Valuations on Cameras and Lenses
The more or less good impression you might get after the assessment of the frame centre disappears immediately when you glance at the values at the edge of the frame. [See the chart above.] The image quality in the area near the maximum relative aperture is weak. The lens has to be stopped down to f/5.6 in order to provide fully useful photos. It means that at the shortest focal lengths you have to stop down by as much as 2 EV. What’s more, the lens’s maximum results are once again hardly impressive. In a case of such an expensive device a level of 40 lpmm, which is reached near f8.0, is simply not enough. If you want to know what resolution level we expect from a lens bought at that price point you might consult our review of the Sigma 18–35 mm f/1.8 lens, which test was performed on the Canon 50D; its sensor provides practically the same MTFs like those you get using the Pentax K-5 so the values are directly comparable. Seeing those results it becomes clear that the more expensive Pentax 20-40 mm fares rather badly.
The unstated assumption behind these remarks is that the resolution of a lens is a prime determinant of image quality, and that if a lens does not reach, in terms of measured resolutions, a certain arbitrary standard, it can’t “provide fully useful photos.”
Let’s consider what a professional photographer, who goes by the name “Qwntm” over at pentaxforums.com, says about the very same lens:
I LOVE this lens. The review(s) are wrong. They must have got a bad sample. This lens is as good as the primes it covers, namely the 21/31/35/40/43 LTD's. And most importantly it has that LTD magic photo dust sprinkled all over it, the shots are rich, lush and have that "artistic" something that we all crave. This lens (and the LTD's in general) make me want to photograph things just to see what they look like through it. This lens covers my 17-50 Tamron with WR and better IQ. I still use the 17-50 for weddings as a backup etc, but this 20-40 is my goto everything lens. With the 15 and 70, it is my travel/landscape foundation lens. I don't find the zoom range limiting at all. It's a very convenient fine tunable normal. If I need wide or tele, then I switch lenses. But for everything else it's perfect. Just buy one. At $695 USD it's a great deal.
So who is right? The reviewers at lenstip.com, with their measurements and their unquestioned faith in the supremacy of resolution? Or is it “Qwntm” over at pentaxforums.com, with his vague insistence on “rich, lush” and the “artistic something that we all crave”?
The view that resolution is either the only or primary determinant of image quality tends to dominate on review sites, which usually almost include a series of measurements. The arguments on behalf of this view include the following:
- Resolution is a measurable property and therefore “objective” and “scientific,” whereas talking about vague qualities such as “that ‘artistic’ something we all crave” is unscientific and arbitrary.
- Resolution is the most important factor in an image because most of the other things (like color, contrast, even bokeh) can be manipulated and/or changed/improved in post.
The issue with the first contention is that it ignores the very plain fact that photography is aesthetic pursuit, and the assessment of images involves making qualitative, immeasurable value judgments. Indeed, the very assertion that resolution is important is itself a value judgement, and therefore, according to the positivistic spirit of the measurebators, would have to be reckoned as unscientific and arbitrary.
The issue with the second contention is that it is not altogether true. First of all, you actually can add sharpening in post. Of course, it’s not as good as the resolution you get from a lens and/or a sensor. But that’s true of post-processed color and contrast as well. The saturation and color you get from a very good lens will always trump what you can get in post. If there are any doubts on this score, consider the following two thought experiments.
(1) If you could get any color you wanted in post, then it shouldn’t be too hard to take an image shot in the middle of the day and make it look like an image shot at sunrise. This would actually be quite useful, as it would allow photographers to sleep in, rather than getting up at four in the morning to get that early morning shot with the great early day colors. But as everyone knows, while it may be fairly easy to turn blue into purple and yellow into green, changing the colors of a midday image so that they look as good as what you can get during the “magic hour” is largely impossible. Yes, you can make approximations; but these approximations never look as good as the real thing.
(2) If you could attain any level of contrast you desires — if, in other words, all contrast, whether attained in post or through the lens in the field were entirely equal, with no ability to distinguish between them — in that case, you should be able to take an image shot on a hazy day and make it look just as good as the exact same vista shot on a clear day. Again, we know this is not how it works. While adding contrast can improve the image shot on the hazy day, you’re never going to make that image look as good as the shot on a clear day. After all, there’s nothing to stop you from adding contrast to to the clear day image! In that sense, you’re chasing a moving target — which leads us to our final consideration. While adding a bit of contrast, a bit of saturation, a bit of the Lightroom clarity slider may improve almost any image, adding a large measure of these things is almost always to the detriment of the image. For that reason, it makes sense to try to make the original image as good as possible, so that you can start off from a better position in post; and the extent to which a lens helps us produce a better raw file, with a better, higher quality set of data within it, the better the ultimate image will likely be.
Thus we can safely conclude that the emphasis which “scientific” reviews of lenses place on resolution is misplaced. While resolution is obviously important, it’s not the sole or even the primary determinant of image quality. Images are evaluated based on aesthetic, non-scientific value judgments. They include measurable and immeasurable attributes.
Now if there are still any doubts on this score, we need only to consult the camera and lens manufacturers to remove them. If the people who make the lenses and cameras admit to regarding immeasurable attributes as important in the development of lenses and cameras, who are we to do deny this? Consider the remarks made by Tetsuya Yamamoto, Manager of Development Sector at Nikon.
