DOF and f-stop

[ishyam79]

All,

Just recently bought a DSLR (EOS 450d) and been playing around in creative zone.
Been a digicam user for quite some time and waiting for this for a long long time.

I have one question regarding the inner working of the lens and camera.
We know that a low f-number (large aperture) gives a narrow DOF and vice-versa, What I have failed to understand and find out is "how does that work".

Keeping focal length same, ignoring shutter speed and other variables and decreasing the f-number -> would mean increasing the aperture diameter -> which would mean more light.
-> doesn't this mean that I can see more of the scene ( I might be silly here).

I am sure I am missing some pieces of the puzzle.

Maybe somebody can help.

Thanks,
Shyam

[Elmo]

First to clarify, which you might realize. When you open the lens (smaller number bigger hole) one need to increase the shutter speed so you always get the one single correct amount of light (based on ISO).

When in focus the lens produces a pin point of light (thousands of them all over the frame) at the exact point of focus. As the distance to subject moves away from the focus distance the point of light (circle of confusion) gets bigger. The bigger the opening in the lens the faster the CoC gets bigger (gets out of focus quicker). Therefore the bigger the hole the smaller the DOF.

[acedrew]

I like this resource for this type of info, technical, clear, and concise: f/stops Functions
If you read that and the next 2-3 pages, it should give you a pretty good idea of what's going on. You're basically changing the maximum incident angle of light off the optical axis that the lens will accept. The wider the angle, the narrower the DOF.
If you really want to learn how and why your camera works, read that guide start to finish.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter

[ishyam79]

Thanks. I guess I will use Elmo's information and read the guide at the same time to understand what is going on.
The guide is brilliant, covers lot of topics - going to be a lot of happy reading now and then practicing it on the field.

[acedrew]

Yeah, some of the data is a little old, and there's not much about digital imaging, but we have to remember as much the manufactures like to tell us about advancements, the basics of photography haven't changed in a long time! I like it because it covers everything into minute detail, if you're not a technical person though, you can just skim it and still get the idea.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter

[dcclark]

Here's a quick correction of a misapprehension:

"Opening up the aperture" does indeed let in more light, but that has nothing to do with how much of the scene you can see. The same scene is visible at all times through the aperture -- the light just has to come in through a smaller hole. Nothing is cut out of the scene by narrowing or opening the aperture. So, thinking of it as "showing more of the scene" is incorrect.

[ishyam79]

Thanks David. These are surely eye openers. Quite funny actually considering that these concepts - (often misunderstandings - are not questioned until they are put in practice.

[Samanax]

Originally Posted by ishyam79

I have one question regarding the inner working of the lens and camera.
We know that a low f-number (large aperture) gives a narrow DOF and vice-versa, What I have failed to understand and find out is "how does that work".

It's the Circles Of Confusion.

The Circle Of Confusion

Tutorials: Depth Of Field

Depth Of Field

[filmoreha]

I was just mulling over these same questions myself last night---more specifically, everything f-stop---and found a great explanation of it all here.

A Tedious Explanation of the f/stop

[Samanax]

Originally Posted by filmoreha

I was just mulling over these same questions myself last night---more specifically, everything f-stop---and found a great explanation of it all here.

A Tedious Explanation of the f/stop

Nice find.