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’?
Continued from part I.
Today, we start with what is probably the most subjective of all of the pictorial qualities of a lens. Generally speaking, ‘good’ bokeh isn’t distracting. Distractions are caused by areas of an image that unintentionally stand out; in the case of bokeh it takes the form of double images, hard edges to highlights, geometric shapes of highlights, and color fringing. Few lenses are optimized to also deliver neutrality in the out of focus ares – this requires correction for spherochromatism and longitudinal chromatic aberration. The very best lenses have a smooth transition from in-focus to out-of-focus areas with no odd artefacts in the intermediate zone; subjects should be recognizable but not distracting.
Ergonomics, build and ease of use
I think this category is pretty much common sense: are the rings located easily to hand? Do the switches move accidentally? Is it robust, weather-sealed and does the hood stay in place? Do the extensions wobble if it’s a zoom? Is it front-heavy? Bottom line: do you feel comfortable using it, or do you have to consciously change the way you shoot to accommodate the lens? A good example is the Nikon AFS 200/2 VR: optically, it’s one of the greats. Ergonomically, it always throws me off balance: it’s not very large, but very front-heavy, and the hood is far wider than it needs to be – so it can reverse over the stubby tripod foot that usually creates mounting issues with bodies that have a vertical grip because there’s no longer sufficient clearance between that and a large tripod head.
Autofocus performance – speed, accuracy and ability to precisely accommodate small changes in subject distance – is a function of both the lens and the camera’s AF system. Most of this depends on the type of motor used to move the focusing group; ring-type coreless DC motors (USM, HSM, SWM etc) can move quickly and in small increments. Lenses which are body-driven will have some backlash in the gear train that can cause slight focus ‘misses’ that become an issue with small changes in subject distance and fast apertures. In such situations, it’s often better to completely defocus the lens and start again to eliminate slack in the gears/ helicoid.
Manual focus and spacing/ gearing
Here’s another highly subjective quality. Some people prefer tight rings, others loose; regardless, backlash/ slop is not desirable because it makes small changes in focus distance difficult. This is usually the result of a helicoid with tracks that are slightly loose, or multiple helicoids not meshing properly. You also don’t want a lens that doesn’t have sufficient throw across the distance range – it’s difficult to hit a precise difference if only a tiny movement is required to go from infinity to 2m, and fast gearing also tends to increase the impact of backlash since tolerances remain similar, but will have a larger absolute effect.
Lenses that have increasing reliance on electronic components – motors, stabilisation actuators, focus clutches, distance confirmation chips, buttons, etc – both have a greater potential number of failure points, as well as limiting compatibility. Put it this way: you can mechanically adapt a 50-year old lens to a modern mirrorless body, but modern DSLR lenses may not even have aperture control. Though these protocols can be reverse-engineered, they don’t always work very well…
System matching/ performance
Optical formulae are always derived with an ideal back focus distance in mind. Changing this will inevitably change the optical performance of the lens. Beyond this, digital photography has introduced a new complication: the sensor surface is not flat and relatively homogenous at the microscopic level, but rather takes the form of a collection of pits – each of which contains a photosite. This affects both light collection efficiency and introduces potential optical issues due to shading and interference effects with the walls of the pits. Though modern sensor designs now incorporate microlens arrays to increase light collection efficiency – effectively focusing light from the sensor directly onto the center of the photosite – they also add another optical element into the system. Perhaps the most obvious example of this interaction is when you look at the corners of older wide angle lens designs; the output rays are not telecentric – i.e. they subtend from the exit pupil of the lens at an angle – which creates not only vignetting/ shading at the photosite level, but potentially also causes added chromatic aberration and color shifts due to interaction with the microlenses. In effect, each microlens for each individual photosite is another optical relay within the system: it too has to obey the same laws of physics as a complete lens. Although offset microlenses can compensate for this to some degree, modern ‘designed for digital’ lenses almost always perform better as they tend to be telecentric to begin with – thus negating any potential radial effects.
Ergonomic and electronic issues aside, it is also for this reason that dedicated lenses tend to perform better than adapted ones – aside from potential planarity issues leading to decentering and tilting. There are of course exceptions to both rules, but in my experience with dozens of different sensors and hundreds of lenses, this is generally the case.
Caution on using converters, filters and adaptors
Again, though there are always exceptions to this rule, additional optical elements will degrade overall system performance because lenses were not designed for to accommodate them in the first place. It’s why using a teleconverter on a fast wide will produce terrible results compared to a normal prime, and why superteles tend to not exhibit very much degradation with teleconverters. It’s also why cheap filters are a bad idea: the glass may not be perfectly planar, the coatings may be poor and introduce flare/ reduce transmission, and if they break or crack – you’re still going to land up scratching the front element anyway. As for adaptors between systems/ mounts – just bear in mind that you are introducing an additional pair of mount surfaces into the equation, both of which may potentially decenter or tilt the entire optical assembly. It is even more apparent with a high resolution sensor; scientific tests by various other sites such as Lensrentals have found this to be the case, and it matches with my own experience. Never mind the fact that you’re going to at very least lose AF and gain size/ bulk…
A note on built in lenses
Built-in lenses take two general forms: zooms in utility (‘Swiss army knife’) cameras and high quality primes (GR, Coolpix A, X2 etc). There are also some good zooms – the RX10 and X Vario come to mind. The higher end of this spectrum tends to perform very well indeed; the GR’s lens has a bite to it that is matched by few – if any – interchangeable lens solutions of a similar size. This is simply because these lenses are almost always designed to match the sensor; the entire optical system is optimized from the start. Practically, this means a GR may well be a better solution than looking for a good 28mm-equivalent for your APSC or 24MP FX DSLR.
Lastly: there’s always that special ‘something’ – an indefinable quality that makes you simply like the quality or rendering style of the output. Quantitatively, it’s a combination of many of these factors; however, it can be harder to put a finger on. Although a good lens may have high resolution, flat field, no distortion, uniform spectral transmission and low chromatic aberration, you might also like one that has moderate resolution, strong field curvature and warm transmission because it renders skin and portraits beautifully. I find that lenses with a very distinctive ‘signature’ tend to be quite polarizing: either you like the look, or you don’t; if you don’t, there’s not much point in buying one since you can’t do a lot to neutralize it. However, the opposite is also true: a lens that’s completely neutral and ‘transparent’ adds no character of its own: it’s all down to the photographer. Transparent or not, it’s still down to the photographer. MT
A future article will deal with the increasingly important topic of sample variation.
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