Aperture Value - WHY?
 

What is the purpose of, and under what circumstances would one use Av? Please explain . .? And is this the same as Aperture Priority?

Grazie

Av and Aperture Priority are the same. Canon chooses to be a little different in referring to it as Aperture Value. Av is useful when it is important to control the DOF in the scene. A constant DOF (Aperture) requires that the shutter speed change if exposure values (EV) change.

hmmm.... not sure . ..
 

Thanks Jeff,

It is not just to do with focal length, its would start looking unatural with bluriness .. please tell me where one would use it over and above the manual settings one already has - if I was shooting a sports car with the shutter speed set correctly for shot and then I'm in control for the rest of the shot .. so again where would I use this? There are other adjustmnts that come along with Av. Using Av/p I would appear to be handing over the control to the circurtry and not in control myself and not knowing what I was doing and being in full control of the camera ..

I'm really having a problem appreciating A/v or p and where I would use it?

Regards

Grazie

Aperture has almost nothing to do with the focal length of the lens. Aperture is the opening through which light passes in a lens. The aperture is expressed as an F number (F2.0, F2.8 etc.). When used in computations, such as determining DOF, it's actual physical size (diameter) is used.

The aperture is adjusted in order to obtain the correct exposure for a given value of light. If a fixed shutter speed is used such as 1/50 a second, when the light value changes the aperture must be changed in order to maintain the same shutter speed. This situation would occur in Shutter Priority (Canon call it Tv, Time Value) when you adjust the aperture.

How would it happen? In shutter priority you adjust the shutter to a predetermined value such as 1/50 a second, the field rate in PAL. When you look through the view finder the camera will adjust the aperture until the zebra bars are reduced or eliminated. The relationship between the shutter speed and aperture would stay the same until the light changes (sun goes behind a cloud) or you zoom or pan and change the scene. When the light value changes the camera is forced to adjust the aperture in order to maintain the correct exposure.

The reverse occurs in Aperture Priority (Av). You select the aperture and the camera automatically adjusts the shutter speed to maintain the correct exposure. By picking the aperture you are controlling the DOF within the scene. If you choose a small F number the DOF be minimized and less objects will be in focus in your scene. This may result in a higher than average shutter speed. If you choose a high F number the DOF will be maximized and more objects will appear to be in focus within your scene. This may force a lower than normal shutter speed (say a 1/15 of a second) and blur moving objects.

In extremely bright light it may be difficult to obtain the desired DOF. If you wish to reduce the DOF in a very brightly lit scene you may need to add Neutral Density (ND) filters to your lens in order to reduce the amount of light entering the lens.

Many photographers are most comfortable using manual (photographer sets both aperture and shutter speed) and controlling both adjustments. It gives the photographer the most control, but requires a lot of attention. An inexperienced or lax photographer, might make incorrect adjustments or choose improper choices of settings. The use of a semi-automatic mode requires less attention, on the part of the photographer, while maintaining important creative and exposure control.

Real World Example please?
 

Jeff .. thank you for that. So, under what "practical" situations would I use it? Come to that, under what real World situations would/have you used it? An example from you may very well clear this up for me once and for all.

Thanks for your patient consideration,

Grazie

When depth of field is critical, and shutter is not. This means no or slow moving objects or people. IE INterviews, product shots, still lifes...

Richard is absolutely correct. If you want a shallow DOF for an interview (want the back ground blurred) you would select an aperture near wide open, such as, F2.0 or F2.8. In Av the camera would automatically adjust the shutter to obtain correct exposure. Some still lifes might require shallow DOF (studio product shots), while other still lifes (scenics, landscapes) might require extreme DOF.

Most sports or action shots may require higher shutter speeds (to reduce blur and motion). If you open your lens up (select F numbers such as F2.0 or F2.8) it will let more light in and the camera will adjust the shutter speed higher to obtain correct exposure.

Limiting Depth-Of-Field
 

Limiting the DOF allows you to control what is in focus and what is blurred. It is a means to force the viewer to focus on what you want.

A couple of examples...

An intimate dinner for two. Camera is focused on a couple having dinner, while the background and foreground are blurred.

Camera is focused on a ringing phone in the foreground, then without moving the camera or changing the focal length, change focus to a person in background reacting to the ringing phone.

Hope that helps?

sadly, the info window in the XL1s viewfinder does not display the camera selected aperture in Tv, nor does it show the selected shutter speed in Av. This makes these options rather useless for me since I can't monitor what auto value the camera meter/logic is setting. Of course, I can always press the manual button to dsiplay the values on the viewfinder, but then, the exposure settings are locked at the value when the manual button is depressed. Have I missed something that would show the computer selected values when in Av or Tv??

AV is a very useful setting if you want to remove the scan line bars from a computer monitor. Select an aperture and the GL2 will vary the shutter speed infinately between pre-set limits. So if you frame up the display and move the camera slowly away from the monitor you'll find a point where the bars stop rolling - and that's the shutter speed to lock in.

AV is also useful for ensuring that portraiture (for example) has the least depth of field, and the softest backgrounds possible under the lighting conditions in which you're shooting. Simply select max aperture (f2.9) and full tele and let the shutter speed decide what's right for the exposure.

tom.