Totally digital macro / microscopes are becoming increasingly common i.e. those that display the image on a screen rather than offer optical eyepieces. Coupled with their accompanying software, they can offer valuable features for the study of a wide range of subjects and can be particularly appealing in an educational setting*. The spectrum of models available spread from cheap toys, to smartphone add-ons, to handheld / stand supported microscopes through to sophisticated modular systems. Quite naturally, a key specification provided by a maker for the potential buyer to judge which model best suits is its magnification (or range). These can typically vary from 0.1X to as high as 7000X.
*Digital compound microscopes are not considered here, i.e. compound microscopes that have a built-in sensor and attached screen rather than an optical eyepiece. They can be regarded as a hybrid that falls between two stools. As they use traditional compound microscope optics, they potentially retain the resolution capabilities of compound microscope objectives (putting aside that the often modest sensor / small screen may limit resolution) but don't offer the benefits of a dedicated digital microscope such as depth of field, extended working distance and handheld capability. Both toys and professional models are on offer.
Also offered, in parallel to digital systems, are the huge range of optical macro / microscopes where visual studies remains the primary observation method. When choosing an optical microscope, the magnifications of the objectives, eyepieces and the total mag for each combination are frequently provided. The experienced microscopist becomes very familiar with what a given optical mag on both a stereo or compound microscope can deliver.
Many potential purchasers of a digital microscope may be coming from an optical microscope background. In which case, a potential source of confusion can arise because the screen mags of a digital microscope are very different from the total optical mags offered on an optical microscope. This isn't helped by the fact that digital microscope sellers don't always state how the mags of their models are defined. My colleague Mol Smith when reviewing the Nurugo Micro add-on for a smartphone was unable to obtain from the makers how they defined its cited '400x' mag. Those unfamiliar with digital microscopes may believe that a model with say a 400x mag has comparable capabilities (in particular resolution) to that of a 40x objective when used with a 10x eyepiece on an optical microscope, when optically an add-on for a smartphone as Mol observed for the Nurugo is typically equivalent to a 10x - 20x hand lens.
After studying a selection of makers' model information, I offer below short notes on interpreting the specifications of digital microscopes when stated and judging how their capabilities compare to an optical microscope.
Shown above is a very simplified schematic of a solely digital microscope i.e. with no optical eyepiece (not to scale!).
The appropriate mags are defined for the discussion below as M, Mo and Ms.
Optics
- Models with either a fixed mag or zoom optics are frequently offered for dedicated digital microscopes (or add-on for a smartphone).
Subject to optics distance - The working distance is a key spec the maker should declare to judge how practical the model will be for its planned use. Many models have a clear collar extending beyond the optics to maintain a fixed and steady working distance.
Optical mag, Mo - This is the magnification factor of the optics alone when they project an image of the subject onto the sensor. It could be regarded as equivalent to the primary objective mag on a compound microscope.
From the various makers' specs studied to date, Mo is not stated for any model irregardless of whether the lens system is interchangeable. For the latter e.g. as sold by Keyence and Hirox, only the screen mags M for each objective are stated.
The value of Mo will either be fixed or variable if a zoom mag system.
It's not vital to know this spec but can be very useful for an optical microscopist to judge how a digital microscope's native optics compare with the compound or stereo microscopes designed for direct visual use that they are familiar with.
Sensor to screen mag, Ms - This magnification is inherent in any digital system because the image on the small sensor is being projected onto a potentially much larger screen. Its value is dependent on the horizontal width of both the sensor Sw and that of the display screen Dw (not the screen diagonal). Ms = Dw / Sw.
An additional magnification will occur if digital zoom (in or out) is a feature of the screen (e.g. on smartphone touchscreens) or of the dedicated imaging software.
The sensor sizes, even of sophisticated digital microscopes, can be quite modest and coupled with large display screens, the value of Ms can be large, typically 40 - 70x or more.
Some makers state the sensor size used, in which case the sensor to screen mag can be readily calculated:
Hirox - Ms is ca. 42 (Sw = 7.2 mm, Dw = 302 mm, models KH-7700, KH-8700).
Dino-Lite - Ms is ca. 68 for a selected model (Sw = 5.7 mm, Dw = 390 mm, model AM7515MZT, 20x - 220x if take the 2592x1944 pixel sensor as the larger 1/2.5" design).
Keyence - Ms is ca. 71 (Sw = 7.18 mm, Dw = 509 mm, modular system VHX-5000).
Screen mag (or mag range) of the system M = Mo x Ms - This fixed value or range is invariably the only type of magnification specified by a maker for a given model. As Ms can vary widely and typically an order of magnitude higher than Mo, the screen mag is not a very meaningful spec, especially if the user chooses a display screen of a very different size to that for which the maker defined the model's mags.
Horizontal field width of subject - This spec is by far the most valuable because it is independent of the screen size used. All the main makers have tables declaring these for each model's mag settings (and working distances.) A potential buyer assessing models is likely to know the sort of subject sizes, or parts of subjects that they wish projecting across whatever display width is being used whether smartphone display, tablet or desktop monitor. The mags can then be circumvented as a means to choosing a model.
