Assorted poppycock: macro zooms,
and format-relative focal length
On this page I discuss a few examples of (intentionally or otherwise) misleading statements and demonstrably false argumentations frequently found in promotional, and sometimes technical, literature on photographic equipment.
For the past five decades, this classic among misleading photographic terminology has been applied to virtually all zoom lenses capable of a reasonably short minimum focusing distance. To put things straight:
- A zoom lens is a lens with a variable focal length. Almost always, the focal length is manually changed by rotating a ring on the lens barrel. In early zooms, focal length was changed by manually sliding a part of the lens barrel forward and back, and in a handful of cases by rotating an external dial. The principle is the same: this manipulation changes the position of some of the lens optical groups, and focal length changes accordingly.
- Most zooms are parfocal, i.e. focus does not change when the focal length is manually changed. Especially in legacy zooms, parfocality was only approximative, and after changing focal length it was necessary to re-focus a little. Even modern zooms typically slightly deviate from parfocality, but this is not evident when using autofocus.
- Some lenses of variable focal length are not parfocal at all, and require major refocusing after changing the focal length. These lenses, properly speaking, should not be called zooms. They are, more correctly, called varifocal lenses. Examples are found among some legacy enlarger lenses used in automated "photolab" machines, and can still be found today among lenses for surveillance videocameras. In the latter case, the focal length and focus are set only once, based on the desired field of view, and thereafter locked at a fixed value.
- Properly speaking, a modern macro lens is a lens capable of a magnification (see below) continuously variable between infinity focus (where magnification is zero) and 1x without using external adapters or add-ons. Legacy macro lenses older than roughly 30-40 years start at infinity and may stop at 0.5x, and require extension tubes or dedicated optical adapters to go higher. A couple of modern macro lenses reach slightly above 1x, and a few reach 2x.
- Some lenses have been called ultra-macro lenses, but this is not a standardized term and has been used with different meanings.
- For lenses that provide a maximum unaided magnification range not exceeding roughly 0.25, I prefer to use the term close-up lenses, and photomacrography lenses for those that start at a magnification at or above 1x and go higher (e.g., the Laowa 25 mm 2.5x-5x).
- Some special-purpose lenses for small-subject photography cannot focus at infinity on their own, or focus at all (e.g. lenses meant to be mounted on bellows). They are generally called bellows lenses. They are designed for a given magnification range and, accordingly, are sometimes called close-up lenses, macro lenses or photomacrography lenses. However, they can often be used outside their design magnification range (albeit not optimally) and their magnification range is a function of the bellows or extension tubes being used.
- At a short notice, I cannot come up with any example of a lens called macro zoom by its makers that focuses continuously from infinity to 1x. Most of these lenses require turning the focal-length ring to a dedicated "macro" position, or to use a dedicated switch or clutch. Thus, their focusing range is not continuous. Their magnification range may even have a "hole" of unattainable magnifications between the normal and macro settings. Even if we are generous, these zoom lenses should rather be qualified as close-up lenses, or more precisely general-purpose lenses with limited close-up capabilities.
- A single legacy zoom lens, the Nikon Micro-Nikkor 70-180 mm, qualifies as a macro lens, and is specifically designed for this purpose. It requires no switching between "normal" and "macro" modes, and reaches 0.75x at the 180 mm focal length (henceforth FL) setting with good image quality.
- The optical quality of the large majority of macro zooms, particularly the cheaper ones, ranges from indifferent to quite poor at their macro setting, and often display poorly controlled geometric deformation and curvature of field. They can be used in a pinch, but certainly cannot compete with proper macro lenses.
