05. The relationship of shutter speeds and apertures to flash

In this information-packed photography class exploring relationships between flash power, aperture settings and shutter speeds, Karl explores four key areas

  • Understanding flash sync speeds
  • The relationship between ambient light and flash
  • The relationship between aperture and flash
  • First and second curtain flash synchronization

Karl demonstrates how each of these can work together to influence the final image.

In this photography class we cover the following:

  • How shutter speed works
  • Sync speed vs flash duration
  • Leaf shutter vs focal plane shutter
  • Shutter speed and aperture
  • The impact of ambient light on an image
  • Controlling flash exposure
  • Combining studio and ambient light


  1. I was under the impression that my strobes pulse repeatedly in high speed sync mode to expose the image. Is that incorrect or do Broncolor lights employ a different system to handle it? Thanks!

    1. Hi Anthony, some strobes do use a repeating pulse but others now use just one extremely long pulse that lasts for the whole duration of the ‘slit’ passing across the sensor.

      1. Okay, so there’s new more efficient technology. With my strobes there is a reduction of max power when you’re in high sync mode due to the pulsing. Is there a max power output reduction with the Broncolor series due to the extended flash time? Thanks.

  2. always been an area to stay away from BCuz its never been explained as well as you just did fir me Karl. THANK YOU My husband is getting jealous Bcuz I spend all my spare time watching you right now hahahaha! “Sorry, I am busy with Karl right now” giggle

  3. very nice explanation , but i am confusing with my canon 5 d mark iv when i work with this first curtain sync its work but when shoot with second curtain sync my camera body not select the second curtain i want to know why its happen because of my canon flash i use older version canon 600ex rt and wireless trigger st ex3rt but when i put flash on my camera the second curtain sync option works, why its not work with wireless trigger

    1. Hi Jaspreet, I’m afraid these sort of technical issues with various camera brands and flash combinations is not something we can help with, you will need to speak to Canon support for something like this.

  4. Section 1: Sync speed
    Best explanation I’ve ever seen on the subject. Simply brilliant and easy to understand.

    Section 4: Not only are you explaining 1st and 2nd curtain synchronization, you shows it in the camera too. Very handy as I use the same camera.

    I’ve surprisingly noticed (as I wanted to follow your steps) that 2nd curtain sync was not available (in the 5D mIII camera nor the speedlite) using Canon Speedlite Transmitter ST-E3-RT and Canon Speedlite 600EX-RT. But when mounting the speedlite directly on the body it was available both places.

    Does it mean that it’s depending on what kind of transmitter/trigger you have if you want to use 2nd curtain sync?

    1. Hi Tom, yes the ability to perform certain tasks on dedicated flashes may be compromised when used off camera with triggers.

  5. At minute 22:05 you explain 3 ways to control flash exposure: aperture, flash power, flash-subject distance; actually there’s a 4th way, which is ISO! Agree?

    1. Hi Giovanni, technically speaking yes but it is not a choice I would opt for as it would require a reduction in quality. For example as you increase ISO then the image quality deteriorates so it is better to work with the others, there is also another option and that is to add filtration over the lens or lights to reduce exposure where necessary but the process of effective teaching is to feed the right information at the right time 🙂

  6. HHHHOOOOO….OMGOSH!!! I totally get that about flash sync and why it’s important. Plus I never knew that the shutter changed the way it operates with a request for a fast shutter speed! No one has ever explained that to me! You just changed my whole world of understanding of my camera. I only thought I knew every mechanical thing about it! I totally get that. That changes so much.

  7. Hi Karl,

    When it comes to rear sync flash, do you set your camera to expose for the ambient light with the long exposure first before you bring in your flash/strobe? So that you can have the subject properly lit while moving and then the flash comes in to stop motion and make the subject in focus? I just want to know how to properly set my camera settings when using this technique.

    Thank you,

    1. Hi Derrious, I usually underexpose my ambient exposure by about one stop and then add enough flash on my subject. You’ll see me apply this technique many times in the “Fashion” section where I’m shooting on location.

  8. Can you adjust remotely (on the broncolor transmitter) the power of your flashes? Or every time you have to adjust the flash power do you have to physically walk over to adjust or have an assistant?

  9. hi sir what is the correct advantage of first curtain synchronisation.

    I mean when it can be used appropriately

    1. Hi Rao, there is no advantage or disadvantage if the shutter speed is high or the subject is not moving. Second curtain is only an advantage on a moving subject but it’s not a disadvantage on one that is not. The exception may be that you get better or higher speed sync with first curtain sync.

  10. Hi Karl,

    I think you may have answered this above, is there an example of where First curtain sync is better than rear or second?

    It seems that cameras default to first so I think there must be a reason for this.

    Many thanks


  11. Hi Nik, yes the answer above mostly covers it but in addition TTL metering (which I don’t use) doesn’t work with second curtain sync.

  12. I only have some overhead lights in my garage “studio”. Is there any significant difference in image quality between raising my shutter speed to kill the ambient light and just turning off the lights? (Other than being able to see what I’m doing)?

    Thanks for such a terrific site. As a newcomer to product photography and flashes in general your hard work is very much appreciated.


    1. Hi Dave, thank you for your kind comments. With regards your garage overhead lights, if you take a test shot without your flashes connected and using your maximum shutter flash sync speed for your camera you can then examine the test shot and see if the garage lights show up at all in the picture. If they were going to show up it would be in gloss highlight reflections, so put something glossy in for the test. If they don’t show up then you’d have no problems apart from them possibly influencing your thoughts about the lighting on the subject when you are setting up. Cheers Karl.


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