Sensor resolution - The resolution of optical microscopes are dependent on the effective numerical aperture of the objective in use. For the resolution of subjects with a digital microscope, the primary optics still have a key role with the appropriate sensor / screen resolution to display this detail. Native sensor resolutions can vary widely, e.g. 640x480 pixels for some budget models, 1600x1200 pixels for some current Hirox and Keyence modular systems, 2592x1944 (5 Mpixel) for Dino-Lite's high resolution range. Some makers use techniques to optimise higher resolution for a given sensor e.g. by using 'pixel shift' and/or selecting lower wavelengths of light.
If the sensor's physical size is stated, the optical mag at a given screen mag setting of the microscope can be calculated as Mo = HFW / Sw. Where HFW is the horizontal field width declared by the maker for that mag. The 20x - 220x Dino-Lite AM7515MZT if it uses the typical larger 1/2.5" sensor (not declared) for its 2592x1944 resolution will have an optical mag range Mo of ca. 0.3x - 3.2x.
For high resolution sensors, the display screen's resolution may limit the observed detail if it does not at least match the sensor resolution. Captured and saved images, if a feature, may offer the full capabilities of the optics / sensor.
Observations
The maker's stated screen mags for each model are only of value if the screen size is defined and if the user adopts the same size. Surprisingly, one of the major makers Dino-Lite do not make it clear what screen size they use. They do state the more valuable fields of view, so together with declared screen mags, it can be readily calculated that the mags are defined for a 20 inch screen diagonal (confirmed after emailing Dino-Lite). Keyence clearly state that for all their models the mags are
for a 15 inch (diagonal) screen and can be recalculated for the larger displays they offer. Hirox systems define their mags for the 15 inch display (or for same image viewing area on larger screen) which forms part of their modular systems.
The very modest primary optical mags in systems with up to ca. 250x screen mags are, of course, why such digital microscopes can be handheld at practical working distances. This has the advantage of better depths of field but comes with the penalty of far lower resolutions cf the same total mags on an optical microscope. It would be near impossible to handhold a system based on the typical medium power compound microscope objective with less than 1 mm working distance! For the low to modest screen mag settings of a digital microscope below ca. 30X, the optics can often be acting as image reduction systems, akin to the sub-1X optical zoom settings often seen on wide range zoom stereos.
The optical specification familiar to the compound or stereo microscope user, numerical aperture (or spatial resolution), is almost never stated, even for the modular systems (nor its range for a zoom system). Keyence do note the max NA for one of their lens ranges although if comparable to zoom optic stereo systems, the working NA is likely to be critically dependent on the zoom setting. For the potential buyer where a given spatial resolution was required for their work, demonstrations of the equipment using subjects of interest may be required.
A key aspect of microscopy is appreciating a sense of scale. Because screen mags can be so uninformative and not comparable to the capabilities of optical microscopes at the same total mag, it could be argued, that anyone using digital microscopes in an educational setting should be particularly wary of using mags to describe what students are viewing. This especially applies if a range of screens are being used e.g. from smartphone to desktop monitor or TV. Instead, the declared horizontal field widths should be used, or even better, encourage students to measure these for themselves. The software provided with many models often allows the screen image to be readily calibrated as a precursor to measurements of subjects studied.
An example of the limited value of screen mags in microscopy education was the BBC TV programme "Miniature Britain" first shown in December 2012 (YouTube clip of equipment set-up). An hour of prime time TV devoted to the wonders of the macro and microscopic world should be applauded and very fine footage and commentary was presented too. The programme solely used a splendid state-of-the-art Hirox KH-8700 modular digital microscope which does have some very neat features, particularly suited for broadcast work.
The presenter remarked it was a "revolutionary new microscope camera that is 7000x more powerful than the human eye" which press reviews the next day picked up on. A visit to the maker's website to inspect this claim reveals that it offers this for the highest mag lens (estimated optical mag 167x), when presented on the display screen (from the specs the sensor to screen mag 'Ms' for the system is ca. 42X). But much of the footage shown was macro / modest micro and didn't use this lens.
Viewers and programme review readers may have thought that such studies were well beyond their pocket, but similar visual studies could be enjoyed by a user of a 10 - 20x hand lens or budget stereo for the macro and a sub-£100 compound microscope for subjects like the tardigrades and plankton shown. Present the results from the 40x objective via a 12.5x eyepiece with attached budget HD webcam onto a typical domestic 40 inch TV and the enthusiast can claim close to a 5000x mag when the camera field of view is a relatively modest 0.18 mm (for my setup). So a rare opportunity was lost to encourage greater interest in hands-on studies of the miniature world around us using very simple and cheap equipment.
Comments to the author David Walker are welcomed (who promises to put his soapbox away by next month).
A number of the digital microscope makers offer a variety of informative literature, notably Keyence who have some superb downloadable documents describing all aspects of the modern digital microscope, their potential advantages over solely optical systems, the underlying technology and the features they can typically offer.