Understanding flash power

To fully understand studio lighting, you need to understand power. Each type of studio light has different power capabilities and it is important to understand how to use this power if you want full creative control.

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 photography class we cover the following:

  • 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 speedlite power

NOTE: This photography class is available with English subtitles.


  1. 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

  2. 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™‚

  3. 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!

  4. 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?

  5. 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.

  6. 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.

  7. 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.

  8. Hi Karl,

    I really appreciate your teaching methodology, its quite comprehensive. The concept of flash power in relation to aperture settings is now clear to me.

  9. Karl, greetings from a fellow Brit from Sevenoaks, Kent but now in sunny California. I studied portraiture at night school in England but nowhere as in depth as your insightful tutorial. I also had a studio in California. My original monolights were 240V Multiblitz Mini Studio 800 and 400 watt, powered by a step up transformer from 110V. I have now added 110V monolights. I found your video extremely helpful, so thank you. I have just measured my Flashpoint XPLOR 400 Pro TTL (Godox AD400 Pro) battery powered monolight against a couple of speedlights and was surprised at the result. The Flashpoint 400 watt at full power gave f22 and the speed light at f16, just a one stop difference and I presume the same output as the Godox AD 200 watt pro. That saves me the expense of adding the Godox AD200 since I have added an S type flash bracket to the speedlight plus wireless receiver allowing me to use Bowens type modifiers. Once again, many thanks for your very helpful tutorial and I also learned that studio lighting has “dual” nomenclature (“joules” and watts – British pun). Peter

    1. Hi Peter, thanks and you are welcome. Also the power output of any light is greatly affected by the type of reflector modifier on it, hence why parabolic reflectors are so good as much more of the energy of the light is focused forwards. Changing the zoom on a speedlite to a longer focal length will also direct more of its energy forwards.

  10. Hi,

    I must admit that I’m mildly confused as it relates to the relationship between “F-stop” and flash power. After watching the tutorial, I have more of a grasp of the concepts. Do you recommend anything to assist in making these adjustment second nature? Further, in making the switch from studio strobes to speed lights, how would I properly calculate the power and “F-stop” needed without a meter.

    – Tramayne Young

    1. Hi Tramayne, It helps a great deal if you have fully memorised the fstop range: i.e. f1.4, f2.8, f4, f5.6, f8 and so on (you can recap this in our Intro course in the essentials section) All ‘stop’ changes work the same, whether it is ISO, Joules power, shutter speeds – they are all either half the light or double the light when making a one fstop change. As you become more proficient you will be able to assess light in as little as 1/10th of an fstop. When you have become fully familiar with exposure in this way it makes life a lot easier.

  11. Again, another fantastic concept explained very clearly with examples. Thanks a lot Karl ?

  12. hello, i think there is some problem with the page, i cant reproduce the videos. I tried in my mobile too and the problem still there. πŸ™

    1. Hi, we don’t have any other customers saying there is a problem, are you still experiencing a problem?

  13. Hi Karl,

    When you applying the ND filter which is 0.9 it says 3 stop strength on it. It means have to increase the light power from 3 steps to get the desired outcome. But you have added one stop increase and secondly another stop. Its bit confused for me… Can you please clarify why you do not increase it by three stops..

    1. he only aded 2 stops because he was already underexposed by one stop. so he needed 3 to cover the ND reduction but because he was lacking one already he only needed to add 2

    2. Hi KR, Yes a 0.9 is 3 stops. So if I had a light set at power 6 and then I put a 0.9 ND filter on the camera then I would need to set the power to 9. to get the same exposure or if I was at f11 on the aperture, then I put the filter on then I would need to change to f4 to get the same exposure. If I only changed by two stops then I would have wanted the picture to be one stop darker than I started with.

  14. Wow! Great job clarifying flash power. I was (am) a little intimidated by the whole thing, but you have an excellent way of explaining everything.

  15. Hi Karl

    What I mean is when I have the flash on the camera. Which mode does the camera have and the flash have to be on in order for me to be able to change the strength of the flash within the camera please? I have put the flash on TTL and the M mode on the camera, but nothing is changing. The flash does work but I have to increase or decrease it within the flash.

    I hope this time is clearer for you to understand.

    Thanks a lot


    1. Hi Darya, as far as i’m aware the only way you can change the flash from in the camera is with ‘flash exposure compensation’ but this is just a minor adjustment on the balance of the TTL, it doesn’t really give you the full versatility of manual mode. For full control you need to adjust the power of the flash from the flash in manual mode on the flash.

  16. Hello everyone. I have just joined and I have got a question to ask. I use Canon 5D Mark iii. My question is this: I use a 580ex ii fish, but when I change it from the camera, nothing happens in the flash? I basically would like to know whether is it possible to do that, and which setting should be on please?

    I really appreciate anyone’s answer…

    Thanks in advance.


