How to Expose for Flash and Existing Light Separately Using Your Camera’s Controls

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Flash photography can be intimidating. Adding artificial light to the existing light in a scene and getting a proper balance between the two requires some basic knowledge and skill on the part of the photographer. If you are not yet experienced using flash, but have an interest in giving it a try, an important first step is understanding how to control exposure of the light from your flash and the existing light (a.k.a. ambient light) separately using your camera’s and flash’s controls.

You probably already have a good understanding of how to control shutter speed, aperture and ISO to arrive at a proper exposure when flash is not being used. Let’s look at how these three variables, as well as your flash’s output, come into play when you are adding artificial lighting to the scene.

Introducing Leon the Lion

It’s been awhile since I’ve been on safari and I’m missing it. So I thought I’d borrow Leon the Lion from my daughter’s room and pretend I was back in Africa. My settings here were ISO 400, 1/50th second and f/8. I shot in manual mode. No flash yet.

1 - ISO 400 f8 1-50 No Flash

Next, I added the off-camera flash. I used manual mode on my flash and fired it through a small soft box at 1/32nd power. The image below is our starting point for an exercise to learn how your camera and flash controls affect how much flash and ambient lighting end up in your image.

2 - Add Flash 1-32nd

Shutter Speed

Changing your shutter speed won’t affect the amount of flash lighting that ends up in your image. This might be surprising to you, but it makes perfect sense. Here is why:

The flash has to fire while the shutter is open. Your camera will have a “maximum sync speed”, which is the fastest shutter speed whereby that can be accomplished. Usually this is around 1/200 of a second (I’m ignoring “high speed sync” for now). Even at full power, the blast of flash happens faster than your maximum sync speed. So, the same amount of flash will illuminate Leon when we reduce the shutter from 1/50th to 1/100th second. Here is the image after the shutter speed change.

3 - 1-100th

Notice that the flash lighting on Leon appears mostly the same as with the original settings, even with the adjusted shutter speed. However, the background has gotten darker. So, using shutter speed, we’ve just controlled the ambient light background separately from the flash-lit subject. There was some ambient light hitting the subject, in addition to the flash, so we don’t have complete control over the separate light sources. But you can see that we have considerable control here.


Now, let’s reset our settings back to ISO 400, 1/50, f/8 and 1/32nd flash power. This time, let’s adjust only the aperture. Staying with this idea of cutting the light in half, let’s adjust the aperture by 1 stop from f/8 to f/11.

4 - f11

Notice that both the flash lighting on Leon and the background lighting went darker than with the original settings. So, the aperture controls both the amount of flash lighting and existing lighting in the image. But wait… there’s more to say about aperture….

Shutter Speed and Aperture Together

With the above aperture adjustment, we left the shutter speed alone. However, we could have made two adjustments, one to aperture and one to shutter speed. Remember, shutter speed does not affect your flash lighting. Here is how you can control each type of lighting separately using a two-step process:

  • First, we get the desired exposure on the flash-lit subject by adjusting the aperture. This also affects the background exposure, which might not be properly exposed yet, but that’s okay.
  • Second, we adjust shutter speed for the background exposure only, without affecting the flash-lit subject.

With these two steps, you essentially control the flash exposure with aperture and the existing light exposure with shutter speed.

What about ISO?

Because your ISO controls the sensitivity of the sensor to all light, it is similar to aperture in that it impacts the amount of both flash and ambient light in your image. To test this, let’s reset our settings to the initial settings of ISO 400, 1/50, f/8 and 1/32nd flash power. Now, let’s reduce the ISO from 400 to 200.

5 - ISO 200

Notice that the aperture adjustment and ISO adjustment look basically the same as both halve the light on the entire image. This means that you can use ISO in a similar way that you can use aperture. However, with ISO, the lower the better in terms of image quality. So, I keep my ISO low. Fortunately, there is another variable we can adjust to control the power of the flash… the flash itself.

Flash Power and Placement

I always shoot off-camera flash in my flash’s manual mode. My Canon flashes can be fired at full power (1/1), 1/2 power, 1/4 power, 1/8, 1/16, 1/32, 1/64 and 1/128. Each of these steps cuts the flash power (the duration of the flash, actually) in half. I have the flash set up to allow further tweaking in 1/3 stop increments.

Let’s reset again to our initial settings of ISO 400, 1/50, f/8 and 1/32nd flash power. Here is the original image back with the original settings.

2 - Add Flash 1-32nd

Now, let’s halve the flash power from 1/32 to 1/64, as shown in this next image.

6A - 1-64th power

As expected, this adjustment mostly leaves the background alone, but reduces the impact of the flash on the subject. You may have some flash hitting the background which will still be affected by adjustments to flash power.

You can also control the brightness of the flash on your subject by moving the flash closer or farther away, changing the angle of the flash, or adding/removing a diffuser. However, these adjustments all change the nature of the light, not just its power. So, don’t use this method to adjust the power of your flash.


Here is what we’ve covered:

  • Adjusting the shutter speed affects the exposure of the ambient light, but not the flash.
  • Adjusting aperture impacts the exposure of both the flash-lit subject and background.
  • You can first use the aperture to control the amount of flash lighting in your image and then adjust shutter speed for the ambient light without impacting flash.
  • Changes to ISO have a similar impact as aperture. Image quality is an issue to consider.
  • Adjusting your flash’s power controls flash output without affecting ambient light, except for flash that spills over onto the background.
  • Positioning and diffusion of the flash should be used to control the nature and quality of light, not the power of the flash.

Try This at Home

You can use this article as the basis of a lighting exercise that you can try at home. Below is the basic set-up I used.

7 - The Setup

Now, if you decide to use a stuffed animal like I did, my suggestion is to not let the neighbors see you doing this. The basic idea is to get an initial decent balance of flash and ambient light using the controls outlined in this article and then to modify each of the variables to see their impact on the lighting. It is the modification of these variables that is the important thing here, so don’t worry about getting a perfect exposure.

Start without the flash to get an initial exposure, then bring it in and try to balance it with the ambient light. You’ll want to work in manual mode on both your camera and your flash. Try it out in dimmer light, bright light, and even indoors. I shot fairly late in the day in cloudy conditions, so my settings will reflect somewhat dimmer outdoor lighting.

Good luck!