There are many concepts in photography that are difficult to wrap your head around, and one that can be especially challenging for someone new to the craft to comprehend is the concept of “a stop of light.” A “stop” is a term you may have heard more times than you can count, though never explained in a manner that allows you to apply it practically in your shooting. We’re going to take a trip in the “WayBack Machine” to 2013 and watch our own Pye break the idea down so that you understand it, internalize it, and are therefore better equipped to manipulate light in your shoots. So…what is it? Watch the video and also check out the bullet points below for a breakdown.
Note: This article was originally written in 2017 and updated in 2021.
“Stop of Light” Video Explanation
Understanding Exposure
Whether you are shooting film or digital, the principle of exposure remains the same. Exposure is the result of a three-way balancing act between three controllable camera settings: shutter speed, aperture, and ISO. I call it a three-way balancing act because when one factor shifts one way, at least one of the other two has to shift the other way to counterbalance the change.
I will go into each component in the next few articles, but for now, here is what you need to know about the three factors:
- Shutter Speed: How long the camera shutter stays open to let light in
- Aperture: How wide is the opening of your lens when the light passes through
- ISO: How sensitive is your sensor to the light
Visualizing the Exposure Relationship
One of the best ways that I have learned in visualizing the relationship between shutter speed, aperture, and ISO is to picture water pouring from a faucet into a cup. The water itself is the light and a full cup of water is considered the “correct” exposure.
Your aperture is going to be how wide you open the faucet. How long you leave the water running is your shutter speed. So let’s take a look at these two things in regards to filling the cup to the brim. If you have the faucet wide open (large aperture), it won’t take long to fill the cup up (short shutter speed). If you close down the faucet to only allow a small stream of water, it will take longer to fill the cup (long shutter speed).
That’s essentially how the aperture to shutter speed relationship works. The larger the aperture opening, the less time it takes for enough light to “fill up” a cup of exposure. The smaller the aperture opening, the longer it takes for the light to fill the same cup.
Now to visualize ISO, imagine adding a pump to the faucet’s plumbing to force the water out faster. Just by increasing the flow rate of the water, you can fill the cup faster without having to adjust the opening of the faucet or extending the time to fill the cup. ISO is similar in that you are artificially “pushing” more light on the sensor by making the sensor more sensitive to the light.
Underexposure and Overexposure
When an image is underexposed, that means that it is darker than a properly exposed image. Going back to our water faucet analogy, that means that you did not allow enough water to fill the cup all the way to the top. To correct this, you either have to open the faucet wider, give more time for the water to fill the cup, or force the water out faster. In camera terms, this means either opening up the aperture, having a longer shutter speed, or increasing the ISO.
If an image is overexposed, that means that it is way too bright. With our water faucet, that means letting too much water flow into the cup and having the excess water spill out into the sink. To adjust for this, simply close the faucet down a bit, shorten the amount of time allotted for the water, or slow the water’s flow rate. The camera equivalence would be closing down the aperture, having a shorter shutter speed, or lowering the ISO.
A Stop of Light DefIned | Summary
- A relative measurement of the amount of light in a particular photograph.
- A Stop of Light is either double or half the amount of light in a particular photo.
- To brighten an image by one stop means you increase its brightness by 100%. Another way to put it is that you double the amount of light your image has.
- To take away one stop of light means, you decrease the brightness of an image by 50%. (Obviously, if you decrease the light by 100%, you have no light and are left with a completely dark image.)
- Increasing the brightness in your image by two stops of light is not an increase of 200%, but 400% because the amount of light has been doubled and then doubled again.
- Decreasing the brightness in your image by two stops means reducing the brightness by 50% and then reducing it another 50%.
Conclusion
So, keeping things simple for now, exposure is simply a three-way balancing act between three controllable camera settings: shutter speed, aperture, and ISO. The right combination of the three leads to a “proper” exposure where the image looks like it has the right amount of brightness. If you change one of the three settings, then you have to change at least one of the other two settings to maintain the balance.
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