I LOVE AHA MOMENTS! So excited about understanding Aperture Stops now – ISO & Shutter Speed is good too (but mostly pumped about Aperture)
Understanding the three elements of exposure and how they change the outcome of the photo is great! Now, on to understanding the scales used to measure these elements so that you can balance all three and properly expose your photo every time!
ELEMENTS OF EXPOSURE
Remember that exposure is the amount of light being let into the camera and it is made up of three elements:
Getting the correct exposure involves adjusting these three elements until they’re balanced. The adjustment of these elements is measured in Stops.
Changing the exposure by one Stop will either double or halve the amount of light captured in the photo. Each element of exposure (Aperture, ISO, Shutter Speed) has their own scale of Stops and some of them make perfect sense but there’s one in particular that makes me twitch a little bit to be honest. Let’s do the easy ones first:
ISO FULL STOPS:
100
200
400
800
1600
3200
6400
12,800
To increase the ISO by one stop, multiply the value by 2.
For example:
ISO 100 x 2 = ISO 200
This will double the amount of light being captured
To decrease the ISO by one stop, divide the value by 2.
For example:
ISO 400 ÷ 2 = ISO 200
This will halve the amount of light being captured
Easy peasy!
SHUTTER SPEED FULL STOPS:
2″
1″
1/2
1/4
1/8
1/15
1/30
1/60
1/125
To increase the Shutter Speed by one stop, divide the value by 2.
For example:
1″ (which means one second) ÷ 2 = 1/2 of a second
This will halve the amount of light being captured
To decrease the Shutter Speed by one stop, multiply the value by 2.
For example:
(1/4 of a second) x 2 = 1/2 of a second
This will double the amount of light being captured
Kind of easy peasy (however, I’d like to find out what’s going on with 1/125 though. I’ll share as soon as I do!)
APERTURE FULL STOPS:
Now for the tricky one!
Just like ISO and Shutter Speed, changing the Aperture by one Stop will either double or half the amount of light being let in. But then why doesn’t the Aperture number double?
Take f/2 for example. Doesn’t it make sense for the next stop to be f/4? Surprise, it’s not! Instead, the next stop is f/2.8. So confusing. But wait, I found the answer! If you get excited about math like I do, check out the explanation after this table. If not, check out the easy way to remember the values for Aperture Stops (also noted as F-Stops) here.
1.4
2
2.8
4
5.6
8
11
16
22
Here’s the secret to Aperture Stops (also noted as F-Stops):
When we say that we’re doubling the amount of light being let in by increasing the Aperture by one Stop, what we’re really saying is that we’re doubling the area of the Aperture opening.
Okay so lets say the diameter of our Aperture opening is 20mm, this will give us an area of 314mm.
If we want to go up the Aperture scale one Stop, that means we are doubling the area of the Aperture opening. So we take 314mm and multiply that by 2. This gives us a new opening of 628mm.
Now, to calculate the diameter of this new area, we’ll have to multiply the value of the previous diameter (20mm) by the square root of 2. This gives us a new Aperture diameter of 28mm.
Lets do this one more time with the next Stop. We’ll take 628mm and multiply that area by 2 to get the next Aperture area of 1256mm.
To get the diameter of this new area, we’ll multiply the value of the previous diameter (28mm) by the square root of 2. This gives us a new Aperture diameter of 40mm.
Great! Good to know, but we can’t just use the area or the diameter of the Aperture opening as our scale because those numbers will change depending on the focal length being used. Take this sketch for example:
This proves that both the size of the Aperture and the focal length of the lens work together to determine the amount of light that is being let in. Check out what we’ve discovered as a result!:
We have the same size Aperture opening of 20mm letting in different amounts of light at different focal lengths.
And then we have two different size Apertures of 20mm & 40mm letting in the same amount of light at different focal lengths.
Alrighty, so how can we calculate a constant unit of measure from this then? Let’s check out how the numbers from these sketches relate:
Here we see that a 40mm Focal Length with at 20mm Aperture will let in the same amount of light as an 80mm Focal Length with a 20mm Aperture as both of these fractions reduce down to 2.
Here we see that a 40mm Focal Length with a 10mm Aperture will let in the same amount of light as an 80mm Focal Length with a 20mm Aperture as both of these fractions reduce down to 4.
The numbers that we’ve reduced above (2 & 4) represent the proportion of Focal Length & Aperture diameter that will let in a consistent amount of light. So how does this come together as a formula then? Check this out!:
Holy, smokes! Formula complete! Now, to put this all together in the Aperture Scale. Let’s look back at this sketch and enter in the value we just calculated for our F-Stop at a 20mm Aperture diameter:
Alright, we know that if we want to double the amount of light, that we have to multiply the diameter by the square root of two. In order to keep our equation balanced, whatever we do to one side, we must do the opposite to the other. This means if we multiply our Aperture diameter by the square root of 2, that we must divide our F-Stop number by the square root of two.
Now we see that in order to double the amount of light, that we must divide our F-Stop by the square root of 2. So then if we want to halve the amount of light, we must multiply our F-Stop by the square root of 2.
Voila! F-Stops calculated! Now to bring this all together in the F-Stop scale and look at the easy way to remember the F-Stop scale:
APERTURE FULL STOPS:
1.4
2
2.8
4
5.6
8
11
16
22
Notice any patterns? Check this out!:
Just remember the numbers 1 & 1.4 and then double those numbers over and over to recreate the scale! Easy peasy!
There we have it! ISO, Shutter Speed & Aperture Stop scales explained. I just love these ‘Aha’ moments, and hope you’ve gotten some out of this article too.
Pro Tip: The term stopping up means that we want to let in more light and stopping down, means we want to let in less light. This applies to all three exposure settings (ISO, Shutter Speed & Aperture).
Here’s the full sunset shot to end things off on a relaxing note…
GEAR USED TO TAKE THESE PHOTOS:
REFERENCES
Some incredible sources that really helped me understand Stops in photography include the Masting your SLR course that I took as well as some awesome YouTube videos. One in particular that I thought was really great was by Dylan Bennett. Thank you for such an incredible explanation.