How Flash Relates to Shutter Speed, ISO and Aperture

 

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One of the biggest lessons that every strobist needs to learn is how to make flash output work with a camera’s exposure settings. It’s intimidating for most, but if you think about it as just something a bit different and understand the concepts then you’ll begin to see that it’s not only very simple but it’s also very fun. With better flash knowledge, you can create photos that most folks can’t.

Now: don’t be scared.

Shutter Speed

Chris Gampat Portraits quickly on Greenpoint (3 of 3)ISO 4001-60 sec at f - 1.4

ISO 400 1/60th f1.4. This image blended the ambient light into the scene to get this look

In more typical settings without the use of a flash, the shutter speed lets more light in onto the film/sensor plane and also helps to emphasize movement. That becomes a bit different with a flash in the equation. The slower the shutter speed, the more ambient light will seep into the scene.
Chris Gampat The Phoblographer Little Miss Rollerhoops portraits (20 of 24)ISO 2501-6 sec at f - 8.0

ISO 250 1/6 sec at f8.0. The fast flash duration here froze the fast moving motion of Jordana’s hula hooping and yet dramatized the movement of the LED hula hoop

If you want to dramatize movement, you’ll typically go for a second curtain or slow sync flash setting. But because of the way that a flash work, it will also help reduce camera shake to a point.

What we’re talking about here is the flash duration: which, in layman’s terms, essentially takes over for the shutter speed of the camera. So if a monolight has a flash duration of 1/8000th then it will stop fast moving motion to that speed.

Don’t believe me?

Chris Gampat The Phoblographer TriggerTrap Flash Adapter review black and white (1 of 1)ISO 2003.0 sec at f - 6.4

)ISO 200 3.0 sec at f6.4

The image above was a three second long exposure with a fast flash duration. Yet it stopped all the fast moving motion because of the flash.

If you know anything about shutter speeds you’ll know that they typically can’t stop super fast moving motion like this.

Summation: Shutter speeds affect ambient lighting in typical and traditional flash usage involving first curtain. When second curtain or slow sync is used then it changes up to emphasize movement.

Aperture

Chris Gampat The Phoblographer Shanny EX600 EF flash samples portraits (4 of 4)ISO 2001-250 sec at f - 3.2

Aperture (f-stops) work a bit different when it comes to flash output. While your shutter speed is directly correlated to the ambient light, the flash and aperture are related here and depending on how your flash is configured they will work in one of two different ways.

TTL

Chris Gampat The Phoblographer Shanny EX600 EF flash samples (1 of 5)ISO 1601-1600 sec at f - 2.5

TTL, known as Through the Lens Metering mode, is something that many photojournalists, some strobists and and wedding photographers typically opt for. Basically it reads the camera information when it comes to the aperture and adjusts its power level accordingly. To do this it reads both the ISO and the aperture,

You can’t really control the output from TTL metering but you can tell it become brighter or darker accordingly from what you see on the back of your camera’s LCD screen. Lots of variables play into this such as whether your flash is in a light modifier, your creative vision, etc.

Manual

With manual flash output you’re directly controlling how powerful the flash is based on its own individual power settings. Lots of strobists prefer this approach. Here, the aperture is directly tied to flash output. Meaning that if your light is metered to be outputting enough artificial flash power that an exposure looks good to you at f4.5, then that’s your base standard. If you open up to f2.8, then the scene will become brighter because more flash output is being absorbed into the scene. Conversely, stopping the lens down to f8 lets less light into the scene.

Summation: Aperture controls how much of the flash output affects the scene when the flash is set to manual mode. Aperture helps to dictate what the flash output will be in TTL mode. The user can use exposure compensation to figure out the rest accordingly.

 

ISO

Shot with the Sony A7r Mk II. A flash was placed towards the top of the scene to give the effect of window lighting.

Shot with the Sony A7r Mk II. A flash was placed towards the top of the scene to give the effect of window lighting.

ISO is quite an interesting setting and one of the reasons why most photographers choose low ISO settings when shooting with a flash is because it gives them more control over the scene. Sometimes you need a higher ISO though.

When a flash is used in an exposure, the ISO controls the overall sensitivity. More ambient light will seep into the scene and more flash output will seep in. Adjusting your ISO sometimes means you need to adjust all the other settings accordingly.

Summation: ISO controls how much ambient light the shutter speed lets in and how much flash output the aperture will let in accordingly. Increasing the ISO increases their sensitivity to light.

Flash Output

Something else I should cover very briefly is flash output. This really pertains to manual flash output more than anything else. All flashes have an output that works in fractions. For example at 1/2 the flash will output half of the power. A great place to typically start is at 1/4 power indoors and 1/16th when outdoors. From there you adjust the flash based on:

  • The ambient light you want in the scene (shutter speed)
  • The depth of field you want (F-Stop)
  • How sensitive the scene is already to light (ISO)

This is really all you need to get started. Let us know if you have any questions.