Re: Which Lens?
Yes, I sort of agree with that.
Now sorry for my usual lecture.
Long focus lenses are hard to use - very hard. Almost everyone I know rushes out to buy one and is then disappointed at first and the reason is poor technique. People hand hold inappropriately and expect razor sharp images. Even atmospheric haze can have a negative impact. People use maximum aperture and expect the lens to be at its best - and so on.
But there's a lot of balderdash being spoken in other responses too.
In general, slower lenses are as good at their full aperture as faster ones are when stopped down that far. If you are going to work at f/8 I can't see any reason for spending 4-5 times as much on a f/2.8 lens as on a f/5.6 lens. With the high sensitivity of modern cameras, unless you're working in very poor light, you simply don't need a superfast supertele lens. Even if money isn't a concern, weight should be. A heavy lens also needs a much heavier tripod than most people carry - the weight limit of the tripod is based on lenses of normal focal length and needs to be significantly reduced when the magnification factor introduced by a supertele lens is considered.
Mirror lenses don't have a variable aperture, it's true, but if they're fixed at f/8 that's just about perfect. (It's also around the point where sensor resolution and diffraction limiting meet in modern DSLRs.) You simply don't need filters for nature photography, so that point is moot. There are very significant advantages in bulk and weight over a "normal" lens with the same specification and the lower price is a major, major advantage. There is a significant disadvantage - the rather nasty "halo" bokeh caused by the central obstruction - but nevertheless it is perfectly possible to make good images with a good mirror lens.
In my somewhat limited experience of nature photography, autofocus is far more of a hinderance than a help - slowing down the response of the camera and more or less ensuring that you miss the shot. Manual prefocus rules.
The big issues here are atmospheric turbulence and mount vibration. Tube currents (caused by differential heating inside the lens) can also be a major issue if the lens is in direct sunlight ... try insulating the outside of the lens tube with several layers of cooking foil. If you're using a supertele lens with image stabilisation / vibration reduction on a tripod, try ignoring the rules and switching the stabilisation on. Otherwise get a heavier tripod. (The tripod alone for my 2800mm f/10 mirror lens, sorry scope, weighs 17 Kg and is if anything a bit on the light side).
But you'll probably do better if you locate the camera closer, using a short tele or standard lens and a remote controller to avoid having to get closer yourself, than if you simply try to use focal length to reel the target in. In astronomy we don't have that option, so focal length for image scale and aperture for resolution rule ... provided diffraction limited optical quality is maintained.
Finally, I don't see any justification whatsoever for using a zoom lens for this sort of work. A prime lens is pretty well bound to perform better than a zoom lens costing twice as much with the same aperture and the same focal length at its "long" end does.
I don't know how much of the prejudice against mirror lenses is due to people trying to justify the £5000+ f/2.8 supertele lens that they've bought, but I'm sure it's a factor. Results should be the acid test.
Last edited by beejaybee; 16-03-2012 at 08:48.
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