Camera Lenses: Technical Excellence

In my previous post I advised to make up an idea about what you actually need, before going and buy a camera. I also discussed the different sources of information needed to make up this opinion, warmly counselling the DPreview website.
A good camera (that is, DSLR) will allow you to capture a good image if provided with a lens fit for your application/subject of interest. For instance, I love astrophotography (more on this in next posts, I promise), which means I should be using wide lenses (in order to capture more light) and fast ones (in order to capture that light in a small time).
But I also like macro photography of flowers and small objects, so that I would need macro lenses too (that is, lenses able to focus objects at very short distances, say 25 cm or even less) in order to capture those tiny details.
And then there are different requirements for landscapes or portraits. In the former case, software programs allow stitching contiguous images of a given panorama together in order to build up a much bigger image in terms of pixels, thus allowing even small cameras to produce very nice results. On the other side, the camera optics should be as aberration-free as possible in order to allow better scene reconstruction. The only disadvantage of this technique is that something may move from one frame to the next one (waves at sea, for instance), thus rendering impossible the image stitching.
The other way is to buy a wide-angle lens (typically, less than 28 mm focal length on a 35 mm sensor) and capture the whole landscape in only one shot. The problem here is that you are resolution-limited by the camera you are using, and if you plan to make wide enlargements of the picture, pixellation of the image will show up.
As for portraits, very typical focal lengths (say, from 50 to 85 mm with a 35 mm sensor size) will come handy, as you want to reproduce a scene in a manner as similar to what your eyes see as possible. Part of this resemblance is also given by a concept defined by a Japanese word: the bokeh.
On Wikipedia (for those like me who do not dominate Japanese) bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image.
In other words, a good photographer is able to picture the subject and its surroundings, and make you see it as if you were there. But, as your eyes cannot focus at the same time objects near and far, a good photographer will also try to emulate this phenomenon by using a small depth of field with his camera, i.e. the subject of his picture will be razor-sharp but everything else will be nicely blurred.
In its turn, a small depth of field is obtained with wide lens aperture values (f values not bigger than 2.8). This means that a desirable requisite for a good lens dedicated to portraits is a wide aperture, which means money.
I could go on and on with examples, the bottom line is: know what you want it for, then you know what you want! More important is where to look for information that will allow you to compare among the huge amount of lenses in the market or discontinued.
A first advise may well be DPreview again: here you find the page for the lens reviews, with plenty of information and comparisons (just like we saw for cameras), but I want to introduce you to an even better website, for the kind of use we have in mind: photodo.
Why do I say that photodo is so better than dpreview? For one simple reason: MTF, Modulation Transfer Function. I beg your pardon if I try to use some “scientifically professional” concept on these pages, but the simple fact that I find it used on a website not scientifically committed makes me believe that amateurs too will take advantage applying it.
Here you find a definition for MTF. Let us see what it means.

Graphical explanation of MTF [Image source: http://photo.net/learn/optics/mtf].
A) original (input) pattern. The pattern profile is represented in C). B) is the image (output) pattern. D) represents the profile of B). The values at the image bottom represent the MTF as a function of the pattern spatial frequency.

Let us consider a comb with teeth with variable spacing. We can assume that the alternated pattern tooth-space-tooth can be seen as a sharp sequence 0-1-0 (or 1-0-1, doesn't matter). If we try to take a picture of this pattern with a camera, it will be converted to a sequence of blacks and whites, the steeper as the detector pixels are tinier (as they better represent the actual pattern) and the teeth are more spaced and opaque.
For any given spacing (spatial frequency) of the teeth, the MTF tells you how good your image represents the actual comb pattern, assuming any value between 0 and 1, with 0 meaning that the comb is actually rendered as a perfectly flat object and 1 meaning that the comb pattern is perfectly reproduced in shape and intensity.

Typical MTF graph as published on photodo for a 50 mm f/1.4 lens.

At photodo they make this simple thing: they take pictures of a standard pattern representing various spatial frequencies through virtually any lens there is/was in the market, then they rate it on a scale of five depending on different parameters but, as I was saying before, MTF is one of these. They are so strict that no lens has earned yet the 5 stars rating but, on the other side, this means that any lens with a rating higher than 4 can be considered as very good, top class or excellent.
You will find any kind of lens, some with the MTF graph, so that you can compare form a pure mathematical point of view the quality of your lenses. Through this link, for instance, you have all of their lenses sorted in descending quality order. I hope you find it interesting and helpful.