03. 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. 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.

  1. 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.

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

  3. 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 .

  4. 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.

    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.

  5. 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.

  6. 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?

  7. 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.

  8. 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.

  9. 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?

  10. 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

  11. 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.

  12. 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.

  13. 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.


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