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.

Comments

  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?
    Thanks.

    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,
    Alexander

    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.

            Cheers.

  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.

    Cheers.

    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. 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.

  6. 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˜€

  7. 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™‚

  8. 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!

  9. 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?
    Cheers,
    Tom

  10. 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.

  11. 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.

  12. 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?
    thanks.
    Angela

    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.

  13. 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.

  14. 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.

  15. 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.

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

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

  17. 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.

  18. 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.

  19. 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

    Darya

    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.

  20. 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.

    Darya

    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.

      1. Hey Karl. How are you? I just love this video. You finally made me understand how it really works. Thanks a lot. I have just 1 question.
        With the 800 w first you put down one full stop and said this will be equivalent a 400 w. Right? But after you put down more 4 full stops and how much power it will be? Thinking about always be a half power and doing it 4 times I think it gonna be 25 w of power. Am I right?

        1. Hi Guigo, yes 800 down one stop is 400. if you put 400 down 4 stops it will be 25. Each stop down is half the amount. Each stop up is double the amount. The same with ISO.

  21. 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!
    Michelle

    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

  22. 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?

  23. 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.

  24. 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?

    Thanks

    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.

  25. 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?

  26. 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.

    Regards

    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.

  27. 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.

  28. 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 .

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

  30. 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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