Reader Michel, referring to a previous post, asks:

Hi Michael, I am struggling with the math of this statement…. “two lights that both say f/5.6 will give you a total of f/8″… Can you clarify?


The “main f-numbers” (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, etc) are the square root of 2 apart (each is 1.41.. times the previous number). This is so that they they are all one stop apart (pi x r squared is the area of a circle). Anyway, math notwithstanding, one stop means double, or half, the amount of light.

So if I were to shoot a photo at f/5.6, and then I repeat it at f/8, I would get half the light in that second picture.

But if at the same time I add a second flash, which doubles the light, I would get net the same amount of light. Two flashes doubles it; one stop higher f-number halves that.

So if you need to set your camera to f/5.6 if you have one flash, you would set it to f/8 if you have two flashes.




An interesting exercise

As you probably know, I am a big fan of shooting in the RAW format. You pretty much have to, because RAW has the following benefits:

  1. Much better quality (more colour bits).
  2. More tolerance for mistakes (e.g. a 2 stop underexposure can be easily fixed).
  3. Decisions such as white balance, colour space, etc are not final. They are mere suggestions and you can change them on the computer before finalising the image into a JPG.

RAW images are like negatives, while JPG images are like Polaroid photos. So if you want to minimize risk, shoot RAW. Stands to reason.

But now I am going to suggest something odd: Occasionally go out and shoot JPG.

Above, my model, the day before flying out to Albania to be with her boyfriend. A JPG image, unchanged (other than the frame and logo), taken at 400 ISO, 1/160th sec, f/5.6 with my small Fuji X100 camera:

Fuji X100 (Photo: Michael Willems)

And why would I suggest shooting JPG? Because it has no tolerance for mistake. Like film shooting.So shooting JPG is an exercise in photographic discipline. You have to meter accurately. Be sure you get the exposure right, Be sure your white balance is correct. Crop properly. Be sure your picture is good in the camera. And all that makes you a better photographer!

So… go for it! A day in .JPG, and no changing the images afterward.

Here’s another “straight out of the camera” JOG from that day: