all.
Mode also matters:
- Manual flash: a smaller aperture will reduce exposure, assuming you’re at or below sync speed. Ambient exposure can still mask what you see.
- TTL flash: if you stop down, the camera/flash may automatically increase flash output to maintain exposure, so the image may not get darker the way you expect.
Also check Auto ISO. If ISO is being adjusted automatically, the camera may compensate for aperture changes, again making the result look inconsistent.
To test this clearly, use a close subject in a dark room, set the camera to manual exposure, use manual flash power, keep shutter speed at or below sync speed, and turn off Auto ISO. Then aperture changes and flash power changes will behave more predictably.
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