I've a problem figuring out how to calculate stops for Interfit EXD 400. In the product manual it says that the strobe has 5 stop range in 1/10 stop increments, but the numbers displayed on the strobe itself aren't stops, it's in watt power value starts with 13 and ends at 400. So how to calculate the stops from these numbers?

  • I updated the question because it isn't an f-stop, it is a stop. An f-stop is specifically stops related to aperture. A stop is simply a doubling of power.
    – AJ Henderson
    Dec 25 '13 at 1:42
  • @akram: May I ask you why do you need to know this? Are you doing shots with digital cameras?
    – TFuto
    Dec 28 '13 at 10:40
  • @AJHenderson: In optics, the term "stop" properly refers to the aperture itself, while the term "step" refers to a division of the exposure scale. Some authors, e.g. Davis, prefer the term "stop" because they refer to steps (e.g., on a step tablet) that are other than powers of 2. ISO standards generally use "step", while photographers normally use "stop". (Source: Wikipedia)
    – TFuto
    Dec 28 '13 at 10:43
  • @tfuto - f-stop is the technical name for referring to the aperture number. "Stop" is used to indicate a doubling of power. My source for this is the Canon 5D Mark III manual where it refers to exposure compensation as having a number of stops that it is being adjusted. Each stop of EC doubles or halves the exposure of the image. It can move in either half or third stop increments. (Page 355 of the 5D mark III manual.) There may be some debate on the terminology it sounds like though.
    – AJ Henderson
    Dec 28 '13 at 18:58

By "Watt power" we basically mean radiant flux, and by "stops" we mean luminous flux. These two have similar definitions, but they are different.

In normal life, it is true that if you halve the "Watt power" and that light is reflected from or refracted through linear (normal, not non-linear optical) materials, then your camera will observe half the luminous flux - therefore if you halve the "Watt power", you create the same effect as if you stopped down your camera 1 stops (by changing aperture or shutter speed).

If you use a digital camera, your life is easy. Since you are using a studio strobe, your shutter is fixed, you are smart so your ISO is fixed ;-), and also your aperture is arbitrarily fixed. So make a test shot, see your histogram. It maybe shows you are off 2.5 stops. So just multiply the wattage power by 2^2.5. You will have a great setup with a few trial-errors, and will quickly learn to be intuitive with the controls.


An stop is simply a doubling of power. If you are at 200 watts and increase to 400, that's another stop. Lighting is drastically dependent on distance and angle to subject, so while the relative contribution of a light will be impacted by increases in stop, the absolute contribution of the light is based on not just the output of the light, but a large number of other factors. This is why flash power is normally measured in either a guide number or watts of total power and why proper metering is necessary to setup lights precisely.

For a flash, fractional power is usually based on the total power of the flash. For example, on my Canon 600EX flash, 1/1 is a full power flash, 1/2 would be one stop below max, 1/4 would be two, 1/8 would be 3, 1/16 would be 4, 1/32 would be 5, down to 1/128. For your flash, 400 watts is full power, 200 is 1 stop reduced, 100 is 2 stops reduced, 50 is 3 stops reduced, 25 is 4 stops reduced, 13 is 5 stops reduced (well technically 12.5, but they probably rounded).

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