What I look for in a lens.
What affects sharpness of an image?
Sharpness is affected by a number of factors: quality of the glass and engineering in the lens, the aperture of the lens, sensor properties like pixel spacing and pixel counts, motion blur or camera shake and finally the resolution, color and dynamic range of the screen or printer, ink and paper you view the image on. In my opinion, the lens, sensor and photographer’s technique are most critical. You can always print on different paper using a different printer to improve results or display the images on a higher quality display, but you can’t go back in time and capture that same moment again. For now, we’ll focus on the lens properties at the time of capture.
No one buys a F1.2 prime lens to shoot at F8. Most certainly you pay for the ability to open up wide to create narrow depth of field or to use in low amounts of light. But the aperture setting also affects the sharpness of the lens and most lenses have a sweet spot somewhere between F2.8 and F8. Go either side of that and you start to see some degradation in sharpness due to aberrations or diffraction. If a particular lens is not sharp enough wide open and you have to stop down to get it sharp, then buying a cheaper version of the same lens could be a better option. Eg. some F2.8 lenses are simply not that sharp wide open and needs to be stopped down to F4 to get the desired sharpness. Make no mistake, they are usually not bad at F2.8 but they are typically better at F4. That makes the F2.8 ability, less usable under certain conditions. Sometimes the F4 lens will achieve that sharpness at wide open but it will cost much less and is therefore a better option. If you buy the F2.8 version and end up shooting at F4 most of the time, then you could have paid less for a lighter lens with similar or even better results. Keep in mind you can still use F2.8 trading some of that sharpness for more light and narrower depth of field, while reducing noise.
Here is an example with screenshots taken from my phone.
F4 lens for half the price of the latest F2.8 lens
At the wide open aperture setting, the F4 delivers better sharpness than the F2.8, thereby protecting you from selecting an aperture setting where sharpness is compromised.
Even when zoomed in to maximum focal length, the F4 delivers more sharpness.
All lenses stopped down to F5.6 for equal comparison and the F4 delivers similar performance compared to the F2.8 lenses. If you like center sharpness, the F4 is better than the latest F2.8 lens.
Zoomed in to maximum focal length, and again the F4 delivers slightly better or similar performance.
Last but not least, we compare the F4 with one of the F2.8 lenses for which we have measurements at F4. Not sure why DXOmark did not measure the latest one at F4. Again, the F4 lens delivers very similar performance.
The only benefit the latest F2.8 lens has over any of the other two lenses, is the image stabilization. If image stabilization is needed or preferred, then it does not matter. Buy the stabilized lens. If not, you can certainly save heaps and get slightly better quality.
Focal length Range
Similarly, when you buy a lens with huge focal length range, then that flexibility becomes the main feature and aperture becomes less important. If you do need to stop down to improve results, then do so, but do make the effort to find out where its sweet spot lies. If you don’t, then you will always believe that the lens is not that sharp because it has a huge focal range when instead you just never used it correctly.
As mentioned, most lenses have a specific sweet spot where they are indeed really good. Some lenses though have a sweet range of apertures and/or focal lengths. I normally look for a lens that gives me good sharpness over a good aperture range for all focal lengths the lens is capable of. This is more important to me than simply having a big aperture or big focal length range.
Center and edge sharpness.
You’ll also find the corner and edge sharpness being affected by the aperture setting, focal distance and focal length. While we usually do not put the subject on the edge of the frame or the corner it still affects subjects when we focus and recompose or for those occasional artistic shots with only half a face on the edge of the frame. This is indeed less of a concern but still worthy to take note of. A bit less sharp on the edges is passable, but some lenses have a huge difference between the center and the edge at certain aperture settings. You do not want to be shooting groups of people with such a lens or lens setting, and find out in post that the two people on the edges are barely recognisable.
So what then is the ideal lens? We get to that on the next page.