AR Farm Gal Loc: Piggott, AR (NE AR)

I've been experimenting taking some moon and planet photos the last night or so with my Canon 7D MK ii and 100-400 IS II lens. That has worked pretty well, but not enough reach for Saturn (plus my lack of experience). In the night I wondered whether I could add my set of 3 Kenko extension tubes to that combine. When I tried that this morning, I could not get it to focus, either manually or autofocus. I tried mounting the tubes to the camera, then to the TC and lens. When that didn't work, I tried mounting the TC to the camera, then the tubes, then the lens. My mind tells me that should work, but I failed to accomplish this. Am I doing something wrong, or is that just not possible?

CHG_CANON Loc: the Windy City

Extension tubes exist to give the attached lens the ability to focus closer, not further away. This is a macro ability, not a telescope. When adding a tube of any length of any lens, the camera / lens ability to focus to infinity is removed.

DWU2 Loc: Phoenix Arizona area

CHG Canon is right - extension tubes are not the same as teleextenders. The former contain no glass; the latter has a glass element.

bleirer

The way I understand things, and I'll be corrected if I have it backwards, is that when you focus at infinity you are moving the focusing lens closer to the sensor. When you focus close up you are moving the focusing lens farther away from the sensor. An extension tube lets you move the focusing lens even further away, meaning the camera can be closer and still be in focus. So extension tubes work because they allow you to move your camera physically closer to the subject, not because they increase the focal length.

Teleconverters and zoom lenses work the opposite. You move that part of the lens farther away to make the image bigger on the sensor, but that is moving a different part of the lens, not the focusing part.

CHG_CANON Loc: the Windy City

More exactly regarding teleconverters / tele extenders, they magnify the image from the back of the host lens over the sensor. The resulting image is like a cropped sensor and an full-frame lens. In both situations, the 'size' of the focused image over the sensor is a circle larger than the dimension of the sensor. The resulting image is the field of view equivalent of a longer focal length lens. For examples, the same physical 2x extender doesn't add / cannot add 100mm to a 100mm lens and then 500mm to a 500mm lens. They simply magnify the image projected onto the sensor by a factor of 2x. When focused and cropped by the sensor from the larger projected circle of light, the FOV is that of a longer focal length lens.

bleirer

It's interesting for me to read about and even partially understand. I know when I look at diagrams of all the lens elements in a telephoto some elements are shaped to converge and some to diverge/magnify, so in a way of thinking all telephotos have converters already built in even to the point that they can be physically shorter than their focal length, and when you add the converter it's like adding another group of lens elements. I believe the net effect of all that is exactly the same as physically moving the nodal point away from the sensor.

I know userid is an expert in this, maybe he will tell us what for.

AR Farm Gal Loc: Piggott, AR (NE AR)

Ok! I get it now. Thank you so much for clearing that up for me. I haven't worked much with extension tubes and then only with some macro shots. I didn't understand the physics of it. I appreciate your help.

Ourspolair

Maybe I am missing something here... Although it is unconventional, wouldn't adding an extension tube to the lens + a 1.4x "magnifier" simply shorten the focusing distance of the combo and change it's effective aperture?

bleirer

Sure, but she is shooting the moon!

gvarner Loc: Central Oregon Coast

Not possible to focus at infinity with that combo.

BobHartung Loc: Bettendorf, IA

Check out the work of Maxis Gamez from Florida. I've done a couple of workshops with him and have seen him use two 1.4 tele-extenders with an extension tube sandwiched between on his Canon gear. He was using an 800 mm lens so he was over a 1000mm equivalent. You might want to drop him an email if you want to confirm what he has done. While you are on his web site you might want to check out his bird photographs–top notch IMHO.

JBRIII

If you have access to any of the long fl, cheap, simple telescope looking lens try one. My wife and I chase solar eclipse and found one she had for a Pentax from decades ago. For an eclipse, it was much lighter and worked well. Amazon sells these that go out to 2000. Mine is 500 fixed which on a Canon 70D, the sun covers about half the sensor, leaving room for corona during totality. Planets would probably still be pretty small at 500, but I think some bands on Jupiter and Saturn's rings would show.
Finally, I believe someone recently said here that the 2000 Fl versions did not perform well, but if you can borrow one?

Nicholas J DeSciose

This was very good advice

JBRIII

Another note, you did not mention (I believe) exactly what your problems were with the planets. The moon is very bright compared to objects other than the sun. While Jupiter is much brighter you may still need to do some long exposures, however without a guide system, limit this to maybe 10 secs, you need to experiment again.
Also, Jupiter and Saturn are now very low on the horizon, where I am not even visible after it gets dark. This presents two problems: 1. the twilight may overwhelm things, 2. and probably more important, you are looking thru a lot of atmosphere which causes all kinds of distortion problems, especially as the time lapse gets longer and longer. I have a Stellina automated photo telescope (camera only, no eyepiece possible) and by the time it gets dark enough for the scope to calibrated, the two giant planets are below the horizon. Astro photographers now use a lot of video for planets, take thousands and thousands of frames, then us software which can select the best by various criteria and stack them. Even then there is the problem that unlike many objects, Jupiter rotates in something like 10 hours, so even over the period of half an hour, what you photograph had the start is not the same at the end for the edges of the planet, if my numbers are right, in 30 minutes, 5 percent of each side, east and west would either be new or disappear into the dark. Some stacking software for astronomy also deals with this. The non-astro stacking software I have played with, does not.

Finally, while very interesting, as others have noted on this site over the last year I have been attending, ASTROPHOTOGRAPHY is a subject onto itself, I believe there is even a forum here just for that.

sippyjug104 Loc: Missouri

My passion is extreme macro and at times I pair a reverse mounted lens with a 2X teleconverter and extension tubes....not to extend the focal length but to increase the apparent amount of magnification.

HOWEVER....adding glass onto other glass has its disadvantages of losing light as well as creating halos and artifacts and diffraction so although I can make it work there are far better solutions.