The Basics of How Flash Works

Updated: March 1, 2022.  In this article I will explain the basics of how your camera flash works. By learning how your flash works and how you can control it, you’ll get better, more natural, flash exposures every time.

This article assumes that you, the reader, understand manual exposure and how a camera meter is calibrated to 18% gray as its “zeroed out” point. This article also assumes that you have looked through the user guide to your flash. The section of the user guide you’ll want to pay attention to concerns how to change the flash Mode and how to get the flash into Manual Mode — as well as how to adjust the power setting once in Manual Mode. Please take a look at that section before you embark on this article. (Many user guides/manuals can be found online now with a Google search. Just in case you lost yours.)

One of the most confusing pieces of technology in a photography student’s path is the “Speedlight” flash. Why is this device so confusing?

First, the interface has a lot of menus, so a beginner unaccustomed to this will invariably be confused by the myriad options on the device.

Second, the default mode that these devices start up in is E-TTL (“Evaluative Through The Lens”) which takes information from the camera to determine the appropriate amount of flash. E-TTL is “automated,” but that doesn’t mean it will automatically produce the best brightness level or blend of exposures. You must know how to manipulate and override the E-TTL and how to mix ambient to suit the situation.

Imagine: An unsuspecting user most likely turns this unit on and fires off a shot and it has that classic telltale “deer in the headlights” look of on-camera flash, such as that seen in the example below. Because of this, a user is displeased with the quality of light from this gadget and the user is frustrated.

A portrait of a man taken with front camera flash.

Third, even though these devices can operate full-manual as well as the automated E-TTL, that doesn’t mean that exposure of flash is going to automatically work well as a variable in our exposure triangle: ISO, aperture and shutter speed. Now we have another variable — the flash exposure!

A diagram of how the flash relates to the camera's exposure controls.

And finally, most folks hope to read a blog article or a manual and gain the understanding of how to operate these devices. The truth is you need to set up the flash and run a couple of TESTS and then things become more apparent. At the end of this blog article I will ask you to put the flash on your camera and run through a series of exercises so you can begin to understand how it all works. Photography is a “doing” occupation/hobby ? there is no substitute for getting your flash and camera out and practicing these steps.

I argue that if you learn how to use these units manually, then it will make understanding other modes and uses a lot easier; with practice you can quickly dial in the look you want based on the lighting scenario you are in and the look you want to achieve. But you must understand how it is playing within our exposure “triangle” — to which we’ve now added another variable: flash!

First, let’s explore how to meter the light from a flash.

Exposures are based off a “constant” which is 18% gray, also called “medium gray”. Our cameras, when zeroed out on the meter, are calibrated to see a proper exposure as an 18% gray tone. In fact, the meter will “change” our image to medium gray. Don’t believe me? Go outside on a snowy day and point your meter at the snow, zero it out, and take a shot. It will be 1.5 to 2 stops under-exposed. This is because the camera wants to see a perfect exposure tone as that 18% gray. Therefore, if you zero out on white, it will change that to medium gray. Same as if you zero out on something jet-black, it will overexpose that tone to medium gray.

In the first image below, I came in close and metered off the white headlight.

Then in the second, I came in close to the car and metered off the black of the fender.

And in the third, I zeroed out my meter off the gray pavement.

A dark image of a car in a parking lot.

Metered off a small section of the white headlight

A bright image of a car in a parking lot.

Metered off a small section of the black fender

A properly exposed photograph of a car in a parking lot.

Metered off the gray pavement in foreground

Because the light gray pavement in the foreground is closest to what the camera’s meter is calibrated to, it gives me the best exposure automatically.

With this understanding, we can assume a flash exposure — a “pop” of light — should also be reading as an 18% gray tone to be a proper exposure, but how do we get there? The unit produces this pop of light at different brightness levels, and those brightness levels are easily metered and/or calculated. We have a few different methods we can use to meter a flash unit. Let’s start with the simplest methods.

One method involves purchasing a handheld flash meter. While most folks don’t want yet another $200+ device in their bag, flash meters are accurate and handy in situations that you need to make fast calculationsThink portrait of a celebrity where you have little time.

But, truly, we can get the information we need from a $4 swatch of fabric! Let’s dive in!

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If you don’t have the bucks for a flash meter, use the reflective meter in your camera, and a swatch of fabric that is about 18% gray. Since thezeroed-out point of the camera’s exposure meter is calibrated to see 18% gray tone as a proper exposure, if you place your gray swatch of fabric in the scene, in the lighting you look to capture — and you point your reflective meter at this tone and zero out on it — you will be creating a shortcut to an incident flash meter! My $4 piece of fabric will get me the information I need if I can’t spring for a flash meter. Below is the 18% gray tone you’d look for in a swatch of fabric from a fabric store. The trick is to switch the Metering Mode on your camera from Matrix/Evaluative/Averaging to SPOT.

Exercise #1: Your Flash’s Output

In this exercise, use a large swatch (1 yard square or so) of medium gray fabric and do your best to fill most of the frame with the gray tone. Make sure you are in a dim location — not a bright room and not outside in the sun. Inside your home in the evening is perfect.

Assuming you have read your flash’s user guide, find the section on Modes and how to put your flash in Manual Mode. Typically, these units will have a “Mode” button and you can cycle to the Manual (M) setting on the flash. Then, there is usually a +/- button that when you depress it you can spin the dial or toggle up/down to adjust the fraction up or down. You will want to put it on 1/1 to start. This is Full Power.

  1. Start off at 6 feet away from the gray target.
  2. Put your flash unit on Manual (M) mode and set the power to 1/1. You’ll want to use a fast shutter speed, like 1/200th to start (but not faster than 1/200th).
  3. Capture some images at a variety of f/stops starting at ISO 100 with the flash on full power. I’ll tell you right now: At 6 feet away most of these units will give you around f/16 at ISO 100 at Full Power on the flash.

What you are looking for is for that 18% gray patch to appear as a mountain range in the center of your histogram. When you see that, you’ll have a proper exposure. Take note of the f/stop you are at when you see this. Our gray cloth test is basically just like metering off the gray pavement in my car example above! We have a known constant (18% gray) that is acting as an exposure target for us.

Some quick notes about power settings on flash units:

  • When you are in Manual Mode (M) on these flash units, the way the device displays its power setting is in fractions
  • Full Power will display at 1/1. That is 100% of the juice this device will pump out in a flash!
  • Half Power will display 1/2. This is 50% of the juice.
  • Quarter Power will display as 1/4. And so forth. This is 25% of the juice.
  • The difference between full power and half power is 1 f/stop in light.
  • The difference between half power (1/2) and quarter power (1/4) is 1 stop.
  • So, if you are on full power and you need to take the brightness value down 2 f/stops, you would go from 1/1 to 1/2 to 1/4 to achieve this. Between each of the full-fraction numbers it is 1 f/stop of light gained or lost, depending on whether you are raising or lowering the power.
  • Let’s say the flash reads f/16 at 6 feet at a power setting of 1/1 and I want f/8 — I need to power the unit down from 1/1 to 1/4 (dropping 2 f/stops).
  • Or, if it reads f/8 at a power of 1/4 and you want f/16, then you need to power UP from 1/4 to 1/1.

In the following exercises, consider this statement: In flash photography, there are 2 exposures happening — the flash exposure and the ambient exposure.

Now let’s tackle the Flash Exposure: Exercise #2

We are going to run some basic tests indoors in our dim room. Put your camera, with the flash attached, on a tripod if you have one — if not, do these steps standing in the same place every time.

  1. Compose a scene in your home. Make sure there is a subject that is about 6 feet away. It can be a lampshade, or a patient person, or some other 3-dimensional object that’s bigger than a breadbox.
  2. Set your Flash on 1/1 power in Manual Mode.
  3. Set your camera on ISO 100, 1/200th shutter, and the f/stop that you arrived at for 6 feet away (probably something like f/16).
  4. Take a photo. Your subject that is 6 feet away should look properly exposed. Don’t worry about the quality of the light or the “look” — we are looking for a good exposure of that subject.
  5. Now, adjust your flash’s power down to 1/2 power. This will reduce the output of the flash 1 f/stop.
  6. So, if you are at f/16 — you’ll then adjust your aperture to f/11. Set your camera at f/11. Keep shutter and ISO the same.
  7. Take a photo. Your subject that is 6 feet away should look properly exposed.
  8. Now, if we want f/4 and not f/11, we need to reduce the power of that flash 3 f/stops, right? f/11 to f8 to f/5.6 to f/4 — that’s 3 stops from f/11 to f/4
  9. Adjust the flash unit from 1/2 to 1/4 to 1/8 to 1/16
  10. Set your camera on f/4 ? keep ISO and shutter the same.
  11. Take a photo. Your subject that is 6 feet away should look properly exposed.

Ok! Great! Notice that in these shots that it looks very “flash” exposed.

Here are mine!

Flash at full power — ISO 100 1/200th f/16 —6 feet away

Flash at 1/2 power — ISO 100 1/200th f/11 — 6 feet away

Flash at 1/16th power — ISO 100 1/200th f/4 — 6 feet away

None of the ambient room light you can see with your eyes is being seen in the images above. They are entirely lit by flash.

Normally, when you make photos in manual mode there is the relationship between shutter speed and aperture and ISO. This is known as reciprocity. It is sometimes referred to as the “Exposure Triangle.” When you close the aperture down it reduces light. To compensate (or “take the reciprocal”) we need to let more light in through the shutter by slowing it down. Or adjust ISO higher or lower. But, when you begin working with flash, start off by separating shutter and aperture in your mind.

Consider that the flash exposure is attached to our aperture value. A second, separate exposure of the ambient light is attached to our shutter speed. When we first start off using flash, it’s good to just put our shutter speed on something fast like 1/200th to black out all of the ambient light in the scene and just work with the flash at first, just as we have in the first two exercises.

In the first exercise above, we wanted to see how at 6 feet away our flash was giving us f/16. We can, therefore, put our shutter speed on something fast (1/125th, 1/200th) and see the flash exposure.

The flash exposure is controlled by the aperture. They are connected together — we can see this in our exercise of adjusting the power of our flash units in full f/stops and matching that with changing our aperture! The reason we can’t see much (or any) of our interior scene is because the ambient light is controlled by the shutter speed. And at 1/200th of a second, that is going to omit nearly all the ambient light in our scene. When we begin to slow the shutter speed down, we will see more of the ambient light in the space. Here is how you can test this:

Controlling Ambient Exposure with Flash: Exercise #3

Go ahead and put your camera back on f/16 and dial the power of your flash back up to Full power 1/1. Have your subject 6 feet away like before.

You are going to take 10 photos. Ready?

  1. Take a photo at 1/200th f/16 ISO 100 with 1/1 power on the flash. Decent exposure? Good.
  2. Now, keep the f/stop, ISO and flash power the same and run the following shots:
  3. 1/125th of a second — take a shot
  4. 1/60th of a second — take a shot
  5. 1/30th of a second — take a shot
  6. 1/15th of a second — take a shot
  7. 1/8th of a second — take a shot
  8. 1/4th of a second — take a shot
  9. 1/2th of a second — take a shot
  10. 1 second — take a shot
  11. 2 seconds — take a shot
  12. 4 seconds — take a shot

My results are below. Notice that the ambient light in the room in my shots doesn’t even begin to really appear until I’m closer to 1/2th of a second shutter speed! That is how dim this space was!

All shots below are at Flash 1/1 Full Power, ISO 100, f/16 — with varying shutter speeds ONLY.

Surely by now you will notice the room getting brighter. If you aren’t seeing any of the room in your own images, you need more lamps on — or you need to keep going (4 seconds, 8 seconds, 16 seconds, 32 seconds) until you see more exposure of the room. This exposure is the ambient light in the room “trickling in” under your shutter and mixing with the flash exposure. The flash exposure might start to seem bright because the ambient light is mixing in and overexposing the total exposure on your subject.

This mixing of the flash and ambient exposure is the key to getting good-looking flash shots. You don’t want your flash to look overpowering, and you don’t want your ambient to be too dark or too light. It’s that sweet spot of a slow-enough shutter and a properly exposed flash!

If the room you are in is fairly bright, you will see this ambient light much sooner than in my example above! You may see it begin to appear at 1/30th of a second. It will depend on the brightness of the ambient light in your situation.

The BIGGEST factor in getting electronic flash to look natural is the mix of ambient light in the image. If there is no ambient light mixed in, things will look “flashed.” Therefore, the variable in our “Exposure Triangle + Flash” (ISO, aperture, shutter speed and flash exposure) that requires manipulation is the shutter speed. We can achieve a well-exposed flash exposure easily once we know how to manipulate the power of the flash to match our aperture — but we must slow the shutter speed down to let more of the ambient light in to achieve a more-pleasing balance between ambient exposure and flash exposure. Or, in some cases, if the ambient light is bright, we may need to speed up our shutter speed. More on that below?


