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Even though a digital image consists of pixels and it would seem that magnifying such an image is not worthwhile, things are not that straightforward. With good algorithms, like those in Photoshop and other image processing programs, the results are quite good. But only if the original image is sharp.
Hence, a sharp 400mm lens can produce better detail than a soft 500mm lens. By scaling the 400mm image to 125% will produce an image with an equal scale - but with a wider field of view. If the 400mm lens produces 25% sharper images than the 500mm lens, the resulting image is also sharper! And such a difference is not at all uncommon between a prime and a zoom.
Hence, if you are looking for maximum reach at a budget, don't overlook 400mm solutions. Two that come to my mind are the Canon 400f5.6L and 300f4L IS + 1.4x TC. Surprisingly enough, Photozone tests say that the 300+1.4 is sharper at the center than the 400. There could be some sample variation, but in any case, all 500mm zooms end up at the same area, in terms of detail.
Photozone tests show the sharpness in terms of line widths per picture height. If you take into account the focal length, you can calculate "line width per target detail". That makes it possible to compare lenses in terms of actual detail that they resolve, when the camera stays at the same location. This is a typical use case for a long telephoto - for one reason or another, you just are not able to get any closer.
Hence, 300f4L IS + 1.4x resolves like a 382mm prime (at f5.6 which is its best aperture!) while the Tamron 200-500f5-6.3 resolves like a 395mm prime (at f6.3) and 416mm at f11 (the best aperture). The Sigma 50-500f4-6.3 resolves like a 379mm prime at f6.3 and 384mm prime at f8 (the best aperture). Suprisingly enough, the 400f5.6L only resolves like a 335mm prime at f5.6 and 358mm at f11. The scale is based on the 200f2.8L, which is used as a reference. The range f5.6 to f8, where it maxes out, is defined to be the reach of a 200mm prime.
Also, use a 1.6x crop camera, with the 400D/XTi being the best in terms of detail - but then you need a sharp prime to start with. However, if the speed and accuracy of autofocus is important to you, then pick a 30D. It has a slight advantage in low light, according to my experiences.
Hence, if absolute reach is important for you, pick a 400/420mm prime solution. If you need a zoom, the Tamron 200-500 seems to be the sharpest available choice.
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