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Posted in : Featured / Photography Principles on February 17th, 2012.
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When you capture an image with your camera, there are three basic things which will affect how that image turns out.  Those are your ISO, your shutter speed and your aperture.

Together these three settings comprise your exposure triangle, and while you can achieve the same exposure value with different combinations of these three components, you won’t necessarily see the same results.

Changing any of these settings will have a different consequence, and you’ll have to adjust something else in order to compensate.  Ultimately your final image will be a trade-off between depth of field (aperture), noise level (ISO) and how much motion you want to capture or freeze (shutter speed).

There are other things which can affect your exposure, but we’ll get to those later on at the end. :)

Ok, so how do I figure out what my exposure should be then?

What you need to do is a process called metering.

Temporarily put your camera into P mode (yes, yes, I know, I said temporarily!), and it should work out a shutter speed and aperture (maybe ISO too, depending on how your camera is configured) that would give you an accurate exposure for your scene (at least, that’s the theory).

If we set our camera on a tripod and point it at a mostly static landscape scene on a very dull day, the camera may have chosen the following settings.

  • ISO400
  • f/5.6
  • 1/60th of a second

If we take a test shot, we can see that we have a nicely exposed image with details in the highlights, details in the shadows, and lovely midtones (that doesn’t always happen in the real world, but for the sake of this example let’s assume your camera metered correctly).

Now if we pop it into manual mode, enter those same settings, take another shot, we can see that we get the exact same image.  But, what if we want to get a shallower depth of field on a subject in the foreground?  Or a much larger depth of field to capture the whole landscape?  Or we want to freeze a moving subject?  Or to capture the motion of it?

Yeah, what about that?

Let’s look at the first example, decreasing our depth of field.  Here you’d have to open up your aperture (remember, smaller number = larger aperture = more light hitting your sensor) in order to get a shallower depth of field.  This has the side effect of letting in more light.  If we look at the following list, we can see our whole stop aperture values starting at f/1 all the way to f/32.

  • f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

There’s our lens at its current setting, right in the middle.  The numbers to the left of this give us a progressively larger aperture with a shallower depth of field, and each allows one stop more light to enter our lens than the number directly to its right.  If we go the other way, the larger numbers give us a progressively smaller aperture, a deeper depth of field, and allow one stop less light than the number directly to its left.

This is a list of our whole stop ISO values.  The higher the number, the more light our sensor is sensitive to, at the expense of an increased level of noise.  Some cameras, such as the Nikon D3s and D4, will go much higher than ISO25600.

  • 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600

There’s our setting, ISO400, highlighted in bold.  Each number to the left is 1 stop less sensitive (less light capturing ability) than the number immediately to its right.  Each number to the right is 1 stop more sensitive (more light capturing ability) than the number immediately to its left.  The further right you go, the more noise you get.  The further left you go, the less noise you get.

Shutter speeds increase and decrease the amount of light in the same way as ISO, by doubling or halving.  There are far more shutter speed options, going all the way from 1/8000th of a second to 30 seconds (and longer with some cameras or external timers).

  • 1/8000, 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8, 15, 30 (duration in seconds, or fractions of)

You’ll see that the fractions aren’t always exactly halves, for example going from 1/125th to 1/60th, or from 1/15th to 1/8th.  These numbers are rounded off simply for the sake of mathematical simplicity.  For all intents and purposes, the numbers above are each 1 stop apart.  As we go left, each shutter speed lets in half the amount of light as the number directly to its right, and as we go right, each shutter speed lets in twice as much light as the number directly to its left.

Remember, when we double or half the amount of light, we are increasing or decreasing the light by a stop.

So, let’s have a look at the settings our camera suggested we use again.

  • ISO400
  • f/5.6
  • 1/60th of a second

We see a nice tree in the foreground of our landscape, and we want a shallower depth of field on that tree so we can blur out the background, in order to be able to really impress that tree upon our viewer.  Our lens opens up all the way to f/2, great, nice and shallow.  If we go from f/5.6 to f/2 and change no other settings, we have the depth of field we want, but we have also gained 3 stops of light (8x as much light).  This means our shot will now be 3 stops overexposed.

Now, we have to lose light somewhere else.

This presents us with 3 options.  We can either lower our ISO, speed up our shutter, or both.  We need to lose 3 stops of light, this means we can take our shutter speed from 1/60th to 1/500th of  second.  If we take a shot now, we can see we get the exact same exposure value (“brightness level”) as we originally did, but with a much shallower depth of field.

  • ISO400
  • f/2
  • 1/500th of a second

The image looks ok, but we do have a little bit more noise than we’d like, so let’s see what else we could do.

