Where to focus?

[ipri]

Hi,

Probably a daft question. When I'm in Manual mode, I can use focus ring to focus on any part of a scene. My NEX3 briefly shows the scene in magnified form allowing accurate focus on any point. OK...if I focus on a point..does that mean the rest of the scene is not in focus ? What are the rules...if any, to go by. Thanks Ian

[Pol]

Re: Where to focus?

Hi,

Probably a daft question. When I'm in Manual mode, I can use focus ring to focus on any part of a scene. My NEX3 briefly shows the scene in magnified form allowing accurate focus on any point. OK...if I focus on a point..does that mean the rest of the scene is not in focus ? What are the rules...if any, to go by. Thanks Ian

There are others who can probably explain in a better way than I can but I'll have a go anyway.

The short answer is no, it wouldn't necessarily mean the rest of the scene is not in focus. It would depend on the aperture setting you're using. Have a read about 'depth of field' and that'll give you more background information.

In short - the wider the aperture (small F number) the more shallow the depth of field So if you use a wide aperture, the point of focus will be sharp but the background will be blurred.

If it's a landscape, you'd be better to use a smaller aperture ( higher F number) ... choose a point of focus but the background will also be in sharper focus.

I'm not very good at explaining these things but I hope that helps a little. Also experiment with apertures at http://www.photonhead.com/simcam/
That site shows the effect of different apertures on an image if you work through the settings.

Cue others more expert than me at explaining these things clearly and succinctly, please.

Pol

[Patrick]

Re: Where to focus?

Hi,

Probably a daft question. When I'm in Manual mode, I can use focus ring to focus on any part of a scene. My NEX3 briefly shows the scene in magnified form allowing accurate focus on any point. OK...if I focus on a point..does that mean the rest of the scene is not in focus ? What are the rules...if any, to go by. Thanks Ian

This is defiantly not a daft question, its a big subject, bigger than most people think.

There is the focal length of the lens to consider, the aperture as Pol has said. Then there is the size the image is to be enlarged to and the viewing distance, all play there part in what appears in focus and out of focus.

In simple terms you choose your focus point, if a good depth of field is required you select a smaller aperture (higher number) if you want shallow depth of field you use larger aperture (lower number) When you stop down to increase the depth of field it increases more at the back than at the front. This means for example if you are doing a landscape and you want front to back sharpness a very small aperture should be selected say f16 or even f22 and then focus around a third into the scene not at infinity. The depth of field will now give what is called acceptable focus (yes that's the official term) through the entire image.
Now if you are working close and want as much sharp back to front as possible the third front has a tendency to be more like a quarter sometime when very close even less.
Now next thing to think about, the focal length, a telephoto lens has less depth of field at any given aperture than a wide angle lens. Wide angle lenses or the wide end of a zoom are often selected to give this extra depth of field, the opposite can be selected for portraits because there is then less depth of field to give out of focus backgrounds.
Another thing to think about is the size of print or viewing screen, smaller appears sharper through the image, larger can look softer where the depth of field is starting to fall off. Next is viewing distance stand further away and the picture starts to look sharper, a good example is these big poster adverts, from across the street very sharp close up and they are not remotely sharp. The same applies with a print or whatever but on a smaller scale.
This now explains the acceptable sharpness term, it comes down to what information the eyes can gather and the brain can process into what it believes is sharp.
The real fact is wherever you focus there is the only one true sharp point, but in practical terms it can look over all sharp.

Their is even more to the subject but it can become very confusing, best to accept what the eyes see.

Patrick

[ipri]

Re: Where to focus?

Thanks Patrick... a comprehensive answer. Ian