Sigma 300mm f2.8 EX HSM Lens

[tornado33]

Good review there Terry. I see you had the same hassle with me when imaging the megallinic clouds, gradients. Its amazing how noticable gradients are when imaging areas of the sky as large as what a 300mm lens can fit in.

What is focussing an AF lens like? As Bert's 300mm Canon lens is manual focus its relatively easy, there is no play and focus is smooth. An AF Zoom lens I have is hard to manually focus as theres some play in the focussing ring when focussing manually, presumably so the AF motor can operate effectively) My 50mm F1.8 is VERY hard to manually focus as there is noticable play in the (tiny) manual focus ring.
Scott

[CometGuy]

Scott,

I just switch the lens to manual focus and turn the focuser by hand to just past the infinity mark (my modified camera is just a touch front focusing at infinity). I then use then take a series of exposures, previewing them on the camera until I get best focus. I examine star images at the corner as these a more sensitive to small changes in focus than the centre.

I've just added an image of the SMC, which is very quick processing job, but gives you the idea. Coma is clearly worse in this lens than the Canon lens , although correction for chromatic aberation is good with the Sigma.

Terry