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How to use Av Mode: Aperture Priority


How to use Aperture Priority  or “Av” mode
What is lens aperture?

In the most basic sense; Av mode or Aperture Priority is the mode in which the camera selects the shutter speed to acquire a user defined exposure in accordance with a user selected lens aperture.
 
When to use Av mode:  Aperture priority is an excellent all around shooting mode.
I use Aperture Priority whenever I want to control the depth of field of my image, and have no concern over the shutter speed. When I look to get a more professional background blur, I will use aperture priority and select a wide aperture such as 1.8 or 2.8 or some aperture near the widest value my lens is capable of.
 
I use aperture priority when shooting sports, I use my 70-200 2.8 Lens,  set the aperture for 2.8 and then meter my subject.  If the shutter speed is over 1/500th of a second, then I know I will get excellent results.  If the shutter speed is blinking 1/4000 (rebel) or 1/8000 (others), then I know there is too much light and my photos will be over exposed.  I then dial in 1/3 or 1/2  aperture  F stop reductions until my shutter speed is appropriate for achieving properly exposed photos.
 

If my shutter speed is too low, I bump my ISO up until I achieve an appropriate shutter speed.  

I also use aperture priority when I am shooting landscape photos.  Using aperture priority allows me to select an aperture between 5.6 and 11, whatever I feel is best for any given situation.  You will need to find the balance that best works for you, but great landscape shots can be achieved with excellent depth of field at apertures between 5.6 and 11. 
 

Below we have a small image menu which will present to you the same scene taken with different aperture values. These range between F/1.8 and F/11.0, this is a useful illustration for the different image results achieved as a result of aperture width adjustments.

Aperture Priority Basics: The lenses you purchase for your SLR are rated for their focal length (the magnification distance) and their aperture values. For instance you have a lens from canon called the EF 70-200 2.8 This means the lens goes from 70-200 mm in focal length, with a constant wide aperture value of 2.8 available. Of course you can also select a narrower aperture to increase the depth of field.

Wider apertures lack background detail: The starting point for understanding aperture is to think of aperture as the clarity of your background. The wider apertures allow for faster shutter speeds, but wider apertures cause backgrounds to be blurred. A wide aperture is inappropriate for a landscape shot, but very appropriate for a sports photo or portrait.
 
When you drop aperture down to a lower F stop, for example from   8.0 to  11.0   you are narrowing the diameter of the hole light passes through to reach the sensor.  Because lenses are limited due to their optics, stopping down too far will result in a softening of your image.  The softening effect is called “diffraction”  and is the result of light bending once it has traveled through the small aperture hole.  Unfortunately, due to diffraction you often times will lose the benefit of an increased depth of field  achieved by stopping down your aperture.  There is a balance and on more recent DSLR’s the balance tends to be between  F8 and F11.
 

Want more understanding of aperture?
Lenses state their maximum aperture rating on them, this means EF 50mm 1.0 lens 1.0 is the widest, yet the lens will be capable of being “stopped down” which means we can reduce the aperture width from 1.0 to 20.0. An aperture of 1.0 would provide a blurry background for our subject, while an aperture of 20 would provide a detailed background shot. Why? When shooting with a wide open aperture, more ambient light is let in because the “aperture diameter” is the widest possible.

F18 Narrow Aperture

F18 Narrow Aperture

Wide aperture F4

Wide aperture F4

The most light is let in, and we get a fast shutter speed. However, light does not travel in straight paths, this is called being “uncollimated”. At a wide aperture, uncollimated light is let in which causes the blurred background to be captured. Aperture blades are a type of collimator, meaning they collimate the light into a straight path, this is achieved by allowing less light in and only the light which is on a relatively straight path as the same focal length as the focused point of the lens. When the lens is reduced in aperture width, the shutter stays open longer and allows more light from the “collminated” selection in from distant objects. The additional light leads to more detail and greater clarity of background (infinity) items .

OK so that’s a lot to think about right? I think the best way to think of it is…. If we are letting a crowd in to a concert, do we get more efficient and better streamlined results with single file lines or do we get better results with a free for all stampede.

Of course the limits of the glass in your lenses and limitations in the designs of our cameras, create a situation where light when over collimated can leave too little light coming in, and thus introduces additional movement from the few light strands being allowed through your aperture hole. There is a “diffraction limited aperture” available to the DSLR user. You will have to accept this as a limitation of your lens, and realize that at a very wide aperture you don’t have large depth of field, and at a very narrow aperture you lose details due to diffraction of the light.

 With all of that said, you now have a basic understanding of aperture.
With aperture priority mode (Av)   you select the aperture you want, wide or narrow.
If you are in a dark environment and need a faster shutter speed, dial like a wide aperture.  If you are out doors in bright sunlight and your photos are over exposed, dial in a narrower aperture.   This will allow less light in and prevent the over exposure.

Av mode allows you to take control of your image, you can control whether you want background blur or not.   You decide how much depth of field you need, and in turn your results will hopefully be in line with what you envisioned.

If you are new to SLR cameras, you should definitely play with Aperture Priority mode.  Its a very creative mode, and will unlock a lot of your understanding of the capabilities of digital SLR’s.  Its hard to just pick up a manual and really grasp what you have gotten into over a point and shoot camera.   You will need to learn aperture , shutter speed, ISO and exposure control.  These fundamentals combine to help you achieve beautiful and creative photographs.