    1. Hi Darya, what do you mean by ‘when I change it from the camera’, change what exactly? If you can be clearer with your question we will do our best to answer it.

  17. Hello,
    1-Is my decision of the “right” exposure subjective or is there a way to say what the correct exposure is? (Based on your first and second shots in the video.)
    2-If I have a speedlight, does the guide number tell me anything that makes this more “scientific” or is it my decision of what the correct exposure is?
    Excellent video, thank you!

    1. Hi Thelma.
      1. It should be subjective especially if you are using a perfectly calibrated monitor to make the assessment but you can rely on tools like the Histogram graph to give you exposure information to check things aren’t blown out beyond white etc, or in the capture tools you can measure the colour values either as a percentage or RGB values, so for example if you had wanted to acheive a pure white but the RGB read R251 G251 B251 then you would know you were just a tiny bit below pure white. Maximum reading is 255 but you are better to start below 255 otherwise you wouldn’t know if you were actually well over 255, you will learn more about this in later chapters.
      2. No forget it.
      3. You will see about light meters in chapter 6

  18. Hi Karl,

    Never afraid to ask what’s possibly a stupid question – I know you said shutter speed doesn’t matter with flash lighting but would you not be able to affect a reduction in f stop when the flash head is at its minimum by having a faster shutter speed?

  19. Hi Karl, excellent tutorial! So high speed sync allows you to use variations in shutter speed to balance the flash with ambient light?

    1. Exactly right again Clive. You’ll see this most commonly applied in my fashion shoots when I’m on location, as in ‘Fashionscape’ in the ‘Fashion’ section.

  20. Great info! In a typical studio what is the starting distance of the main light usually? Is it 4, 6, 8 or 10 feet? And lastly if you have to move the main light back to reduce power is the floor marked at all if so in what increments?


    1. Hi Edmond, I’m not going to answer that question until you have watch the rest of this course or at least up until chapter 25. If you feel you need to ask me again at that point then we’ll discuss it as you should learn some fundamental information along the way that I need to make sure you absorb so you can master lighting.

  21. Hi Karl
    Can you please clarify – if your flashgun is 2,5 stops slower then your Siros light does that mean that flashgun is approximately 150 joules?
    Also I tried to calculate what minimum power of light I should have for product photography and for example if I need aperture 11 then light power should be 50 joules and that fits even to flashgun. But what do you think the minimum power of light source(I mean flash type) that can be used for product photography?

  22. Hi Karl,
    Did I hear correctly if I heard you say you where shooting at 1/60 of a second? Or was it 1/160 of a second? I guess you will cover the lamps sync speed later and how to relay to the sync speed.


    1. Hi Tom, yes that was 1/160th of a second just to make sure I cut out any ambient daylight from the shot but I will be covering more in this and sync speeds in later chapters.

    1. Hi Kamel, you can only increase the shutter speed to the maximum speed that it will still synchronise with flash but as explained and you will see in a later chapter, this has little effect on the exposure of the flash it only reduces the ambient exposure. Please see the later chapter.

  23. Excellent tutorial. I’m glad you showed the relation to speed lights also. I work with speed lights, so it helps understanding the difference in power between the two types and the limits they can achieve. I can appreciate why you would buy strobe flashes.

    1. Hi Geoff, thank you. The main reason is the use of modifiers and then of course the extra power to get through the modifiers (as you lose light) and also convenience of consistency, ease of use and the batteries not draining.

  24. Thank you Karl , Excellent education . You make it simple through your poised & clear teaching and bring these constellations of tips and tricks which are so practical like opening a pandora box. Will surely spread the word. Sometimes while you are teaching i pause the video and almost ready to ask you a question : ) then it flashes ! its a video . Thanx once again for bring this education so live and spreading u wisdom of experiences .

  25. I’ve just finished the second video. This is exactly what I’ve been searching for. I love your style of teaching!!

  26. Hi Karl,

    Does the modeling light really affect the shot ? I was under the impression that due to the power of the flash and the sync speed, the modeling light has no effect. Plus, I felt that the flash unit temporarily switch it off during the firing.

    1. Hi Anan, if you have bright modelling lamps like 650w and shooting at apertures such as f8 or larger then yes the modelling lamp can definitely become a problem. It is a common mistake that photographers make in the studio by not accounting for the modelling lamps. Which can then cause unwanted colour casts or add blur to an image when you would have expected a frozen sharp shot from the flash.

    1. Hi Trond, you certainly could reduce the amount of light captured by one stop if reducing the ISO was an option. On my Hasselblad definitely yes as 50 is the default ISO but on the Canon the default is 100ISO and dropping to 50 actually causes a slightly lower quality image than at 100. Each cameras sensor has a ‘sweet spot’ ISO, I believe on Nikon it is 200ISO.

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