Understanding Flash Power

Before you can achieve top-quality results with your studio lighting, you need to understand power.

Each type of studio light has different power capabilities. To harness those capabilities and give yourself full creative control, you need to make flash power work for you.

This photography class covers all you need to know about flash power, including the relationship between f-stops and studio light power, why it’s important (but not necessarily crucial) to have a large range of power, and the factors that influence power.

In this class:

  • Joules and what they mean
  • The relationship between f-stops and studio light power
  • Factors that influence power
  • Achieving the correct exposure
  • Power ranges and how to overcome a limited power range
  • Power output comparison between studio light power and speedlight power

Questions? Please post them in the comments section below.


  1. Hi Karl,
    Thanks for the great lessons. I learn a lot.
    I use Godox 200 with 120 cm Octabox. Do you think it is enough power to “fill” this box with light?

    1. Hi Thank you. It depends on many factors such as shooting aperture, ISO used, distance etc. Just remember the power of lights is in stops just like ISO or Aperture full stops so a 200 light is one stop less than a 400 or two stops less than an 800. So to get the same amount of light as an example if you were using an 800 light on full power at f11 ISO 100, then with a 200 light you would be at f11 at ISO 400 or at f5.6 at ISO 100.

  2. Hi Karl,

    How about flash lights that don’t have large power spread? Like a maximum of 3 and a minimum of 0.1.
    Is the rule of 1 full decrease equals a full F-stop decrease still representative? Or do you need to divide the total amount of stops your lens is capable of by the amount of power the light can bear and do some calculation to match a 10 range light?

    Loving the courses, especially the business ones!

    Thanks in advance,

    1. Hi Alex, nearly all studio flash lights are designed to work in quantities of an f-stop. For example many years ago before flashes were as precise as they are now, many were only available to change in 1/4 or 1/2 stop adjustments only. As technology improved 1/10th of an fstop became common. As far as I’m aware (regardless of total range) nearly all brands operate that one whole number change equals one f-stop change. As I don’t know which brand of lights you’re talking about I would say your best bet is to take a picture with the lights with the camera at f8 (shutter speed at highest sync speed in a relatively dark room). Then put the lights up by one full number (for example from 2 to 3) then close the aperture on your camera from f8 to f11 and if the exposure looks about the same then you know that the power of the light changed by one stop.

      1. Hi Karl,

        Thank you for your comment & advice! I’ll definitely try it out!

        I’m in possession of a very basic set of lights: Godox E300. It’s not high quality but sure does the job finding what suits best for me and to go from there for future decisions. I’ve been able to work with the Profoto b10’s and man are they lovely to use. Such creative freedom! But still quite out of the budget haha. Therefore, I’ll be looking for something in between as an intermediate level which still has a good power range for its market price.

        Any recommendations by coincidence?

        1. Hi Alexander, I can only recommend brands we’ve used which are Elinchrom (good middle of the road) Profoto (good but more expensive) Broncolor (top but expensive). I’ve got a couple of Godox 600s that seem OK but I’ve not used them much we only purchased them to show in our videos that other brands can be used to yield the same results. 600-800s are plenty powerful enough for most people and if the power doesn’t drop low enough then a good ND filter on the lens will do the trick.

          1. Noted! Thanks again, Karl.

            Much appreciated.


  3. Hi Karl,

    First of all, this series about how light works is fantastic.

    With the same aperture, is there a difference in the illumination on the distant background when using a light on power 5 with a 3 stop filter as opposed to using it on power 2 with no filter? I tried comparing the images on the video and it looked slightly different but I wasn’t sure it was due to video compression.


    1. Hi Squiggle, Power 5 is 3 stops brighter than power 2. The ND filter would remove 3 stops of light so it would be exactly the same in all respects as on power 2 with no filter. The only slight discrepency can be sometimes that when reducing the power of lights they might not be exactly the amount you requested but usually they are very close.

  4. Hi Karl. Really enjoying all the content.
    I have a question. Im starting put with my own home studio, and I am using just the light of my flashes. As it happens, if I take out the picture without the flash, my pictures are a black because I have taken all the ambient light off with my camera settings. At the time I cannot afford a light meter, so is there a way to know how to get the correct exposure before taking the picture? I do not trust my camera lcd display and it obviously takes too long to take the pictures into my laptop just for checking the exposure, I’m also not tethered to my laptop. So as I said, if I try to expose with the camera light metering, it will be surely underexposed before the flash do its work. Is there something I can do to try to get the exposure and light power correct without having a light meter. Of course Im going to get one as soon as possible.
    Thanks a lot for your classes and you patience with all this questions.
    Greetings from Mexico!

    1. Hi Momo, yes this should be relatively easy. First of all set the desired aperture that you wish to use for creative depth of field. Let’s say that’s f8 for example. Then you have your shutter speed to block out ambient light and that shutter speed can be set up to the maximum sync speed which is usually around 1/200th of a second. Then you have ISO which the default is usually 100. Now if you take a picture with no flash or other lights the picture will be dark. Put the flashes into full manual mode and now you will be able to set their power between 1/1 (full power) or 1/2 (half power) 1/4 (quarter power) and probably all the way down to 1/256 power. Start on 1/16th power and look at the result, if it’s too dark go up to 1/8th power if it’s too bright go down to 1/32th power – It’s that simple just think of the manual power up or down just like turning the sound up or down on your radio until it sounds/looks right.

