Exposure triangle explanation please.

Started Jul 9, 2013 | Questions thread
Guidenet Forum Pro • Posts: 15,748
Exposure Values

AnthonyL wrote:

yooperguy wrote:

Perhaps this will help?


Seen that and dozens like it, so you increase the shutter speed, what happens to the triangle? How does the triangle itself help? What am I looking at? What do the sides represent? What do the apexes represent? Which way is more or less?

There's no geometric concept to it. It might not even be a triangle. What they are is three out of four considerations that contribute to your choice of camera settings. For instant, At ISO 100, what combinations of shutter speed and aperture will give you the "correct" exposure. EV100 --> f/stops and Shutter duration stops with consideration to subject luminance in stops. Each full stop in one direction either lets in double the amount of light or effectively causes the sensor to be twice as sensitive to light for a simplistic method of explanation. Go the other direction and you're halving light.

To be able to effectively manage this you need to have memorized all full stops in aperture and shutter speed (actually shutter duration) also ISO, but that's easy.

Example: Lets say you've metered the scene and your light meter, whether in hand or in camera, reported a proper exposure would be attained if you chose to set the ISO to 100, shutter speed at 60th of a second and aperture of f/8. Those are all full stops. Now lets say you don't like that aperture. Everything else is fine, but you bought that nice 85 f/1.8 to blur the background in portraits and isolate your subject.

So you know you want f/2 which is the widest full stop your lens can do. You're at f/8 now, so you need to open up 4 full stops, right? Each full stop lets in double the light as the previous stop. So, you go:

f/8---> f/5.6---> f/4---> f/2.8---> f/2. That's letting in 16 times more light. Get it? You're doubling, doubling again, doubling again and doubling one more time. So that's 2, 4, 8, 16 times more light.

Now if you're going to let in 16 times too much light to get to the aperture you want and decided was the best for you, you need to let in 4 stops or 16 times less light somewhere else. You pick Shutter speed. You're at a 60th of a second now, remember. You want to move to a faster shutter speed four stops. That would be 16 times less light. That balances you opening aperture four stops. Let's do it

60th --> 125th --> 250th --> 500th of a second.

Your new exposure is ISO 100, f/2 at 500th of a second.

That's the exact same Exposure Value as you started with at ISO 100, f/8, 60th of a second. Exactly the same within normal errors. Also notice that isn't not always perfect between stops like when we went from 60th to 125th. We somehow got bumped that 5th of a second. That's ok.That's the way it is and is the reason for memorizing all full stops. If you don't, how will you know what to set the aperture at if you had to stop down three stops? Notice also that our new 80 f/1.8 is not a full stop. It's a third of a stop wider than the f/2 which is the full stop.I tend to round it out to the nearest full stop so whoever I'm teaching can get it easier. Calculating in half or third stops is a pain. Just memorize the full stops.

There's also scene stops and you can add light to a scene by stops with a flash. If you can't get to a shutter speed or aperture you want, you can add light or increase ISO or both. As long as you understand and balance the stops you open by the stops you close. Make sense

I hope I didn't over or under explain it all. Please let me know, Anthony. Have a great afternoon.

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Cheers, Craig
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