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Lesson 4: Back to Basics, Obtaining Proper Exposure

July 13th, 2010 · No Comments

Let’s take some time and get back to basics.  Exposure is obviously important yet so many are befuddled on how to obtain proper exposure.  I really encourage people to get back to basics and practice techniques on obtaining Exposure.   In the Digital World, the ability to review on the spot will make learning/re-learning  and retaining these techniques light years faster than Film (remember taking notes of all your settings and then trying to pair those notes up days later after getting your pictures back from the processor?)  To help aspiring photographers with this and remind those who learned these concepts in the Film Days, I will re-visit some exposure techniques in this lesson.  Before we dive in, if you’re in the process of switching from Film to Digital, don’t be afraid.  Exposure principles are exactly the same.  The only difference is the camera writes to a sensor now instead of a 35mm film plane.

One of the most important things to realize with exposure is there isn’t one single exposure that is correct for a given situation.  There are actually MANY correct exposures for a given situation.  Exposure is based on 2 principles (3 including ISO):

1) Aperture  (determines your Depth of Field, how much is in focus)
2) Shutter Speed  (how long the shutter is actually open, letting in light)
3) ISO  (makes your sensor more sensitive, FASTER SHUTTER SPEEDS!)

The following chart shows many possible shutter speed/aperture combinations that will give the same exposure.  The major difference between all of those combinations is the Depth of Field (amount that is IN FOCUS).

Given that exposure is based on shutter speed and aperture, let’s dive into these individually to get a better understanding of how they both affect exposure.  Shutter Speed is all about time.  Every camera has a shutter and when you depress the shutter release button (main camera button to take the picture) the shutter opens for a set amount of time letting in light to record the image.  The amount of time the shutter is open is known as your shutter speed (i.e. 1/500th, meaning the shutter was open for 1/500th of a second).  Aperture, on the other hand, is all about the amount of light the lens allows in the camera.  The picture below shows what is going on inside a lens based on what aperture you choose.

When you choose a wide open aperture (i.e. f1.8, f2.8 or f4 depending on the lens) this lets in the largest amount of light possible which means the shutter is open for LESS time (faster shutter speed).  When you choose a “stopped down” aperture, the lens lets in LESS light which means the shutter has to stay open for MORE time (slower shutter speed).  The most important thing with aperture is not that it ONLY affects your shutter speed; it determines your Depth of Field (DoF).  In its simplest explanation, DoF determines how much is “in focus” in your image (from front to back).  A small DoF means that very little will be in focus in your image.  A large DoF means that more and more will be in focus in your image.  While this can be hard to comprehend at first, I’ve found that most 1-on-1 participants get this by the following rule.  The smaller the f-stop number, the smaller the DoF, the larger the f-stop number, the larger the DoF.

Now that we know what makes up Exposure, how do we put this to good use?  One of the quickest ways to get out of those dreaded AUTO Modes on the camera is to start small and go into Aperture Priority mode (AV on a Canon D-SLR).  AV is a great starting point because it gives you full control of your DoF which is so vital to your image.  In AV mode, you set the aperture and the camera chooses the correct shutter speed that goes with that aperture.  If you are after fast shutter speeds, set your aperture to wide open or close to it.  If you are after a large DoF for flowing landscapes, stop down to an aperture with a larger number and more will be in focus in your image.  This may or may not sound simple but with digital go out and play with these settings and review your images right away in the field to see what is happening when you choose different apertures.  Seeing is believing and digital allows us to see these affects right away.

ISO does play a role in exposure.  By making your sensor more sensitive, you can speed up your shutter speeds in low light situations.  ISO doesn’t play a role in DoF but a perfect situation where ISO can help is photographing delicate wildflowers when wind is a factor.  In some cases, you want a large DoF which as we’ve learned above, will slow your shutter speed down.  By increasing your ISO, the shutter speed will speed up and may give you fast enough speed to get the shot without the wind affecting your image.  That’s just one example of how ISO can help.  To read more on ISO, please see Lesson 3 on high ISO.

Top 3 things to take away from this lesson:
     1)  Aperture and Shutter Speed make up Exposure
     2)  Start by shooting in Aperture Priority mode to control DoF
     3)  Practice in the field; review affects of your changes RIGHT AWAY

Stay close to the Blog for Lesson 5, The Ins and Outs of Shooting at Night.

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