Lens sharpness is of high importance for most photographers. When a picture is sharp, they see more details. When they see more details, the viewers' neurons fire as if they're saying "Oh this really captures the moment, I love this picture." There are lots of published reports on lens sharpness. Manufacturers and independent testing corporations have their own MTF results that describe a lens' characteristics in terms of contrast and resolution. The more contrast and resolution a lens can reproduce, the "sharper" an image will project to the film or sensor. Every lens has a best and worst sharpness depending on the location (center or corner) of the projection, and it is very dependent on its aperture. In general, almost all lenses have the best "sharpness" in the center at the sweet spot of the middle f-stop (for full frame, that is about f/5.6-f/8).
While there are numerous published MTF, there is not a standard when measuring the quality of blur (aka "bokeh") in a lens. This is mostly because the quality of a blur is very subjective. Below are two pictures using two different lenses (courtesy of http://www.rickdenney.com):
Don't look at the wine-- both are equally sharp. Look at the background blur. A majority of the people on internet forums will say that the background blur for the top picture is very busy and distracting, while the background blur for the bottom picture is more uniform, more creamy and soft. The top one has a lot of bokeh balls that look like donut rings, whereasa the one on the bottom has more creamy, soft, smushed artistic look. Ken Rockwell has a pretty good graphical explanation
here. Of course, there will always be a minority that prefers a busy looking bokeh (with donut ring shapes, weird shapes, inconsistent inner fill, etc).
While lenses optimized for sharpness are great for action/sports, landscape, and architectural work, lenses that yield soft creamy bokeh are great for portraits and for artistic purposes. So while it's interesting that while many people spend lots of money buying the sharpest lenses, there are a few who will spend even more buying the lens with the softest bokeh money can buy. Below are two artistic bokeh examples I found that I like:
Courtesy of Rogvon. This using a 55mm Voitglander which yields an amazing creamy soft bokeh.
Courtesy of Dustin Diaz. This is using the famous Nikkor 200mm f/2 which yields a huge bokeh. Note that many new Nikkor G lenses tend to exhibit a ring around the bokeh ball. This is a very typical 200mm f/2 bokeh.
According to Google Trends search, more and more people are looking at lenses not just based on the criteria of sharpness (which is of high importance for most work out there), but also bokeh characteristics. Lately, I've been hearing the term "Hollywood Style Creamcheese Bokeh". It really means the same thing as just "bokeh", it's just that the word Hollywood emphases the blurring and transition techniques commonly used in Hollywood movies.
What lenses do portrait photographers use? They use the lens with the most pleasant looking bokeh so that they can obliterate the background into soft and creamy cheese to make their subjects pop out. The best bokeh lenses are ones that are f/2.8 or below (which happens to be the minimum aperture for pro-line lenses), usually 85mm and above. The biggest bokeh shape is achieved by 1) grabbing the longest lens you can get your hands on 2) opening up the aperture from f/1.4 to f/2.8 3) setting the lens focus distance to MINIMUM while moving your body to aim at the subject 4) making sure the background light source yields a nice blur 5) use LIVE-VIEW on the camera because bokeh cannot be seen on the viewfinder. Note that even if you try all of the above, you will not always get a consistent result. The reason is bokeh is not only sensitive to focusing distance and settings, but also dependent on background light intensity! This is why the lens that yields a nice bokeh one day may not yield the same the next day-- a lens will have a tendency to yield bokeh one way or another, but that tendency is not guaranteed 100%.
Below I have put up a result of different bokeh characteristics of the different lenses I used. The background green and blue bokeh balls are my router and cable modem lights. By taking the bokeh of a light source at night time, I am isolating the shape, transition, and fill of individual bokeh looks for more objective viewing. I need to do this at night because during day time, all the light balls will be smushed together, and the background blur will usually look pleasing regarding of the lens used. Note that I don't own all of the lenses below. Thanks to everyone who lent me their babies.
Click here to see it in details:
Personally, my favorite is the Nikkor 24-70mm f/2.8 used at 70mm. It is a very versatile zoom with the quality of a prime lens, and the bokeh shape is mostly consistent and the fill is solid. There is a slightly line/edge on the rounded bokeh balls, but that is acceptable IMHO. There are not many lenses with great bokeh AND sharpness-- this is one of them.
My second favorite is the Nikkor 105mm f/2.8. It has a very low minimum distance, thus small objects can have huge looking bokeh. It can also obliterate the background into art. The contender 70-200mm f/2.8 is an excellent bokeh lens, but I didn't have one to play with at the time.
The Lensbaby Composer (the 3G is a typo) is a very good bokeh lens when used at f/2.8 and beyond.
The softest, creamiest bokeh is the Sigma 50mm f/1.4 at f/1.4. The edges are super soft, it's like an angel painted the balls! However, f/1.4 on this lens aperture yields very soft images.
A peculiar bokeh here is the old 1980 Nikkor 50mm f/1.8 AI-S. It has 7 blades, and you can see the heptagonal bokeh at a higher aperture. This can be useful for certain types of shot (product with shapes). I wouldn't use it for people or pets though, but you'll see this peculiar bokeh characteristic frequently in older pictures, especially from the 60s to 80s-- the era Nikon ruled the SLR 35mm world.
Lastly, it's important to note that none of these examples have ugly donut shape bokeh like ones from the 18-200mm f/3.5-5.6G VR (I had it for 2 months and sold it off). This is a hideous lens that yields ugly busy background blur. There are numerous other really bad bokeh lenses, mostly those with higher than f/2.8 aperture (e.g. 3.5-5.6, 4-5.6, etc etc). I didn't have the pleasure of testing+trashing them here, but if you Google for their names and the word "bokeh", you can see how bad they are.
