Your flash and your lens’s aperture are directly correlated. First off, know that your shutter speed generally controls the ambient light in an exposure while your aperture controls your flash’s exposure. ISO controls overall sensitivity in an image. But then your flash’s power varies. Many people use TTL. But if you’re using manual flash output, then consider this: if your flash is fixed at 1/4 output, and you vary your aperture, the flash will either illuminate more or less of the image that your camera captures.
So how does this relate to TTL users? At a given ISO, your flash can only be so effective because it judges not only the distance that your subject is away from the lens but also your aperture. That’s why sometimes your image might be too dark despite using exposure compensation. The reason for this is because your flash only has so much power output–in fact it’s probably less than 1/10th of what a monolight (studio light) may have. The counter is to raise your ISO settings, but the veterans may tell you to never go above ISO 400 when using a flash. And in general they’re correct because that’s how you can capture the most specular highlights in an image. But sometimes you have to.
After the jump, we used the Phottix Mitros flash with the Odin TTL triggers in conjunction with the Canon 5D Mk II and Tamron 90mm f2.8 VC (which we’re currently reviewing.) The flash was in the same position fixed at 1/32 output while the camera was fixed at 1/200th at ISO 100. The only thing variable was the aperture. The results are just how much your aperture can affect an exposure.
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