# How to calculate stops for a Watt based strobe?

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. Commented Dec 25, 2013 at 1:42
• @akram: May I ask you why do you need to know this? Are you doing shots with digital cameras? Commented Dec 28, 2013 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) Commented Dec 28, 2013 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. Commented Dec 28, 2013 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).

From the user manual

Digital display shows the power output from 13 being the minimum to 400 the maximum, decreasing in units of 20 down to 200 (1/2 power) then in units of 10 to 100 (1/4 power) then to units of 5 to 50 then in units of 3 to minimum power. Each unit is equivalent to 1/10th of an f stop.

So I'm fairly certain it goes:

Display Ratio Decimal*
400 1/1 10.0
380 1/1 -0.1EV 9.9
360 1/1 -0.2EV 9.8
340 ... 220 (in 20s) 1/1 -0.3 to -0.9 EV 9.7-9.1
200 1/2 9.0
190 1/2 -0.1EV 8.9
180 1/2 -0.2EV 8.8
170 ... 110 (in 10s) 1/2 -0.3 to 0.9 EV 8.7-8.1
100 1/4 8.0
95 1/4 -0.1EV 7.9
90 1/4 -0.2EV 7.8
85 ... 55 (in 5s) 1/4 -0.3 to 0.9 EV 7.7-7.1
50 1/8 7.0
47 1/8 -0.1EV 6.9
44 1/8 -0.2EV 6.8
41 ... 22 (in 3s) 1/8 -0.3 to 0.9 EV 6.7-6.1
19 1/16 6.0
16 1/16 -0.5EV 5.5
13 1/32 5.0

I'm guessing/interpolating on the final two values. But I don't see any other way to make it to a five-stop range of 1/1 to 1/32, while still using units of 3.

*Decimal power assuming the power settings are using 10.0 as maximum power and counting down in stops, rather than 1.0 being minimum power and counting up in stops.

As you know, light from the flash falls off with distance. Modern cameras and flash can make measurements and set the exposure. No such automation, no problem, try this work-around.

First run some tests and discover the “guide number” for your flash unit. Actually, you might find that the “guide number is published in the flash manual or on the web. If you know the “guide number, you simply estimate the subject distance and divide, and the answer is the f-number setting. Be aware, there is two sets of guide numbers, one for those who work in feet, and those that work in meters.

Run a test to discover the guide number:

In a space that is typical of your working locality, set your camera to 100 ISO, now shoot a series, subject at 10 feet, Shoot using all the f-numbers.

Now, evaluate for the best exposes frame, say it is the one shot at f/16. Now, multiply 16 X 10 (f-number by subject distance). In this instance the answer is 160. You have just discovered the “guide number” for your flash (English distance units). For metric folks, say 2 meters @ f/16, “guide number” is 2 X 16 = 32.

OK, you are ready to shoot a subject 20 feet distant. “Guide number” 160 ÷ 20 = 8 (shoot at f/8.

For other ISO’s say 200, “guide number is 160 X 1.4 = 224. For 400 ISO “guide number” is 224 X 1.4 = 313.

A word of caution, this method works but location changes will induce an error, we are talking ceiling height, distance to walls etc.