Monday, August 3, 2015

Sharpness Comparison: Nikon DX 18-200 vs. FX 24-120 f/4

A friend of mine and I have each owned the Nikon 18-200 DX zoom.  Which, for being an 11x zoom, is a pretty impressive lens.  But we both felt that it never delivered what we really wanted out at 200mm.

Especially as we moved up to the FX bodies (he's on a D750, I'm on a D600), and started shooting with prime lenses (the Nikon 50/1.8G and the Tokina 100/2.8 are my main lenses, both are stupendously sharp, even wide open).

In comparison to the primes, shooting with the 18-200 was just disappointing.  Obviously, an 11x superzoom has to make a lot of compromises, and the test data backs that up (DxOMarkdpreview,

My friend pointed me at the 24-120/4, and he's picked up a copy (kitted with the D750), and I rented one from over this last weekend.  It's a fantastic walkabout lens, and is super sharp for what it is.  It's not the 28-70/2.8  But then, it costs 2/3 as much, and zooms nearly twice as far in trade for the loss from f/2.8 to f/4.0.

What I noticed the day I picked it up is that at 120mm, it felt sharper than the 200mm.  Maybe it's better VR, the much shorter length reducing camera shake (I have horrible hand-shake for a photographer).

And so after shooting with it for the last 6 days, and loving it, I decided that I needed to shoot some benchmark photos in controlled circumstances, with the same subject, in a short timeframe.

I don't have a snazzy photo target and lab space for this, so I reparked my motorcycle and used it's fairing graphics and bolts as my sharpness indicators.

The first test was to setup the tripod and shoot the bike from a distance, and simulate a situation where I'm short on lens, and going to be cropping down to a subject from too far away.  This is my "zoo" and "motorsports" scenario.

24-120 @ FX 120mm f/6.3 1/125sec ISO110
18-200 @ FX 200mm f/6.3 1/200 ISO400
 I think that in this case, the 18-200 edges out the 24-120, but you need to be looking at a fairly large image to tell that (the thumbnails on the screen are equivalent to me).  Pixel-peeping gives the edge to the superzoom, but not by a huge margin.

And the advantage comes in when you scale the larger 200mm image down to the same pixel dimensions as the 120mm (after cropping down to the same subject):

24-120 @ 120mm, cropped
18-200 @ 200mm, cropped and scaled down to same pixel dimensions as the 120mm
But again, you need to be looking much closer than this to really see the difference:

24-120 @ 120mm
18-200 @ 200mm
 In this case, the 18-200 clearly edges out the 24-120, but it took a bunch of zooming in to make it apparent.

And even then, I'm not sure that I care that much, as at normal viewing sizes, they're pretty comparable.

But then I realized that there's another use-case, one where you're not stuck trying to photograph something with what's really too short of a lens for what you want.  If you can put the whole image to use, and therefore get closer or farther from the subject, so that when the picture is taken, it's filling the frame, what can you get?  (ie, perform the same composition through the viewfinder, and just use that).

24-120 @ 120mm
18-200 @ 200mm (FX)
18-200 @ 200mm (DX, for 300mm equivalent)
Here, there's about 50% more lines of resolution in the two FX-composed photos (as is expected), and they make a huge difference (although again, you need to be viewing at full screen to see it).

After scaling them all down to the same pixel dimensions, and zooming in, however, the different is stark, and the 24-120 handily out-does the 18-200.

24-120 @ 120mm
18-200 @ 200mm (FX)
18-200 @ 200 (DX for 300mm equivalent)
However, it's also _much_ closer (10m @120 vs 16m @ 200 vs 25m @ 300 for the same field of view).  So perhaps that's just cheating...

But, overall?  I'm definitely going to upgrade to the 24-120.  It's far better than the 18-200 over their overlapping areas that are equivalent (18-80mm on the DX lens).  The measurements are that you have about 50% more resolution on the FX zoom than on the DX zoom.  For the range that the 24-120 doesn't cover, but needs to be cropped down for, it's close enough.

And that's before taking into consideration the constant f/4.0 aperture.  The 18-200 is only close to that at 18mm, and there it has far worse vignetting in the corners, and can't be used at FX, and it very quickly drops to only f/6.3.

The 24-120 is going to give me an FX lens that covers the entire range that the 18-200 did, and almost always do a better job, and only when I've reached a point where I should be using a 300+mm lens am I really going to be giving anything up.  And in those cases, I should just be using a big lens, anyway.  At least for me, I just don't need that long of a lens very often.  And when I do, I know it well enough in advance that I can go rent the exceptionally sharp 70-200/2.8 or 300/4 for about $120/week.

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