I've been meaning to do this comparison for a while and since the weather outside is cold, grey and damp, I decided to finally do it. This is a test to see how the Fuji 18-135mm zoom lens compares to the two Fuji star primes, the 23mm f/1.4 and 56mm f/1.2. I set the camera to ISO 100 (jpg. only setting) to get the sharpest image I could and compared the two lenses.
So a distinct advantage for the primes, not huge and nothing that a bit of Photoshop sharpening couldn't deal with, but significant none the less. The 23mm does exhibit a slightly bigger difference than the 56mm which is interesting. I did the same comparison with the 56mm at f/11 (see below) and it is still the prime that wins in terms of sharpness, though by slightly less.
So it's the old Zoom versus Prime dilemma. Primes are faster and sharper (most of the time) but zooms (though slower) are more convenient and arguably more useful in many picture taking situations. If I was shooting exclusively in a studio then I'd shoot with the primes, but I'm usually out walking in the countryside so the zoom has obvious advantages. Particularly at this time of year when changing lenses in a cold wind with cold fingers isn't exactly fun!
The Fuji 18-135mm is a very good zoom lens, the image stabilisation being especially good. This means that I can use slower shutter speeds for my normal work than I would normally. So, in a situation where I'm shooting images with very few moving parts (which is most of the time) the zoom works just fine, but for ultimate quality and a wide fast aperture, primes still rule. And I suspect that will always probably be the case unless someone revolutionises lens design.
Sharpness is also not a fixed, unchangeable constant. I mentioned above about post processing sharpening in Photoshop, or indeed any other software. Below is the same 23mm comparison shot above with selective sharpening added.
I only sharpened the guitar pickup, but as you can see there is now no difference between the two. The zoom image has also not degraded because of the sharpening either. Since I do add sharpening to all the images I upload to image libraries, this is a fair representation of how I work with images.
So assuming that a lens doesn't have significant CA, fringing or soft corner /edge issues, a little careful post processing can pretty much 'equalise' lenses, certainly in terms of how sharp they are and how they will reproduce. And I've always checked out any image samples I've found for a lens I'm interested in and added my own sharpening to see if that lens will do a job for me.
This is one of the reasons I am very enthusiastic about the 18-135mm zoom, because it does sharpen very well. Since the amount I've applied to the sample above is very much in line with what I would do anyway, hopefully I've demonstrated that using a zoom in preference to primes doesn't really compromise the image quality I seek to achieve.
I'm not putting forward the view that it isn't useful to know which lenses produce sharper results, or that there is really not much point to manufacturers striving to make the sharpest optics they can. I firmly believe that is a goal all lens makers should strive towards. However, it is important to exercise caution in reading too much into differences between lenses. For example, I would never contend that my Nikon 100mm Series E or my Sony 85mm SAM lenses were supreme examples of the skill of lens designers, but both sharpen up very nicely and on a printed page I defy anyone to spot the difference between the £190 Sony 85mm and the £650 Fuji 56mm.
However, as ever I do this testing to appreciate what differences there are between my lenses and can then use them with no surprises. And yes I have to sharpen some lenses more than others, but knowing just how each lens performs and what I have to do to get an acceptable image I can upload is important to me. A little knowledge may be a dangerous thing, but the more I have the less problems I seem to encounter.
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There is an interesting article on the BBC website HERE, which deals with many of the issues above.