02-27-2017, 05:51 PM
Diffraction limiting will be caused by too small optics for the focal length, resulting in soft images. If you want to stick around f/5.6, then by implication the longer you go, the bigger the aperture gets, and the heavier the optics.
I've attempted to use astronomical scopes at prime focus for terrestrial use, and for any degree of focal length you probably will need a tripod to help stabilise and aim, and hope your subject stays still long enough to manual focus. So it does depend if you will be walking around with it, or would you be fixed in a hide where you can rest the equipment?
I quickly decided 1325mm on an APS-C sensor wasn't fun to hand hold. With my more normal kit, I have hand held a Sigma 120-300/2.8 with 2x for 600mm f/5.6, but that combination was very heavy for the image quality, I found I actually preferred using the 100-400 mk1 instead.
I've not used one myself, but I think one of the Sigma 150-600mm lenses would be a consideration if I wanted a longer zoom these days. The C version is very affordable, and the S is still not going to break the bank, although there may be a bit of risk to your back... Tamron also do similar but I personally wont consider them as their zoom ring is backwards compared to Canon.
I've attempted to use astronomical scopes at prime focus for terrestrial use, and for any degree of focal length you probably will need a tripod to help stabilise and aim, and hope your subject stays still long enough to manual focus. So it does depend if you will be walking around with it, or would you be fixed in a hide where you can rest the equipment?
I quickly decided 1325mm on an APS-C sensor wasn't fun to hand hold. With my more normal kit, I have hand held a Sigma 120-300/2.8 with 2x for 600mm f/5.6, but that combination was very heavy for the image quality, I found I actually preferred using the 100-400 mk1 instead.
I've not used one myself, but I think one of the Sigma 150-600mm lenses would be a consideration if I wanted a longer zoom these days. The C version is very affordable, and the S is still not going to break the bank, although there may be a bit of risk to your back... Tamron also do similar but I personally wont consider them as their zoom ring is backwards compared to Canon.
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