  • Your flash exposure is controlled by your aperture.
  • Your flash and ambient exposure (total exposure) is controlled by ISO.
  • Your ambient exposure is controlled by the shutter speed.
  • Aperture will also control total exposure — but shutter speed won’t control the FLASH exposure.

A Note on X-Sync: On many cameras, you cannot fire a flash unit beyond what is called X-Sync and the X-Sync speed varies among cameras. Most often that speed is no faster than 1/200th of a second. Therefore, if you are running tests for flash, shoot at 1/125th or 1/200th and no faster! If you shoot faster than the X-Sync the result will be a “half image” where you’ll see the curtain of the shutter in the image. Consult your user guide or the internet for your camera’s exact X-Sync speed.

Here are a few Best Practices that can help you nail better flash exposures:

  • Run that initial test to know at 6 feet, ISO 100, what your flash will give you in terms of f/stop at Full Power.
  • When you approach a scene that you’d like to photograph with flash, start with the f/stop you want (“your intended f/stop”), based on how much depth-of-field you desire.
  • At that f/stop, meter the scene and determine what shutter speed you will need. (Aperture Priority is fine for this, too.)
  • Look at the distance you are from your subject (keep it easy and get your subject 6 feet away at the beginning) and then dial the power of the strobe down to match “your intended f/stop.”
  • Example: If you chose f/5.6 — and your flash outputs f/16 at 6 feet, then power down from 1/1 to 1/8 — 3 stops down.
  • Take a shot at the shutter speed you metered for “your intended f/stop” and the flash powered appropriately.
  • If there is too much ambient light, speed up the shutter speed. If there is not enough, slow down the shutter speed.

Let’s do a photo breakdown using these exact steps!

When I approached this scene, I knew only 2 things ahead of time: The f/stop I wanted, and what my strobe would output with my preferred soft-box at 12 feet away. I knew my light would be pretty far from the model and up high in the sky, about 12 feet away. So, I ran my initial test of the strobe with the soft-box from 12 feet and dialed in what power setting would give me my intended f/stop of f/5.6. BOOM! I have valuable info! I have pinned down TWO variables in our list of variables (ISO, shutter speed, aperture, and flash output)! Now I only have ISO and shutter speed to contend with?

When I got to the location, I knew I wanted dusk lighting, and not mid-day lighting, so I got there about 45 minutes before dusk. I took an Aperture Priority reading of the overall scene at f/5.6 and it told me I needed ISO 800. At the time of my first reading, things were bright, so my first shutter speed reading was 1/250th ? way too bright. That would be more ambient than I want and I could see that brightness with my eyes. I needed the sky to get darker and the overall brightness to dip down. So, we waited.

I set my camera at ISO 800, f/5.6 and every 10-15 minutes I would take an ambient reading and I’d see the shutter speed was dropping as it got darker. I wanted to shoot no slower than 1/60th of a second since I was hand-holding the camera. Once we got to 1/60th, I began to shoot. The ambient eventually dipped to 1/30th and I wanted to cut 1 stop of ambient out of the exposure, so I kept it at 1/60th (one stop faster than 1/30th) so that my key light was the Main Thing and the ambient light would act as my fill at 1 stop under.

Start with what f/stop you want.

Power your strobe to that value based on your tests.

Take an ambient reading at that intended f/stop to get an idea of what ISO and shutter speed you are at.

Adjust shutter speed accordingly.

And? experiment!!

Here are a couple more articles on using flash in photography:

The Basics of Flash Photography

Making Flash Look Natural Indoors


  • Jeff McLain

    Jeff McLain is a photographer, videographer, digital technician, and location sound mixer. After his photography education, Jeff got his start as a freelance photo assistant in San Francisco working on editorial, catalog and advertising shoots. His skills in Photoshop and computing allowed him to help photographers bridge the gap between the film days and all-digital workflows, and he stood at the forefront of the advent of the career of the "Digital Technician." Jeff then moved laterally to video capture. Locally, he is the Director of Arrowroot Productions, LLC, a commercial videography business. His background as a multi-instrumental musician has also benefited his understanding of sound design and audio capture, which can be a technically challenging aspect of film-making. He is regularly called by filmmakers and television networks to record sound-for-video. He has been a freelancer for over 20 years and takes a 'real-world' approach to his perspective of the photographic and video industry and the skills needed in today's market.