Nex-6 & Background Focus Question

Started Apr 10, 2013 | Discussions thread
boardsy Senior Member • Posts: 2,215
Robert, meet Depth of Field :-)

Hi Robert,

Background Defocus is Sony's simplified term for changing aperture size which controls Depth of Field (= background blur), and the NEX may possibly do some camera processing magic  (= "photoshopping" the JPG)  also.

Depth of Field (= the amount of the image in focus) is a function of aperture size (the changeable lens hole that allows light in),  focal length, distance of subject to camera and to background, and sensor size, but let's focus on aperture for now.

For any given focal length, let's say you focus on something, like your friend in front of the building. Now the wider the aperture (depending on the lens, widest could be f1.7, f2.8, f3.5...) the less around your plane of focus will be sharp i.e. sometimes only a sliver will be sharp, like your friend's face, or even just the eyes, and everything else blurs away gradually front and back of that (= "shallow Depth of Field"). Like this:

Now closing down the aperture hole ("stopping down") expands that thin plane of sharpness to include more detail front and back of the focus point, to gradually include e.g. the building behind your friend - this could be at e.g. f8 like here, nearer and farther parts of the building are (roughly) sharp (= "deep focus"):

At a certain point smaller than f11 optical diffraction will also blur the image, putting a limit on how small you can make your aperture. And at the other end if you have a very large aperture lens like f1.4 or f1.2, for a close subject only a millimetre might be in focus wide open, and even then the image may not be very sharp. You can try to blur backgrounds from RAW or JPG in photoshop etc. but it won't look the same, and you definitely can't change blurred detail to sharp after taking the shot! The blurred effect increases the closer you are to your subject and the farther the subject is from the background.

A mode (aperture priority) is the best way to control all this - just dial the aperture size up & down to your taste, leave EV at 0, and the camera will select shutter speed automatically. The LCD will show you what's in focus, magnify it if necessary. It's easy and is my most-used mode. In low light you might have to widen the aperture more and use higher ISO to keep up shutter speed fast enough to prevent camera shake blur (not the same as defocused blur!).

By the way, blurred backgrounds are a very nice effect that smaller-sensor camera's can't so easily achieve, and can be used artistically, to islolate a subject, for portraits, to calm down busy backgrounds etc.

[edit - apologies for x-posting with Justin/zackiedawg]

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