Fundamentally, lenses are pretty simple: they take light from a scene and focus it so that your camera can make a picture. Practically speaking, though, choosing which lens is the right lens is surprisingly complex. Choosing lenses is one of the most powerful choices you can make as a photographer.
In this tutorial we’re going to look at what makes a good photographic lens. You'll learn what characteristics define a good lens and how to choose lenses that match your needs. There is no perfect lens, at any price: even the best lenses have issues. The key is to choose lenses that maximise the characteristics you want and minimize the characteristics you don't want.
What Happens When Light Enters a Lens?
The simplest lens possible that can be used to project an image is a single convex lens. When a ray of light hits the lens glass it is refracted, changing the direction of the light. With a convex lens, all the rays of light will converge at the focal point of the lens. The distance between the lens glass and the focal point is called the focal length of that lens. Although the exact positioning of the film plane varies between manufacturers, the film plane is placed at the focal point so that the focussed light from the lens creates a small, projected image of the scene.
While all this is great in theory, it assumes perfect lenses and ignores many of the more esoteric properties of light. A simple convex lens just won’t create a satisfactory image due to optical aberrations.
Optical aberrations are caused by two things: the imperfect geometry of lenses and the differences between how different wavelengths (colours) of light pass through them. In a simple convex lens, these aberrations will be incredibly noticeable. However, by adding additional elements, each designed to correct for specific aberrations, lens manufacturers can minimise their effects.
Adding additional elements isn’t a perfect fix. Every time light is transmitted from one lens to another a little bit of light is lost. The more elements in a lens, the more light and colour information that is lost in transmission. There is a trade off between correcting for optical aberrations and preserving information.
Our Simon Bray has an excellent article on the fundamentals of lens design, if you want to know how more about lenses are made. Two of Dave Bode's courses, A Photographer's Guide to Light and What Every Photographer Should Know About Lenses, go into depth on how light and lenses work.
Edge to Edge Sharpness, Optical Aberrations and Elements
Modern lenses are designed to do two things: create the sharpest possible projection across the entire image plane and minimise optical aberrations. Compared to older lenses, they use a lot more, and a lot more complicated, lens elements.
The focal plane of a lens isn’t perfectly flat. Due to spherical aberration it actually has a subtle curve. Digital sensors, for the most part, are flat. This means that if the centre of the image is perfectly focused, the edges won’t be. In order to have edge to edge sharpness, lots of different elements need to be added. Many modern lenses will have upwards of ten elements. For example, the Sigma 50mm f/1.4 (which is widely praised for sharpness) has 13 elements.
Not all elements are created equally. Complex lenses, like aspherical elements, are added to compensate for the aberrations present in spherical elements. It’s very easy for manufacturers to get caught in a pattern of adding more elements to fix the problems created by the previous ones!
While there is nothing inherently wrong with high element count lenses they do reduce light and colour transmission, which affects how the resulting images look.
3D Rendition and Micro-Contrast
Older lenses have far fewer elements than modern lenses. These makes them more prone to optical aberrations and, in particular, it means they often have very soft edges. On the other hand, they can have some incredibly pleasing characteristics. Unlike resolution or sharpness, these characteristics are a lot more difficult to measure with a chart.
Lenses with fewer elements are generally better at rendering details, so they images they create appear to be more three-dimensional. Obviously, all details are actually rendered as a two-dimensional image, but the appearance of three-dimensions relies on subtle details that are lost by the over-correcting of optical aberrations and the light passing through too many elements. A lens with only a few elements produces images that have a greater apparent separation between background and foreground subjects and more accurate drawing of geometric shapes. As elements are added to a lens, the three-dimensional effect diminishes. This is what’s responsible for the flat, clinical look of modern lenses. Canadian photographer Yannick Khong has a great article that digs deeper into this issue.
Micro-contrast is the small amounts of tonal and colour variance between details on a subject. It’s an incredibly subtle effect but it is often what sets excellent glass—like Leica, Zeiss, Canon L series, medium-format or large-format lenses—apart from cheaper lenses. Micro-contrast is what gives areas of consistent colour a realistic and accurate texture. It is among the first detail to be lost when light passes through too many elements. While it’s not essential for a great photograph, it is one of the things that adds character to an image. Again, Yannick Khong has written a detailed take.
The Hard Decision
So on one hand we have modern lenses with great resolution, edge to edge sharpness, minimal aberrations and lots of elements. On the other, we have older lenses with fewer elements, more aberrations, soft corners and great (but hard to measure) three-dimensionality and micro-contrast. Which is the better lens?
As with so many things, the answer is: it depends. Modern glass is more expensive. You can get older lenses with fewer elements at a bargain on the second hand market. If budget isn’t a major factor though, the choice gets harder.
What it really comes down to is what subject matter you’re working with. If you are shooting portraits with a low aperture, all the background-foreground separation will come from depth of field rather than any lens effects. You might notice a little two-dimensionality in some facial details and the loss of tonal variation in the blurred background, but you should be okay.
If, though, you’re a landscape photographer, you might want to get as much three-dimensionality and texture as possible in your photos. As long as you’re working with narrow apertures, the loss of sharpness at the edges shouldn’t be too noticeable.
You need to work out for yourself where your needs fall on this spectrum. There will always be trade offs, so identify the qualities you want most in your lenses, and then choose the ones that have them.
