Managing Two Exposures in the Same Photo With Flash

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Learning DSLR

8 min readAug 3, 2011

One thing I’ve enjoyed practicing this summer is incorporating flash with outdoor daytime photography. I usually have my flash with me so why not use it? On a bright day, you may not think you need more light but a flash can bring clarity to the shot without looking unnatural. It serves to balance your subject with a bright background while also removing unwanted shadows.

Mark Wallace and the team at AdoramaTV just released the first video of their “second season” and this topic was well covered. Start here if you have time (approximately 10 minutes):

Type of Flash to Use

If you only have a built-in flash, that’s fine. It’s probably the only time it produces desirable light as you don’t need or want much diffusion.

You’ll probably need all the power the flash can produce given it’s during the day. To give yourself more creative options, this means you’ll want to invest in an external flash. I have a Canon 580EX II which is the best external on-camera flash unit offered by Canon. I definitely recommend it and insist you buy it in the USA to avoid getting ripped off by Canadian retail options. The same unit goes for $475 at B&H (New York) vs. $600 @ Henry’s (Ontario).

Controlling Ambient and Flash Exposures

As pointed out in the video, there’s really two exposures to consider when you use your flash. The ambient light is one exposure, and anything hit by your flash is another. What’s not obvious is that you can separately control these exposures to create a lot of variance in the final photo.

If you’re not careful, it’s easy to properly expose your subject while having the background be too bright. This is especially true if your subject is in the shade where it should be to avoid high contrast light on people’s faces. Once you add flash, and aren’t too far away, you have the opportunity to contribute light on your subject so that they balance out or exceed the background brightness. This is a must if your subject is backlit but it’s useful on any bright day.

Technically speaking, your flash fires very quickly (e.g. 1/1000th of a second). So quickly that your shutter speed setting has no real impact on the quantity of flash bouncing back to your camera. The rapid flash burst speed is effectively the shutter speed for the flash exposure. Your camera shutter speed only impacts the ambient light and thus you can vary it considerably without negative consequence on your main subject’s lighting (there are limits to this of course).

Aperture and ISO are the other two parts of the “exposure triangle”. These two settings equally impact the ambient and flash exposures so they’re not as useful in this scenario.

Finally, there’s the strength of your flash which you can vary up and down via flash exposure compensation (FEC) or by manually setting your flash strength. Flash power won’t effect ambient exposure as your flash can’t light up the entire outdoor scene. It therefore gives you separate control over flash exposure in the same way that shutter speed discretely affects the ambient exposure.

Let’s recap:

Shutter Speed: Only impacts ambient exposure
ISO & Aperture: Impact ambient and flash exposures
Flash Power: Only impacts flash exposure

You might read elsewhere that “Aperture controls flash exposure” but that only applies to those of you using manual flash. If that’s the case, you’d have to also factor in distance from subject. I’m assuming that we’re all using the automatic mode of our flashes (i.e. TTL or through-the-lens metering). You can read more about this on Neil Vn’s blog in a post titled, “When Aperture Does NOT Control Flash Exposure”

Canon vs. Nikon

Once you have the theory portion memorized, you have figure out how to find your initial settings and how to vary them to control the end result. To explain the first step requires that I point out a difference between Canon and Nikon cameras. This point is well covered in the video above but it’s not well illustrated by the sample photos shown.

The important difference is that Nikon shooters must use Manual mode of the camera to tweak the ambient and flash exposures separately. Canon shooters, however, can stay in Aperture Priority mode. Canon’s Aperture Priority mode never takes flash into account when it meters available light. If you use exposure compensation, it will only impact the ambient exposure. If you use a Nikon, exposure compensation in Aperture Priority mode will affect flash power as well as the ambient light.

In Aperture Priority mode of a Canon, the only way to vary the flash power is to use Flash Exposure Compensation (FEC). To access it, click the lightning bolt icon with +/- or look for that symbol in the Quick Menu. Compensation is measured in stops of light (i.e. +1 stop = double the light).

Step 1: Expose for Ambient Light First

The best way to start is with your flash shut off completely. You want to expose for your background first before introducing another variable. First, however, you’ll want to set a reasonable starting point for your ISO and Aperture which impacts both the ambient and flash exposures.

