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Exposure – Aperture


There are many different settings you can make to get the proper exposure.  It just depends on how you want the photo to look. 


For example let’s say I am taking a photo of a flower in program mode and the camera’s meter tells me it is using the following settings:  Aperture of f5.6, Shutter speed of 1/400sec. at an ISO of 100.  While these settings give me a perfectly acceptable photo, it is not what I had in mind when I took the picture.


Blossom BunchWhat I really had in mind is for the the background to be out of focus so the main subject really pops.  Which control of the exposure trilogy do we have to give us less Depth of Field?  If you said Aperture Priority mode give yourself a gold star. 


If I set my camera to Aperture Priority (A or Av) and dial in F1.8 or f2.0 the camera will automatically set the shutter speed to 1/3200 sec.  If I choose F2.8 it will select 1/1600sec.  Are you starting to sense a pattern in the shutter speed.  In each one of these examples f5.6/400, f2.8/1600 and f1.8/3200. What would the Shutter speed be if I had set my aperture to f4 right between f2.8 and f5.6?  That’s right the shutter speed would have been 800. 


 Each one of these apertures lets in twice as much light to your sensor as the previous higher number.  The larger the number the smaller the opening in your lens and the less light that hits your sensor or film.  Some of the more common stops are f1.8, f2.8, f4, f8, f11, f16, f22 & f32.  Your lenses might not go this high or low and may have numbers in-between, giving less than full f-stops to work with.


Why do some zoom lenses have a range of numbers on them like 18-200mm f/3.5-6.3?  This means that when the lenses focal length is set to 18mm the largest aperture that can be used is f3.5 and if you zoom in to 200mm the largest you can go is f6.3.  If your zoom lens is a 28-75mm f2.8 this means that the maximum aperture of f2.8 is available at any focal length.


Let’s talk a little about Depth of Field (DOF)


The simple definition of DOF is:


Shallower Depth of Field (Lower aperture number) = Blurrier Background

Greater Depth of Field (Higher aperture number) = Sharper Background


Another term you may hear when the discussion of shallow depth of field or a blurry background is bokeh.  This is pronounced Bok-ahh.  Now you can impress your friends and neighbors with your new found photography lingo.


Below are five images of an ornamental butterfly, all taken on a tripod from about 12 inches away.  All were taken with the same lens a 50mm f1.8 and the same camera.








This shot was taken using f1.8.  Notice the flowers in the background are very out of focus.  The flowers are about a foot behind the butterfly, if they had been further back they would have been even more out of focus.















This photo was taken at f4.0.  Notice how the flowers are slightly more in focus.  Still not overpowering our subject but getting there.

















Now we get to f8.0.  The background is almost in focus.















And the final shot here was taken at f16.  Kind of distracting isn’t it.  I like the first the best.  If the subject had not been so bold against the backgound it would appear even more cluttered.











In the example below, I took several angled pictures of a ruler with the focus point on the 6 inch mark, on the left side.  Then I only changed the aperture for each example.  The first is f1.8, note how out of focus the 4 and 8 are.  The second is at f4.0,  now I can now tell the 4 is a 4 and can almost make out the 8.  The third is set at f8.0, notice how the in focus portion just keeps increasing.  In the final example at f22 it is very sharp.  Click the image below for the full size comparison.



Depth of field is influenced by a few different things.  First is the aperture, as shown above, but just as important is the cameras distance from the subject.  Second is the closer you are to the subject, the shallower or narrower the area of focus will be.  Third is the focal length of the lens, meaning the DOF will be shallower the more you zoom in, assuming the aperture stays the same.


In the case of digital cameras the size of the senor also influences depth of field.  The larger the sensor, the shallower the depth of field will be at a given aperture and distance.  This is why compact digital cameras are so great at macro (close up) shots, because they have very small sensors and a high depth of field. 



So what aperture should I use for different situations and why.  To isolate our subject from the background, we want a wide open aperture so we will use f1.0 to f5.6 for these shots. 



BeachcombingMost lenses are their sharpest at around f8 to f11.  These settings are good when it doesn’t matter if the background is out of focus or super sharp, but you want your subject to be nicely focused. 



For landscapes use the lower apertures of f16 to f32.  This will give you the greatest depth of field or sharpness throughout the entire photo. 

There are many papers written on how optics behave at different apertures.  If you search for them on the internet, you will find very technical and detailed explanations.  Here is a basic explanation.  At wide open, let’s say f1.8, a very large portion of the lens’ surface is being used, including the outer edges.   The edges are not as sharp as the center.  At a middle aperture, of let’s say f8 to f11, only the sharpest “center” portion of the lens is being used.


Take a look at the Ruler example again.  In the test shot taken at f1.8, notice the shadows in the corners?  This is called light falloff or as it’s often referred to Vignetting.   The f8 example does not have this as it is using the lens’ sweet spot at the center.


Story time!   


mclane-frogIn my transition from compact digital to DSLR I was not happy with the close-up photos I was now getting.  I used to be able to hike around a local park and get great shots of flowers, frogs, bugs and interesting plant life.  When I went back to the same location with my new DSLR I was stumped as to why my shots were so out of focus. 




Now I know it had to do with the reduced DOF of my larger sensored DSLR.  How did I get my shots to be closer to the ones I took with my compact?  You guessed it, I decreased my aperture by using a higher setting.  However, there was a trade-off to be made.  Since I had to change my aperture, from let’s say f2.8 to f8, to get the subject as sharp as I wanted, I had to give up the blurry background I also desired.  One thing I could have done is use a tripod to steady the camera while taking the photo at f2.8 or f4.


This is what you deal with when you try to take a close-up shot of someting like a butterfly, but you can’t seem to get the whole thing in focus.  Try stopping the lens down another click or two.  If you can’t get what you want in focus at f2.8 then try stopping down to f4 or f5.6.


Rusty BellIf I change my aperture from F2.8 to f8, my shutter speed may have changed to a setting that is really to slow for me to get a handheld shot.  A general rule of thumb is to keep your shutter speed equal to or greater than the focal length of your lens.  For example, If you have your zoom set at 100 mm, your shutter speed should be 1/100 sec or higher.  Remember, you can also change the third side of our trilogy, the ISO speed which will help in handheld situations.


In the next section we will discuss Shutter Speed and how you can use it to create interesting photos.



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