Warning: This is a controlled rant alert.
When I first started with a DSLR, one of the first websites I found was Ken Rockwell’s lens reviews. Chances are, you’ve seen his site too – seeing as they rank highly on Google – and he’s pretty darn good.
Nevertheless, there is one article on his site that I beg to differ with: Lens Sharpness Doesn’t Matter.
Well, Ken – sorry to disagree with you, but sharpness does matter. Sure, lens sharpness may be overrated – especially when comparing some of Canon’s mid-range zoom lenses to L series offerings with shockingly similar performance – but it is a topic worthy of some finesse.
Thing is, sharpness is not an all-inclusive definition of a lens. Sure, a lens may be sharp in its center, but that may be the only place it can claim that fame.
Two, older EF-mount lenses prove that point:
|Canon 22-55mm f/4-5.6 USM|
|Tamron AF Aspherical 28-200mm f/3.8-5.6, EF mount|
Though both of these lenses are no longer on the market, they’re generally quite affordable on eBay – making them appear to be quite attractive to the DSLR filmmaker looking for a dirt-cheap starter lens.
If lens sharpness did not matter before, these two lenses ought to change the course of things. There’s a certain manner that both of these lenses handled areas out of focus and those areas supposed to be in focus that the line became quite blurred (pun not intended) between what was what.
That, and vignetting (darkening of the edges) occasionally proved to be an issue as well.
Cases in point – from the Tamron:
It’s not that this photo isn’t sharp (though there is enough purple fringing on all the edges to make it appear as if the camera was shaking slightly during the exposure), but the sharp areas and blurry areas all look about the same.
Technically speaking, the difference between these areas are known as the “circle of confusion” (explained very well here: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm), but – for the purposes of this basic explanation – let us just call it for what we see in the photo: It is a circular, muddy mess, with vignetting to boot.
Mind you, this is a photograph taken in RAW, not video or .JPG, so you can imagine what the results would have looked like with the extra compression (or reduced scan lines as the 5DM2 uses for 1080P). Not so good.
The Canon didn’t fare much better:
Aside from obvious fringing, the out-of-focus areas in harsh sunlight look as if the camera was mounted to a pogo stick. Granted, I could have put a gradual ND filter on it for these photos, but I doubt if I would have improved anything other than the exposure.
It’s neither sharp, nor pretty – it’s muddy.
In closing, lens sharpness does matter, though the overall sharpness of an image is more a result of whether the lens does a nice job of separating the out-of-focus areas from those that are in focus.
Be mindful of what you’re buying, and remember this when you find a low-end lens for cheap: