Very often we wonder why sometimes our photos are washed out and soft and sometimes they are sharp and with plenty of contrast and saturation. After all we are using the same camera and lens, right?
Not quite so…
You see, as everything in this world lenses responds to physical laws. If you understand how and why lenses work you will be able to know their limits and explain those unattractive photos 😉
Optics for the uninitiated
To understand why some lenses produce better results than others you’ll need to learn some basic concepts about optics. By no means I’m an expert on this area but I’ll describe them to the best of my ability. I encourage you to read about this topic, there’s plenty of information in Internet.
In a nutshell, lenses work by refracting light waves. Some will disperse the light and some others will concentrate it. In our case we are interested in the lenses that refract light into a “focal point”, hence concentrating the light. Why? because these lenses have the particular property of producing real images which we will capture in our film or sensor 🙂
When we say that light is refracted we are saying that it changes direction. This change is explained by Snell’s law. In short, this law demonstrates how light is affected when moving between two media (air and glass) with different refraction index. Knowing the refraction index of both air and glass and the angle of the light we can predict where the ray of light will be directed. This law enables us to build lenses specifically to concentrate light rays in a focal point to produce an image that can be captured by a film or sensor.
There is another property that affects light when passing through two media with different refraction index, it’s called reflection. When light hits a glass in an angle part of the it will be reflected. The higher the angle the more reflection it produces. This is not good at all because we want the maximum amount of light to pass through the lens (imagine a mirror as a lens, duh!). Normal glass reflects 4% of the light every time it passes from one media to another. In a lens we pass from air to glass and then back to air, we loose 8% of the light!
Light is also absorbed by the media that passes through, decreasing the amount of it.
Chromatic aberration is also explained by Snell’s law. You see, so far I’ve been talking about ray of light as a single entity, but in reality light is a sum of waves with different wavelengths. Remember the experiment with a prism that produces a beautiful rainbow? What the prism is doing is refracting light differently for each wavelength to produce a rainbow. Camera lenses are no different, they will produce different refraction depending on light’s wavelength. This will translate on different focal points for each wavelength causing what is known as fringing. At certain point these differences will show up on the photos; the pictures taken will be less sharp and will have colored boundaries.
How do we fix all of this?
Today’s lenses have very advanced design to overcome most of these undesired effects. For example, to reduced chromatic aberration we have achromatic lenses used to correct blue and red light, or apochromatic lenses to correct red, blue and green. These last ones use fluorite crystals to correct three wavelengths with two lenses. But fluorite is expensive to produce so the industry designed Ultra Disperssion glass (UD). These are not as effective as fluorite ones but they are significantly better than normal glass. And less expensive to produce.
To minimize reflection we have multiple coating applied to the lens surfaces. How does coating a lens help reduce reflection? I will quote Canon Science Lab to explain it:
“Surface reflection can be reduced by applying coatings to the lens surface. You might think that coating the lens surface would block light, but in fact it increases light transmission. This is because light is reflected first by the coating surface, and then by the lens surface itself. The light reflected by the coating surface and that reflected by the lens surface have a phase difference of twice the coating thickness. If the thickness of the coating is one quarter of the wavelength of the light to be suppressed, light of that wavelength reflected by the coating surface and light reflected by the lens surface will cancel each other out. This reduces the overall amount of light reflected. In short, coatings make use of light wave interference phenomena to eliminate reflections.”
Quote from Canon Science Lab
Today, applying multicoating with different materials and using nano-technology results in 99.9% of light transmission.
Old, spherical, lenses produce a big amount of lens aberration because they are unable to converge parallel rays on light in a single focal point. To overcome this the industry came up with a type of lens called Aspherical (non-spherical). These lenses are very complex and require extremely low manufacturing tolerance.
But, what does all of this mean?
The good news, lenses today have advanced designs that eliminates most of the problems seen on earlier times. The bad news? These technologies are expensive, very expensive. This is why the best lenses cost and arm and a leg…
Phew! I’ll go back to take photos now. Cheers!