How Shutter Speed and Aperture Relate to Flash

This illuminating tutorial is guaranteed help you take better photos.

In this information-packed photography class, Karl breaks down the relationships between flash power, aperture settings and shutter speeds.

You’ll learn about:

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

You’ll learn how each of these can work together to influence the final image.

In this class:

  • How shutter speed works in photography
  • 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

If you enjoy this class, check out Understanding Flash Power and Fast-Flash Jelly Headshots.

Questions? Please post them in the comments section below.


  1. Hi Karl, a rather theoretical question. Why is it the case that the camera sensor only catches flash light within the sync time and none of the ambient light? Would that be different, if the ambient light would be brighter than the flash? Cheers, Gerald

    1. Hi Gerald, it does capture the ambient light as well it just depends how much ambient light there is and if it’s strong enough to record.

  2. HI,Karl
    Is it technically difficult to support Google Translator?

    Because of the language barrier, it is difficult for me to understand all lecture content.

    The integrated translator does not deliver the correct language, but it helps to understand the lecture content.

  3. What abou changing ISO? doesn’t it affect the flash?

    1. Hi, yes ISO is the base of the exposure level, the sensitivity of the recording material. But generally speaking in a studio environment when we have all the light available to us then we put ISO at 100 or 64 to ensure the cleanest possible images. But if your lights are a bit weak then you can increase ISO to compensate.

  4. Hi!
    Super informative as always!
    When working with fast shutter speeds, as you menthion in the video – e.g so fast that the shutter curtains are following each other and letting light in as a they go down/up – can this effect my choise of focus area and/or metering mode?

    1. Hi Jesper, thank you. No it doesn’t affect your choice of focus area or metering mode as those functions take place before the shutter moves.

  5. Hi Karl,

    Which shutter speed is recommended to achive the blur effect while using the rear sync flash?

    Best regards.

    1. Hi, it would totally depend on the speed of the movement of your subject. I think in this example I mentioned the shutter speed but can you remind me at what time in this video you are referring too please.

  6. Karl,

    It appears that you use 1/160 shutter speed on most of your shoots. Since sync speed is less of an issue on medium format cameras, how did you decide on that speed?

    1. Hi, the only purpose of ‘sync speed’ other than allowing the camera to time correctly with the flash is for the sync speed to be fast enough to cut out any ambient light that you don’t want. Often at f11 or f16 the amount of ambient light would be so low I could choose a sync speed of 1/50th and wouldn’t see any difference. The cameras in this video were 35mm with a maximum sync speed of 1/200th, however I’d noticed a problem with the sync on one of them as I had a shadow line from the shutter appear at 1/200th so I dropped to 1/160th to solve the problem. On medium format yes you are correct I could sync up to 1/2000th if I needed too. The times when that is useful (as you will see in some of our other classes) is when you are shooting fashion with flash outdoors in bright conditions, then that faster sync can allow me to cut most of the daylight out if I want to and have the scene look underexposed but the flash at the correct exposure. I show a way around this for 35mm cameras in another class by using ND filters you can see that here: As the shutter speed doesn’t affect the flash exposure (if it’s within the sync speed) then the only thing you ever need to know about sync speeds is ‘can it cut out enough of the ambient light that I don’t want?’ if it can then that’s all the sync speed you need. If it can’t then can you shut the blinds, darken the room, turn your modelling lights down or wait until later in the day? It’s the fundamentals of what a sync speed is meant to be doing that is the most important and that is cutting out light other than the flash.

  7. Hi Karl. I’m just a bit confused here, if I’m deciding to use some ambient light say for an outdoor shoot and wanted to add some flash. If there was some fast action needed to be caught. Would hss work or would the flash cause any issues as what I’m understanding is it would take longer (the burst) of course I fully understand if I was completely cutting out the ambient light but if I was to have both for action shots. Would the high shutter speeds in HSS catch the moment… of course I know without the flash yes it would as it’s a high shutter but just wondering would flash cause any surprises with motion… this may be a silly question but I’m just trying to understand how it works as opposed to just shooting and then saying great stuff it worked. Thanks, Brad

    1. Hi Brad, I’m afraid your question isn’t quite clear but I’ll do my best to put some answers down.
      1. Generally speaking a burst of flash is faster than most shutter speeds and certainly faster than the usual 1/200th sync speed of many cameras.
      2. On cameras with HSS or medium format cameras you can shoot at higher sync speeds so that you can cut out more ambient light if you need to, as explained in this video at higher shutter speeds the shutter is now working as a ‘scanning slit’ across the sensor on the 35mm cameras (not on Medium Format cameras) and as such the flash burst has to last for the entire time it takes the ‘scanning slit’ to pass across the sensor – this is how HSS works, therefore essentially your flash burst ‘appears’ equivalent to the shutter speed you selected.
      3. In situations where there is very little ambient light then you can rely on the flash to do all of the ‘freezing’ for you as there is not enough ambient light to really affect or cause any ‘ghosting’- see some of my fashion on location shoots/classes in Iceland where the model is running in the green dress.
      4. There would be no surprises with HSS, the amount of motion captured would be the same as the shutter speed would capture ordinarily. The downside of HSS is that you have less control over the amount of flash exposure level but this isn’t usually a problem.

