THE NOT SO TRIANGULAR EXPOSURE BUCKET

Now that we've taken a look at aperture, shutter speed and ISO, our attention should turn to using them together to create perfect exposures. 

You may or may not have heard of the exposure triangle, a tool that's supposed to help with exposure.

A triangle has three sides. There are three exposure variables. I suppose someone decided it logical to tie the two together. So, placing one setting on each side of the triangle, you end up with something that looks like this. 

If you can look at that and understand how the three variables work together to build the proper exposure, great. You can stop reading and go make beautiful pictures. 

For the rest of us, that really doesn't get the job done because other than three variables and three sides, triangles and proper exposure have little in common. As you may gather, I'm not particularly a fan of the exposure triangle. I feel its confusing and misses the point, especially for beginners trying to process how this all fits together. 

I've developed a technique that seems to help people understand how the three variables fit together a bit better. I call it my exposure bucket.

We've all filled a bucket with water. Imagine being able to fill your bucket with either a garden hose or a fire hose. Obviously if you fill the bucket with the fire hose you'll open the valve and immediately close it as the bucket will fill rapidly. However, with a smaller hose, like the garden hose, you'll need to leave the valve open longer. 

In my example, your aperture equates to your hose diameter. Are you filling the imaginary exposure bucket with a large aperture that will cause it to fill rapidly or a smaller opening that will take longer to fill.  

If you recall this image from my aperture post, it provides a good visual for that firehose vs garden hose relating to aperture. 

f5.6 maximum aperture (garden hose) lens on left - f1.2 maximum (fire hose) lens on right. 

Can you see how a lens with a large aperture can be compared to a firehose and the smaller maximum aperture compared to the garden hose? 

In the same way that aperture represents the size of the hose, the camera's valve is its shutter. How long does the valve need to remain open in relation to the aperture?

When looking at these two settings in the simplistic formula, most people can begin to understand the  fundamental relationship between shutter speed and aperture settings. 

Shutter speed and aperture work together directly to build proper exposure. A reciprocal relationship exists between them meaning that by increase or decreasing on of the settings, you can compensate by doing the opposite with the other. 

To help you understand how these settings work, we'll step back in time to a simple, mechanical cameras for a few minutes. Electronically controlled apertures and shutters have allowed the manufactures to build a lot of shutter speeds and aperture settings into the cameras that weren't always there. So starting with "full" f-stops and "full" shutter speeds might simplify things at the start.

On the manual film camera below you'll see that the aperture is currently set to f5.6. On either side of f5.6 are f8 and f4. These are full f-stops. A full f-stop is an f-stop that lets in either twice as much, or half as much light than the stop to either side of it.

I know this might sound confusing but lets look at it deeper. By moving my aperture from f5.6 to f4 I would double the light entering the camera. By opening it to f2.8 from f4 I would double it yet again, and so forth. Conversely, by stopping the lens down to f8 from f5.6 I would be allowing only 1/2 as much light in. Stopping down to f11 cuts it in half again. So, moving the aperture one stop in either direction cuts the light in half or doubles the light. This is known as a full stop.

Now looking at the shutter speed dial we can see the camera is currently set to 1/125th of a second. On either side of that are 1/60th and 1/250th. Just like with aperture, changing the shutter speed one setting will either allow twice as much light, or half as much light to enter the camera. Its a bit easier to comprehend with the shutter speeds due to the numbers being essentially half or double that of their neighbor.

The fact that each f-stop as sell as each shutter speed change will either cut the light in half or double it, the reciprocal nature of the two begins to become apparent. 

If we move our aperture ring to f4 we've allowed twice as much light to enter through the lens. By changing our shutter speed to 1/250 we cut the exposure time in half so this exposure is exactly the same as 1/125 at f5.6. Likewise, f8 allows 1/2 the light to pass through the lens as f5.6 but if we were to change our shutter speed to 1/60 we double the exposure thereby once again creating the same overall exposure. 

Because of the reciprocal relationship between aperture and shutter speed, moving either the shutter speed or the aperture one full stop is commonly simply referred to as "a stop." You might hear someone say something is a stop overexposed. Or you need to a stop more exposure, or a stop less exposure. 

Please realize there is no specific relationship between f5.6 and 1/125. That just happened to be the camera settings when I pulled it from the drawer. In brighter light, or dimmer light those settings might be widely different. 

If you pick up your camera and begin to compare the shutter speeds and aperture settings you'll likely find you have many more options. Between 1/125 and 1/250 you'll likely find 1/160 and 1/200 and between 1/125 and 1/60 you'll likely find 1/100 and 1/80. These smaller increments are 1/3 stops. You'll almost assuredly find the same 1/3 stop increments on your aperture as well. These 1/3 stops, just allow for more precise control through electronics. The same reciprocal nature of shutter speed and aperture exists with 1/3 stops as it does with full stops. 

Have you wondered what happened to the third side of our triangle? Lets go back to our exposure bucket for a moment. Hopefully now you can begin to understand how we can change the size of our aperture and the timing of how long the shutter is open to properly fill our imaginary bucket. And having it properly filled is important. Too much light and our bucket overflows and makes the picture too light. Too little light means our bucket isn't full - our picture is dark.

So how does ISO effect any of that. ISO changes the size of our bucket. ISO changes the amount of light we need in the first place. By raising our ISO we make our camera more sensitive and don't need as much light for a proper exposure. Lowing our ISO requires more light. 

Proper exposure is always a juggling act. At any given time for any particular situation, one or two of the three variables will take priority. Which one depends upon your needs. For example, for most handheld shooting my first priority is keeping my shutter speed high enough to prevent camera movement. I then need to look at my aperture and ISO. Can I achieve the sharpness or depth of field I'm looking for at the aperture dictated by the ISO in use. If I'm photographing something that depth of field is of little concern then ISO probably becomes my secondary setting as it will directly effect the noise of my image. If I need greater depth of field, perhaps aperture becomes secondary and I sacrifice a bit of noise to gain the depth of field that will ensure sharpness throughout. 

Another example might be when I shoot real estate. My camera is on a tripod and shutter speeds are no longer the priority. My priority in this situation becomes aperture. I want to ensure good depth of field throughout the room. I also need that depth of field to remain constant as I create various exposures. Quality dictated by ISO becomes my secondary concern for real estate photography and shutter speed tertiary since my camera is on a tripod.

If I'm photographing insects in the wild my shutter speed and aperture become my higher priorities because both I and the insect can move so shutter speed is important. Aperture is critical as well because depth of field in macro photography is very limited so keeping the lens stopped down is almost a requirement. A small aperture and high shutter speed mean high ISO is likely going to be required. If my bug gets a bit noisy I'm ok with that.

At this point I often get the "how do I even know where to start?" question. One of the wonderful features of digital camera is their instant reporting of what your image looks like. The simple answer is push the shutter button. Is the picture to light? Too dark? If its too light, you've overflowed the bucket. You need to stop the aperture down, or speed up the shutter, or make the camera less sensitive  by lowing the ISO. Possibly some small amount of adjustment to all three. Too dark, just the opposite.  

Happy shooting. In an upcoming post I'll talk about the various shooting modes on the camera and when to use them.