Full Version: Astromical lens 560mm F 7.1 fixed
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While buying a S/H  FA* 300mm F4.5 yesterday the guy showed me his pride and joy lens the monster M42 Optic 560 mm F 7.1 fixed, which he uses for birding, This lens is only a 2 element couplet design that sells for 475 euros . When he showed me the photos I was gob smacked, approx. 20x30" prints, JPGs only, supersharp  great colours,great bokeh,competition winners!  He then mounted my K5 on it and I took a couple of photos of flowers to look at home. Hear is the JPG (with  little contrast boost but nothing else) The irony is that he has just switched to a Nikon D7100 with and yes, a Sigma 300mm F2.8 and he was obviously despairing at where his sharpness had gone with his new system. He looked very perturbed actually.  


I'm confused - and never really taken the time to understand optics (though I did take physics in college). How does 2 elements produce such a lovely image ?

Astronomical optics tend to be optimised to produce good results at infinity only. They don't have to worry about close focus. Doublet designs give some degree of CA and other aberration correction, and the more exotic the glass used, the better that can get. They can also use more elements too, with the price rapidly rising as the complexity does.

Out of interest, which Sigma 300/2.8 was it? The 120-300 OS isn't critically sharp at f/2.8 but does improve stopped down a bit. I do find AF/OS of a lens to be very useful though, so astronomical scopes wouldn't be my choice unless I had to camp on a single spot.

I've got a 600mm f/5 I can fish out with examples if you want an example of a budget scope (current typical price £250 with basic manual mount/tripod).
Sorry Popo I can't tell you what model his new Sigma was other than it was a 300mm 2.8 with a monster front element, both  of his lenses were in camouflage jackets. However there was no doubt about the quality of his images. The image posted here doesn't do the lens at all justice.


Dave, if you are curious about the telescope: in astronomy, they specify the telecopes by their aperture size in mm. Therefore, you have to look out for an 560mm:7.1=80mm type refractor. If you google "80/560 telescope", you will find some in the price class you meantioned.


By my own experiences, the optical performance is not to be underestimated. The drawback is size, weight and max. aperture when compared to proffesional prime lenses.

 Hi Chris ,

                thanks for your input, I am raising the subject because I'm frankly surprised at fact that these telescope lenses produce such sharp images from such simple construction and seemingly so little is known about them. From the practical standpoint,as you say they are very limited by their long and unwieldy dimensions but given that Pentax brought out an equally long and unwieldy lens for 7,000 euros with only 3/4 of a stop more and AF  to better them, it seems that telescope lenses can have their place amongst those who do serious birding on a more reasonable budget.  

Last update, the guy phoned me to say that he has now adjusted the AF and is now very impressed his D7100 and Sigma 300mm 2.8 combination. All's well that ends well!