      Please see the other chapter in this section on ‘Flash duration’. Kind regards Karl.

  8. Hi Karl,

    This knowledge is so interesting and fascinating to me, but it’s also a little bit confusing.

    So I can see from all the demonstrations that flash has the ability to freeze motion, but I also know that shutter speed affects motion blur in pictures and can also freeze motion.

    Can please clarify the difference between freezing motion with a flash and with the shutter speed.

    1. Hi Michael, you can freeze motion with your shutter speed if it is continuous light and it is bright enough. You don’t need a fast shutter speed with flash because the flash will freeze the motion and the problem is the camera getting it’s timing correct to trigger at the same time as the flash and this is limited by the flash sync speed which was explained in this video, I hope you watched that part?

    2. BastianLai

      Hi Karl,
      Is it possible to obtain a first curtain and then a second one later at the end ?
      Thank you.

        1. BastianLai

          Hi Karl,

          I am expecting to get a precise & clear face at the beginning then the head movement to obtain the blurred effect & finally a 2nd flash which is required to obtain another precise & clear face at the end of the movement my idea was 1st flash- blurred movement – 2nd flash.,..but I may have misunderstood sthg.
          Thank you for your time & explanations

          1. Hi, then there are two options you need to shoot in an almost dark studio and open your shutter manually, fire your first flash manually, turn your continuous lights on and then fire your second flash manually. Alternatively you can have the camera trigger the first flash for you by using the camera in ‘B’ mode and the flash will fire at the start and then when you let go of the shutter just before you will need to fire the flash manually with another trigger.

  9. Hi Karl,

    Can I replace a fast flash duration of 1/8000 of a second to freeze a motion with a flash in HSS mode with a shutter speed of 1/8000 of a second and still freeze the motion?


    1. Hi, yes but there are limitations in both the amount of flash you will actually have in terms of power and this isn’t a reliable way to work if you are serious about fast flash work in the longer term.

  10. Hey Karl!! Wow I am learning so much from this class, thank you for everything you have put into it. My question is for the external speed light control setting, I’m not finding that in my camera anywhere. I shoot Sony A7iii and I’m not finding anything on YouTube for it. I have a strobe Einstein 640w with a trigger and receiver, do I need something special to be able to do the 1st and 2nd curtain shots? I cant seem to find the settings your changing in your camera, thank you!!

    1. Hi, glad you are enjoying the classes. What do you mean by the speedlite control setting? When you put a approved speedlite on the hotshoe the camera should see it is there? You then choose to use the speedlite in manual power or TTL and you set your sync speed ect. If you are using a trigger there is nothing to set you just use your camera in manual mode and the trigger will fire when you press the shutter button and you control your studio light on the studio light. For things such as first curtain or second curtain sync (sometimes called rear or front) you will find a setting in the camera menu to choose which one.

  11. Nick

    HI Karl. 1st off (& most importantly) – these courses are absolutely 1st class, thank you I’m learning soooo much!! 🙂
    My question is around shutter operation at speeds faster than the camera’s sync speed when you get the sort of ‘scan’ effect over the sensor. As the speed the 2 parts of the shutter can move is clearly limited by the mechanical aspects of the camera I understand why this is required. What I was wondering is at shutter speeds faster than the sync speed does the time the shutter is actually ‘working’ remain fixed at the ‘sync speed’? ie the reduction in f stop being linked to the reduced time each part of the sensor is being exposed to light? For example: if the sync speed is 1/250th sec, at that speed the whole sensor is essentially exposed in one go to the image for 1/250th second (in the same way it would be exposed for 1 second at a shutter speed of 1 second). Then at 1/500th sec the shutter scans over the sensor, still taking 1/250th second to cover the whole sensor but because of the ‘slit’ size each part of the sensor effectively only receives 1/500th second of light. Hope that makes sense…I’m a proper newbie so still wet behind my ears I’m afraid! 😉

    1. Hi, no if you go above your camera’s official sync speed when using flash you will have a black stripe across your image.

      1. Nick

        Thanks Karl. Fully understand that bit (getting a black stripe with a flash) – the question was around the time span the shutter is actually in operation, with or without flash….so the thought that the exposure of the sensor is shorter at speeds faster than sync speed (with corresponding f stop reduction) but the time it actually takes to expose the whole of the sensor remains at that of the sync speed. Sorry, I’m not really explaining myself very well.
        For example:
        – 1/250th sec (= sync speed): whole sensor exposed at once for 1/250th second
        – 1/500th sec: slit scans over the sensor. Size of slit means each part of the sensor is exposed to 1/500th sec light. Total time to scan the sensor from top to bottom = 1/250th second
        – 1/1000th sec: smaller slit scans over the sensor. Reduced size of slit means each part of the sensor is exposed to 1/1000th sec light. Total time to scan the sensor from top to bottom = 1/250th second

