Introduction
How do we select a lens for a subject? This depends on many factors. Sometimes, one of those factors may take more of a priority over another. Say, for example, I am a landscape photographer. However, what if I am a landscape photographer on an extreme budget? Well, I might suggest looking for the most bang for buck camera and lens combo one could probably buy. A older Nikon DSLR such as a D800 in great condition, with perhaps a reasonably priced mid ranged zoom, or maybe an older mirrorless body with an accompanying lens suited to their shooting style. If money is no object, or say, less of a concern, the game changes a bit. They might be splashing out on a Nikon Z 8, or a Sony a7R V etc and the very latest zoom or prime kit. The same is the case when we select a lens for shooting. The priority is balanced with budget and intention, how the image will be used and displayed etc. This said, even despite the D800 being quite old now, it has phenomenal dynamic range and image quality. (In capable hands, I still consider it world class. It is only budget now, because it is older).
However, getting back to lenses, what if we really think about why some lenses are better suited to certain shooting applications? If we put initial cost of owning a lens aside, how do we then select a lens after that? Should we shoot landscapes with a lens tuned for portraiture? What about the other way around? Should we try shooting the night sky with lenses that were never intended for that purpose? What happens if we do these things? Why can’t lenses be perfect for every subject?
Balance
A lens designer is an architect of optics that must balance science with art. This isn’t dissimilar to a photographer. We hopefully balance technical craftmanship with art, to hopefully produce an image that produces an idea, a feeling, a journey, an expression of emotion. Someone I speak to often about lens design - someone who knows way more about this than me, once told me that lens design is a balancing act; a veritable horse trading of parts - a give and take in a very real sense. The designer can trade and optimise one thing, which will perhaps be to the detriment of another optical property. And this is why, truly great lenses often fall into different categories, of course by design. This is exactly why, we should at least consider which lenses we are using to shoot which subjects. Would another lens work better? Many of us do this by looking at reviews - I will myself admit I do enjoy doing this. Despite doing so however, for many years I have begun to question the efficacy of tests for a lens clearly designed to be shot (or is optimised overall) for infinity shooting, being tested with a close up scene in a studio or a garden? What would that really tell me about a lens that was clearly optimised for something else entirely?
We have never lived in more sophisticated age than we do presently for lens design, and it has only gotten better. Lens design is dramatically improved vs decades ago by computer aided design, and relatively speaking, manufacturers’ are able to provide us with excellent lenses compared to thirty years ago for reasonable prices. 35mm format photography now approaches medium format in quality when optimal lenses and technique are melded together. The designer has access to tools they simply did not have in those times past.
Aberrations
All optical equipment has to contend with the properties of light, with regards to how it refracts through lens elements. An aberration simply refers to the deviation of a light ray through a lens causing blurred images, or areas of an image which are blurred / degraded in quality. There are many types of aberrations which can hamper final image quality that the designer must contend with and balance. The big ones are, vignetting, distortion, field curvature, chromatic aberration, comatic aberration, spherical aberration and astigmatism. Many types of aberration improve when stopping a lens down through it’s aperture range, however some do not - which dismantles the old cliché that all lenses are the same by f/8: this is simply not true.
Vignetting - This is a peripheral shading (darkening) of the side frames and corners of the image. This tends to affect faster lenses more and always improves when stopping down, due to the aperture blades of the lens obstructing the outer light path as the lens is stopped down.
Distortion - An obvious optical problem is when the lens distorts things like vertical or horizontal lines or objects. Lens distortion is different from perspective distortion (the distortion we see when using an ultra wide lens on a person close up). Distortion can either be barrel or pincushion, or sometimes a mixture of both.
Field Curvature - This occurs when light rays do not attenuate to the same focal plane, we are left with potential dead spots were the lens isn’t sharp (or it is less sharp). Field curvature can come in different types, wavy etc.
Chromatic Aberration - There are two types. ‘Longitudinal CA’ occurs when different wavelengths of colour do not converge at the same point after passing through a lens, also known as bokeh fringing. The second type, called ‘lateral CA’ occurs when different wavelengths of colour coming at an angle focus at different positions along the same focal plane.
Comatic Aberration - Also known as ‘coma’ is an aberration that occurs when light rays from the edges of the image pass through glass elements over the changing shape of the lens elements, they vary in magnification and become stretched out. This aberration only affects off axis light. In astro work, stars on the periphery / corners of the frame can literally look like little comets.
