Thank you for explaining all that i do appreciate it, its not an easy situation to find an answer to and my lens does not help, not because its rubbish or low on IQ quite the reverse, the 55-250 has really thrown the gauntlet down and is a Brilliant lens in every department except for length
I have been through every lens test and reserched every lens and put them against the 55-250 STM and honestly/truthfully there are not many that are better or even equal, there is just no point putting a longer lens on my camera if the image is going to be closer but less sharp or even out of focus.I have had bridge cameras before one of them had a 600mm eqv lens, but its IQ was very very poor so that is a route i dont want to go down, a scope is no good either as i need to keep what i do portable - just me handholding my camera.
I think a crop frame with perhaps 30mp might be the answer, as i kno my lens can cope with it.
Regards
Pagman

There is a very large gap between the 55-250 STM and any other faster or longer lens versus budget, in regards to similiar IQ. The 120-400 or 150-500 lenses from Sigma just aren't going to be good matches for the 55-250, especially on the 7D. Nor will the 100-400L first version...
The best options, ignoring budget, that will give you IQ and quality are:
Tamron 150-600
Sigma 150-600 (either version)
Canon 100-400L II
Canon 70-200 II with a 2x TC MKIII
(probably missing some other obvious alternative?)
Anything 300mm-ish just isn't really any kind of increase in reach, so that leaves you with 400mm+, or try a 1.4x TC on the 55-250 and see what you can get. For what you are shooting, that combo might actually work.
owns a Lumix FZ1000 bridge camera and he swears by it, he sent me a crop he did of a jpeg as he doesnt kno a lot about cams - RAW etc, i did a tiny bit of work on the jpeg in LR4 and it does look honestly quite good, at least as good as the pics i was getting out of my Nikon d7100 with 70-300VR lens.



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, although what blind deer have to do with it I don't know. Toilet roll tube, poor mans extension tubes, for manual only lenses. Just be careful not to get any added tilt though. Get that Blue Peter Badge.
