DPReview.com is closing April 10th - Find out more

Minimum Focus distance

Started 1 month ago | Discussions thread
bclaff Forum Pro • Posts: 13,922
Re: Effective F#

Chris Wolfgram wrote:

bclaff wrote:

When you focus closer than Infinity your f# changes; this is called the effective f#.

The simple formula, ignoring pupil magnification, is

effective f# = f# * (1 + m)

where m is magnification (note that m is zero at infinity).

When you use an extension tube you increase that maximum magnification of the lens and lower the effective f#. So AF gets less light and doesn't do so well.
Normally this isn't noticeable but your lens is quite slow probably slow enough to be near the AF limit as it is.

Okay, but here's two things to consider...

First. It happens with my 800 F11 (a fixed aperture lens) both with, or without the extender tube. It's just that without the tube, it happens at like 19.6 ft (where the MFD is listed as 19.5ft, and with the tube, it happens at about 14.6ft, where the MFD, is 14.5 ft. I mean, this latter number is not published anywhere, it's just how far I need to be to get the small blue box on a birds eye.

Also, this happens with my Sigma 150-600, both with and without the extension tube, albeit, it has a much shorter MFD to begin with at 11.5 ft.... Or about 6ft with an extension tube.

Sure, but in all those cases consider that higher magnification means higher effective f# means potentially worse AF.

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow