When people talk about a lens being soft at a certain focal length, what exactly does it mean??
Can it generally be seen in a normal size photograph or is not visible until image is blown up or is it just seen from pixel peeping!!
Would be nice to see an example of a soft and sharp image to see the difference and wether Jo blogs doing it for a hobby photographer needs to worry about it!!!
1. That a zoom lens will be sharper at some focal length settings (or a range of settings) than at others. It seems quite common that consumer level zooms are typically softer at the long end, but even professional grade (or expensive at least) lenses can still suffer the same issues.
2. Define "normal size photo". A 6x4 print as you might have once got in the old days would rarely reveal problems unless it was taken from a heavily cropped original. But print at 12x8 or 19x13 for the wall, for example, or larger still for a gallery and you might find the quality to be unacceptable. Pixel peeping is kind of like making a 30x20 print, or maybe even larger, and then viewing it from ridiculously close, which can reveal flaws you might not see at more conventional presentation sizes or viewing distances. However, if you want to make a judgement on lens quality, such as when buying one
second hand, I personally would want to see 100% crops without edits so that I could make a fair evaluation. Tiny little 800 pixel webshots can conceal a multitude of sins.
I seldom print, with most of my photos being displayed on a 40" 1920x1080 TV or 17" 1920x1080 laptop, so I need my pictures to be of sufficient quality to produce files which can fill those screens in one or both dimensions and still look sharp and clear. If they don't then they are of no use to me.
3. Here are two photos from my 100-400 at 400mm and f/5.6. The first one I'll only show at 100%, because it is plainly sharp to begin with, sharper still with a little tweaking, but would make a near useless photo when shrunk to 800 pixels....
The second example is with quite similar settings, but with my 7D instead of my 5D2. At 100% and before edits it is an abomination....
But give it a few tweaks and shrink it and you could almost call it usable....
I don't know why it was so crap to begin with - shake (the shutter speed is slower and the magnification of the 7D file higher), misfocus, atmospheric muck - maybe a combination of all the above or something else, but as I said earlier, you might think it was a decent photo if you didn't know the ugly mess to begin with.
You can do an awful lot more with a properly sharp image in the first place, like crop it mercilessly or print/display it as huge size, but with a ropey image to begin with your options will be pretty limited. That's why I'd only be interested in 100% crops when judging a lens. An 800 pixel wide web shot is not going to tell me much at all.
My own experience of the 100-400 is that it does sharpen up further when stopped down, with the optimum being at around f/9, but when you're trying to shoot sports/action in low light f/9 isn't really the answer, so if those things interest you you really need the lens to be sharp (enough) wide open and at all focal lengths - or accept that you might not be able to make very large prints/displays from them.
Oh, sod it, here's the first image, full frame resized to 800x533. Enjoy.
