Relating ISO, Aperture & Speed

[Lewis Family]

Newbie: Need a little help with a stubborn question. While I am working my way through the Lenspiration Roadmap, one thing continues to hold me back. Specifically, it is the relationship among these three basics: aperture, speed and ISO. Some photos appear to use wide aperture (sometimes to affect DOF) with faster speed while others use slower speed with more narrow aperture. Throw in the fact that ISO also can be (may need to be) adjusted and it becomes a three-way dilemma. I have a fairly good grasp what each one does individually, but my question regards the proportioanl relationship among them. Can you help enlighten me or point me to a place where I can get a better understanding?

[Carissa Pentimone]

I’m also interested in the answer to this question.

[Ezra Morley]

@Jamesstaddon?
I wrote an article quite a while back about a totally different subject, but it somewhat addresses your question. Feel free to take a look at it here: https://www.lenspiration.com/forums/topic/easy-to-read-user-guide-for-youngno-yn-560-iii-speedlight/

The idea of “stops” is an important one when dealing with the exposure triangle. Each of the 3 parts of the triangle: (aperture, shutter speed, and ISO) affect the exposure the same way. i.e. When you change your ISO 1 stop higher, your photo will get brighter, just the same as decreasing your shutter speed 1 stop makes your photo brighter, and opening up your aperture 1 stop makes it brighter. Theoretically every one of those will affect the brightness of your image exactly the same. So it doesn’t matter which one you use to make your photo brighter, if you go 1 stop, your photo will get one 1 stop brighter.

What makes it interesting is that each part of the triangle not only affects exposure (brightness), but like you said, aperture affects DOF, shutter speed affects motion blur, and ISO can make your photo noisy if it’s pushed too far. So the tricky part of the exposure triangle is to balance out the 3 components till they do what you want them to do.

Let’s take a theoretical example that happens in real life quite often. Let’s say that I want to take a picture of a flower in the flower garden with my 50mm f/1.8 lens. My camera is set to Auto Mode, and it chooses these settings:
Shutter speed: 1/250
Aperture: f/5.6
ISO Speed: 200

I snap a picture, and the exposure is right on, not too bright, not too dark, just perfect. However, with the aperture set at f/5.6 the DOF is pretty large, and I don’t like how it’s trying to bring the nearby flowers into focus too. So I decide to change the DOF by adjusting my aperture value to f/2. Now remember, I already know what my picture should look like in terms of exposure. So I just need to keep my triangle balanced to achieve the same exposure with my new aperture setting of f/2.

Let’s switch our camera over to Manual Mode now, and plug in our settings that we have so far.
Shutter speed: 1/250
Aperture: f/2
ISO Speed: 200

If we snap a picture now, our photo will be WAY overexposed. We’ve messed with one point of the triangle, but failed to balance it out with the other 2 points.

Let’s do the math. f/5.6 to f/2 is 3 stops. f/2 -> f/2.8 -> f/4 -> f/5.6 So if we change just the aperture, our photo will be overexposed by 3 stops. To compensate, we need to adjust the other 2 points of the triangle, (shutter speed and aperture). Using the other 2 points, we need to darken the photo by 3 stops so that we have an even exposure again. So let’s adjust the ISO speed from 200 down to 100. That makes our photo 1 stop darker, but we need 3 altogether. So let’s adjust our shutter speed 2 stops. 1/250 -> 1/500 is 1 stop, and 1/500 -> 1/1000 is 2 stops. So our final settings should be as follows:

Shutter Speed: 1/1000
Aperture Value: f/2
ISO speed: 100

Now we’ve got our nice blurry background by changing the aperture, but we’ve balanced out the exposure so that it stays the same by using the other points of the triangle.

Hopefully that helps you out a little bit, feel free to ask if you’re still unsure about something!

Note that if you had used A/Av mode, instead of M Mode you could have just changed your aperture, and the camera would automatically have adjusted the other settings for you.

Also, most cameras allow you to change your exposure settings in 1/2 stops too, so you can get even finer control over the balance of the exposure triangle. (My Pentax camera allows me to change settings in 1/3 stops!)

[Jinny Schober]

That’s really helpful! Thank you for explaining, because I had been guessing on what to do with the settings! 🙂

[Carissa Pentimone]

So, I understand all that. How do you know what a full stop is and what a 1/2 stop is and such?

[Lewis Family]

Precisely what I’ve been looking for. Certainly, experience has a lot to do with making it work, but at least I can now grasp the underlying formula. Thank you very much.

[Ezra Morley]

Great! Glad you found it useful. 🙂

@biblebee, that question is something that you’ll have to learn with experience. 🙂 I honestly don’t really pay much attention to stops when I’m actually taking photographs. I just adjust my settings till I’ve got the exposure right, and whatever special effects I want, (such as low DOF or motion blur), and then I just shoot. Stops are only really needed when you’re trying to convert your exact exposure settings as you adjust the rest of the triangle. I rarely do the math, I just play it by ear, and adjust till it’s right.

For me, the advantage of being able to adjust exposure in 1/3 stops is merely an easy way to fine tune my exposure, not to aid in mathematically calculating the correct exposure. 🙂

[Carissa Pentimone]

Okay, thank you!

[James Staddon]

@jamesstaddon?

Sorry, I went AWOL this week….took a few days off to rest and refocus while I was in California after the Sacramento Family Conference. 🙂

I think I use stops more mathematically than @buddingphotographer does.

If it’s of any help, this diagram shows you what a stop is for each one of the 3 exposure trio elements (Aperture, Shutter Speed, ISO): https://www.slrlounge.com/wp-content/uploads/2015/04/photography-shutter-speed-aperture-iso-cheat-sheet-chart-fotoblog-hamburg-daniel-peters-11.jpg

• This reply was modified 54 years, 8 months ago by .

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[Ezra Morley]

Neat! Looks like a lovely place to be!

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[James Staddon]

Ok @vince, so I’m not planning on taking on this question during the webinar today per our planned conversation on Monday, but if there’s time and you’d like to bring it up via Live Chat today, feel free!

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