I tested three lens combinations:
- Sigma 400mm alone
- Sigma 400mm with 1.4x Tamron-F teleconverter
- Sigma 600mm alone (well, using FD-EF adapter without the optical element)
Here are the full frame shots (downsampled, click to see larger... but not full-res):
Overview: Sigma 400mm f/5.6 Telemacro HSM
These shots don't really show anything notable for full frame. Yes, the 600mm has less contrast, but we expect that from a mirror lens. The 600mm seems to have more vignetting too, but again, that's expected.
One thing I noticed was that both lenses performed about the same in terms of focusing -- as in, it was a pain in the a**, but in this static situation I had over 50% accuracy on nailing focus. Obviously, the f/5.6 of the 400mm wide open makes the viewfinder brighter, but in this case I didn't find that a brighter viewfinder made focusing any easier.
Next up, a comparison of 100% crops. All were from the center of the shot. These were converted from RAW with 100% sharpening in Bibble and all other settings off.
As expected, the 400mm alone is the sharpest, followed by the 400mm with TC, and the 600mm decently behind. I'd also say the 400mm alone has the best contrast and color; again, this was expected. So, if you don't need 600mm of reach, the 400mm will definitely be your best option.
But what if you do need 600mm of reach? For instance, if you shoot birds, you pretty much need whatever reach you can get. Which lens combination will give you the best reach if you want to see something really tiny or really far away?
I tested this by taking larger portions of each of the images above and upsampling them to all be at the same resolution as the 600mm image. The image below is the composite of these three images (click to see it at 100% crop):
In terms of best reach, the 400mm with 1.4x teleconverter gives the sharpest image. Not by a lot, but you can definitely tell in areas of fine text (like the "invent" in the HP logo). But, on the other hand, the 600mm does slightly outperform the upsampled 400mm. Again, not by a lot, and the 400mm has much better contrast, but I can definitely see more detail on the 600mm image.
Of course, these results are the lower end of 400mm image quality. Although Tamron-Fs aren't that bad, they are a lower-end teleconverter, and a Pro TC should be at least marginally better on a super telephoto. Also, these were shot wide-open; if I could have stopped down the 400mm, it would have sharpened up even more. I'm not exactly sure how much more it would have sharpened up, but I suspect it would be noticeable.
So, in the end, yes, you are much better off with the 400mm lens than the 600mm mirror lens. This was my original hypothesis back when I got the 600mm. Plus, the 400mm lens gives better contrast, bokeh that isn't donut shaped, and autofocus. It seems like the 400mm would be a no-brainer compared to the Sigma 600mm.
Until you look at cost.
The 400mm (without TC) goes for $250+. The 600mm goes for right around $100 if you have to purchase the adapter with it. In fact, mine just sold for $70 including shipping without the adapter. Ultimately, it comes down to how much money you can spend and if you are willing to tolerate the quirks of the 600mm.
Personally, I'm happy to have the 400mm, but the 600mm was a great stepping stone until I could afford it.