Flash Photography

Understanding Flash's Guide Number (GN)

Guide Number, usually abbreviated GN, determines power rating of flash unit that describes how powerful flash unit is and how far it can shoot. In another word, GN specifies the power of an electronic flash in a way that it can be used to determine the right f-stop to use at a particular shooting distance and ISO setting. GN is mainly used to calculate how your camera’s aperture should be set to get proper flash light. When there were no automatic flash units available for photographers, they used to do a manual calculation using the GN and the distance of the subject from the flash unit. GN is usually given in reference of ISO value 100. For example, Nikon D90's built-in flash has a Guide Number of 12 meters or 39 feet at ISO 100 in auto mode (i-TTL). And in Manual mode, it has a Guide Number of 13 meters or 43 feet. GN is slightly less in i-TTL mode than in Manual mode because it spends energy to fire pre-flashes before the main flash goes off (to meter correct amount of the light for the subject). Basically, to determine the right exposure at any given ISO settings, you would divide the guide number by the distance to get appropriate f-stop. We can write the mathematical formula as,

Aperture = GN / Distance

Using D90’s built-in flash as an example, at ISO 100 with its GN of 43 feet in Manual mode, if you wanted to shoot a subject at a distance of 10 feet, you would use f/4.3 (43 divided by 10), or close to this value in practice. Similarly if subject is at 5 feet, an f-stop value f/8 would be used to get proper exposure. But in practical, we may need to increase the ISO few stops to adjust with f-stop and the distance of the subject.

When we are talking about the Guide Number, we didn’t mention about the Shutter speed and we only talked about the Aperture. It is very important to remember that Shutter speed controls the brightness of the ambient light and Aperture controls the amount of light from the flash. We use Guide Number calculation to help control the flash exposure. Today, GN is mostly used for comparing the power of various flash units, rather than actually calculating the exposure value to use. You don’t need to be a math genius to see that an electronic flash with a GN in feet of, say 98 at ISO 100 (like the SB-600) would be a lot more powerful than the Nikon D90’s built-in flash. Using that GN as a reference, Nikon SB-600 allows you to shoot up to 22 feet using f/4.5 at ISO 100 easily.

Please refer to your flash user manual to get the exact GN value.


Camera Exposure Compensation Vs Flash Exposure Compensation

Today, I am going to talk about two different types of exposure compensations; camera exposure compensation and flash exposure compensation. They both are different and work differently to some extent but not always (depending on what camera body you are using). When I was learning about flash photography, I found this topic little difficult and confusing to understand but I will try to explain it as clear as I can. Let's talk about them briefly one by one and see how does it work differently in different camera bodies.

Camera exposure compensation: Camera exposure is set on the camera body and affects both ambient and flash exposure for Nikon; but only ambient exposure for Canon cameras.

Flash exposure compensation: Flash exposure is set on the flash unit itself. Flash exposure compensation affects flash output only. Changing the flash exposure doesn’t affect ambient exposure.

Nikon Vs Canon (Exposure compensation)

Nikon camera allows you to change the exposure compensation which sets the overall exposure (flash and ambient exposure) even if you are in a Manual mode. But in Canon camera, you can’t dial exposure compensation in a Manual mode.

With Nikon, the camera exposure compensation for the ambient light and the flash exposure compensation are cumulative when ambient light is low and flash is using as the main source of the light. For example, if the camera exposure compensation is set to +2.0 EV and the flash is set to -2.0 EV, it would cancel each other resulting 0.0 EV for the overall exposure compensation.

But when the ambient light is strong and you are using the flash as a fill light, camera sync speed (using Auto FP High speed sync decreases the power of the flash output) and apertures and other settings come into play to determine the overall exposure with the combination of the flash exposure and the camera exposure compensation.

But with Canon, flash exposure compensation and the camera exposure compensation aren’t linked together, as they are with Nikon. So with Canon, in a Manual exposure mode, you can only set the flash exposure compensation which doesn’t affect the ambient exposure.

Why do we need exposure compensation?

Camera metering sensor tries to map everything inside the frame (provided your metering system is in the Matrix metering mode) into 18% middle gray. If your subject is white with the white background, camera auto metering system tries to map it to 18% gray and the result will be underexposed. Similarly if your subject is dark around the dark background, metering system gets confused by the dark tonal color and tries to convert everything into 18% middle gray and hence you will get overexposed subject. To balance the color tones in these kind of scenario, you have to use the exposure compensation to tell the camera “Do whatever you think is right and add or subtract extra light to adjust the subject tonal color to get the perfect color tone mapping”.

How does exposure compensation work?

When you are in an Auto Exposure mode or semi-auto Exposure mode like Shutter Priority mode (S) or Aperture Priority mode (A), dialing the exposure compensation tries to adjust the color tone by adjusting the Shutter speed or an Aperture depending up on which mode you are currently in. If you are using an Aperture Priority mode, changing the exposure compensation adjusts the Shutter speed to balance the overall exposure, and if you are using the Shutter Priority mode, exposure compensation changes an aperture value to adjust the exposure.

