Wednesday, May 5, 2010

Inexpensive Remote flash

Picked up a Cowboy Studio remote flash trigger to play around with. Compared to some they are much less expensive. The only problem I can see is that I have to figure out the guide number and f-stop accordingly. There is no TTL information passed across the wireless link.

I have a speedlite 420 EX, which claims to have a guide number of 42 (meters based) according to the manual. A guide number is used to determine the f-stop used to take the picture. So a guide number of 42 tells you that at 10 meters away, at ISO 100, you would use f 4.2 (42 / 10). At 3 meters away (around 10 feet) you would use f 14. If you were shooting at ISO 200, twice as fast as ISO 100, you should double the f stop. The distance is from the flash to the subject, not the camera to the subject.

Now the 420 EX will zoom based on the lens length when it is attached to my camera. It defaults to 35 mm, which has a guide number of 31. That 42 before was when it was in the telephoto mode of 105 mm. Since none of the TTL information is available at the flash, then I should use a guide number of 31.

All this sounds complex, but basically just take the guide number and divide by the distance and that gives you the f-stop at ISO 100. Or, since I now deal with digital, I can take a picture, see how it looks and adjust accordingly. All of this guide number division was much more important to calculate when shooting with film. Now with digital, you have instant feedback.

And the reason for all of this? Hopefully more macro photography work. Of course, when working with bees, you usually only get one chance so figure out the correct f-stop.