For the past couple of decades, the concept of magnification in macrophotography has been obscured by numerous misleading, often plainly wrong statements frequently found in the promotional materials for camera lenses, and sometimes in more technical literature. To sort out the truth and discard the rest, we need to go back to basic principles. Magnification (M ) is the ratio between image size (Si , i.e. the image of the subject projected onto the sensor) and subject size (Ss ), i.e.:
M = Si / Ss
The above formula is a complete definition of magnification. It tells you all you need to know. For example, it tells you that magnification is a dimensionless number: if you have mm at the numerator and mm at the denominator of the fraction, they cancel each other out and the result is just a number, devoid of any measurement units. Thus we can say, for example, that M = 1 (which we usually write as 1x, where x means times, although the x is mathematically not required). For example, the formula allows us to answer the following questions:
- Does magnification depend on sensor size? Look at the formula. Do you see the sensor size anywhere in the formula? No? Then magnification does not depend on sensor size. A 1x magnification is always 1x, regardless of sensor size. The lens knows nothing and cares nothing about the camera mounted at the rear of the lens. Placed at a given distance from the subject, the lens projects an image of constant size onto the focal plane, regardless of the size of the sensor you place at the focal plane. It still projects an image of the same size on its focal plane even if there is no camera at all at the focal plane.
- Does magnification depend on focal length? Look at the formula. Do you see the focal length anywhere in the formula? No? Then the magnification does not depend on focal length.
As an example, Olympus announced in 2016 the 30 mm f/3.5 macro lens for Micro 4/3 cameras.
Multiple web sites, including Olympus' own, erroneously report a 2.5x maximum magnification for this lens. This stems from a habit by Olympus writers of promotional materials of specifying magnification as a factor relative to full frame (henceforth FF), i.e. the 135 film frame format, which of course is pure nonsense. Maximum magnification is a dimensionless constant for a given lens and distance between lens and subject, and is completely independent of sensor size. A 1x lens always produces a 1x magnification, whether on full-frame, APS-C, Micro 4/3 or any other format. What changes with sensor size is the field of view at the same magnification, which is a whole different thing.
The Olympus 30 mm f/3.5 macro lens produces a maximum magnification of 1.25x. Period. No ifs, no buts and no sensor format equivalents. Don't let anyone tell you otherwise or try to confuse you.
Olympus, and OM System as their heirs are presently known, have frequently stumbled on this pitfall, including in the descriptions of their 60 mm macro (erroneously described as reaching 2x, but in reality reaching 1x) and the future 90 mm macro (erroneously reported as capable of 4x magnification, while in fact only reaching 2x).
Expressing magnification as a format-relative number, besides showing that the writer is ignorant of the definition of magnification, opens the door to all sorts of errors and misunderstanding, especially if the "relative to FF format" part is left out (as often done) in the specifications and in the calculations based on the latter.
Format-relative focal length
Sometimes, to give an idea of the angle of view produced by a lens, the actual focal length is compared with the focal length required to produce the same angle of view on a full-frame sensor. This is often shortened by saying, for example, that a lens with a focal length of 300 mm designed for use on Micro 4/3 is equivalent (in angle of view) to a 600 mm on full-frame.
For one thing, it is becoming questionable to continue to regard 36 x 24 mm film and sensors as the "golden standard". While senior photographers may have started out with film SLRs and can still be expected to remember the once ubiquitous 36 x 24 mm film frames, the younger photographers who started out directly with digital cameras are unlikely to have ever handled a can of 135 film. Camera sensors smaller than full frame are now very popular and no less "professional" than full frame, and cameras with sensors larger than full frame will likely become more affordable in the near future.
The problem with expressing focal length as a format-relative number is that, like format-relative magnification, this is nonsense from the point of view of optics. A 300 mm FL lens always has a 300 mm focal length, regardless of the sensor size and regardless of whether there is, or there isn't, a camera at the focal plane. The lens does not magically become a 450 mm if mounted on a camera with a smaller sensor.
One of the consequences of using format-relative focal length is that it easily leads to a wrong estimate of the working distance. For example, a 30 mm Micro 4/3 lens is said to be format-equivalent to a 60 mm on FF. The 30 mm, approximated for simplicity to an idealized thin lens, at 1x has a working distance of 60 mm (i.e. twice the FL). If we work backwards from the FF format-equivalent 60 mm FL, we would instead expect that at 1x the lens will have a working distance of 120 mm, which is poppycock if applied to the 30 mm lens. The working distance of the 30 mm at 1x, just like the focal length of the lens and its magnification range unaided by accessories like extension tubes, remain the same regardless of sensor size.
