Using extension tubes with teleconvers

Many of us use teleconverters to get a bit of extra reach. However, extension tubes, when combined with teleconverters allow us to get just that bit more reach. They say a picture tells a thousand words, so here we go.

The gear:
Sony A850
Minolta 1.4 x APO TC (original)
Teleplus 2 x MxAF TC MC7
Kenko extension tubes (12mm, 20mm and 36mm)

Method:
I set up a test subject around 10 metres from the camera.
Tripod used.
Camera set to Aperture priority, set to shoot at maximum aperture.
ISO set to 200
Autofocus

The Results:
Converted from RAW to JPG, then downsized to 1024px across, a dash of smart sharpen.

300mm
f4 1/8000


300mm + 1.4TC
f5.6 1/4000


300mm + 1.4TC + 12mm tube
f5.6 1/2000


300mm + 1.4TC + 20mm tube
f5.6 1/500


300mm + 1.4TC + 36mm tube
Will not focus - image is beyond the reach of focus limit.


300mm + 2TC
f8 1/500


300mm + 2TC + 12mm tube
f8 1/320


300mm + 2TC + 20mm tube
f8 1/320


300mm + 2TC + 36mm tube
Will not focus - image is beyond the reach of focus limit.

300mm + 1.4TC + 2TC
f11 1/80 (Camera would not autofocus - manual focus used)



The results don't really need much explanation. However, I must add that the lighting was changing as I was swapping converters and tubes, so the shutter speeds may not be reflecting the true situation. The tubes do "steal" a bit of light, however they do not affect the maximum aperture that you can use.

I'll also add that you will lose infinity focus when you use extension tubes. The combination of 300mm lens + 1.4TC + 12mm tubes will focus to 20 metres. Using the 300mm + 1.4TC + 20mm tubes will five you focus up to around 10 metres.

Note that since extension tubes contain no glass, they will not degrade image quality.

I hope this demonstration was useful.

Cheers, Frank

Hi Alex

I used the Lens, TC, Tubes configuration. You can't put the APO TC's between the tubes and the camera. Won't fit because of the design of the APO TC's. However, the Teleplus TC is able to be fitted either before or after the tubes. I went for the similar configuration to the Min TC for this test.

I must admit that I didn't even consider that quality could be affected by the order that they're stacked. If I have time tomorrow, I'll give it a try.

Cheers, Frank

I always thought adding ex.tube between the lens and camera will not change the magnification.At least no more then shifting the lens closer to the subject by the size of the ex.tube-s 12-20-36mm.In your case the lens is fixed to the tripod while keeping the 10m distance, so you actually moved the camera from the subject,or?
Perhaps somebody can explain to me where do I go wrong?

Edited by Pavel - 31 May 2011 at 14:09

Pavel - No, the camera was not moved during the exercise. Interestingly, the focus distance scale on the lens does show a "shift" as I increase the size of the extension tubes. This verifies what Alex has written above.

Cheers, Frank

Tubes do 3 things, all by moving the lens further away from the body:

1. Increase magnification.
2. Decrease minimum focus distance.
3. Decrease DOF.
The longer the tube, the more change you get. The longer the lens, the less change you get.
You also lose infinity focus and some light, the longer the tube the more light loss.
Adding the TCs means more light loss and magnification, but I don't think they change the MFD or DOF.
There may be some kind of chart out there that lists characteristics for different combinations, but all I really know to do is experiment and try to remember which combo is good for what.   

I understand that teleconverters ´steal´light due to the extra glass. But surely extension tubes without additional optical obstructions should maintain the amount of light... or have I missed something?

BTW, the idea of mixing tubes and TC is pretty cool. I have both but assumed that tubes were just for macro, but this proves me wrong.

berlin steve wrote: I understand that teleconverters ´steal´light due to the extra glass. But surely extension tubes without additional optical obstructions should maintain the amount of light... or have I missed something?

Steve:

Extending the lens from the sensor decreases the light available to the sensor due to an inverse square relationship (the projected area covered increases at the square of the distance extended and thus the light available to a given area is reduced). So there is definitely an exposure penalty with extension tubes. I believe teleconverters "use more light" for a similar reason in that they are essentially "beam expanders", though that expanded area may be cut off as part of the teleconverter design. The loss due to light transmission of the glass is actually a small part.

Pete

Edited by Pete Ganzel - 01 June 2011 at 23:18