Depth of field

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Tin man

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Tom

I'm at my daughters in NJ will be home this evening face a question ref depth F2.5 have to get the book.  Seems my lens does not allow what the book tells me to do. When I look for the F stop it does not gothat low.

Later. Will beon the train later..

Jim

Trying to get background and foreground out of focus.
 
Tin man said:
Tom

I'm at my daughters in NJ will be home this evening face a question ref depth F2.5 have to get the book.  Seems my lens does not allow what the book tells me to do. When I look for the F stop it does not gothat low.

Later. Will beon the train later..

Jim

Trying to get background and foreground out of focus.
Click to expand...
Every lens is rated by their maximum aperture. An f stop is the size of the opening divided into the length of the lens. So lets say you have a 100 mm lens and the opening (aperture) is 50 mm wide. Divide 50 into 100 and the answer is 2. Therefore the f stop for this lens is f/2. Another way to think of it is that f/2 is an algebraic equation. F stands for focal length. So substitute the focal length (100) for f and the equation becomes 100/2 which equals 50, the aperture of the lens.

Every time you focus a camera on something there is an area in front of and behind the subject that is also in focus. That is called the "depth of field" of dof. If you want to have the foreground blurred and the background blurred that means you want a shallow dof. There are three ways to get a shallow dof. The first is the one you are struggling with. The larger the aperture the shallower the depth of field will be. Now your lens doesn't go down to f/2.5, that's why you can't find it. The bigger the aperture of a lens the more expensive the lens is. You are probably shooting with a kit lens that only goes to f/3.5 or so. So open your lens to f/3.5 to get the maximum dof you can get with that lens.

The second way is to have the subject very close to you. And the third way is to use a telephoto lens.

So if you get a 200 mm telephoto, put the subject very close to you and open the lens all the way up then you will completely blur both background and foreground. 
 
Tom

Thank you for the info, I have not used a 35MM in over 35 years. Starting all over........I know its not a 35mm

Jim

Have to hunt down the Rand McNally problem
 
Of course it is not a 35 mm but photographic theory still remains the same whether it be film or digital.

On the other hand if you want everything in the photo to be in focus (large dof) then use a wide angle lens, stop it down to something like f/16 and focus on something that is not very close.
 
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