        Cheers Karl,

        1. Hi Nick, what makes you think that the total time the sensor is scanned from top to bottom remains at 1/250th of second even on faster shutter speeds such as 1/1000th? I also don’t understand what you are trying to ascertain in terms of the usefulness of this, if for example the flash burst lasts for only 1/2000th of a second and bursts during the period of a slit then of course we have a black line. There are other technolgies, for example leaf shutters in medium format lenses work in a different way which means I can sync at any speed on most medium format cameras as there is no slit and the whole sensor is exposed whilst the very brief flash burst goes off. There is also something called HSS which through clever trigger timing of the trigger and forcing the flash to burst for very long durations (longer than the time it takes for the slit to move across the whole sensor) then you can actually work with shutter speeds higher than the sync speed. For example with some HSS you can shoot at 1/4000th of a second but the timing of the flash burst lasts longer than the entire scan so during it’s whole pass over the sensor there is still light coming out of the flash, however this results in often weaker recorded light due to the small slit and the flash power has to be increased to compensate which is often OK because at full power most flashes are on there longest duration but this is usually still around 1/1000th of a second so they have to be forced into bursting light out for a longer period. It’s quite a complicated process but what I’m trying to understand here is what it is you are trying to achieve? If you want faster sync then HSS or medium format are your only off the shelf choices to not get a black stripe across the image. Some mirrorless cameras can sync at 1/320th because they are recording electronically with a timing cut off of the sensor rather than just a physical shutter.

          1. Nick

            Many thanks for taking the time to reply Karl, it’s very much appreciated.
            It wasn’t so much what I was trying to achieve as to whether it was in fact possible or even worthwhile using faster shutter speeds than the sync speed to freeze motion (without flash) if faster shutter speeds were, by design, simply reducing the overall light exposure rather than the time the sensor (in totality) was being exposed to the image. My interpretation of your response is that this is the limitation of a ‘standard’ camera and the only way to get truly sharp freeze motion shots would be either to use a flash (and appropriate shutter speed to overcome any ambient light) or to use a medium format camera which would allow ‘true’ faster shutter speeds.
            Many thanks again Karl.

          2. Hi Nick, what you are saying here is partially correct however if you have enough light such as very bright daylight or extremely bright artificial continuous light such as HMIs then it is possible to shoot with your fastest shutter speed such as 1/8000th on some cameras and freeze very fast movement such as birds in flight, helicopter rotor, liquid flying through the air etc etc but of course as you already understand the faster the shutter speed the less light being exposed on the sensor so you need a lot of light or a bigger aperture or higher ISO to compensate and you may not want to do those things. In a studio environment we don’t worry about the shutter speed to freeze things, instead we use a very fast burst of flash instead which on the fastest studio flash can be a fast as 1/10,000th of a second, this light is also usually brighter than continuous light so we just use the 1/250th of a second sync speed and then the whole sensor is exposed at once and that sync speed is usually fast enough to cut out any ambient light from your modelling lamps or interior lighting especially if your are at f11 or smaller.

          3. Nick

            Sorry – missed the 1st bit. I thought the seed the camera could physically move the shutters hit a wall at the sync speed…if you’re saying that the time to scan the sensor increases at increasing shutter speeds beyond sync speed (rather than simply decreasing the width of the slit) this makes sense and answers the question!

  12. Hi Karl, Frist off I just your got classes, I find it VERY informative and amazing of all the knowledge at my fingertips. I might be jumping ahead of myself , but can’t you also adjust the the flash exposure by adjusting the ISO if need be. Such as going from ISO 100 to say ISO 400, which would be 2 stops.

    1. Hi, thank you and glad to hear you are enjoying the classes. Yes you are probably jumping ahead as that would have been covered in earlier classes. As a rough guide our learning levels go sort of left to right in the menu and left to right in the thumbnails, so if you skip a thumbnail or level then you may miss information that has been covered in an earlier class. We welcome any comments and answer all training related questions in the comments sections below each video (as you can see) often though questions are also asked that are then covered in the very next class. All the best Karl.

  13. This class was amazing. Karl I don’t know how to configure my sony alpha 7 camera so that it does the flash at the beginning or at the end.

    1. Hi, in your menu settings you should have something called first curtain sync or second curtain sync or it might be called front or rear. It should be in the flash sync settings if it has that ability.

  14. Hi Karl,
    Thanks! I understood the relationship between the ambient light and shutter speed. Basically if I leave the shutter speed longer, it will let more ambient light in. I re-looked at the video and understood this time.

    Thanks for the help! Loving the classes.


    1. Hi Tummavg, I think I’ve already asked you to watch the following classes which should have given you the answer as the answer is in the classes? Are you watching these classes properly?
      Once you have watched the classes above then this section of classes will make more sense to you and they cover various situations where you would need to sync your flash at different shutter speeds based on the surrounding ambient light. Do not watch these classes first, please watch the other 2 classes first as it appears you are missing some valuable information.

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