To support my explanation, I have included few photographs I have taken (using Nikon D90+SB-600) during my experiment. All these photographs were hand-held shot and taken in a Manual mode using front curtain flash. You will see blurred words due to camera shake but it has nothing to do with today’s experiment and it's finding. All of these photographs were taken using same aperture value, shutter speed, ISO and focal length and the values are:

Shutter speed : 1/4     Aperture : f5     ISO : 200    Focal length : 70mm

#1 Manual Mode (Camera Exposure 0 and No flash unit attached)

Camera Exposure Compensation 0 and No flash attached

Camera Exposure Compensation 0 and No flash attached

#2 Manual Mode (Camera Exposure -2 and No flash unit attached)

Camera Exposure Compensation -2 and No flash attached

Camera Exposure Compensation -2 and No flash attached

#3 Manual Mode (Camera Exposure +2 and No flash unit attached)

Camera Exposure Compensation +2 and No flash attached

Camera Exposure Compensation +2 and No flash attached

#4 Manual Mode (Camera Exposure 0 and flash exposure compensation -2)

Camera Exposure Compensation 0 and FEC -2

Camera Exposure Compensation 0 and FEC -2

#5 Manual Mode (Camera Exposure 0 and flash exposure compensation 0)

Camera exposure compensation 0 and FEC 0

Camera exposure compensation 0 and FEC 0

#6 Manual Mode (Camera Exposure 0 and flash exposure compensation +2)

Camera exposure Compensation 0 and FEC +2

Camera exposure Compensation 0 and FEC +2

#7 Manual Mode (Camera Exposure -2 and flash exposure compensation +2)

Camera exposure Compensation -2 and FEC +2

Camera exposure Compensation -2 and FEC +2

#8 Manual Mode (Camera Exposure +2 and flash exposure compensation -2)

Camera exposure Compensation +2 and FEC -2

Camera exposure Compensation +2 and FEC -2

If you look at the picture #1, #2 and #3 closely, you will hardly find any lighting difference and the exposure even though I told camera to set 0 EV, -2 EV and +2EV camera exposure simultaneously with no flash unit attached. And in other pictures (#4, #5, #6, #7 and #8) where I used the flash unit attached in camera hot shoe, the camera exposure compensation and the flash exposure compensation are cumulative. This is true only with the Nikon camera. But if you are using Canon body, they both are separate entity and they both work entirely separately without affecting one another. Again, in Canon camera body, the camera exposure compensation controls the ambient light and the flash exposure compensation control the flash output power separately to control the overall exposure.


If you are shooting with a Nikon camera body using an external flash unit (mainly in a Manual mode) and trying to balance the exposure, never use the exposure compensation but rather use the Shutter speed and an Aperture to balance the ambient light and use the Flash Exposure Compensation (FEC) to balance the flash output power to your main subject.

But if you are using Canon camera body, you can use them independently to balance the ambient light and the flash output to control the overall exposure.

Inverse Square Law In Digital Photography

The title of the blog suggests that it would a math or physics article but please read until the end and I promise that it would be a interesting read once you find its connection with photography. Today’s topic is practically very important if you are dealing with flash light or studio strobe in your day to day photography. So, without wasting any time, let’s get right into it and see how can we apply the law into our photography world.

In physics, an Inverse Square Law is defined as "any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity". And mathematically, we can write this law as,

Intensity=1/d2, where d is the distance between the light source and the object.

Inverse Square Law

Inverse Square Law

If you have read the definition carefully, it mentioned “any physical quantity or strength”. That means anything on this earth can be related with this law (for e.g. force, energy, light etc). But today we are going to relate the law with photography, and that means we will talk about the light source and its relation with the distance to the subject. We just looked at the mathematical definition of inverse square law, which shouldn’t scare you even if you are not a mathematical genius. Let me try to decode the formula and explain it in a simple language. Basically, the formula says: the intensity of the light is inversely proportional the distance to the light source, which means the light gets brighter when we are close to the light source and light gets darker and darker as we increase the distance from the light source. I am pretty sure that you would agree with me on this statement and can be related to your daily experience as well, right?

We just discussed the essence of the formula but let’s try to define it again with more precision. And, that is: an object that is two times far from the light source will receive a quarter of the illumination. In photography world, this means, if you move your subject from 1 feet to 2 feet away (double the distance) from the light source, you would need four times as much light to get the same exposure. It’s because according to the Inverse Square Law, light intensity at the 2 feet distance will be 1/4. This extra light can be achieved by changing either Aperture, or Shutter speed, or ISO value by two full stops and compensate the exposure. If you change the aperture, you would need to wide open the lens by two full stops and if you plan to change the Shutter speed, you would need to decrease the shutter speed by two full stops to allow enough ambient light to get the proper exposure. Similarly, you can also change the ISO value by two full stops to get the same exposure.

This light fall-off behavior is pretty interesting if you do more research. As you move the subject far and far from the light source, light fall-off is not that much significant as it is in the beginning. Let’s say you get 100% light in 1 feet distance from the light source, we can form the following table to analyze the behavior of the light with the distance.

