change in DOF using 2X converter?

Started Mar 27, 2013 | Discussions thread
brightcolours
Veteran MemberPosts: 9,103
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Lots of nonsense being written.
In reply to ladodger, Mar 28, 2013

ladodger wrote:

how does DOF change when using a TC-20 with 300mm lens.  Is it as simple as calculating DOF without converter and then simply taking half?  For example if DOF without converter is 10 feet in front of subject and 10 feet behind, is new DOF 5 feet in front and 5 feet behind?

There is no such thing as "DOF". It all will depend on print size, distance of viewing and so on.

So "DOF" does not start and stop anywhere, there is only one focal plane that is in focus, and how much in front or behind that seems to be in focus depends on all those factors.

You can, however, talk about when lenses have similar DOF. And that depends on the size of the aperture.

With a 300mm f2.8 lens, you have a 300/2.8 = 107mm aperture.

If you stick a 2x TC on that lens, the FOV changes to that of a 600mm lens. The aperture remains a steady 107mm though. What changes is the focal length / aperture ratio: 600 / 107 = f5.6.

The DOF will be the same (same aperture), the exposure time will not (f5.6 instead of f2.8). And the FOV will not stay the same either (600mm instead of 300mm).

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