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

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

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

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

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

Conclusion:

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

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.