Understanding Exposure

This is a continuation of my previous post on where I used the analogy of a rain gauge (Rain Can Teach Us Photography) to understand photographic exposure. Here we dig deeper into understanding what photographic exposure really means in terms of real photography. 

You might have heard of the concept of exposure triangle. This concept explains the interplay between three independent aspects of photographic exposure namely:

1. f-stop

2. shutter speed

3. ISO sensitivity

Item #1 is more often incorrectly referred to as aperture. Although f-stop involves aperture, saying that f-stop is equivalent to aperture is photographically wrong because aperture alone totally ignores the effect of focal length in the intensity or amount of light that hits the sensor. I have covered this in detail in this post: Understanding Your Lens (Part 3). Please read that post if you have difficulty understanding this concept.

The main reason that I am discussing this supposedly understood-by-all-photographers concept is because it’s actually misunderstood by a lot of photographers – myself included until after reading and conducting experiments. Before I start, I would like to acknowledge a friend, Dan Bridges, for introducing and helping me understand this concept. It is not really a difficult concept to comprehend but it will surely change the way you think because we are accustomed to thinking in terms of the exposure triangle.

Let me start by saying that the exposure triangle is not entirely correct. Yes, you read that right. In fact, when you talk about exposure it’s really just the first two items: f-stop and shutter speed. ISO is not really a part of exposure and you will soon understand why … hopefully.

Begin by understanding that the amount of light in the scene that you are trying to capture is fairly constant throughout the entire exposure. Unless you are doing very long exposures in a disco bar or covering a concert gig, the ambient light is practically unchanging. Therefore whatever light that comes into the sensor chamber is basically controlled by your chosen f-stop and shutter speed.

Let me repeat that: the amount of light hitting the sensor is only affected by f-stop and shutter speed.

So why is ISO sensitivity not part of the equation? Because for a particular camera, ISO sensitivity is constant. You do NOT have any control of it. Surprised?

The immediate reaction is, “of course I can control my ISO”. Yes, cameras let you change the ISO but you are not really changing the ISO. It’s not real. You are lead into thinking that you are changing the ISO when in fact you are not. What you are changing is not the ISO sensitivity of your sensor but the brightness of the image. What you are changing is technically the gain.

The ISO sensitivity of any given camera is FIXED at manufacturing time. This is called the native or base ISO. It can not be modified at all. It is very important then that you know the base ISO of your camera. Read your camera’s manual. The Nikon D700 for example has a base ISO of 200 while the D800 has a base ISO of 100. We say that the D700’s sensor is more sensitive than the D800’s. If you think of sensors as water containers, differences in base ISO is like differences in the height of the containers. Same opening size but different heights. A sensor with higher base ISO is like a shallower container. It means that an ISO 200 container will fill up quicker than an ISO 100 container. Here’s an illustration:

Image

Allow me explain further. If you pour water at the same rate into two containers where the only thing different between the containers is their height, they will obviously have the same water level after any given time. Photographically speaking, same water level means same light level meaning same exposure. However, if you continue pouring water at the same rate, there will come a time when the shallower container will overflow and then water will start spilling for that container. As we have said, a higher ISO is like a shallower container. The sensor with higher base ISO will overexpose quicker compared to a sensor with lower base ISO. Water (light) will spill more quickly for the shallower container (higher base ISO). This means that if for a given ambient light, f5.6 at 1/125s is just enough to fill up an ISO 100 sensor to its brim, the same f5.6 at 1/125s will overexpose the ISO 200 sensor causing light to spill somewhere else (blown highlights).

As you can see, exposure is really NOT about the ISO but the amount of light that gets into the sensor. The base ISO is more of a warning label telling you not to overexpose your sensor or else light will start to spill. If a water container says it has a 100ml capacity, you would not want to pour 200ml of water into it. Makes sense?

Recall that base ISO is fixed and it can NOT be modified. It follows that the only way to control exposure is by f-stop (container opening) and shutter speed (total time that you are pouring water into the container).

Now let’s try to understand brightness. This is different from exposure. The definition of brightness is as camera-specific as that of base ISO. For the sake of simplicity, let’s say that an empty sensor produces a pure black image and a completely filled up sensor produces a pure white image. Since different sensors with different base ISOs fill up at different rates, it follows that they have different definitions of what is black or what is white.

Now here is an interesting outcome: Supposing that f5.6 at 1s is just enough to fill an ISO 200 sensor. It means that for that sensor, f5.6 at 1s produces a pure white image. That same f5.6 at 1s though is not enough to completely fill up an ISO 100 sensor. Therefore the same exposure will produce a slightly darker image for the ISO 100 sensor simply because the sensor is not completely full. Note that they have exactly the same amount of gathered light but they are producing totally different images. To produce the same pure white image, the ISO 100 sensor will have to be exposed longer at 2s for the same f-stop of f5.6.

Here’s another interesting fact: You can actually make the ISO 100 sensor in the example above produce a pure white image at the same f5.6 at 1s exposure. How? By artificially filling up the sensor until it’s full by adding “something”. If light in a sensor is like water in a container, you can make the water reach the top by boiling it. The amount of water will be the same but the act of boiling it has made it fill the container to its brim (and possibly spilling some of it). This “act of boiling water” is what happens when you increase the ISO in camera.

Increasing the ISO in camera does NOT add light to the sensor at all. It does not increase the exposure. It only artificially fills up the sensor with something. It boosts the signal. Unfortunately boosting the signal boosts everything including noise. The problem with increasing the ISO is not the act of boosting the signal itself. The main problem is that sensors have inherent noise in them already – signal or no signal. In darker areas where there is no light (signal), noise is still present. That is why if you boost the darker areas of an image, what you are boosting is just noise because there is no signal. Noise is more pronounced in darker areas of an image at high ISOs. This is why you do not test the high ISO performance of your camera in good light. That’s cheating. You should test high ISO in low light.

And now we finally arrive at an interesting consequence. Supposing that you are shooting in low light and you have chosen ISO 1600 so that you can hand hold your camera at a shutter speed of 1/125s at f5.6. Since exposure is only affected by f-stop and shutter speed, you can actually shoot at your base ISO, say, ISO 200 at the same 1/125s at f5.6 without affecting the final image. Of course when you look behind your camera’s LCD, the image will be very dark and you won’t probably see anything. However, when you get to your computer, you can use the exposure slider in Lightroom or Photoshop to boost the signal and arrive at the same image as the camera-boosted ISO 1600 image. The reason this is possible is because the exposure is the same. It’s still f5.6 at 1/125s. You either choose to boost the signal in camera by increasing the ISO or shoot at base ISO and boost the signal later in the computer. The advantage of doing this boosting in the computer is that modern software are smart enough not to boost highlights that are near clipping point. Your camera is not that smart and it will boost everything thus causing blown out highlights.

Disclaimer: that last paragraph is not always true. Some sensors behave differently. Sensors are actually more complicated than just a simple container so experiment with your camera.

This discussion won’t be complete without covering fake low ISOs that are in every camera. For example, the Nikon D700 has a base ISO of 200 but it also has Lo 1 which is equivalent to ISO 100. This lower fake ISO allows you to shoot at longer exposures. Since we know that the sensor ISO is fixed, fake low ISOs won’t actually gain you anything. The longer exposures will only cause areas of highlights to blow up. This is no different to applying the same ISO 100 exposure, say, f5.6 at 1/125s to an ISO 200 sensor. The ISO 200 sensor will be overexposed by a stop. So the same thing happens when you use a lower fake ISO and increase the exposure. Your sensor will be effectively overexposed.

To summarize everything:

1. Exposure is only affected by f-stop and shutter speed.

2. A camera’s base ISO is more of a warning label saying do not exceed your exposure beyond this point. It is a fixed value.

3. Increasing the ISO in camera only boosts the brightness of the resulting image. It does not increase the sensitivity of your sensor. With some cameras, you are better off boosting the image later using a photo editing program.

4. Fake low ISOs will do you no good. If you need a longer exposure then use a longer exposure using your base ISO. The consequences will be the same: blown highlights.

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