Note that if you set your ISO to 100, you’re going to need a powerful flash. Start somewhere between 400 and 800 and bring it down if you can. Removing unwanted noise in post is not that bad these days.

Setting the correct aperture value is a matter of taste. Especially for portraits, I often want a shallow depth of field (e.g. f/1.4 or f/2.8). These big apertures are good as they let in a lot of light which reduces the stress on your flash.

If you ever increase your flash power and it makes no difference, you may need to open your aperture more or boost your ISO. If you can, you can also step closer to your subject.

Now it’s time to find the best shutter speed:

Canon cameras:

Set your camera to Aperture Priority (Av)
Point your camera at the background or use spot metering if you know how to use different metering modes of your camera
Take a photo and look at the results
If the background is overexposed use exposure compensation (e.g. -1 stop) and try again. If it’s underexposed, then go the other way (e.g. +1 stop).
Vary exposure compensation by 1/3rd stops (1 click) up or down until you get what you want.

Nikon cameras:

Set your camera to Manual mode
Point your camera at the background or use spot metering if you know how to use different metering modes of your camera
Adjust your shutter speed until you zero out the exposure meter (i.e. look for a scale in your viewfinder as you change shutter speed and move the dash so it’s at the zero mark)
If the background is overexposed, speed up your shutter speed. If it’s underexposed, then slow down your shutter.
Vary your shutter speed by 1/3rd stops (one click) up or down until you get what you want.

Step 2: Turn on Your Flash and Take a Test Shot

Your on-camera flash or pop-up flash has its own auto mode. For Canon it’s called “E-TTL” and in Nikon land it’s “I-TTL”. All that means is it will send light out before the shot to make a determination about required flash strength. It will automatically account for for the ISO and Aperture you’ve selected earlier.

Even in you’re shooting in Manual mode, your flash will still be in TTL (auto) mode unless you change it to “manual” as well. So Manual mode isn’t fully manual with a flash — that’s a good thing.

The only setting you might need to flip on before the next shot is “high speed sync” (Canon) or “Focal Plane” (Nikon). This is required if your selected shutter speed is above your camera’s flash sync speed. For most DSLRs, the flash sync speed is either 1/200th or 1/25oth of a second. Be sure to read my earlier post about flash sync speed if this concept is new to you.

What’s brilliant about TTL is that you’ll likely get a near-perfect exposure on the first try. “Perfect” might not suit your tastes however. Personally, I don’t want my daytime outdoor portraits to look like I used flash. I might therefore decrease my flash exposure compensation by a stop when using it to fill shadows. But if I want to “overpower the sun” (i.e. make my subject brighter than the background), then I might do the opposite and crank up my flash exposure by a stop or more.

Step 3: Season to Taste

Now that you have your second test shot, you need only vary the “independent” settings as needed to find the look you’re seeking.

Vary shutter speed to control ambient exposure
Vary flash exposure compensation (FEC) to control flash exposure

As I mentioned earlier, it’s also generally accepted that aperture is a way to control flash exposure but that’s really not the case with TTL flash. That said, a very really small aperture like f/16 will probably not work as you’ll hit the practical limit of your flash unit’s strength.

For example, if you set your flash exposure compensate to be +3 stops and it doesn’t seem to have the desired brightening effect on your subject, you may need to open up your aperture (e.g. open up from f/4 to f/2.8). You will need to speed up your shutter to keep your ambient light in check but at least you’ll be letting in twice as much flash light.

Alternatively, you can increase your ISO and make a similar shutter speed adjustment. Doubling your ISO will double your camera’s sensitivity to the flash output.

Some Recent Examples

Just yesterday I was meeting up with some friends in a park in Montreal. I put my flash on the camera right away and got great results with minimal trial and error. I found the background was quite bright so I took exposure compensation down by a full stop. Some shots looked bright so I brought down flash exposure by 1/3 or 2/3 stops.

As you can see from these examples, it’s not always obvious that I’m using a flash at all. The light can be a bit flat but the only way around that is to get your flash off the camera — that’s a whole other topic.

My friend Adam Khodadeen also posted a great example of how overpowering the sun can save a shot ruined by dappled light-and-shade. Here’s the before and after: