Discovery Park of America





Flash Photography

Photography requires light.  Good photography usually requires a lot of light.  If you've read the article on this site about exposure, then it probably makes sense that proper exposure can be achieved with a little light striking the film or sensor for a relatively long period of time or with a massive amount of light striking the film or sensor for a much shorter period of time.  That statement makes use of two of the four factors of exposure: light and time.  You can fiddle around with the other two factors, aperture and sensitivity, but the fact remains—the more light you have, the less time you need.  That said, it is also true that the more light you have, the less sensitivity you need; and the more light you have, the higher f-stop (narrower aperture) you can use.  The reason why I placed so much emphasis on the time factor (shutter speed) will hopefully become clear later in this discussion of flash.


Ambient Light

Ambient light (often referred to as available light) is all light that exists in a scene prior to any light introduced by the photographer.  The most important form of ambient light is the sun.  Other forms include incandescent light bulbs, fluorescent lighting, and candles.  In ordinary photography, a light meter inside the camera measures the total amount ambient light and selects (or at least recommends) some combination of aperture and shutter speed that will result in a proper exposure.


Scene Brightness

During the day, there is a major, major difference in brightness between most outdoor and indoor environments.  The sun is an immensely powerful light source, and there is little on earth to rival it except for the sort of lighting used in music concerts and movie studios.  You've probably heard stars saying that you have no idea how bright those lights are until you've been on stage with them actually pointed at you.  Further consider the fact that while wearing sunglasses is common outdoors (even recommended to avoid eye damage), it is not necessary indoors.

The human eyes can adapt to a wide range of light levels by the dilating and constricting of the pupils.  This process takes a little time, so we have all noticed how dark a house can seem even with the lights on after having spent a good deal of time outside on a sunny day.  Conversely, even on an overcast day you may find yourself squinting a bit when you first go outside after spending a few hours indoors.  With all this in mind, we can come closer to conceptualizing how different the average scene brightness is outdoors vs. indoors (during the day, obviously).  Now it should be easier to see why indoor photography is so often done with flash.  It evens the playing field so to speak.

Before this discussion proceeds any further, please note that insufficient scene brightness is not the only reason to use flash.  Bright scenes with excessive contrast can benefit from flash supplementation (called fill flash, which is discussed later).  For this reason, the use of flash is also extremely common outdoors.  That may be the most important thing you ever learn about flash.


Flash Power

The basic purpose of flash is to emit a powerful burst of light for a very brief duration.  A typical duration would be 1/1000th of a second, but the duration can be much shorter.  In general, the duration of the flash burst is what determines what is referred to as flash power.  This is actually a misnomer since by definition power is the rate at which energy is transferred.  For this reason, a more accurate term for flash power would be flash energy.  Flash power is what, in turn, exerts an effect on the exposure of any photograph taken with flash.  Bear in mind, however, that any ambient light may also affect exposure.


Flash Exposure

Flash exposure here refers to the component of overall exposure that results from the firing of the flash.  Conversely, ambient exposure refers to the component of exposure that results from any ambient light present.  Whenever a photograph is taken without flash, ambient exposure is the sole component of exposure.  If flash is used to take a photograph in complete darkness, then flash exposure is the sole component of exposure.  In most flash photography situations, the final photograph will have resulted from a combination of flash and ambient exposure with the ratio of flash to ambient controlled either by the camera and flash or by the photographer.

The greater the flash power, the greater the exposure if all other exposure factors are constant.  There is one caveat here.  The relationship between flash exposure and shutter speed is not straightforward.  For reasons that will be covered in more detail later, the flash duration must always be significantly less that the shutter speed.  Hence shutter speed has little or no effect on flash exposure.  Basically, this leaves flash power, aperture, and ISO as the remaining factors for the control of flash exposure.  With flash exposure, there is one exposure factor that is not relevant to ambient light photography and for that reason was not mentioned in the article on exposure.  This factor is the flash-to-subject distance.


Flash-to-Subject Distance

If your camera has a built-in flash or if you are using an external flash unit mounted on your camera, then the flash-to-subject distance is the same as the distance from the subject to the camera.  You can estimate this distance, measure it, or read it off the lens after focusing.  Most of the time you won't need to know the flash-to-subject distance, but it can be important to remember that the farther away the subject is from the flash, the less exposure will result from the same flash power.  If your flash works automatically (details to follow), this should make you aware that there is a maximum distance beyond which the flash cannot sufficiently illuminate the subject.  Advanced flash units will display this distance on a scale of some kind.  If not, then information about this can probably be found in the instruction manual, either for the flash unit or for the camera.

If you ever find yourself using a flash that does not work automatically or if you have a flash unit that has a manual mode that you want to use, then flash-to-subject distance is something you will definitely need to know.  Such a flash unit may have a scale of some kind that will aid you in manually selecting a power level for that distance in conjunction with the aperture and sensitivity you are using.  Without a scale you will need to know the guide number of the flash.


Guide Number

The guide number is the most common means of rating the maximum power of a flash unit.  If you want to know if one flash unit is more powerful than another, then compare their guide numbers.  The higher the guide number, the more powerful the flash unit.  In a sense, the guide number is a distance.  It will be given in feet or meters or both, along with the ISO speed for which the guide number applies, usually ISO 100.

The guide number can also be used to determine proper exposure factor settings.  The equation

Distance = Guide number / f-stop

relates the shooting distance and f-stop in use to the full-power guide number at ISO 100.  Solving for the f-stop, you get

f-stop = Guide number / distance

for when the distance is known and you need the f-stop to use.  If you are using an ISO other than 100 and/or a power level other than full power, the guide number needs to be adjusted before using it in one of the above formulae.  This can be thought of as the effective guide number adjusted for ISO and power level.  The effective guide number for an arbitrary ISO and power level is given by the expression:

Click here for the derivation

When shooting at full power use 1 for the power, otherwise enter the fraction or its decimal equivalent.

The Internet is rife with guide number calculators that will do these calculations for you, but this is the only web site that gives the formula rather than tables enabling you to adjust for ISO and flash power level.  You can also get a guide number calculator app for your tablet or smart phone.


Flash Coverage Angle

Another important property of flash is its angle of coverage.  This is the angle at which light spreads out when emitted by the flash.  While this angle can be quoted or measured in degrees, it is most often quoted in terms of equivalent focal length.  If you are using a built-in flash unit on a 35mm film camera or full-frame DSLR, it most likely has a coverage angle equivalent to a 28mm lens.  It is important to be aware of this if you ever use a lens wider than 28mm in conjunction with the internal flash because vignetting may be noticeable in such cases.

Vignetting will not occur with a lens focal length narrower than the flash coverage angle, however light (henceforth flash power) will be wasted.  Many external flash units offer a zooming flash head which can be adjusted either manually by the photographer or automatically via communication with the camera and lens.  This conserves battery power and actually increases the guide number as the zoom setting is increased.  The amount of increase in the guide number varies by model and may be found in the manual.


TTL Flash

TTL (which stands for through-the-lens) is a method of automatic flash exposure control.  Some form of TTL flash is used by most automatic cameras.  It works by enabling the camera to control the flash duration.  The camera knows the f-stop setting and the ISO value.  In many cases the camera also knows the shooting distance.  Thus the camera knows how much light should be allowed to strike the film or sensor.  Enhanced communication between camera and flash allows the flash to be turned off or 'quenched' at the precise instant when the right amount of light has passed 'through the lens'.

Cameras of different eras and of different manufacturers implement TTL flash in different ways.  There may even be more than one TTL flash mode available on a given flash unit.  In my opinion, these differences are too subtle to be of interest here.

The basic concept of TTL flash, as described thus far, is sufficient for us to move on to the most important point I want to make about TTL flash photography.  With TTL flash enabled, you will receive an automatic level of exposure, even when you use the 'manual' mode of your camera.

In ambient light photography, when using your camera's manual mode the camera will use whichever aperture, shutter speed, and ISO are currently set regardless of the amount of available light in the scene.  This can result in underexposure or overexposure.  With TTL, however, the flash will supply just the right amount of light to properly expose the scene (within the limits of the distance range of the flash).  What this means is that the photographer can use any aperture for the desired depth of field and sharpness, any ISO within reason, and any shutter speed within certain limits.

Recall from our discussion of the guide number that the range of the flash changes with different apertures and ISO sensitivities.  If the photographer sets some combination of aperture and ISO that exceeds this range, he or she will probably receive some sort of warning from the camera or flash unit.  So, manual mode isn't so manual when using TTL flash.  This is a very powerful concept, not just because it allows for the use of a wide range of apertures.  Using manual exposure mode with flash also enables us to control the ratio of flash to ambient exposure.  Read on.


Balancing Flash and Ambient Exposure

This subject consists of separate treatments for indoor and outdoor flash photography.  At the conclusion of the previous topic of TTL flash, we were considering how TTL flash behaves in conjunction with a camera's manual mode.  The combination of TTL flash and manual exposure mode is most useful indoors, where the ambient light will not overexpose a scene with any likely combination of aperture, shutter speed, and ISO.  Nevertheless, whether indoors or outdoors the key to controlling the ratio of flash to ambient exposure is the shutter speed.

Recall that earlier we observed that shutter speed has no effect on flash exposure.  This is because the shutter speed is longer than the flash duration.  Effectively, the film or sensor is exposed to the entire flash burst regardless of the shutter speed.  Later we will learn that there actually is a limit to how fast our shutter speed can be while using flash.  Also recall that a flash photograph has two components of exposure, ambient and flash.  If you think this is a long article, check out Flash Pictures are Double Exposures.  Or you can just take my word for the fact that while shutter speed has no effect on flash exposure, it does affect the ambient exposure.  This is because the ambient light was present the entire time that the shutter was open.  The longer the shutter is open, the more ambient light combines with the flash to produce the overall exposure.

Why is this important?  In my opinion, it isn't terribly important except in a large room, auditorium, or indoor public space.  Since the distance range of a flash is limited, the entire room may not be lit by the flash.  Imagine a group photo in a convention hall.  Suppose the human subjects are a few feet from the camera and are perfectly exposed by the flash, but that the back wall of the auditorium and anyone more than a few feet from the camera appear to be in near darkness.  We've all seen photos like this, and they can be fine.  But sometimes, it might add value to the photo to use a longer shutter speed.  The longer the shutter speed used, the more the ambient light will bring up the brightness of the background areas that were beyond the range of the flash.  This can give a better idea of what the overall scene looked like to the people who were there.  Consider using a tripod if you go with a shutter speed much less than the reciprocal of the focal length.

Even in smaller rooms, I've heard the argument that since people invest in all kinds of lighting in their home, including ordinary ceiling lights, track lighting, table lamps, etc., it can be undesirable for a photographer to come along and (POOF!!!) blow away all of that expensive ambience with a powerful flash burst.  Whatever...  Okay, use a longer shutter speed, and let some of the ambience seep into the photo.  Even in TTL flash mode, some units have a feature called flash exposure compensation (FEC) which allows the amount of automatic flash exposure to be increased or decreased.  In such a case, try cranking it down some.

Indoors, we sometimes use ambient light to supplement our flash as described above.  Outdoors (during the day), we more often use flash to supplement the ambient light.  This is called fill flash.

The sheer power of sunlight is capable of creating scenes with a high degree of contrast.  There is a definite limit to how much contrast can be accurately recorded by a film or sensor.  The vast majority of the time, we are unlikely to point the camera in a direction where the sun is in the photo.  So where does this contrast come from?  Clearly it comes from bright objects and dark objects in the same scene.  Sunlight shining directly on light colored clothing is a good example of a bright object in a scene.  In fact, sunlight is so powerful that even colors like blue and red reflect a lot of light, not to mention tan, yellow, pink, etc.  The dark objects in a scene are the objects inside shadows.

The shadows cast by objects such as trees and buildings are areas which are much darker than areas exposed to direct sunlight.  The human eyes are capable of perceiving a much greater degree of contrast than a camera's film or sensor, so it is rarely apparent to the untrained observer exactly how dark these shadow areas will appear in a photograph.  They are so dark because such high contrast scenes strain or exceed the dynamic range of the medium.  The solution is to reduce the contrast.

There are two ways to reduce contrast.  We can reduce the brightness of the brightest objects in the scene, something which is not possible in direct sunlight because the light source is not under our control.  The other thing we can do is to brighten the darker objects, something which we can do with flash.  This is called fill flash.  TTL flash is really good at this.  Just turn on the flash.  If your flash unit supports flash exposure compensation, try dialing down the flash power a little, particularly if the subject to be brightened is close to the camera.

The results from using fill flash are best when the ambient exposure is the same as what would have resulted without the use of flash.  Achieving this ideal is sometimes problematic due to an equipment limitation known as sync speed.


Sync Speed (X-sync)

While shutter speed ordinarily has no effect on flash exposure, there is a maximum shutter speed which can be used in conjunction with ordinary flash.  This shutter speed is called the sync speed.  The limitation of sync speed exists because the entire flash duration must take place while the shutter is fully open.  Due to the principle of operation of the focal plane shutter, the shutter itself is not fully open for its entire operation cycle.  Furthermore, at very fast shutter speeds the shutter is never fully open at all.  At such speeds, ensuring that the entire film or sensor frame is equally exposed to the illumination of the flash is problematic if not impossible.

