New To Photography:Depth of Field and Aperture - why is it so?

Previous: New To Photography:Why do we recommend faster (more expensive) lenses over 'kit' lenses Appendix A - Explanations of Shutter, Aperture, ISO etc Next: New To Photography:Appendix B - Raw (and White balance)
This page is a chapter in the book Appendix A - Explanations of Shutter, Aperture, ISO etc.
This post explains why DoF changes with aperture. You don't really need to know why,
just that it does. We have put this information in the appendix for those who have an
interest in how things work.

To visualise why aperture affects DoF we show how a point source of light projected
through a large and small aperture lens has a smaller or larger distance of sharp focus.

When light is projected in this fashion it is called the depth of focus and is the corollary of
depth of field (imagine the light going the other way).

In the diagram you can see the length of the depth of focus is longer for the small
aperture lens.



In this example the distance of sharp focus is defined by a tolerance of how far the red line
is away from the centre line. This tolerance equates to a technical term called the circle of confusion.
The circle of confusion is used by camera manufacturers in the design and
development of cameras and lenses.

The circle of confusion is a topic you can research in your own time and is beyond the scope of NTP.
Previous: New To Photography:Why do we recommend faster (more expensive) lenses over 'kit' lenses Appendix A - Explanations of Shutter, Aperture, ISO etc Next: New To Photography:Appendix B - Raw (and White balance)

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