The hypothetical camera that we’re using has a minimum ISO value of 100.  So, with our camera set back to ISO400 at 1/60th of a second, when we set our aperture to f/2 we again need to lose three stops of light, and we want to get rid of some noise.  So, we can drop down to ISO100.  We would like to go lower, but this is as low as our camera allows us to go.

This only loses us two stops of light (see the list of ISO values above), but we have a much cleaner image than we did before with less noise.  Now we can bump our shutter speed from 1/60th to 1/125th to lose that last stop of light.

  • ISO100
  • f/2
  • 1/125th of a second

If we take another shot now, we’ll see that once again we have the exact same exposure value, with our shallow depth of field, but with less noise than the image above.

Ok, fantastic, but there’s this really cool waterfall behind the tree that I’d love to get in focus, and I’d really like to be able to capture the motion of the water a bit if I can, what can we do?

Well, as we know, decreasing the size of our aperture (larger number) will increase our depth of field.  The lens we’re using goes all the way up to f/22, and that’s probably plenty enough depth of field for what we want.

If we’re still at the settings of our last shot, and we go down to f/22 in order to maximise the depth of field we can get from our lens, we have lost a whopping 7 stops of light.  Now, we could bump our ISO up to ISO12800, and get the following settings.

  • ISO12800
  • f/22
  • 1/125th of a second

We’d get a well exposed image, same as all of our examples above, but we’d get far more noise than we would at ISO100, and lose a lot of detail.  It wouldn’t look anywhere near as clean as the other images we’ve shot.  We also would still be shooting at 1/125th of a second, which wouldn’t allow us to capture much motion from the beautiful waterfall.

So, let’s start again, if we keep our ISO at 100 in order to minimise the amount of noise, we change our aperture to f/22, then we need to gain 7 stops back on the shutter speed.  At 1/125th of a second, we need to slow the shutter all the way down to 1 second in order to get that 7 stops of light back and get a good exposure.

  • ISO100
  • f/22
  • 1 Second

Now we have a really deep depth of field, allowing us to capture the tree in the foreground of our scene, the lovely waterfall behind it, the clouds in the sky several miles behind that and have everything in focus.  As our shutter is open for an entire second, the speed of the water in the waterfall means we have captured the motion and the path of the water falling down it and along the river.  As our ISO is very low at only 100, we also have a very clean image with virtually no noise, so we can see every detail.

Remember, we have not moved our camera, the light has not changed much, it’s a fairly dull day, so even though the sun has moved, the clouds have given us some consistency.

Given that the light has not changed, and our scene has not changed, every combination of settings listed above will give us a “correct” exposure in this instance, given how our camera initially metered the scene.  Every time we changed one setting to give us a certain effect, we changed another setting to compensate for the increase or decrease in light recorded by our camera.

Which one is the right image depends entirely upon you and what you’re trying to capture.  Whether we increase or decrease the depth of field, capture more or less motion, or have more or less noise in our image are creative decisions, and it’s a balancing act that allows you to get the results you desire.

This is one of the reasons why automatic exposure modes often aren’t a great idea, and why you don’t always get the results you expect!

My camera has a whole bunch of other numbers in between all the ISO, aperture and shutter speed values you listed above, what are those?

These will either be 1/3rd or 1/2 stop increments, depending on which camera you use and how you have it configured.

The principle is the same, you lose a 1/3rd of a stop of light by adjusting one thing, you have to gain it back by adjusting something else, you’re just dealing with more specific amounts of light, depth of field, noise and motion in order to really nail down the shot you want to get.

Before you go, earlier you said there was something else that could affect our exposure?

Yes, I did.  There is one other main method of changing your exposure and that is by the use of neutral density filters.  Sometimes you want to shoot at a shallow depth of field in the middle of the day and capture some motion at the same time.  When your ISO is as low as it’ll go, if your image is still overexposed, the only other reliable way to decrease the amount of light entering your lens is via  a neutral density filter.  On a bright day, even at f/22 and ISO100, you may not be able to get anywhere close to a 1 second exposure, and you may want to go even slower if you want to capture some motion.

When I say a “reliable way”, I mean predictable and measurable.  Neutral density filters are typically available at varying strengths from 1 stop all the way to 10 stops (and sometimes even more!), and will allow you to shoot at wide open apertures for decreased depth of field, and longer shutter speeds to capture that motion.

Circular polarising filters will also decrease the amount of light entering your lens, and allow for longer shutter speeds at smaller apertures, but the amount of light lost on polarising filters will vary from brand to brand and from price point to price point.  Typically, the better the brand, and the more expensive the filter, the less light is lost.

This might make the cheaper filters sound more appealing if you’re looking to lose more light, however it comes at the expense of a loss of sharpness and a much increased risk of flare due to the lack of high end coatings used on the filter.  Your best bet is to get the highest end circular polariser you can, and if you need to drop the light even more, stack it with ND filters.

But, we’ll get deeper into using both of those in other articles.