Extension Tubes

[RogersDA]

I've been playing around with some Kenko extension tubes lately. I have using them individually and stacked. For kicks I put the stack in combination with the 28-300L (f/3.6-f/5.6).

Smallest aperture setting on this lens is f/22. However, when stacked with the tubes the smallest aperture shows as f/40. Why? I assume that there will be a lot of optical distortion at f/40? I am stilling shooting raw and the computer I have access to does not allow me to process images at all.

Also, is there any way to get the autofocus to work better? With this lens (or the 17-40 or the 24-70) the autofocus is constantly searching (sounds like a rapid oscillation about a very narrow range) when one or more of the tubes is attached. I understand the the Canon requires f/5.6 or wider for enabling autofocus. Do the tubes cause the lenses (such as the 28-300) to loose autofocus when extended to 300mm? That is, at 300 mm does the widest aperture go from f/5.6 to say, f/8, when one or more of the tubes are attached?

[ziggy53]

David,

I moved this thread to the "Accessories" forum where I think it is a better fit for the topic.

An extension tube is nothing more than a device to position the lens further from the imager or film. The effect is to enlarge the size of the image on the image plane and therefore magnify the image. Since there is no optic involved, the image "cone" is simply allowed to expand.

Since the projected (real) image is now spread over a larger area, the photons per unit area are reduced. The net effect is magnification at the expense of brightness.

Most folks don't use autofocus for either macro or very close focus, rather they use a fixed focus on the lens and then vary the distance from the front of the lens to the subject by moving the camera with respect to the subject. You can even purchase a focusing "rail" (or "rack") to assist. the process.

An example:

http://www.bhphotovideo.com/c/product/379022-REG/Novoflex_CASTELCROSSQ_Cross_Q_Double_Rail_Focusing.html