Guide Numbers—How Do They Work Again?

Started 1 month ago | Questions thread
OP rfsIII Regular Member • Posts: 344
Re: Guide Numbers—How Do They Work Again?

This is great info and thanks for explaining.

But what is still tripping me up is the idea that if I have one SB800 pointed at my subject then add a second one how can that give me more than double the amount of power?

Seems like voodoo.

antares103 wrote:

First, you need to normalize the guide numbers. Some ISO, same angle of coverage, same units of measure, etc. With all else being equal, a GN of 250 would be 4 times more light than that of one that is 125.

This is because when you double the distance, you make the width of the resulting beam twice as wide, AND twice as tall. Together, this makes the area four times the size. To make the whole area the same brightness, you need four times as many photons.

A one stop change in distance is 1.414 times the original distance. Or .71-ish times the original distance (in the other direction). If you note, f/7.1 and f/14 are one stop changes from f/10.

Post (hide subjects) Posted by
BAK
BAK
BAK
BAK
BAK
BAK
BAK
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow