Modern Lens Sharpness

[DeMorcan]

First of all, I do not believe maximum lens sharpness is the best reason to buy a lens for most people. Although, it can be for some thing. Still it is a subject much discussed in the forums. There is much old information and even incorrect information floating around. The resolution charts are from 2 Olympus Pro lenses. One is the excellent 12-100/4 and the other is from the 25/12 a lens with gives wonderful rendering.

Previous knowledge said a lens was sharper stopped down 2 or 3 stops:
First if you look at the top graph, you will see at 12/mm the lens is sharpest wide open at the distance tested. The limitation of this lens test is that it is tested at one distance. The 100 end is actually shaper at f4 when focused near infinity. The other ranges of the zoom re sharper 1 stop higher. Then do to diffraction they get steadily less sharp as the aperture is closed down. Seeing the near straight line at 12mm, this lens is diffraction limited. A normal lens, the lines straightens at the top as the other focal lengths do. This is do to lens design and costly correction to make a lens diffraction limited. At this limit the physics of photon movement sets the maximum sharpness and nothing can be gained beyond that in optical design although AI can add false detail. Technically, this lens is not totally diffraction limited. I am using the term loosely without the math that shows no lens is fully diffraction limited.

Previous knowledge said a prime was sharper than a zoom:
Now we go to the bottom graph. Lets look at a Pro lens designed for 1 length so it does not have to have the compromises of a zoom. Comparing at f4 and f5.6 the zoom is sharper in the center. You will see at F/2 it is sharper than 1.2 so 1½ stop does make it sharper. First this begs the question why is a prime that is less sharp than the zoom have the same cost. First off is 3½ stop more light gathering. Apparently this comes at the cost of sharpness. Which is not a given. This lens could have been deigned to be sharper. It was designed for rendering. Leica built their company on 2 types of lens. Slow lens sharp across the field and lens that have beautiful rendering. You got either one or the other when you purchased their lens and none of the middle ground try to do everything lens which always does some things half assed.

The rest of the story:
Now we look at the middle graph - edge sharpness. Again looking at 25mm notice the difference between the center and edge sharpness. On the zoom at f/4 is about 8 or 9 lines per mm difference. The less this is, the sharper the edges. At 5.6 where it is sharpest it is is about 10 lpmm difference. Lets go to the third graph. This lens is said to use feathered bokeh (which does affect sharpness. Look at f/1.2. They edge and center are both very near the same. This lens was designed for the look that it gives wide open. That is where many people (including me) use it. It is not comparable in sharpness to the F/2 or the zoom. Really, not even close. It is smooth magical look lens out of camera. The early users of it and where I learned of it from were Leica 50/2 APO lenses users which except for the fast Canon dream 85s were the best rendering lens in the world. And my favorite lens ever even over the 1.4s or 0.95s. The rendering of the 1.2 is different than the APO. But they are both high quality renderings. If this lens is closed to f/2 it behaves like a typical fast lens. The difference between the edges and center are 12 to 15 lpmm which gives a softer background much like many fast lens. That is what this lens is built for.

Summary:
Modern lens can be sharp near or at wide opening. Many zooms are sharper than fast primes. You must choose between sharpness or rendering when you purchase your lens. Bokeh is very dependent on distances from camera to subject to background. If you use a sharp slower lens, and the right distances, you can get a dof that will look the same. I have taught people how to do this with kit lenses and it is easy. However the out of focus area will be busy. This means a single line (branch, etc) may show up twice and have hard edges. This is the effect of maximum sharpness. There is another class of low light lenses. A good affordable example is the Panasonic 1.7s compared to the Olympus 1.8s. I have had all of them. The Panasonics are sharper. They are great for inside groups where you want everyone's face as sharp as possible. The Olympus lens give a smoother rendering. At one time I had a 15/1.7 and a 17/1.8 because although they were similar focal lengths, the rendering of the 17 was beautiful, but the sharpness was quite a bit behind the 15 when shooting events or in clubs. The 25s the differences are there, but not quite as extreme. I use th1 1.7 for landscape type shots. The 1.8 is smoother, but 2 higher priced options (1.4and 1.2) are so much better at rendering. The 45s, the 1.7 is very much sharper than the 1.8. I also think the out of focus areas are ugly for how I use this length. I do not use it for landscapes or sharp across the frame images. I use this focal length for separation and like the smooth rendering of the 45/1.8. For the cost and what it does, it really is a great deal today.

This is in no way to say one lens is better than another. It is to say lens are different. And by knowing the differences, you can better chose the lens that is best for you. Which when all is said and done, the lens that does what you want the way you use if is best for you regardless of any test results or other people's opinion.