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Tuesday, January 8, 2008

Lens sharpness in the digital world

Primes vs zooms!
Pro vs consumer!
Film vs Digital!
Photographers have been arguing the same things for years in regards to lens sharpness and resolution. Now we have some new mud in the waters and that is all the possibility of sharpening in post processing. Is it worthwhile to use your prime lens now when you can just sharpen the pics from your zoom a bit more? In fact was it even worthwhile in the days of film?

I have done a small test to compare the differences between three different classes of lenses in terms of sharpness and contrast and then tested the effect of digital sharpening of the results to see if the increase in sharpness is worthwhile in the new world of post processing and USM. For this test I have chosen to use the equipment I have lying around at home, big surprise!
As my consumer level (cheap) zoom I've used the Canon 28-90 f4 - 5.6 that came with my original film EOS Rebel, aka EOS 300.
For the upgraded zoom I used my Canon 28-70 f3.5 - 4.5 series II which though dated is very highly regarded and in my opinion a great bargain if you can find one.
And representing the primes a Pentax 55mm f1.8 which is quite possibly the most beautiful lens I've ever seen. Sad I just described a lens as beautiful I know.

The test consisted of me shooting the back of a cereal box from the same position at f5.6 and f11 with the 3 lenses on my Rebel loaded with Agfa APX 100, a fine grained and sharp black and white film, and scanning with the trusty HP S20 film scanner at 2400ppi. The results you will see below. You will probably ask why I didn't use a dslr well first because I don't have one but I believe film better illuminates the question because you can compare the original un-sharpened scans (representing the end of the line in the analog days) to the sharpened versions showing the effects of the digital post processing. This way it acts as a comparison of not only the three lenses but also analog vs sharpened. I'll say up front that there are flaws to my methodology, perhaps I should have used TechPan and a drum scan or perhaps a 1Ds mk2, well then do your own test and show me the results I'd be curious to see them. With no further delay onto the results, first a re-sized shot of the full frame for reference. This is a cereal box lying on its side and you're unlikely to understand the writing unless you know Czech or Hungarian as I live in central Europe.


Following are crops taken from the center at 100% with no adjustments made and the scan left at default values and no sharpening.

Kit lens f5.6

Mid level zoom f5.6

Prime f5.6

I have to admit I was quite surprised so see such a big difference between the lenses at f5.6. Granted at smaller print sizes the differences become smaller and smaller. What we are looking at here are pixel level scans and at typical monitor dpi this would be like looking at a 24x36 inch print this close. Now lets look at the same crops but at f11.

Kit Lens f11

Mid-level f11

Prime f11

Stopping the lenses down to f11 really levels the playing field. Both zoom lenses show substantial improvements in sharpness and contrast. I would say that the prime has actually a tad less contrast than it did at f5.6 which seems reasonable for a prime. Also note all these crops are from the center of the frame, when you move off to the sides there is a drop in sharpness especially with the zooms at f5.6 while at f11 they hold onto the detail better. I'm not going to post the crops from the sides for two reasons, first because if the film plane and cereal box are not exactly parallel then any loss of sharpness could be due to depth of field and not the lens itself. Second because I think this post is going to be quite long as it is. Now lets move on to the second part of our test, the effects of sharpening.


I've decided to use the same crops from the lenses at f5.6 which shows the biggest differences and add an equal amount of sharpening to each. Here they are...

Kit Lens f5.6 sharpened

Mid-level f5.6 sharpened

Prime f5.6 sharpened

With an equal amount of sharpening added to each crop I think it does help to mask the fuzziness of the inferior lenses but the better lens still has the advantage and to me looks the best. I must say I was a bit heavy handed with the sharpening and it's left some artifacts that ruin the nice clean look they had but its all in the name of science isn't it? And now for the final challenge here is the mid-level zoom sharpened and the prime un-sharpened.


Mid-Level zoom plus sharpening

Prime no sharpening


I would say that careful sharpening can improve almost any image. In the final challenge adding sharpening to only the zoom brings it a step closer to the prime but I'd say the prime still looks better. It has more contrast and is clean and less tired. It looks good without trying is what I mean. Of course in the real world the advantages of post processing can be applied to any lens you might have and so the prime retains its advantage in absolute image quality. I still do most of my shooting with my mid-level zoom because it's simply more useful to me and in the real world outside of the lab I'm only limited by lens sharpness about 1% of the time. Enjoy your shooting!

About Me

Amateur photographer extraordinaire!!

thoughts and opinions about photography from the advanced amateur perspective