I can see how an old photographer who used 35 mm film cameras for most of his life may still find it necessary to think in terms of full-frame equivalents, even though he may now be using only a Micro 4/3 camera. I may unkindly think that such a photographer would do better to hang up his camera in a closet and call it a day. I may even suspect that many long-time professional photographers prefer to use full-frame digital cameras for no other reason that in this way they don't need to change their mental habits to accommodate different focal lengths and different sensor sizes. To me, this shows how dangerous habits can be, once established. The world changes fast, but habits do not. As a former biologist, I cannot avoid thinking of Darwin's often-forgotten reasoning that, in a changing world, it is neither the strongest nor the most intelligent of the species that survives, but the most adaptable.
As for myself, I used 35 mm film cameras for several decades, and afterwards switched to digital APS-C, Micro 4/3 and full frame. I still use both Micro 4/3 and full frame, but I do not need to think in terms of format equivalents. I instinctively know what a 300 mm and a 23 mm look like on Micro 4/3, and a 24 mm and a 135 mm on full frame, without doing any conscious mental math. Besides, zoom lenses are now the rule except at extremely short and extremely long focal lengths, in very fast lenses, or in lenses for special applications (including macro lenses), and this eliminates much of the advantage of being able to visualize the results before mounting a given lens on the camera. I find most discussions of format equivalents, with all the pitfalls they involve, annoying and fruitless.
Incidentally, everyday terms like macro photography may be imprecise, or plainly wrong. For example, consider the term photomicrography, which is universally accepted to mean photography through a microscope. The apparently similar term microphotography means a completely different thing, and is not a synonym (albeit on occasion it is mistakenly used as such). By analogy with photomicrography, photography of small subjects with a camera and lens could be called photomacrography. However, the story goes that a writer of photographic promotional material, tasked with writing a blurb about a new macro lens, made up the term macrophotography because he did not know any better. Like many other mistakes, this incorrect term stuck fast, and became predominant.
Nikon used to call its dedicated macro lenses, designed for a range of magnifications between roughly 0.1x and 20x, Macro-Nikkor (with a hyphen) or Macro Nikkor (without hyphen, for small lenses lacking a sufficient space for the complete name). Subsequently, instead of continuing to call newer lenses for small-subject photography Macro-Nikkor, they switched to Micro-Nikkor (initially with a hyphen, then dropped the hyphen after a number of years). The earliest Micro-Nikkor lenses, like the 70 mm f/5, are very similar in all respects to the Macro-Nikkors, including the lack of a focusing helicoid, so I can see no valid reason for the name change. As a result, in the Nikon world, to make macro photography you need a Micro lens. In the wider world, a microlens is a lens of microscopic size, e.g. the microlenses covering pixels in a general-purpose camera sensor.
Since the term photomacrography has no established meaning except as an infrequently used synonym of macrophotography, I prefer to differentiate between macrophotography and photomacrography, and to use the former for photography at magnifications up to 1x (since it is now far too late to correct the original sin of this term), and the latter above 1x (because there is otherwise no available term for distinguishing between the two magnification ranges). In addition, a common way to use a general-purpose camera lens at high magnification is to reverse the lens, so photomacrography is - kind of - a reversed macrophotography where the image is bigger than the subject.
Admittedly, my choice of terminology is both subjective and non-standard. However, even a non-standard term can legitimately be used, if defined with sufficient precision at the beginning of a discussion, and if no exactly equivalent standard term is available as an alternative.
This choice of terminology was convenient as long as most macro lenses had a 1x maximum magnification, but it is becoming questionable, now that camera lenses focusing from infinity to 2x, thereby straddling the borderline between macrophotography and photomacrography, are becoming more and more common.
Basic photographic concepts like magnification, focal length, what is a macro lens, and what is macro photography have been widely twisted, distorted and misunderstood in promotional materials, and sometimes technical literature. You need to know the basics to avoid being hopelessly, and sometimes intentionally, mislead by literature that tries to make these simple ideas look like complex, nuanced and arcane knowledge that requires mere mortals to unquestioningly accept as golden truth the arbitrary interpretations and misunderstandings of the self-appointed experts and high priests. See for example "Principles and methods of obscurantism" for practical applications of the same obscurantist principles in scientific literature.