" I am getting results at 800mm (1280mm with 1.6 crop factor of the smaller sensor), that are, I think sharper than hand held at 400mm (640mm) with the lens only."
I ended up buying both the 2x extender and new 100-400 lens and it turned out to be a good decision. In short, the image stabilization on the mark II lens is indeed amazing even with the 2x extender and well worth the investment.
I had read numerous online posts from "experts" that an extender will make the images look soft and that you will lose contrast. "They" also said you could get just as good a result by not using the extender and blowing up your images by 2x. I decided to put this to the test. I want to be able to take wildlife headshots that show whisker detail from far enough away that there's no danger of getting my face ripped off or getting gored. So I set up a resolution test chart 75 yards away in a somewhat open forest that had dappled light, probably the worst conditions that I could expect to be successful in. I then took multiple photos and varied the following: Old and new lens, no extender, 1.4x and 2x extenders, f stop, ISO, tripod vs. hand held. Shutter speeds were slow at reasonable ISO settings due to the light and gave a good test of real world conditions. I found the following:
1.Mounted on a secure tripod and without an extender both the old and new lens gave approximately the same maximum resolution (about 12 lines per inch) at 75 yards, the resolution limit is driven by the number of pixels in the camera, not the lens.
2. When using the extender, even when mounted on a tripod, you will need to set the shutter to high speed continuous and take 4 to 6 images so that you get at least a couple good images. Breezes and shutter vibration will cause some movement during exposure.
3. The 2x extender was more useful than the 1.4x extender. With a 1.4x extender and the lens set to 400mm, I could resolve about 20 lines per inch, with the 2x I could get up to about 25 lines per inch when mounted on a tripod.
4. The real difference between the two lenses is the image stabilization. A hand held shot with the old lens at 800mm was laughable; it was just one big blur. The image stabilization with the new lens makes hand held shots possible if you are leaning against a support with your elbows tucked in and control your breathing (or rest the lens on support). You will need to set the shutter to high speed continuous and take 5 to 10 images. I got about 50% that looked at least OK and about 20% that were really good even with shutter speed under 1/100.
Here is a jpg copy of a test shot I did handheld. It’s lost some resolution in the jpg conversion process but you can get the idea. This was the best of a half dozen images and was shot at 1/50, f11, ISO 320. The target is 20” across so you can think of it as a head shot of a mountain goat or grizzly bear. It’s good enough for printing to 8x10 or smaller or for use on the web. I found the best tradeoff of ISO, f stop, and shutter speed at 800mm to be ISO 320, f11, and as fast a speed as lighting will permit. That was true whether handheld or using a tripod.
As you can tell from the resolution chart, I won't be getting any bear photos showing whisker detail any time soon, even 25 lines per inch won't capture that.
