What is a "Sharp" lens ???

Matty777

Matty777

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Whilst it may seem a strange question that has an obvious answer, but what makes people say "Mine's a SHARP copy" ??

What I mean is, I know if an image is clear or blurred but at what level magnification is considered sharp.
I have just bought a Sigma 17-50 f2.8 EX DC OS, it's renowned for being sharp. I have taken some sample shots and they look great (mostly for my level...lol) They look sharp at fill screen (21") sort of view, but look kind of soft at "Actual Size", where pretty much one eye almost fills the screen (18mp)

So is sharp at "viewable" shrink or at actual size?? Anyone care to enlighten me as to at what point sharp gives way to softening????
And exactly what is a 100% crop....is it a crop of image that is actual size and fits say 800x600 ???

Thanks, I may have worded all that clumsily! :thinking:
 
Also forgot to add, is it just about front or back focusing??

I did a test with the usual coloured pencils, low angle, full zoom, f2.8 and it did focus clearly on the one I chose, so doesn't look as it it has any front/back focusing issues.

Just more what should I expect??
 
If the image is still clearly defined and sharp when zoomed right in it's a sharp lens :) Put simply, can you count the eye lashes on someone thats in focus? Usually, using a lenses widest aperture gives a softer image than stopping it down a bit (ie. using f4 instead of f2.8). As you use more lenses you'll see the difference.
There can be slight differences between lenses of the same model, some better than others, due to manufacturing tolerances. Add to this the variations between cameras and results may vary (slightly).
100% crop. When viewing a full sized image it needs to be squashed so it can all fit on the screen. When posting online, some sites may resize or compress the image reducing image quality. If you zoom in on the eye and crop you may get an image of ~500x700 pixles. This can be viewed full size and web browsers won't need to squash it to fit. Basically its cropping (not resizing) so the bit you see can be viewed at 100% on screen.
You've got the front/back focussing bit covered, Front focusing is when the focus point is in front (closer to camera) than where it should be. Back focussing is where it focusses behind what you focussed on.
 
Andy_S said:
Front focusing is when the focus point is in front (closer to camera) than where it should be. Back focussing is where it focusses behind what you focussed on.
Click to expand...

Don't forget though that the area of sharp focus will always extend further behind the actual focus point than in front of it.

This has often confused a lot of people.

.
 
Thanks for detailed replies.

Think mines fine, just seems one mans Sharp is another persons soft.
 
Not quite... Sharp is, generally speaking, sharp across the board, but people have different thresholds.

Another thing to remember is that lenses and cameras will vary according to the tolerances to which they have been made, I.e. Accurate focussing to +/- 5% or whatever. If somebody with s -4 camera buys a lens which is 'out' by +4, they're going to think they have the sharpest lens in the world, when in fact it's actually out by4%. Stick that lens on another body, and it could turn out much worse. Which is why we should all have each of our lenses calibrated properly.
 
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