As implied above, the sync speed is the fastest shutter speed at which the entire flash burst can take place while the shutter is fully open.  Unless your camera has a feature called high speed focal plane sync (or FP sync), then the sync speed is the fastest shutter speed at which flash can be used at all.  Due to the way high speed FP sync works, the effective guide number of the flash diminishes greatly with each successively higher shutter speed used.  Since balancing flash and ambient exposure outdoors frequently involves both a high shutter speed and high f-stop, cameras with high sync speeds are still preferable.  This is because the higher the f-stop used, the more flash power is required, and extra power is exactly what you don't have with high speed FP sync.


Color Temperature

What color is a yellow object with blue light shining on it?  You'd never believe me if I told you.  Maybe you would at least believe that it wouldn't be quite the same shade of yellow anymore.  What color is a white wall with yellow light shining on it?  I think you would agree that it would be yellowish.  In photography we want things to be the same color in the picture that they appeared in the original scene.  So if we are going to bombard a scene with light in order to achieve a proper exposure, it is important that we do so with light of the right color.  The color of light as it applies to photography and certain other fields of study is referred to as color temperature.

Visible light comprises a relatively small range of wavelengths along the electromagnetic spectrum.  At one end of the visible spectrum is red, and at the other end is violet.  Each color of the rainbow (so to speak) corresponds to a specific wavelength or frequency.  However, in practice we almost never encounter light of a single wavelength.  Instead, naturally occurring light is generally a combination of different colors resulting from a characteristic proportion of intensities associated with various wavelengths along the EM spectrum.  These proportions are called color temperatures because they are emitted at specific temperatures by ideal objects called black bodies.

We have established that the apparent color of an object is related not only to its inherent color but also to the color of the light shining on it.  It is also the case that the apparent color of an object is affected by the characteristics of that which perceives the object.  The perceiver may be a pair of human eyes, a photographic film, or a digital sensor.

Films are designed to produce a balanced and faithful rendition of the color spectrum in response to the proportion of color intensities associated with a particular color temperature, usually 5500K.  A color temperature of 5500K means the proportion of color intensities radiated by an ideal black body at a temperature of 5500 Kelvin (5227 °C or 9440 °F).  This value closely approximates the accepted value of daylight.  For this reason, most films available today are called daylight-balanced films.

Digital cameras have a setting called white balance by which the camera or the photographer can effectively control the response to various color temperatures.  Consult your camera manual to find out more about the white balance settings that are available to you.  The automatic white balance setting (usually labeled 'AWB') is a safe bet unless you decide otherwise.

The subject of color temperature does not relate solely to flash photography, but a discussion of color temperature (and hence white balance) as it relates to ambient light photography is outside my intended scope.  This information is included within the flash photography topic to play counterpoint to the increasing abundance of advice on the Internet against using indoor flash.

The plethora of individuals out there preaching sky-high ISO/tungsten white balance indoor shooting are no doubt capable and knowledgeable.  Their articles almost invariably include some very compelling examples of their indoor technique.  Still, I think that in their writing they have somewhat lost perspective in that they place far too little emphasis on a few important facts:

  • Indoor photography without flash generally requires shutter speeds that are too slow for the vast majority of candids.

  • Most of their readers do not own cameras and lenses sufficiently advanced to achieve the desired quality at extremely high ISO sensitivities and wide apertures.

  • Anyone attempting to mimic their technique with a mainstream camera may have difficulty achieving a realistic-looking white balance in-camera.

  • Their articles may give readers the impression that it's okay to use cell phone cameras for indoor photography even though they do not have flash capability.  In other words, leave your point and shoot at home.  This is why the average photos on Facebook don't look as good as they did five years ago.

Let's resume our discussion of color temperature by describing how it affects the photograph.  The two most common types of lighting other than sunlight are fluorescent and incandescent.  Fluorescent lighting has a somewhat lower color temperature than daylight and is said to lend a greenish cast to photos.  Incandescent lighting has a much lower color temperature than daylight and leads to an orange hue that most people would recognize.  ("The flash didn't go off in that one.")

With digital cameras, you can compensate for fluorescent and incandescent lighting by selecting a white balance setting specifically for the type of ambient lighting present (or simply use AWB). You can also compensate later in image editing software, although certain limitations apply when mixed lighting was present.  In film photography, the simplest method is probably to nuke the ambient light with your flash.  Image editing is also an option because digital scans are almost always part of the analog workflow these days.  Nevertheless, as you might have guessed from some of my comments above, I still prefer the nuclear option.



As is the case with most camera settings, experimentation is ultimately the most effective method of deciding what works.  This is probably because the best photo is the one you like the best, not necessarily the one the literature says is the best.  If you aren't occasionally getting a photo you really like, you'll probably lose interest in photography.  Then you'll stop doing it.  Then it won't matter.