  5. OK – now I know EXACTLY what I want for Christmas!

  6. Hi Karl, I have one strobe light Quadralite ATLAS 600 PRO TTL (600Wat). I’m using it in home studio which is rather small, about 20m2. Would you recommend to use the same power strobe light for fill light as the key light ? Would be better for controling a effect with lights of the same power ?

    1. Hi, it doesn’t really make any difference if I had a 300W lamp then I know it’s one stop less powerful than a 600 but I also know I can turn the power down on a 600 by one stop so resulting in the same power, or put an ND gel on them or a different modifier. All of these things (as will distance) affect the power of the light.

      1. Thanks Karl

  7. Two last questions Karl. In terms of distance (in m or cm) what will be equivalent to 1 stop of light difference? Does the brand of the lamphead affects this? (I guess 400j it still should be 400j independantly of the brand…but im still asking this because with all your ears of experience maybe you have seen some exceptions or particular cases).

    1. Hi, it depends on the modifier you are using but two different brands with 400J with the same modifier should put out the same light but often it’s not the case, I’ve measured the values and they aren’t always equal! The power of the fall off in light based on distance also depends on the modifier used as they all spread the light differently but as an example with a bare bulb light (no modifier) if you move it twice the distance away it will be 2 stops less powerful. – https://en.wikipedia.org/wiki/Inverse-square_law

      1. Yep, I asked the question too soon…learned this in the next chapter but thanks anyway for your reply πŸ˜€

  8. Hey everyone! probably what I’m going to say is obvious…but anyway, by watching the chart of aperture values that indicates 1 stop of light change I realized that I only need to know the first two : f 1 and f 1.4 for knowing them all, the following values are just the double of the number before the last aperture value:

    The next value in the sequence will be f 2 because the number before my last aperture value is 1 and 1 x 2 = 2. Now I have f 1, f 1.4 and f2, then my next aperture value will be 2.8 because now f 1.4 is the number before my last aperture value and it continues the same way: f4, f 5.6, f 8, f 11 (which will really be f 11,2 though guess in this point it was rounded to the nearest tenth), f 16, f 22, f 32, f 44, f 64 and so on (I don’t know if the last 2 values actually exist on cameras but I guess it will be like that).

    1. Hi, yes this kind of works, the last two values are f45 and f90 so it still works but their are of course the half step apertures inbetween these. You’ll find in time that you memorise all the numbers anyway! πŸ™‚

      1. Oh I see! well now it totally make sense for me, at f 11 it was rounded to the nearest tenth, remember the value I said it should of been was 11.2? If you continue with the same process but without doing the rounding, the values will be the following: f 11.2, f 16, f 22.4 (rounded will be 22), f 32, f 44.8 (which rounded will be 45), f 64 and f 89.6 (rounded will be 90) πŸ˜€ … But yes, you are totally right Karl, sooner or later I will end up memorizing all these numbers πŸ™‚

  9. Hi Karl!
    I have a question…
    How much flash power does one need for product photography when photographing in a small home studio? I was thinking of buying three lamp heads… should I get them in different powers?
    Thanks in advance!

  10. Hi Karl,
    Really enjoying the training so far. So you’ve explained how using an ND will help get the desired DoF & correct exposure when constrained by the low-end power of the light unit. Without looking at the cost difference between a 400j and 800j light unit, my initial thoughts when constrained by the high-end power would be to fix exposure by using ISO. Modern cameras deal with an increase of 1 stop ISO no bother when at the lower end of the range like you are at ISO 100.

    I don’t think this was covered in the video (if it was my apologies) but is there any impact of this besides image quality and risk of introducing a little bit of noise?

  11. Hi Karl!
    I am thinking of investing in a home studio, to start to shoot products.
    I have a Canon camera (t5i)
    I don’t have such a wide budget and was wondering what brand of flash would you recommend buying for the beginning. Should I buy a strobe flash + a normal one? What’s most crucial and useful as a beginner?
    Thanks in advance

    1. Hi HC, I’m afraid we are unable to make good recommendations on equipment that we haven’t used. The top brands are broncolor, profoto, other reasonable brands could be Elinchrom, Godox.

  12. Thanks Karl for a another fantastic tutorial! Query: Are all better caliber studio lights adjusted manual during a shoot or can they be adapted TTL metering some how? Or better question: Should I be using my setup with manual power settings rather than TTL? I am presently using a 2-speedlight setup with softbox modifers as I am just getting into studio indoor work. Thanks!

    1. Hi I would say from a learning perspective get into the habit of using them in full manual and forget TTL. Most studio lights don’t have or need TTL you just set the power based on what you see in the same way you control the volume on your ipod based on what you hear.

  13. Hi Karl this was very helpful and clear to follow. You teach in a very pleasant and comprehensive way about each topic.

    I have one question regarding product photography lighting set up. is there a lesson or a video you created which explain the ideal light set up for a home based studio for Product Photography?

    1. Hi Angela, yes we have lots of product photography videos in the ‘product’ section head to the top menu and browse through those classes.

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