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Message: <blockquote data-quote="Paul Corfield" data-source="post: 1244992" data-attributes="member: 19132">My scope is a 600mm F7.5 but with a 2X converter it goes to F15 and if I treble the power it goes to F22.5 etc. The shutter speed isn't that much of an issue. It's dull and cloudy here today and with a 2x converter (1200mm F15) I'm at 1/100 sec ISO800 in my back garden, probably be higher if I was out in the open. On a bright day the speed would be well up.Only thing to think about if using a mirror lens like a Mak across water is that any sparkly highlights on the water will look like little rings. The photo below is an example of what I mean.Post some ideas of scopes you are looking at before you buy.Paul.</blockquote>