Distance Intensity of Light

1 1 (100%)

2 1/4 (25%)

3 1/9 (11%)

4 1/16 (6%)

5 1/25 (4%)

6 1/36 (3%)

7 1/49 (2%)

8 1/64 (2%)

9 1/81 (1%)

10 1/100 (1%)

So when you move the subject from 1 feet to 2 feet from the light source, light intensity will be decreased by 75% but when you move further from 2 feet to 3 feet and 3 feet to 4 feet and so on, light fall off is very minimal and at the end there is no difference at all. It clearly demonstrates the importance of a distance and shows how it affects the behavior of the light. You might find this helpful in the future whenever you are dealing with lights, especially flash lights or studio strobes to illuminate your subject.

Auto FP High Speed Sync Mode

Flash photography is a very exciting topic and confusing too at the same time. It offers variety of techniques to produce excellent photographs, but also requires you to learn its technical terms, understand those techniques and apply them properly. Earlier, we discussed about different types of flash sync modes and when should we use them. Today, we are going to discuss about another flash technique/mode, Auto FP High Speed Sync mode. But before jumping right into the topic, let’s discuss about what is the flash sync speed and how does it work?

What is Flash Sync Speed?

Flash sync speed represents the fastest shutter speed any camera can use with a flash. Different camera offers different flash sync speed, and over the time, flash sync speed has been increased significantly due to advancement of technology in digital world. Nikon D90 has a sync speed of 1/200th sec, whereas D7000, D300s and D700 offer 1/250th sec to 1/300th sec. Similarly, Nikon D3X and D3S have a sync speed of 1/250th sec. When you are using a built-in flash unit or an external flash unit, you can’t use a shutter speed faster than the camera’s flash sync speed. It is as simple as that. If you need to use a flash light beyond the flash sync speed, you need to use a feature called Auto FP High-speed sync. This situation may arise when you are shooting in a bright daylight and need to use a flash as a catch light or fill light for the shadows. When you use flash in such condition, you would need to use a shutter speed faster than the flash sync speed to compensate the exposure from the flash light.

After this brief intro to flash sync speed, let’s get into today’s topic, Auto FP High Speed Sync. FP stands for focal plane and it represents the type of shutter used in DSLRs camera. A focal plane shutter is two precisely timed curtains that move vertically between the lens and the camera sensor allowing or disallowing the lights hitting the sensor. The reason to use two precisely timed shutter curtains is to increase the performance of the shutter speed. The shutter speed is determined by the timing between the start of the first curtain opening and the start of the second curtain closing. This is important to understand that the entire sensor will be exposed to the light (the condition where the first curtain completely opens before the second curtain begins to close) for the duration of the shutter speed that matches the flash sync speed, also called maximum normal flash sync speed.

When you are using a faster shutter speed than the normal flash sync speed, the second curtain (also called rear curtain) starts closing before the first curtain (also called front curtain) is completely opened; thus never exposing the entire sensor to the light at any one time. If you keep increasing the shutter speed higher and higher, this will result in a very narrow 'slit' of light that travels across the sensor. And in this case, if there is not enough ambient light, you will get a dark area at the bottom of the picture. This is because when the camera takes a picture, image is recorded upside down on the focal plane, and as the second curtain already started closing before the first curtain completely opens, sensor will not get enough light to brighten the top part of the image which is actually a bottom area of the subject. To overcome this issue, Nikon developed a technique called Auto FP High Speed Sync Mode (not available in all models of DSLR). Auto means camera will automatically use high speed sync whenever it’s needed, or otherwise it will use normal standard sync speed. In a normal flash sync mode, the flash is fired at once with its full power. But in case of FP High speed sync mode, flash is divided into burst of flashes and fires continuously throughout the travel time of small window which travels across the camera sensor. And because of that reason, when you are using very fast shutter speed like 1/8000th sec for example, you will have to consider the power of the flash unit as the flash power is stretched more for narrower slits and it may not be able to brighten the picture or subject area.


Different camera has a different menu option to setup Auto FP High Speed Sync mode. If you are using Nikon D90, you can go to the CSM menu and go into the option e5 to set Auto FP ON, and if you are using D700, you can go to e1 to enable High speed sync mode.

Here is what Nikon says about Auto FP High Speed Sync Mode

Auto FP High Speed Sync is a flash mode used for fill-flash photography under brightly lit conditions. It will fill in and open up shadowed areas in order to portray the greatest detail in subjects. It's also ideal when using wide aperture lenses, and because it allows fast shutter speeds up to fastest shutter speeds on compatible Nikon D-SLRs. It is often used for action-stopping sports photography. And for portraits, you can open up your lenses to their full aperture in order to isolate your subject against a blurred background without overexposing the image.

When Auto FP High Speed Sync is selected, the flash will fire for the duration of the shutter curtain's travel, thus syncing with the camera's shutter speed when that speed is set higher than the camera's normal sync speed.

The loss of a flash power is the trade-off when you are using a flash in a High speed sync mode.