Fowl Mouth
Just a guy with dogs
What about a TeleVue Powermate? I wonder how they would operate photographically. Maybe no advantage regarding field curvature?!? At least they are only $300...(only)
Thanks for the test. It's interesting that you were able to improve corner sharpness (field flatness?) in a couple of the barlows by modifying the camera distance.
Imagine the image leaving the barlow as an ever increasing cone. By moving the barlow slightly further from the camera, the cone covering the sensor is enlarged and the softer parts of the image at the edges of the cone can be made to fall outside the edges of the camera sensor. This also has the effect of increasing the magnification because less of the image is now hitting the sensor. It works with teleconverters too. I found with my Kenko 1.4X teleconverter that I can put a 60mm macro tube between it and the camera and the magnification is boosted to 2X.
Maybe I misunderstood, then. Does this change the focus of the corners that still fall inside the edges of the camera sensor, relative to the center?
In theory, by moving the barlow further from the camera you are using more of the sharper central part of the glass. If the barlow produces the right shape cone then you could end up with a perfectly sharp image from corner to corner. Some barlows produce a small cone and some are bigger. A barlow with a small cone is harder to correct because you have to move it a very long way from the camera to see much of a difference. My 1.5X Apo barlow is a bit like that which is why there wasn't much improvement. The GSO produces a wider cone and so it corrected the sharpness much more than the 1.5X apo.
Paul and others -
Thank you for this thread!! I've been considering adding an "Astro" scope to my bird photography kit for several months and have narrowed it down to either a StellarVue or Tele-Vue. Since I have Canon gear and both Canon teleconverters (1.4x & 2x), it was helpful to know that I probably won't have to invest in Barlows or other in-line lens magnifiers right away.
Chris
One of the only telescope shops in my area has a floor sample of a Stellarvue SV102ED for a reasonable price and I'm considering that one. I've looked at their SV80ED Raptor which is one of theirs which uses the carbon fiber tube but the focal length (560mm) is what I can get now with my 400mm f4 DO + 1.4x TC.... I'm really looking for a scope with will give me a high quality image with a magnification factor of 30 - 40x and I think the larger SV102ED will enable me to get there without too many more pieces in the "optical" chain.
I try and shoot as close to f7.1 as possible because the diffraction index on the 7D is f6.8. This is a shot made with the DO and the 2x, and it holds up ok...