Friday, November 23, 2007
You Say Diffraction, I Say Motion Blur
Don’t get me wrong; diffraction is a problem and it does exist. The laws of physics can’t be changed –if you send light through a small opening it will have e tendency to bend. Wikipedia has an excellent article on diffraction if you want to dig deeper into the effect.
Unfortunately I think that a lot of photographers are too quick to blame diffraction for soft images, so they open up the aperture on their lens, get a sharper image, and then think that they’ve defeated diffraction by using a smaller Fstop. But by dropping down to a smaller Fstop (moving to a larger aperture) the exposure is also changing, so that the flash has to turn on for a shorter amount of time. Maybe the problem isn’t diffraction; maybe the real issue is motion blur due to a flash duration that’s just too long…
Take a close look at the image that I’ve included with this post. I shot it at F14 and 4x with an MPE-65mm macro lens and an MR-14EX set to a 2:1 ratio (too keep the light from looking flat) and with the flash assembly turned 45 degrees off axis. That shot, even though it’s at F14, has a lot of detail and all I had to do was brace the camera so that there was no movement.
Flash macro really is nothing more than a form of stop motion photography, and keeping the flash duration to a minimum can really make a difference in the quality of your images. To learn more about high speed photography and how flashes work see the HiViz web site.