A Practical Primer to F-Stops, Shutter Speeds, and ISOs

The goal of this post is to give a practical demonstration of the effects of F-Stops, Shutter Speeds, and ISOs and how they affect each other.  I'm skipping over much of the technical information so as not to bog down readers with too much information too soon.  I'm a big believer in seeing something work and then finding out why it works once I have a solid understanding at the superficial level.  It is tricky to understand how these things relate to one another, but it is easy to see the relationship with a very simple exercise.  To make the effects very obvious, I used the extremes of what my camera and lens will allow.

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Below you can see what happens when the f-stop is changed from 4.5 to 29, but the shutter speed and ISO are left the same.

F4.5, 1/5 seconds, ISO 100

F29, 1/5 seconds, ISO 100

By changing only the f-stop, the photo became very dark (underexposed).   For the photo to be exposed correctly at F29, the shutter speed can be lengthened from 1/5 seconds to 8 seconds without changing the ISO.

F29, 1/5 seconds, ISO 100

F29, 8 seconds, ISO 100

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Below you can see what happens when the f-stop is changed from 29 to 4.5, but the shutter speed and ISO are left the same.

F29, 8 seconds, ISO 100

F4.5, 8 seconds, ISO 100

By changing only the f-stop, the photo became very bright (overexposed).   For the photo to be exposed correctly at F4.5, the shutter speed can be shortened from 8 seconds to 1/5 seconds without changing the ISO.

F4.5, 8 seconds, ISO 100

F4.5, 1/5 seconds, ISO 100

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Below you can see side-by-side comparison of the effects of the f-stop when the exposure is correct.

F4.5, 1/5 seconds, ISO 100

F29, 8 seconds, ISO 100

At F4.5, only the yellow mug in the front is in focus, but at F29, all three mugs are in focus without changing the focus on the lens or the distance of the camera from the mugs.  As the f-stop number "increases"¹, more of the shot can be in focus.  As the f-stop number "decreases"¹, less of the shot can be in focus.

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Below you can see see the effect of the ISO is changing from 100 to 1600 without changing the f-stop and shutter speed.

F29, 8 seconds, ISO 100

F29, 8 seconds, ISO 1600

By changing only the ISO from 100 to 1600, the photo became very bright (overexposed).   For the photo to be exposed correctly with an ISO of 1600, the shutter speed can be shortened from 8 seconds to 1/2 seconds without changing the f-stop.

F29, 8 seconds, ISO 1600

F29, 1/2 seconds, ISO 1600

At this point, if the ISO was changed back to 100 without changing the f-stop and/or shutter speed, the photo would be very dark (underexposed).

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Below you can see side-by-side comparison of the effects of the ISO when the exposure is correct.

ISO 100

ISO 1600

There is much more "grain" with an ISO of 1600 than with an ISO of 100.  You may need to click on the images to see the difference in graininess.

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The photos above were taken using a tripod and a still life subject, so the chance of motion blur from a slow shutter speed was slim to nil.

Below you can see a side-by-side comparison of a shot with and without motion blur.

Still Air

With Wind

The settings on the camera were kept the same in both shots, but the shutter speed was slow enough to capture motion blur.  Motion blur can be desirable or undesirable.  To freeze motion, the shutter speed needs to be quicker than the motion.  To capture motion, the shutter speed needs to be slower than the motion.  As shutter speeds get slower, the chance of capturing motion blur increases.  As shutter speed gets faster, the chance of freezing motion increases.

As a general rule, if you are working without a tripod/monopod, you should not let your shutter speed be slower than 1/60 seconds because it's around that speed that you can freeze your own motion as a photographer holding a camera and clicking the shutter release button.  Below that shutter speed, it's generally better to start using a tripod/monopod.  Of course, all rules can be broken depending on what you want to do.

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In summary:
F-Stop affects how much of a shot is in focus.  ISO affects the graininess of a shot.  Shutter Speed can show motion blur or freeze motion.  All three affect exposure, and you will have to make choices and prioritize which aspects of a shot are more important.  As the F-Stop number "increases"¹ (4.5 to 29), the shutter speed quickens (8 seconds to 1/50 seconds), and the ISO number "decreases" (1600 to 100), there is less light entering the lens and being captured, and the photo may be underexposed (too dark).  As the F-Stop number "decreases"¹ (29 to 4.5), the shutter speed slows (1/50 seconds to 8 seconds), and the ISO number "increases" (100 to 1600), there is more light entering the lens and being captured, and the photo may be overexposed (too bright).

If you would like to see these effects for yourself and learn through experience, you can recreate the scenarios used in this demonstration and see the relationships for yourself.

• You will need a camera that allows you to set f-stop, shutter speed, and ISO.
• Create a setup that has objects at different distances from the camera such as with the mugs in the example shots.  
• Keep the camera on a tripod with the focus set to manual.  
• Only touch the camera to change the f-stop, shutter speed, and/or iso.  Make only one change and then adjust to compensate for the change.  
• Use the timer on your camera for slow shutter speeds (slower than 1/60 of a second).

If you do not have a camera that allows you to set these, just remember the priorities of your camera's settings and be sure to read the manual.  Regardless of what camera you have, you should still read the manual so you can use your camera to its fullest extent.  Manuals are a wonderful source of information, and knowing the tool you're using is the first step.

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More Resources:

• Manual Photography Cheat Sheet
• cameraDemo
• A Tedious Explanation of the F/Stop
• Understanding The Relationship Among Aperture, Shutter Speed, and ISO 
• Wikipedia Article on F-number

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¹Technically, F4.5 is larger than F29, but when you are using your camera, you see numbers like 5, 6, and 13.  If you'd like the tedious explanation of f-stops, literally, please check out this article - it's truly wonderful but a bit dense.