How Do You Change the Brightness of a Flash?

I’m one of the rare ones; I actually read the instruction manuals. I want to make sure I know my way around the camera, so I know how to get what I need from my camera when I need it. I’ll admit, however, that the first time I read the part about Flash Exposure Compensation (FEC) and Manual Flash, I skimmed over it. And when I saw that the instruction manual for my speedlite was as thick as the one for my camera, I couldn’t believe it! The first thing I did was look to see how many languages it was printed in (answer: only 2, same as the camera). Reading my camera manual helped me understand my camera, but even the English half of the speedlite manual looked like a foreign language to me.

I’ve seen good photos shot with a flash and I’ve seen bad ones. Through patience, study, and practice I began to understand my speedlite, and to stop being intimidated by it. This post will have a few experiments you can try that will help you feel less intimidated by your speedlite as well. In some cases I will ask you to change the exposure and flash output manually. I’ll tell you where to set those exposures, but you might have to grab your instruction manual if you’re not sure how to manually set the exposure.

The quality of a flash photo has a lot to do with the brightness of the flash. In fact, one really great thing about flash photography is the ability to have one exposure for the part of your photo lit by the flash, and another exposure for the rest of your image. To help you get a grasp of this idea, I’m going to show you 5 ways to change the brightness of a flash. Some will only work when shooting with a speedlite (or speedlight); others will work on a smartphone.

Shooting with flash doesn’t need to be as intimidating as the size of the manual. If you experiment with these tips, you will be able to control the brightness of your flash in no time.

#1. Flash Exposure Compensation

This is by far the easiest way to begin shooting flash (don’t confuse easy with mediocre results). Canon has had the FEC system in place for a while, so Canon’s system is a bit easier to use and more widely available than Nikon’s. But even if you don’t have Flash Exposure Compensation, this section is important since the principles of how flash works are mostly the same when shooting in manual mode.

Before a flash fires, it determines how long it should fire to get the necessary exposure. The longer the flash fires, the brighter it will illuminate the subject. Flash Exposure Compensation works a lot like exposure compensation: turn it down one stop and the flash effectively gets darker; turn it up one stop and it gets brighter. In the series of images below, I fired the flash directly at my camera through a diffusion panel. The shots progress from FEC -2 to FEC +2.

FEC

Often, only part of your photo will be illuminated by the flash. Just like with a normal exposure, there are times you will want to correct what the camera is “thinking”. If the flash isn’t giving enough light, dial it up a stop. If it is giving off too much light, dial it down a stop. It’s ok to peek at your screen – in fact, I find it helpful. If you have FEC, try it. Take a series of pictures and dial the FEC up or down to see how it changes the photo.

To demonstrate how this works, I purposely placed a model in front of a bright window so that the camera would underexpose her. Without a flash, she would appear completely black. For each shot I started with an exposure of 1/125 second, f/8, ISO 100 (This is my starting point for every set of photos in this article). If you want to try along with me, put your camera in manual mode and use the same exposure settings I did (this way you avoid other changes that could alter the brightness of your flash – since this article is about changing the brightness of the flash, it’s not really important if the view outside is exposed correctly). For the series below, you can see that I started with the FEC set to 0. I thought it was a bit too dark, so I moved it up to +1. When shot at +2, you can see how my model starts to look too bright.

FEC2

#2. Manually Change the Flash Output

Manual Flash works a lot like FEC because you’re still changing the power of your flash. However, you are also telling it not to make its own exposure decisions. Manual flash, like shutter speed, is read as a fraction from 1/1 (full power) usually down to 1/64 or 1/128. Getting the right exposure dialed in takes a little trial and error, but I usually find I can get it figured out after a few shots.

In the series of photos below, I started with my flash set to 1/4 power. I felt it was a little too dark, so I took another at 1/2 power. The final shot was at full power for this speedlite. If you have more windows or lights where you are shooting, you may end up with a model that is overexposed at 1/4 power. In that case, you will have to dial it down to 1/8 or 1/16 until it starts to look better. Manual flash works well if the distance from your camera to your subject is constant. If you are moving around a lot, or the light is changing, I suggest using flash exposure compensation instead.

Power

Manual flash doesn’t have to be any more complicated than this. Play around with increasing and decreasing the power of your flash and notice how it changes your picture. One thing you may notice when trying this out is that your camera will cap your shutter speed much lower than you are used to. Mine caps at 1/250 second. This is due to the flash sync speed. (If you aren’t familiar with flash sync speed you might want to check out this article on Flash Sync Speed and High Speed Sync.)