Evaluating a Lens: Selection Criteria and Essential Questions
Let's go through all the qualities and points to consider when evaluating a lens. I've broken it down into four sections: technical quality, aesthetic quality, construction, and feel.
Technical Qualities of the Lens
Technical quality is a good starting baseline.
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Focus: Does the lens have autofocus, and if so, what kind of autofocus? Newer methods are usually faster quieter, but not always. What focus modes do the lens allow?
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Focus breathing: Does the focal length change dramatically depending on where you're focusing? How will this affect any images you plan to make?
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Colour rendition: How does the lens render colour? Is it incredibly accurate it or does it mute some tones?
- Lens coatings: How are the lens elements coated? Coatings are an effective method for minimising chromatic aberration but they can negatively affect light transmission. If you're shooting black and white, less coated elements is better. For colour, you need multiple coatings to deal with aberration.
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Coma: How are off-axis point sources of light rendered? Does the lens suffer from a lot of comatic aberration or do they remain points?
- Distortion: Barrel and pin cushion distortion affect the straight lines in your images and wide angle lens are more prone. How are the edges of the image distorted by the lens? Is the distortion too much for your needs?
- Light falloff: How does light fall off towards the edge of the image? Is there a heavy vignette or is the image evenly exposed?
- Use with filters: Can you use the lens with standard 77mm screw on filters or is it an unusual size, needing adapters?
- Flare and ghosts: How does the lens react to off-axis light sources? Do they cause dramatic lens flares and ghosting? Do you want this effect?
- Lateral colour fringes: How much does the lens correct for chromatic aberrations? Are all your images going to end up with colour fringes or will they be barely visible?
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Macro focusing and minimum focusing distance: How close can the lens focus?
- Sharpness: How sharp is the lens wide open? What about at its sharpest aperture? Is it sharp from corner to corner or just in the centre? Do you even need tack sharp images?
- Spherochromatism: Similar to lateral colour fringes but caused by a different optical property. Will they be a problem?
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Sunstars: How will bright light sources look at smaller apertures? How many blades is the aperture diaphragm? That's what determines the appearance of sunstars.
Aesthetic Qualities of the Lens
A lens might not be technically perfect but still have aesthetic character that makes it worthwhile.
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Lens family: Is the lens based on a classic design? There are plenty of imperfect-but-aesthetically-pleasing lenses that have become cult favourites. Maybe a particular photographer used this lens in their most famous works?
- Bokeh character: How do the bokeh look at wide apertures? Do they add or detract from the kind of images you want to create? Are they unusual or interesting in any way?
- Microcontrast: How does the lens render small changes in colour? Does it flatten them out or leave microcontrast intact?
- 3D-ness: How does the lens render 3D objects? Are things compressed so they appear on the same plane or do they maintain their fullness?
Lens Construction
The lens housing is almost as important as the lenses themselves.
- Material: What's the lens made of? Metal lenses are hardier but plastic lenses are cheaper. Do you need tough lenses?
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Focal length: What's the lenses focal length? Is this a focal length you want to work with?
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Angle of view: What's the adjusted focal length of the lens if you're using something other than a 35mm/full-frame camera? Is that too wide or tight for your needs?
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Maximum reproduction ratio: For macro shots, what's the ration between the size of the subject and it's reproduced size on the centre?
- Hard infinity focus stop: Does the lens stop focusing at optical infinity or can you focus past that? Do you need to be able to quickly focus to infinity or can you eyeball it as you work? Are you going to be working in a wide variety of environmental conditions where optical infinity will noticeably change?
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Focus scale: How small are the focus increments? If you're going to be doing a lot of manual focusing, the smaller the better.
- Depth-of-Field scale: many older manual-focus lenses have markings on the barrel to help you do hyperfocal focussing.
- Filter thread: What size is the filter thread? Is it common? Do you have the filters that fit it? Is it metal or plastic?
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Mount: How is the lens mounted to the camera, and is it compatible with your camera? Is it a plastic or metal mount? How far does the flange project into the body?
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Size: How big is the lens? You do have to carry it. Some lens manufacturers like Sigma and Sony make big lenses while others, like Leica and Voigtlander, make smaller lenses.
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Weight: How heavy is the lens? Again, you have to carry and work with it.
- Caps: How does the lens cap attach? Is it easy to remove? Is it going to fall off easily?
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Hood: Does the lens come with a hood to protect against lens flare?
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Case: Does the lens have a case? Is it capable of protecting the lens?
Lens Feel
This last measure is entirely subjective. How does the lens feel when you use it? Is it balanced with your camera? Do the aperture ring and focusing controls move smoothly? Is it a nice beefy portrait lens, good for working slowly? Or is it a diminutive pancake lens, perfect for street photography? Lens feel is, in some ways, the most important characteristic. You can read internet reviews all day long about 'perfect' lenses but it's the lens in you hands that you'll use in the real world. It has to feel good.
Wrapping Up
In this tutorial I’ve looked at what makes a good lens, or rather, what characteristics can be used to define a good lens. Whether you prioritise edge to edge sharpness and resolution over the more subtle three-dimensionality and micro-contrast is up to you. Certainly modern many-element lenses have a place, as do older and simpler lenses.
Finally, it’s worth noting that I’ve been tacitly focussing on prime lenses. It’s almost impossible to have a functioning zoom lens with only a few elements. For zoom lenses, you’re always going to be leaning towards flatter images.
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