Understanding Exposure – Part 4: Putting It All Together

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This is the final part of my four part series, Understanding Exposure. This series of articles attempts to explain everything a photographer needs to know about how the light gets into their camera and how their camera uses that light to record the scene.

In part 1 we discussed ISO. We followed that up talking about Shutter Speed and Aperture in parts 2 and 3.

Part 1: ISO can be found here

Part 2: Shutter Speed can be found here

Part 3: Aperture can be found here

In this final installment, I’d like to wrap things up with some real world examples of how it was important to prioritize one, be it ISO, shutter speed or aperture over the other two.

You may remember in part 1 we spoke of ISO and I talked about how I had to boost the ISO up to 1000 to achieve a shutter speed that would allow me to hand hold my camera and capture the image of the two young bucks. The tradeoff encountered was that the resultant image had some noise caused by the higher ISO. In that case it was worth it because there wasn’t any other way I could have captured those young bucks without boosting the ISO.

Click on any image to make it bigger

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Conversely, when I shoot landscapes, I generally want as little noise as possible in the image so I purposely shoot with the lowest ISO available on my camera to ensure that digital noise i.e. grain is minimized.

Following along that train of thought, when I shoot landscapes I usually want as much of the scene as possible to be in focus so I choose a smaller aperture or, if you remember our discussion in part 2, to achieve the smaller aperture, I used a large f-stop number. So remember, to increase the depth of field, make the aperture smaller by increasing the f-stop number.

Remember in Part 3 I spoke that, because of the optical effect called diffraction, it’s not good technique to stop your lens all the way down to it’s smallest aperture. It’s best to use an aperture that is 2-4 stops down from the lens’s widest aperture.

Most lenses have a sweet spot where the depth of field is adequate and the light is least distorted by the diaphragm resulting in a super crisp, in focused, high depth of field photograph.

Usually this magic sweet spot is two to four stops closed from wide open. This can vary, depending on the lens, but usually it’s around f/5.6 to f/13.

If you look at the landscapes that I shoot you’ll see that I’m usually shooting at f/8 to f/11. I found with the Nikon Lens I often use in my landscapes, that f/8 to f/11 offers me good depth of field and a crisp in focused picture.

Now, because I’m using a low ISO and a smaller aperture when I’m shooting my landscapes, the shutter must remain open longer to expose the scene properly. Often so long that I can’t handhold the camera hence I need to use a tripod much of the time.

That’s the trade-off that I’m talking about. I decided that I wanted a noise free, high depth of field image so that means that I must use a low ISO and a small aperture (high f-stop number). The low ISO means that there will be minimal gain applied by the camera which means the sensor will need more light to expose the scene. The smaller aperture lets in less light then if I used a larger aperture. So since I’m using that ISO which needs more light and a smaller aperture which needs more light, I must compensate by keeping the shutter open longer to allow enough light to get through to the sensor.

 

1/5th sec, f/11, ISO: 100. In this landscape of the Wind Sculpture I shot at my lens’s sweet spot of f/11 and ISO of 100. To do that I had to use a shutter speed of 1/5th of a second. Too slow to hand hold so I had to use a tripod.

 

1/125 sec, f/11, ISO: 100. In this scene I was fortunate in that the sun was bright enough for me to handhold the shot but I chose to use a tripod anyway to get the crispest shot possible. With the f-stop set at f/11, my lens’s sweetspot, you can see that the entire scene is in focus, front to back.

 

What about times when you need to freeze action by using a fast shutter speed? This usually will be the case in sport and action photography. Usually the photographer will choose the fastest shutter speed that will freeze the action than decide how much depth of field is needed. If it’s a fast action sport, you’ll probably want a bit more depth of field so that you have a little room for focus error. If the subject happens to move out of the point of focus, you still can get a crisp shot if you have enough depth of field.

Now comes the tradeoff. This time you’re compromising the ISO because you may be forced to increase it to expose the shot properly. Of course on a very bright outdoor scene, you may not have to go up too high, if at all, but in less liberally lit scenes, you may have to increase the ISO to the point where some noise is introduced.

1/2500 sec, f/5.6, ISO: 1600. In this image the photographer wanted the action to be frozen so he used a very high shutter speed. Even with a relative wide aperture of 5.6 he still needed to push the ISO to 1600 in order to get the scene exposed correctly. Photo courtesy of SXC.hu

Well, that’s it. My hope is that reading these four articles you learned how ISO, shutter speed and aperture affect your picture and what the advantages and tradeoffs are when you favor one over the other two.