Visualizing lens aberrations — why do it?

Three people are sampling the same wine. The first says, “I like it.”

The second says: “Pretty good older California Cab. Fruit is fading, but there’s a lot of bottle bouquet. Brown edges. Mint, oak, and tobacco.”

The third says: “Alcohol about 14%. Maybe half a percent of residual sugar. Sugar/acid balance is a little high. Seems to have been stored properly. Not corked. No chemical aromas. Not too hot. No malolactic fermentation. No herbaceous notes. No bitterness. Tannin is resolved. Limousin oak. Bacon, coffee, mint, tobacco, blackberry.  It’s not going to get any better, and it may be a little way past the peak. I give it a 17, and we should drink it now.”

All those perspectives are valid. But as you go down the list, you find that there’s a deeper understanding of the experience, and a greater likelihood that that person will be able to make informed purchase decisions. Ther’s a continuum of experience and knowledge, from simple enjoyment to analytical assessment. Each person responds truthfully and meaningfully, but with increasing levels of insight and vocabulary. That insight enables not just richer appreciation, but better judgment, especially when deciding what to buy, when to drink, and what to pair it with.

The same spectrum applies to photography and, specifically, to understanding lens aberrations and rendering.

At one end, a photographer might simply say, “I like this lens.” That’s valid. Emotional response and aesthetic satisfaction are legitimate reasons to use a piece of gear.

Further along, another might say, “It’s sharp in the center, a bit soft on the edges wide open. Bokeh is smooth. There’s some chromatic aberration and a bit of vignetting, but nothing that can’t be corrected.” This level of analysis often comes from real-world use and casual testing, and it’s already quite useful for making informed decisions.

At the most advanced end, a photographer might say, “Wide open, there’s significant spherochromatism and some undercorrected spherical aberration, which gives a soft glow and enhances apparent bokeh smoothness. The field curvature is inward, and astigmatism kicks in off-axis. There’s some residual lateral CA. The PSF has a noticeable flare tail in defocus. Stopped down to f/4, all this tightens up, and the MTF curve flattens out to give even contrast across the field. This lens would suit portraiture at f/2 and landscape at f/5.6, but not architectural work.”

This kind of technical understanding leads to predictive power. It allows the photographer to select the right tool for the job, understand what compromises are acceptable, and even correct for certain flaws in postprocessing. Just as the wine connoisseur can describe why a wine tastes as it does and whether it’s worth cellaring, the technically informed photographer can choose and use a lens with precision, getting more consistent results and often saving time, money, or disappointment.

That deeper knowledge does not preclude appreciation; it enhances it. As with the wine, the enjoyment remains, but it’s enriched by understanding.

How did the most well-informed wine taster get to that point? Like the other two, she probably tasted a lot of wine. But that level of awareness requires more than that. One path to knowledge is something called component tasting. The way that works is that person setting up the test starts out with some generic wines and adulterates them with things that skilled wine tasters are supposed to be somewhat quantitative about. Some common adulterants are:

• Tartaric acid. To increase acidity and help tasters identify sourness and freshness.
• Citric or malic acid. Sometimes used alongside or instead of tartaric acid to demonstrate different types of acidity.
• Glycerol (glycerin). To increase the perception of body and viscosity without affecting sweetness or alcohol. It’s naturally present in wine in small amounts.
• Ethanol. To increase alcohol level and illustrate warmth, body, and aromatic volatility. Food-grade ethanol is usually diluted to simulate real wine conditions.
• Sucrose or glucose-fructose solution. To raise residual sugar and demonstrate sweetness and balance
• Tannin powder. To demonstrate astringency and bitterness.
• Oak chips or oak extract. To introduce vanillin and lactone aromas associated with barrel aging.
• Volatile acidity (acetic acid). Added in small, controlled amounts to demonstrate flaws or complexity depending on concentration.
• Sulfur compounds (e.g., SO₂).

In the posts to follow, I hope to do something similar for lens aberrations to what component tastings do for wine qualities. I plan to show the visual effects of various aberrations, alone and in combination. I hope that careful study of the images that I will post will help people learn how lens aberrations affect images, how they interact, and will ultimately assist them in making sound purchase and use decisions.