Color Rendition in Lenses

Blooms SM11-2159

What do I regard as the most important quality of a lens? Is it sharpness, focal length, speed, control of distortion and abberrations? No, it's none of those things (though they may all have their place, particularly sharpness). The most important quality of a lens, as far as I'm concerned, is color rendition. Assuming that the lens is not blatantly soft, the lens’s color rendition will have the biggest effect on the images it produces. A lens of moderate sharpness with great color rendition will (generally speaking) produce better images than a lens with great sharpness and mediocre color rendition. Yet lens buyers tend to pay far more attention to sharpness than they do to color rendition. This is yet another example of mistaken priorities.

One of the reasons for the lack of appreciation for the importance of color rendition is the tendency to consider sharpness the most important element contributing to image quality. Impressive images tend to have a "snap" or a "pop." The image jumps out of the screen and makes the viewer say, "Wow!" It is assumed that this snap or pop is caused by sharpness; and while sharpness contributes to the effect, vibrant, stunning colors seal the deal. Consider the following three images:

Image 1:


Image 2:


Image 3:


All three images produce “nice” color, but the second and third produce richer colors. Nor is it merely the case that the first two were taken about the same time whereas the last was taken several years earlier. Now some of these colors can be manipulated in post, but would never be able to get identical colors out of all three images, unless you greatly desaturated the color. Each lens produces a slightly different color scheme. Image 2 has the most striking scheme. It is in some ways the least realistic yet the most striking. Image 3 is also very good, with slightly brighter colors, yet still with rich blues and solid greens. Image 1 produces somewhat washed out greens and paler blues (though in this image it looks nice). The lake is lighter in color, less rich in hue. Image 1 was taken with the Pentax DA 18-55/3.5-5.6; image 2 was taken with the Pentax K 28/3.5; and image 3 was taken with the Pentax DA 12-24/4.

Some of these colors could, of course, be changed in post; but they can't be changed to whatever one might please. Most color changing tools are global: they change one color by changing all colors. This is true of white balance controls in Lightroom, and of general hue and saturation controls. Some photo imaging software allows you to target color: but again, the targeting covers an entire gamut, so that if you change blue, you end up changing all the various hues of blue within the image. There really is no substitute for getting the color right in the camera. A lens with outstanding color rendition will produce colors that are bright, rich, vibrant and intense without looking over saturated or bleeding all over the place. When people try to improve color rendition in post, they end up with over-saturated images that look no better than images subjected to over-sharpening.

Color rendition is strongly influenced by the quality of coatings in a lens. Lens coatings are added to improve light transmission through the lens and to reduce reflections. Light lost during its journey through a lens means less light striking the camera's sensor. Reflecting light leads to flare and loss of contrast. So coatings affect more than just color rendition. They reduce flare and improve contrast. But there is a price to these added benefits. Although coatings improve light transmission, they don't necessarily improve all the colors in the spectrum equally. Some portions of the spectrum (e.g., the green or blue part of the spectrum) may be improved (or worsened) relative to spectrum's other colors. If the differences between various colors is great enough, the lens will have a color cast. But even if the unevenness of light transmission is not great enough to be easily noticed, if not as much of the blue part of the spectrum is getting through the lens as the red and green parts of the spectrum, this means the sensor will record less information about blue than about the other colors. Less information about blue means less information to work with in post. What is lost is not recoverable: no amount of PP work can replace it, though its affects can be mitigaged.

As an example, consider the two shots (heavily cropped) of a church window, taken with different lenses:



The first shot was taken by the DA 18-55, an inexpensive kit lens; the second by the DA 12-24, an expensive ultra-wide zoom. The blue in the window is rendered far better in the second shot. Why? Because the coatings of the DA 18-55 block out a lot of the blue light, so that spectrum of light is not as well recorded as in the DA 12-24. And if we look at the graph recording how the DA 18-55 transmits light, we find that we see in the image is confirmed by the numbers compiled by lenstip:





The numbers along the bottom of the graph correspond to the spectrum, as per below:



Note that in the graph, the percentage of light transmission falls around the aqua section of spectrum (500nm), and continues to plunge as it passes through the blue toward the violets at 400nm. The light transmission of dark blues drops preciptously toward 65%. Hardly impressive numbers. So much light partitioned off at the blue end of the spectrum will lead to pale, uninspiring blues, as seen in the photos above. Even the darker greens don’t fare so well, which is why Images 2 and 3, taken with lenses with better coatings, produced richer greens.

As an example of what a lens with first-rate coatings would look like, consider this graph of the Pentax DA 40/2.8 (a lens not used in any of the photos posted):



Notice how much higher the light transmission is across the spectrum with this lens than the 18-55! Color rendition does matter. Don't be fooled by the myth that the color lost through bad or mediocre coatings can be recovered in post. No amount of over-saturation can make up for color data lost in the lens.

Here’s another example—

DA 18-55mm lens:


Pentax K 28/3.5 lens:


Or—

Tamron 75-300:


Pentax DA* 300/4:


Now this comparison might seem unfair, as one photo is taken in sunlight, the other in shade; but it should be noted that colors normally come out brighter in the light.

Another comparison—

Tamron 75-300:


Pentax K 200/4: