This might seem like a very obvious question, but the moment you try to define a set of criteria to quantify ‘good’, you soon realize there’s quite a lot more to lens performance than immediately meets the eye. So, for those of you without the ability to try a large number of lenses – let alone samples of the same lens – how do you know if the one you’ve got is ‘good’?
When evaluating a new optic, I look broadly at several categories; this article will explain them a bit further. It’s wroth keeping in mind that most of these evaluations are relative and/or subjective; something that might perform well on a larger, lower-density sensor and balance properly on a large DSLR might not do so well on a very high density crop factor camera without a built in grip. It is therefore important not to consider ultimate resolution alone, but how well the system works together as a whole. A pancake zoom on a GM1 is a match made in heaven; you can mount an Otus on it, and it’ll deliver better resolving power, but it won’t be practical to use simply because of balance and manual focus/ stability/ ergonomic issues. Remember: the job of a lens is to collect and relay information, in the form of wavelength
(color) and spatial position to the recording medium. The less it affects the information along the way, the better.
This is perhaps the most important and most easily defined category. Resolution at maximum aperture (and various points in the zoom range, if applicable) is obviously the priority; but one also has to consider whether the lens is symmetric: optical designs may intentionally be compromised (e.g. for long zoom ranges) to deliver high centre performance at the expense of corners or edges; however, they won’t intentionally be sharper on one side than the other. That would be an indication of manufacturing issues. The better the lens, the higher the resolution across the frame it will deliver at faster apertures. It isn’t difficult to make a f5.6 kit zoom with a moderate focal length range that delivers decent performance across the frame, but it’s quite something else if it delivers the same resolution at f1.4 and in the corners.
Correction for aberrations
In order for a lens to deliver high performance in the corners and at fast apertures, it must be well corrected: this is to prevent different wavelengths of light focusing at different points – both laterally and longitudinally. If the focal plane for different wavelengths shifts laterally, then you’ll see spectral separation in the form of lateral chromatic aberration, or blue-red/ green-red fringing. If you see it in front and behind your plane of focus, that’s longitudinal chromatic aberration. Lenses designed to minimize or eliminate this for visible wavelengths are usually given an Apochromatic or APO designation. Note that this doesn’t say anything about spreading/ ‘smearing’ of point light sources – that’s coma, and it’s possible to have an optical design that suffers from coma without having chromatic aberration (and vice versa). Aberrations that cause different wavelengths to focus at different spatial points for a given focus distance setting will land up reducing resolution because they affect the lens’ ability to resolve fine detail structures – i.e. edges – giving the overall impression of being a bit soft.
This is a property of a lens that has to do with its ability to resolve the very finest detail structures – i.e. of low contrast and high spatial frequency – a lens with ‘good’ microcontrast is able to do this for structures that are close together/ fine and similar in luminance. Although you can increase the apparent gross contrast of a lens by sharpening, it’s almost impossible to ‘fix’ microcontrast because you cannot generate information that simply wasn’t recorded by the sensor to begin with.
Macrocontrast and flare
Overall contrast – recorded luminance between brightest and darkest areas of a scene that take up a significant portion of the overall image width – is quite dependent on the coatings of the individual elements; in order to maintain high contrast, you need to not have any stray light bouncing around inside the lens and landing up on different portions of the image than the area from which they came. A lens that has high flare has high internal reflections and poor coatings; this will affect both global (macro) and micro-contrast. It is especially important if you’re shooting into the light, as this will exacerbate the problem. The very best lenses show very little to no flare and have very good coatings.
I consider this to be the least of the optical maladies, since it’s easily corrected for in post processing – dark corners are not really a big deal so long as they aren’t completely black.