#3 Change the Distance from the Flash

This is a process of changing the brightness of a flash that doesn’t require an SLR, speedlite, or any other fancy equipment. You can do this with a point-and-shoot camera or a cell phone. (If you google “changing distance” and “flash photography”, you might see something called the inverse square law. Don’t read it unless you really love math!) Here’s the simple version: back away from your subject and your flash gets less powerful. Get closer to your subject and the flash gets more powerful (we can save the talk about the inverse square law for some other time).

I’ve snapped plenty of blown out pictures on my iPhone shooting a quick picture to send off to my mom to say, “Look Mom! I’m eating ice cream and you can’t stop me any more!” Backing up a little won’t necessarily make an award winning picture, but at least my mint chocolate chip won’t look more like split pea soup.

With a DSLR and a speedlite, you can try the same experiment we did with Manual Flash. Instead of adjusting the flash power to change the brightness, move the flash closer or farther from your subject and look at how the brightness changes.

changing distance

#4. Adjust your Aperture

This can be a bit of a tough idea to grasp, but you can change the brightness of your flash by changing your aperture. Your flash fires for a very short amount of time; in most cases, faster than your fastest shutter speed. Therefore, your shutter speed doesn’t really have anything to do with how bright the light from your flash appears in your photo. Instead it is controlled by your aperture. You’ve experienced this: when someone shoots a flash at you in the middle of the day, you don’t really notice. But at night, when your pupils are open to let in more light, it hurts!

To practice changing the brightness by changing the aperture, go back to the experiment where we were manually changing the flash output. Leave the power on the flash at 1/4, but this time change your aperture. Notice how the brightness of the flash changes. If you’re using a pop-up flash, this is one way to gain control of the brightness of the flash. (If you are shooting in manual mode and you change the aperture, you have to be sure to adjust your shutter speed accordingly in order to keep the rest of your photo from changing exposure. I wasn’t paying close enough attention when I shot this series of photos. I forgot to adjust my shutter speed when I changed the aperture, so the entire photo was affected by that change. Switching to aperture priority mode would have helped me avoid this mistake.)

Aperture

#5. Zoom the Flash

You will also find that a speedlite has a zoom feature for the flash head. Zooming in allows the flash to send out a tighter beam of light. This can be useful if you are too far away from your subject to get adequate light. It can also be used for creative purposes. In the shots below, I purposely backed up from my subject so that my flash would be underpowered. Watch how changing the zoom of the flash head changes the brightness of the flash.

zoom

Often, your camera is automatically set to zoom the flash as you zoom your lens. You can override this, as I did in the series of experiments above. I generally leave the automatic zoom on, and only turn it off when necessary.


Working with flash can be tricky, because you are introducing a new light source that you control. This means controlling the amount of light falling on a particular area of your photograph. It can seem overwhelming at first, but as you begin to experiment with the brightness of your flash using the methods in this article, you will begin to understand how you can change the brightness of your flash and the impact it has on your photos.

In this post, we only focused on changing the brightness of the flash. By firing the flash directly at the model, you may have noticed that the light isn’t always flattering. Knowing how to quickly change the brightness of the flash is the first step in making more advanced changes (such as bouncing the flash) so that you can create more pleasing light on your subject.

Interested in learning more about portrait photography? In our 30-day online portrait photography class, we teach posing, flash photography, touching up portraits in Photoshop, and much more!

Comments from the I.P. Community

  1. says

    Great article, as always. But you should do a chart of some sort (like the quick reference quide for Lightroom you produced) to show the direct relation between aperture and manual flash power. Almost forgot. You could also include a graph including Kelvin temp settings and corresponding WB settings.

      • Bill C says

        Not true, Leonardo. ISO will change the sensitivity of camera’s sensor to all light sources. The sensor will be unable to tell what light is ambient and what light is flash.
        Try it for yourself. Try it in a place where there is little if any ambient light.

  2. says

    You can also use shutter speed (up to the sync speed – higher if your flash has high speed sync) to adjust the ration of the brightness of the subject with the brightness of the background.

    • Bill Cullen says

      While reducing the shutter speed does allow more ambient light in, the “flash brightness” is unchanged, and that is which is the subject of the article.

  3. alvin says

    Simple yet great write up, ISO should also be mentioned as it has its effect in brightness of image, looking forward for more. Thank you

  4. says

    Thanks for this. Good article. My flashes (Nikon SB600 and SB700) both behave strangely sometimes when in TTL mode. You set the FEC to say -3 and then shoot. Immeadiatly after shooting, I look at the flash’s lcd display and it has changed my selected FEC value to some other value like +1 (or anything else). I don’t know if it did this before or after the shot (i suspect before since I don’t get the results I want a lot of the time).

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