A question about focus

was'nt sure where to post this - so move it where ever if you want.

You might think abit of a stupied question But i'll ask anyway - I took this picture today, and just wondered why the front part of the bush/flower what ever it is would have blurred! when the back part came out ok'ish'.




Got this one to which maybe seems a little better? But still not perfect

In the first shot - the nearest branch is out of focus as it's much nearer than the one (just above) that you've focussed on.

In the second shot - there's less distance between the nearest and furthest branch so you've less difference in terms of focus.

In simple terms - To maximise how much 'depth' is in focus - use a smaller aperture setting - however for correct exposure a smaller aperture will require a longer shutter speed which might result in motion blur.

Focus falls back.
So if you focus on a banch, then some of the ones behind it may be in focus (depending on your f number) but the ones in front will not be in focus.
I think!

If you had wanted all the branches in the first one in focus then you would have needed to focus on the closest one then narrowed your aperture (higher f number) so that more depth was in focus.
I think!

We can apply this to people. Focus on the closest person and make sure your aperture is narrow enough to catch the other people.
I say this - but I am rubbish at taking pictures of more than one person as I can never get the focus right!

The laws of optics is that the DOF extends from 1/3rd in front of the point focused on, to 2/3rds behind it. In other words, you have twice as much DOF behind your point of focus than in front of it. Therefore there is sharpness in front of the focus point as well as behind it.

If using a DSLR you should use the depth of focus preview button on the camera. A much forgotten but very useful tool.

Steve

DOF = Depth of Field. Steve is absolutely right to say about the third front two thirds back rule. Depth of field is controlled by three things 1) The aperture you are using - wider apertures (lower numbers) gives you less DoF 2) The focal length of the lens - generally lesser focal lengths will give you greater DoF 3) The subject to camera distance - the closer you are the less DoF you have


An experiment you maybe woul like to try assuming you have a car with a long bonnet such as a Focus/Astra/Mondeo and not a flat front like some people carriers. On a day when the weather is a bit dull park your car with a fair bit of space to the front and drivers side. Using a standard lens that came with your camera set the focal lenth to approx 30-40mm. Now walk away from the car at 45 degrees to the front drivers side. Go far enough away so that when you look through the viewfinder you can see all the front of the car across the frame and all the way to the back end of the car and maybe even a little behind. Set your camera on a tripod or a wall so that it is rock steady. Ideally set your camera to manual focus so you have control and not the camera. Looking through the viewfinder carefully focus on the pillar that runs down the side of the windscreen (the windsreen will be to the right of this and the drivers door to the left).

When you look through the viewfinder you should see that this is focussed sharp but already you can see the car drops out of focus as goes off the back and right at the front. If you set your camera to 'aperture priority' so you control the aperture, the camera will control the shutter speed. Take a series of pictures starting with the widest aperture maybe on a typical zoom f3.5/f4.5. Then take one at the next aperture f4.5, f5.6, f8, f11. As their is a fixed amount of light the camera needs for your picture, as you change aperture, the shutter speed correspondingly changes and in this case will become slower to let in the same amount of light but over a longer period.

When you have finished have a look at the finished results on screen. At widest aperture, the pillar and area immediatly surrounding it is in focus. As your aperture decreases (the number gets higher) you will see the bonnet and eventually all the front right across to the other side get sharper. The same as the car extends back. By f8 or f11 even behind the car is probably sharp. It should be that this depth of field increases one-third to the front for every two thirds going back.

If you want to see the effect close up put a spaced out row of oranges, apples or mugs on a table. At a slight angle focus on the second or third mug. As you decrease the aperture you can see the one at the front and those further behind become in focus.

Sorry for the long diatribe, but if anyone is still reading, getting control of your picture so people look at what you want them to is one of the biggest issues in creating your photograph. That's the art bit. The exposure is merely a functional technicality to get to the art in most cases. I don't mean it to try and be clever, but if you look at my entry to photo of the month, I have used shallow depth of field to focus on the man in the mirror. Phil has used it on his landscape shot posted today because he wants you to see everything in focus as most landscapes do. I used f4.5 or thereabouts, phil probably used f11 or f16.

I really hope this helps. If not I'll shut up

I just realised as this tends to be a dog-focussed forum... stand your dog at 45' and focus on the nearest eye... you will have to be at f8 before the dog is in focus nose to tail...

Further to what everyone else has said, check out this guide on Depth of Field

Excellent explaination BC.

Steve

Ok thanks all ~ Think i understand, Will get practising