Monday, December 13, 2010

Distance and DOF

In the previous post we discussed DOF and I mentioned that the F Stop is only one reason for the blur effect in your photos.  Distance also plays a major part.  So looking at the photo of the headphones again, you can see that in image 2 we have the headphones nice and sharp but the table blurs as it goes into the distance. 

There is nothing wrong with this, in fact it places more emphasis on the headphones, where I want you to look anyway.  More to the point, it’s aesthetically more pleasing to look at

Ok so getting back onto the subject.  Take a look at the diagram I have drawn up, look very carefully at it and try to understand what’s going on.  Don’t worry, I’m going to explain it anyway below!

Looking at the image you can see the camera and, represented with grey lines, the viewing angle you would see if you were looking through the camera.  The camera is pointed at an object (in this case a sculpture).  The camera is simulating a similar angle I used to take the photo of the headphones.
The blue dotted line represents the Focal Plane.  This is the point on the subject that we are focused on.  The sharpest point of focus on the resulting image.  Remember the focal plane is always parallel to the image plane (the recording plane, film or ccd chip).
The two purple lines represent the F Stop we have chosen, in this instance it’s f/8.

The measurements are not accurate nor are they important for what I am trying to explain to you.  F Stops and distance charts can be found all over the internet if you are interested further

When we focus on an image, the F Stop represent the sharpest point on the image and depending on what F Stop number we have chosen, the image will be sharp an equal distance behind and in front of that point of focus.  If you were to stop down to f/4 then the lines would come closer together reducing the amount that is in focus.

Take a look at the image below.  This should help you understand F Stops and Distances.  Once again I should remind you that the amount the lines are apart is purely arbitrary.  I’m just explaining to you what, in theory, is happening when you use the different F Stops.

It’s much easier to understand what’s going on now that the camera is level with the ground.
Once again the camera is pointed at a subject, a stylish man about town.  The blue dotted line represents the point of focus. 
As you can see there are groups of lines spaced equally apart from the point of focus.  (Equally in front and behind the subject)

You can see that I’ve clearly coloured and marked the lines as:-

Yellow       f/1.4       very close together
Green       f/2.8
Orange     f/5.6
Purple       f/8          far apart

To reiterate what we learned in an earlier post, the higher the F Stop the greater the amount of focus (sharpness) in your image.  When I say ‘greater’ I’m speaking of the distance in space between you and the subject you are photographing.  Look at the above diagram again, if it didn’t make much sense before, it might do now.

What is that funny looking symbol that looks like a number 8 on it’s side?  Well that symbol represents infinite space.  On my diagram I have shown the F Stops up to f/8.  Lenses normally go up to f/16 and some go even higher.  So why did I stop at f/8?  Well first of all, I’m not being very technically accurate, I’m just trying to explain the fundamental principles.  Secondly, if you stop down to f/8 you’re image is going to be pretty sharp, beyond that your image will be infinitely sharp.  The sharpness of the final result really depends on the quality of the lens you have.

If you have any questions about this post then please leave a comment and I can help you further.

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