How do you select your aperture in your photos?

Hello.  I'm new here and a bit new to photography with a 'professional camera'.     OK, I don't know if what I have is a professional camera.  I've been shooting with an iPhone for the longest time and I just got a Sony A7r and my wife says it's a professional camera.  But I'm trying to understand when you are in Aperture priority or I guess manual mode for that matter, how do you determine what aperture you are going to use? 

Thank you for your help! 

The most important points, using the aperture ring, are two.
1, make sure you hit the right ƒ for a correct exposure and
2, control the DoF.

With a bit of practice you will be fluent in the selection.

G'day Storz

The aperture settings in the camera are used for three main things ..... a) exposure control in conjunction with the best shutter speed, -or- b) to create / determine the Depth-of-Field that you want for the scene, and/or c) to determing the level of image sharpness from the centre to the corners of the photo

Looking at these in reverse ... sharpness - if you use a magnifying glass to look at a subject, you will quickly see that the edges of the image are blurry when compared with the middle. Camera lenses are much better, but as we are a fussy lot !! we want good edge sharpness as much as possible. So comes the recommendation to use a mid-range aperture for good image quality, and to be aware that when using the lens at wide-open (small aperture numbers) then the IQ may not be perfect

Secondly- when you have a subject that needs more or less Depth-of-Field, then you can run the apertures down to F16, F22 if needed, or out to F4 maybe if you do not need DoF

Finally, it is alwayas a balancing act (like a child's see-saw) to balance the Aperture to the best Shutter speed so that the exposure is correct and the end result is what you wanted

Remember also that the aperture number is the bottom of a fraction ... the diameter of the aperture hole is a fraction of the millimetres of the lens focal length. So small numbers F2, F4 etc are 1/2 - 1/4 of the focal length and large numbers F16, F22 are 1/16 or 1/22 of the focal length

Hope this helps
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/

It depends on how much blur (Depth of field) I want behind the subject I'm shooting

Depends on how much blur I want and what I'm photographing.  

So is there an aperture you use most when you don't care about background blur?  
Thank you all.  

The widest aperture ( lowest f/number) provides most background blur (if that's what you're referring to).
But remember that the amount of background blur depends not only on aperture ,  but also on distance from you to subject and distance from subject to background , and focal length of lens (from same camera position) .

Wider aperture (e.g.  f1.4 , f/2 , f/2.8 ... ) : more background blur
Smaller aperture (e.g. ... f/11, f/16 , f/22 ) : less background blur

Closer camera-to-subject distance             further subject-to-background distance :  more background blur
          Further camera-to-subject distance , closer subject-to-background distance : less background blur .

Shorter focal length (from chosen camera position) :  less background blur
Longer focal length (from same camera position) :     more background blur

Hope that helps .


Storz wrote: So is there an aperture you use most when you don't care about background blur?  
Thank you all.  

Blur can also occur in the foreground.  So Background blur isn't my deciding criteria ... Depth of Field is.

The first thing I ask myself before shooting anything is, "do I have any depth of field concerns?"  The answer is usually yes.  I usually want either a deep or a shallow DOF, so aperture will be my priority and I'll set the aperture small or large accordingly.  If the answer is no, then I'll use something like f/8 or f/11. 

The second question I ask myself is "do I have any shutter speed concerns?"  If the answer is no, then I'll be working with aperture priority, as above.  If the answer is yes, as in stopping motion or as in showing motion blur, and the answer to question 1 is also yes, then that's a problem that I'll try to resolve either by working completely in manual mode and adjusting ISO, or by enabling auto-ISO.

Easy - do I want the background blurry or sharp?  F2.8 vs f/16

Depends on how much light there is, what I'm after in the shot, and so forth.  

   3 words:  Depth of field

Portraits - depends on how much crap is behind the person I'm shooting or how much I want in focus.  Landscapes, usually f/8 to f/11, or if sunny out, then f/16.  

Finally getting a grasp on all of this, thank you all for your help.  

For me, it's all about the background blur.  Most of the time, I'm pegged around f/8