Spherical Aberration - This occurs when light rays focus on different planes after passing through a spherical surface. Rays that pass off axis refract more than rays passing horizontally through the centre of the lens and thus cause this. The rays that pass through the elements off axis refract so much they can focus in front of the intended focal plane. This can cause a blur / glow in images at fast apertures. Sometimes this is desirable in small quantities and is balanced with other aberrations to optimise a lens for a specific task.
Astigmatism - Known as the ugliest optical aberration. Astigmatism occurs when the lens elements fail to focus image lines running in different directions in the same plane. This one doesn’t really go away as we stop down the aperture of a lens (I have previously spoken about how not all lenses are the same at f/8, and this is one of the reasons). As we stop down, the increasing depth of field can mask a lens with some astigmatism in it, but often not fully account for the problem. Astigmatism causes softness to the edge frames in many lenses, in astrophotography it produces winged seagull shaped stars at the edges.
Lenses for Landscape Photography
Loch Garry - Nikon Z 8 with 24-70/2.8S
Generally speaking, most landscape photographers want lenses that portray a very detailed and realistic interpretation of the scene. Landscape photographers need an accurate representation, and where possible, a lens which imparts nothing onto the image other than the scene in front of it. This includes fine detail, such as in grasses and rocks at near and far distances. Generally speaking, a good landscape - tuned lens is optimized to provide as much low, mid and high frequency MTF structure as possible. Controlled aberrations, sharpness balance across the frame (rather than just centrally), control of coma, astigmatism, flare and other optical phenomena are important to shooters in this genre. Lenses for landscape photography use tend to be f/2.8-4 lenses at their maximum apertures. The are usually designed to work best at mid range apertures such as 5.6-f/11 where they will be used most of the time. Lenses for this genre benefit from being sealed from dust / moisture to protect from inclement weather conditions. One thing I would highlight here is, there is a great deal of confusion from our community regarding lens speed. People will often balk at the suggestion of using a fast aperture lens for this genre of photography, because it is likely, mostly, going to be used stopped down in the field. It is understandable why they think like this, however it is a fallacy - we must consider that some aberrations do not go away as we stop down. Sometimes the faster glass just works better overall, even stopped down.
Lenses for Portrait Photography
Boy and his Balloon - Nikon D800 with 85/1.4D prime
Lenses for this genre have completely different aims. The best ones are obviously designed and built to make people look good. Out of focus areas have much more priority in the design, things like spherical aberrations and coma are adjusted in different ways than in landscape lenses to balance the background out of focus elements, with the sharpness of the subject and the focal plane transition areas. Lenses for this genre tend to have faster maximum apertures in order to be able to produce large areas of defocus (bokeh) in front and behind of the subject. These lenses tend to be optimised for use at their widest, or wider first few apertures, whilst still being able to balance scene fine detail when stopped down at close range.
Lenses for Astro Photography
Orion over the Quiraing, Isle of Skye - Nikon Z 8 with 14-24/2.8S
This genre is by far the most testing for any optical equipment. Point light sources, which are essentially what all starlight is to us, is a torture test to even the best of lenses. The three big ones in astro photography that we want to see controlled; are astigmatism (winged stars on the corners of the frame), coma (comet shaped stars out with the central axis) and chromatic aberrations (purple / green halos around areas of high contrast). If the designer can balance this with a good level of sharpness, especially across the frame from wider apertures, and a reasonably fast aperture to boot, the lens should excel on starlight.
Dual Personality Lenses
At the Arcade - Nikon D810 with 24/1.4G, a true dual personality lens
There is a very real situation whereby a lens can be a master in two disciplines. Let us consider, a fast aperture prime lens, for example the 50mm f/1.2S Nikkor. Clearly built as a portrait lens, and designed to work on people, it produces beautiful rendering at it’s first few apertures and at close ranges. However, use it at infinity, stopped down and it technically speaking is an excellent landscape lens also. There are many examples of such lenses. An older design that this applies to, is my 24/1.4G nikkor. Wide open and stopped down to the first few apertures, this lens is a bokeh, focus transitional rendering piece of magic. However, use it on a landscape at f/5.6-f/11 and it is extremely capable. I find this quite curious, and I have often bought lenses with this personality in mind. I used the 24/1.4G for many years as both an environmental portrait and landscape lens. What I like most about a lens like this is it’s ability to give a cinematic look in pictures such as the one I have shared above, and also be very proficient and showing vast amounts of detail and contrast when used stopped down on landscapes. It is perfectly capable in both disciplines. Needless to say, I like owning lenses that can do this, and as much as I like using zoom lenses for landscape photography, I love using primes. on people / objects. I keep the zooms for landscapes now.
NB - I shot most of my landscape work with prime lenses up until very recently.