If you’re losing light internally, or it’s being reflected out again at each lens-surface interface, then you’re not collecting it at the imaging plane. It is impossible to make a lens surface that has 100% perfect transmission, though it’s possible to minimize losses to the point that the vast majority of light makes it to your recording medium. Lenses with low flare and high contrast typically also have very high T (transmission) stops; the closer the T stop to the f stop (geometric aperture, or focal length divided by effective entrance pupil), the better the lens. Very good prime lenses have T stops that are usually within 0.1 of the F stop – f2/T2.1, for instance – whereas kit zooms that have an f stop of 5.6 may well have a T stop of 8 or lower. Not all f stops are equal – look at the shutter speed to give you clues.
Transmission also affects color rendition: a lens needs to have equal transmission across all wavelengths of the spectrum in order to record an accurate reproduction of the scene; these are neutral. Lenses with color casts are attenuating portions of the spectrum, resulting in an overall shift in colors. Though the sensor or recording medium can affect this, you can generally see the difference between a very neutral lens and one that’s attenuated in a certain color. Neutrality is obviously desirable because it means you’ve got more information to work with later.
We change gears a bit here and look at the behaviour of the plane of focus as the lens is stopped down: if it moves backwards or forwards, then the lens exhibits focus shift. This means that you need to consciously adjust your focused distance in order to get the theoretical depth of field you expect; there can be significant differences between maximum aperture and say f4-5.6. Better lenses will employ floating elements to automatically correct for this as the focusing group is moved to the desired distance; this is computed as part of the lens’ optical formula.
This is when the apparent magnification of a lens changes as you change focus distance; frequently, lenses will shorten their effective focal length in order to focus closer – this is so they require less helicoid extension and therefore can be made more compact. Although a consumer superzoom may reach its labelled maximum focal length at infinity, you will find that at minimum focus distance, your 18-300 might well be giving you magnification equivalent to 150mm. Let’s just say there’s a reason those 300mm supertelephotos require long barrels, but can only reach around 1.5m minimum focus distance: they do not shorten in focal length. The very best lenses do not exhibit focus breathing at all – cine lenses come to mind; they are designed this way to enable changes in focus plane (‘pulling focus’) to not change the composition of the scene. Few stills lenses are designed to compensate for this, partially due to size, partially because you can always recompose between shots – something that isn’t always possible in a video sequence. In fact, the only stills lens that immediately comes to mind that does not exhibit focus breathing is the Zeiss 1.4/55 Otus Distagon.
Ever have issues with your corners not being sharp, but something towards the front or rear of the frame being in focus instead? This is due to field curvature. If you see a resolution test where the sharpness (resolution) drops off alarmingly towards the edges of the frame, it’s probably because of this; it means that the plane of focus isn’t so much a flat, two-dimensional plane as a section of a much larger sphere. It isn’t always a bad thing because it can create a more three-dimensional effect to the image by making the edges effectively more out of focus than they should be. But it is worth noting for flat-field work – e.g. macro and reproduction.
Straight lines should render as straight lines: if there are a lot of elements in the lens to correct for other aberrations across various focal lengths, then chances are they’re going to introduce some nonlinear projection. Some types of distortion – simple spherical pincushion or barrel – are easy to correct for; others like ‘wave’/ ‘moustache’/ or ‘sombrero’ types are trickier, and worse still, tend to vary with focal length. Of course, extreme distortion of any type – simple or not – is not desirable as correction may well result in a change in effective field of view.
Performance at distance
‘Macro’ lenses are designated as such not just because they have shorter minimum focus distances, but also because the optical design has to be optimized for near performance where the object and image distances are relatively similar. This requires (usually) extra glass and nonlinear helicoid movements at closer distances; which of course costs more. There’s a reason why a cheap 50/1.8 with an extension tube does not perform the same as a dedicated macro of the same focal length. By a similar token, not all lenses (including some macros) do well at infinity, either; faster lenses usually start to be compromised at longer subject distances. Very few lenses are good throughout the range, and most will have a ‘sweet spot’. It’s important to take this into consideration if you’re wondering why your fast prime does fine at f1.4 and 3m, but not f1.4 and infinity.
To be continued in part II. MT
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