Do a web search for “Canon 2x extender” and you will find quite an assortment of articles, tests and discussions on whether it is worth using. The problem is that adding an extender means adding more glass which means more image degradation. Actually, it stands to reason that the more glass you put in front of the sensor the more the image will degrade. The question I had is whether the amount of degradation was still better than shooting with 1/2 the focal length (without an extender) and cropping to get the same sized image as with the extender. By the way, the tests done here were using the revised Canon 2x II extender.
So the real question here is whether an image shot with the extender can equal the cropped image shot without it. The problem will lie in the number of pixels we need for the resulting final output. If we are going to create a print then we need to think about what size will be required. I’m using and 8.2 MP camera which is probably the lower limit you want to use and perform any cropping. Image size measures 3405 x 2332 pixels. If I shoot an image with a 400mm equivalent focal length using a 2x extender (in this case I am using a 200mm lens) and then shoot the same subject using a 200mm focal length (no extender) how much cropping will be required to achieve an equal size subject on print.
Actually, it’s simple math. I need to increase the size of the image shot at 200mm by 2 times. That doesn’t sounds so bad until you actually crop the image and realize how many pixels you have to throw away!
This screenshot shows the full size 200mm image with a crop rectangle over it. 
That is quite a bit of cropping on an 8 megapixel image. Would there be enough left to create a decent sized print? Would the image quality still be good?
And here is the 200mm image after it was cropped. 
Of course it is quite reduced for this blog page, but I can say that at full size its looks very good on my monitor. I was actually quite surprised. The only problem is that if I were to print this image at 240 dpi it would be limited to a 5 x 7.
Here is 400mm image (taken with the 2x extender). 
It looks surprising comparable to the 200mm image after it was cropped. I do notice some very minor loss of detail, mostly off center (evident on the branch), but after I did this test and started looking at the 2 images I had to keep looking at the original files to remember which was which. But the big advantage with the extender is that I have the full pixel count to print with. So instead of being limited to a 5 x 7 I can now print a much larger 15 x 10. That’s a huge difference.
Now keep in mind that in this test I did some level and sharpening adjustments to get the best possible image just like I would normally if I was to print these. It is interesting to note that although the level adjustments for both images were similar, the sharpening was not. Of course that makes sense when you consider the fact that sharpening works on the analysis of pixels and there are twice as many in one image versus the other. It is interesting to note, however, that once I tweaked them individually for the best possible quality they were almost identical when completed.
Now before I trumpet the use of a 2x extender I must also bring to light… the light it needs. With the 2x extender you lose 2 stops of light. And to improve image quality using the 2x extender I stop it down one more stop. The 200mm image was shot using an F-stop of 4 (on a 2.8 lens) and the 400mm image was taken with an F-stop of 8. That makes a significant difference in shutter speed. The lighting was similar when both images were shot although probably not identical. The 200mm image had 1250 msec while the 400mm image had only 125 msec. That’s a huge difference! In reality there should only be 2 stops of difference due to the 2 stop difference in aperture, but in reality that is what it came out to. If you are shooting relatively stationary subjects with a tripod or good image stabilized lens then it’s not a problem. I guess I was lucky in that there are not many squirrels that are going to be still enough to capture with a 125msec shutter speed.
Bottom line is that the 2x extender does have a usefulness. It does give 2 times the reach when you have lots of light and don’t need a really fast shutter speed. And if you need the reach on a bright sunlite day you will get some beautiful large images. But when the light is not good or you need a fast shutter speed then leave it off and try to get closer.
