Originally posted by VE2CJW Derek, you have clearly described my problem. Most of my experiments were done near sunset or on an overcast day with the geese quite far away on the water, good recipe for disaster. Since I have only 300mm to shoot, I had to crop and the result was worse. Looking at your pictures with the 170-500, I can't say that I'm impressed since they look a lot like most of my cropped pictures. I don't think I'll go with a long zoom but will probably explore longer older primes. Thanks for the clear explanations. I'm including a picture taken with my 55-300 that was cropped to get the effect of 600mm. It's not too bad but I need to get closer but there is a limit to cropping. Take care.
Mike.
Mike it is not easy to tell with web based photos but to me the foreground in the cropped geese shot looks sharper than the birds.
Accurate focus will make a huge difference to a cropped image. If you haven't already, check the focus adjustment/calibration with the lens. With long lenses I also recheck calibration with a target set at the distance I most shoot at, especially close to infinity.
Further improvements in image quality can then be gained through, best aperture for resolution/lens, movement/shutter speed, lower iso = better sensor signal to noise & dynamic range, accurate exposure for no blown highlights or excessive shadow noise. All these things are amplified with a heavy crop and make a huge difference to the final image.
Once again looking at the geese shot, the exif shows 300mm 1/400 f8.0 400iso and I believe taken on a tripod. For example it may be possible to lower the shutter speed, you are stabilized from shake and distant subjects don't show as much movement, 1/200 would gain a stop to then reduce to 200iso.
I believe that most lenses when shot at their ideal parameters will yield good results, the only problem being a very narrow window for those parameters. Looking at the Sigma 135-400 @ 400mm f11 would be similar to your 300mm f8 resolution wise. So you will gain some length but only if the light is better will the quality be close to equal. On the other hand a quality prime like the DA300 has a fairly equal resolution from f4 - f11 and @ f4 should beat the peak f8 of your 55-300, all with an extra 2 stops up your sleeve.
In this instance, the 300/4 gives the best performance/weight ratio and an older 400/5.6 or 400/4 perf/price. For everything else (500/4.5 600/4 500/5.6 etc) there's mastercard.
Cheers, Dré.