amfoto1 Loc: San Jose, Calif. USA

I agree with CHG_Canon... While you can use extension tubes in combination with a teleconverter, it will NOT accomplish anything close to what you want.

In fact, MACRO extension tubes do just the opposite. They increase magnification by making the lens able to focus CLOSER. They do this without optics... by moving the lens farther away from the camera body. Once the extension tube is installed, the lens will no longer be able to focus to infinity, which is precisely OPPOSITE what you want trying photograph the moon and planets!

Teleconverters use optics to increase magnification, essentially changing the lens focal length. A 400mm lens with a 1.4X installed makes for a "560mm combo".

To make the shot of the moon in the background of the composite image below I used a Canon EF 2X II teleconverter on EF 500mm f/4 lens and a 1.6X crop sensor camera (a 1000mm combo that thanks to the crop sensor camera behaves like a 1600mm lens would on full frame). Camera used was a 6MP Canon 10D. The most difficult part was getting a steady shot with so much effective focal length. The moon image was cropped to some extent. I forget exactly how much, but still had to soften the moon image a little for this composite. (Camera used was a Canon 10D, only 6MP that doesn't allow for a lot of cropping!)



There is one small exception to the above info about using extension rings for telephoto work. Due to their design, you can't stack Canon's 1.4X and 2X teleconverters to use them on a lens. The protruding front element of either teleconverter cannot fit into the rear of the other teleconverter. However, it is possible to stack them if a 12mm extension tube is used in between. Many of Canon's modern lenses will still be able to focus to infinity in this case, because the lens is actually designed to be able to focus slightly "past" infinity... i.e., there's no hard "infinity stop" in the focus mechanism (this is done because it's not needed with an autofocus lens and because an infinity stop requires precise optical calibration and in powerful telephotos might be affected by ambient temperature changes).

Below image is a test shot I did with the same EF 500mm f/4 lens, 1.4X II and 2X II "stacked" with 12mm extension between the teleconverters. (A 1400mm combo that the crop sensor camera makes act like a 2240mm lens would on full frame.) The deer in the image is approx. 1/4 mile away. This test shot mage is not cropped. However, image quality was pretty heavily compromised using two teleconverters. It looks "okay" in small size and Internet resolutions. But any larger than how it's shown here, the loss of IQ becomes pretty apparent. I wouldn't print any bigger than maybe 5x7".



Due to the loss of image quality, stacking teleconverters on a long lens is not something I'd use seriously and certainly wouldn't help you with what you're trying to do. Your best bet would be renting a longer focal length lens (Canon 800mm?) to use with a teleconverter, or perhaps renting a high quality telescope and adapting it to be able to use with your camera.

petrochemist Loc: UK

Teleconverters don't change the focusing distance of the lens they're used with, but make the image bigger.
Extension tubes move the lens further from the sensor making it focus closer also making the image bigger.

You can combine both, and will get both effects bigger images & closer focus. The effect of extension is dependant on the focal length of the lens they're used with so the order the two are connected in will change the results. With a long lens the focus shift from extension tubes will be less significant but it's still going to be enough to stop you focusing on subjects miles away. For any sort of astro-photography you need infinity focus so extension is right out!

It's possible confusion might have stemmed from Canon who refer to teleconverters as 'extenders' NOT the same as extension tubes but close enough to potentially cause confusion.