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The Exposure Triangle

May 19, 2018  •  Leave a Comment

In the photography world, exposure is critical.  From the old days of film to the current days of digital imaging, the amount of light that reaches your camera’s film/sensor is of huge importance to any photographer. Photography is literally writing with light, after all.

But you knew that.  And if you’ve been a photographer for any longer than a day, you’ve probably heard of the “exposure triangle”.  It’s simply a way of expressing the fact that exposure is controlled by three settings on your camera: shutter speed, ISO, and aperture – with each setting represented by a side of the triangle.  But if you haven’t yet read a full description of the triangle – well you’ve come to the right blog – keep reading…

The point of the exposure triangle model is to emphasize the way that these three settings work – or, more accurately, work together – to control the exposure of your image.

First, a word about exposure terminology:  Exposure is measured in terms of “stops”.  I won’t get into the optics and math related to this.  Just know this: Changing your camera’s settings to allow 1 more “stop” of light will double the exposure.  Changing your camera’s settings to allow 1 less “stop” of light will cut the exposure in half.

With that in mind, here’s the exposure triangle:

 

Shutter Speed

Shutter speed of course represents the amount of time that your camera’s sensor is exposed to the light coming through your lens.  Shutter speed is measured in units of seconds, and can of course vary greatly, from really fast (for example, 1/500 second, 1/1000 second, or faster) to really slow (for example, ½ second, 1 second, 10 seconds, etc.).  If you double the shutter speed (say, from 1/200 to 1/100), you double the exposure (you add a “stop”).  If you halve the shutter speed (say, from 1/100 to 1/200), you cut the exposure in half (you remove a “stop”).

Straightforward, eh?   That’s why I started with it.

Shutter speed primarily and most commonly affects your ability to freeze motion (or to create blur, if that’s what you’re going for).  It also allows you to collect light for a longer period of time, as with night sky photography, for example.

When shooting handheld (i.e. without a tripod), a common rule of thumb is that your shutter speed should be no shorter than 1 over the focal length of your lens.  (That’s the "35mm equivalent focal length", but we’ll cover that in a future blog post.)  So, if you have a 200mm focal length, you really shouldn’t hand-hold your camera with a shutter speed slower than 1/200 second.  This is just a rule of thumb, and doesn’t factor in your equipment’s “vibration reduction” or “image stabilization” abilities, which sometimes allow you to shoot at even slower shutter speeds.

 

ISO

Your camera’s ISO setting controls the sensitivity of your sensor to light.  The range of ISO settings can vary with each camera, but common settings are 100, 200, 400, 800, etc.  Those numbers are significant, because doubling the ISO number doubles the exposure (adds a “stop”), and cutting the ISO number in half will cut your exposure in half (removes a “stop”). 

Lower ISO values are almost always desirable, because image noise increases as ISO increases.

Still straightforward?  Nothing too complicated so far.  But keep reading…

 

Aperture

The subject of aperture gets a little complex, I think just because the notation and the math tends to throw people.  Aperture values represent a ratio between the lens focal length and the diameter of the opening.   By convention, aperture values are written as “f/N”, where “N” is this ratio.  (For example, if you have a lens with a 10mm focal length and a 5mm opening, the value of “N” would be 2, and the aperture would be written as “f/2”.)

As with everything else, the aperture settings available on your camera/lens can vary, but standard settings are:  f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.  Why these particular numbers, you ask?  (Let’s just say you asked.)  The “N” values are powers of the square root of 2.  (1.4 is the square root of 2 to the 1st power, 2 is the square root of 2 to the 2nd power, 2.8 is the square root of 2 to the 3rd power, etc.  Don’t believe me? Check your calculator.  Just note that some values are approximate.)   Why is the square root of 2 involved with this?  That’s a little too heavy to get in to here – but I encourage you to read more about that online if you’re interested.

Anyway, based on the standard scale of aperture values above, we can once again equate these settings to the exposure.  Moving your aperture setting one value to the left on that scale (for example, changing from f/2.8 to f/2) doubles the exposure (adds a “stop”).  Moving your aperture setting one value to the right on that scale (for example, changing from f/2 to f/2.8) halves the exposure (removes a “stop”).  The expression “F-stop” comes from this relationship, by the way.

The most popular effect of an aperture change is your image’s depth of field.  See my previous blog post regarding that.

 

Anything Else?

One additional thing to note is that the sample values I used above are just examples.  All 3 exposure settings have intervals between the standard ones.  For example, f/3.5 may be available, falling between f/2.8 and f/4.  These additional settings just allow for more fine adjustments.

Also, remember that some lenses are better than others, so the total light gathered by one lens with a given set of exposure settings may differ (probably slightly) by the light gathered by another lens with the same settings.

 

Yeah, Ok… So How Does All This Work Together?

Let me give you an example:  The largest aperture (smallest “N” number in the “f/N” notation) supported by a lens will vary.  More expensive lenses support larger apertures.  Lenses that support a maximum aperture at the lower end of the scale (e.g. f/2.8, f/2, or lower) are called “fast” lenses, because large apertures like that allow you to use a fast shutter speed.  That’s a good example of how different types of exposure settings affect one another.  By using a large aperture (a small f/N number), you are collecting more light, and that allows you to keep your shutter speed short in order to freeze motion while still getting a good exposure.

More generally, understanding the way exposure settings relate to one another allows you to make adjustments and achieve the image you’re going for.  For example, if you are shooting at ISO 100, at 1/200 sec, at f/8, but you want a little bit deeper depth of field, you can “stop down” your lens to f/11, but you’ll of course lose a stop of light.  To account for that, you can either increase your ISO to 200, or lengthen your shutter speed to 1/100.  Either way, you’re maintaining the same exposure.   But there’s a trade-off here – increasing your ISO may add noise to your image (unlikely at ISO 200, but you know what I mean).  Likewise, lengthening your shutter speed might result in motion blur if your subject is moving.

These are the types of decisions that photographers (good ones, anyway) make for each shot.  It becomes second nature after a while.  And modern cameras have shooting modes which help simplify this – but that’s a blog post for another day.

 

So That’s The Triangle?

That’s the triangle.  It’s really not complicated, but it’s central to photography.  As with most photo topics, you can dive down a rabbit hole into discussions about optics, sensors, etc.   We won’t do that here – I don’t have enough room, time, or expertise for that.  Just understanding how these settings interact and affect your images is all you really need to know.

 


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