There are some kinds of lens that we cannot appreciate only by the indicators, or in this case the numbers that are shown by the data. There's something more that can be realized by the lens, but it cannot be measured by numbers....
Since I started the post considering a review of a Pentax lens, I will conclude with a very interesting statement made by Pentax (currently owned by Ricoh) concerning the philosophy behind the company and the brand. This philosophy, incidentally, vindicates the lens discussed at the beginning of the post. It’s not about numerical values, insists Pentax, it’s about “resolving factors that can’t be expressed by numerical values.” In a discussion about Pentax’s new full-frame camera, the Pentax K-1, we find the following comments:
In the age of advanced technologies, PENTAX still places great importance on human sensibilities, something that is in complete opposition to much of today’s technological advancement. For PENTAX, this is the primary source for attaining exceptional image quality.
How this applies to the color reproduction of cameras (and presumably lenses as well) is rather surprising:
[T]he PENTAX K-1 inherits PENTAX Blue, a term affectionately used by Pentaxians (PENTAX camera enthusiasts). A technician in the image-processing team explains: “We are trying to create memory colors, rather than faithfully reproducing original colors. In particular, blues and greens are the colors that decide the impression of scenery. That’s why we work hard to reproduce rich, dramatic tones of these colors.” Although today’s SLR cameras provide a high-saturation mode in their default settings, the saturation level is more subdued now for most makers than it was earlier. In fact, PENTAX is probably the only manufacturer to offer such high saturation and rich tones. “PENTAX’s Bright and Vibrant modes are often avoided by other manufacturers, because higher saturation and richer tones essentially result in more noise. However, it is our strength in image-processing technology that lets us maintain the same expression of noise regardless of a selected shooting mode. This is a tradition we are all proud of.” Even though it may not sound technically logical, it is the mission of PENTAX engineers to deliver the colors desired and favored by Pentaxians. Today, a camera is a cluster of high technologies, which continue to advance day by day. PENTAX believes, however, that, no matter how advanced technologies may become, image quality is always something perceived by the human eye, and is the product of human sensibility.
So the Pentax view of the matter, which is shared, at least in part, by Nikon (and probably by all the other camera and lens manufacturers), is that in order to make a full assessment of photographic gear, we need to be looking at actual images, not just test charts and numerical evaluations. This means having the courage to make qualitative value judgments, rather than hiding behind resolutions scores and other so-called “scientific” data.
To conclude, I’m going to try to demonstrate the thesis of this post with a series of images from a lens that illustrates the position, advanced by Pentax, that image quality is something perceived by the human eye. Pentax’s DA 21 f3.2 Limited has often been panned by the critics. Again to quote lenstip.com:
There is another problem, though. Most of standard zoom lenses offer us 21mm focal length. The purchase of a prime lens makes sense only when it offers something more than a zoom. This “something more” mainly consists of a better fastness or/and higher image quality. Here, unfortunately, such advantages are debatable because a lens like the Sigma 17-70 mm f/2.8-4.0 DC Macro OS HSM at 21 mm is not slower than the Pentax and the quality of images is the same, momentarily even better. What’s more, the Sigma is cheaper.
The reviewer at lenstip.com is basing his opinion solely on the fact that the Sigma 17-70 is sharper along the edges than the DA 21 and is a little faster in terms of aperture. He is not considering such things as lens contrast, color rendition, rendering, or the artistic “something” referred to by “Qwntm” in his review of the DA 20-40. On the contrary, the lenstip.com reviewer explicitly disavows such “subjective” considerations:
Even if sometimes it is difficult to find a rational reason of buying [the Pentax Limited lens] – like in the case of the DA 21 f3.2 model, tested here, which, optically, is bested by the cheaper Sigma 17-70mm – you don’t always take only purely rational aspects of a purchase into account. I don’t mean here that mythical image vividness which might be important to some users but I certainly don’t fall for it. I rather mean the pure joy of owning something truly original, untypical and made in a less cliché way than most of the equipment available nowadays on the market.
In other words, our numbers obsessed reviewer considers it irrational to purchase and use the DA 21. The only partial excuse he can make for the lens is that it is well made, and one may take joy in using an instrument with fine build quality. But as for any “mythical image vividness,” he’s certainly not going to fall for that!
Now it just so happens that, as Pentax itself has confirmed, the DA 21 is not designed solely to do well on test charts! As Pentax has said:
The Pentax Limited Lens series is designed for distinctive visual description by combining a series of mechanical and numerical evaluations with human assessment of test-shooting samples. As a result, this lens produces smooth, natural skin tones and beautiful out-of-focus backgrounds at open aperture in portrait applications, while assuring crisp, high-contrast images at closed-down aperture in landscape photography.
In other words, the DA 21 is not designed to impress numerologists such as the reviewers at lenstip.com; no, they were designed to make images that look delectable to human perception! Has Pentax succeeded in this goal? Perhaps a glance at some actual images would give us a better idea whether Pentax has succeeded in meeting this goal. I have posted some of these pics before, but since they provide evidence that the DA 21 does very well in “assuring crisp, high-contrast images at closed-down aperture in landscape photography,” I’ll post them once again: