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This section questions what defines a good lens now that very powerful image editing software is available to correct many of the failings of lenses, raising their image quality to rival those of far more expensive lenses. Can a not-so-good lens be as good as a good lens by using such software? Can you save money by buying a not-so-good lens and have it match the performance of a better and much more costly lens? I will tell you that answer now and that answer is "it mostly comes down to resolution with bokeh making up the rest". This page is here to explain why I have given that as an answer.
It is the fixing of the fixable faults of lenses that gives you real cost benefits. A not-so-good lens can perform as well as a better lens once the fixable faults of the lens are corrected by the software.
To correct lens faults you will need to be shooting in raw mode. Only with raw images is the editing software best able to work, especially when it comes to correcting what is called "chromatic aberration". You can shoot jpeg as well as raw, if you want the convenience of having jpegs ready to use, but if you want good quality jpegs then you need to use the highest size and quality setting. And with jpegs you might need to do some extra sharpening. That is because you should never choose a sharpness setting that might be higher than the one that is best for the scene because unsharpening a jpeg that is too sharp is very difficult where exaggerated edges and halo effects it might be showing. But some camera settings require jpegs, but again, do not make these sharper then is bes for any type of scene you may be shooting.
Colour hue is very easy to fix and even if you use very simple image editing software then you will have done colour corrections many times. There is likely an "auto correct" function that not only corrects colour but brightness and contrast as well. In the old days of film cameras, a lens that showed true colours on film was considered to be a better lens but with digital cameras it does not matter any more.
Chromatic aberration (CA), to some degree, will be present in all lenses. Some lenses show CA much more than others. It is here that you can make a real saving since image editing software is capable of detecting that colours are being focussed to different places. The raw image contains separately details for each of the three primary colours and will adjust the scaling of the images so that the colours focus at the same place. Some lenses, called "APO" lenses, are designed to focus colours at the same point but these lenses are very expensive. And with the good imaging software capable of fixing this, this extra expense is not worthwhile. So a quite poor lens can appear to show very little CA after the image is fixed.
Distortion will be present in most zooms at some focal lengths. It even shows in some fixed lenses. Here again, the software can fix it and save you money from buying a better lens that shows less distortion.
Vignetting is where the image gets darker as you approach the corners. The software can fix that as well.
So with colour hue, CA, distortion and vignetting fixable in images from less expensive lenses, this gains us a lot. But it is still not enough to give us a "good" lens.
The main thing that photo editing software can't fix is poor resolution. It can improve it through the process of fixing CA, if there is significant CA to fix, since it will remap the images of the three primary colours and in so doing, move the light and dark edge details to a place where they coincide with the other colour images. This will definitely result in better resolution. But som