Date: 2010 Oct

Fig 1. V434B stereomicroscope, $450

MicroscopeNet.com is a distributor based in Canada of generic, no-name microscopes and accessories. It's similar to the California-based Amscope; both apparently import unbranded products from China.

Amscope implies that you'll be getting branded products but without the brand label: "This microscope is made by the same technicians and on the same production line as optical instruments for Leica, Zeiss, Nikon and Olympus". "Its retail value is about $3,000", says Amscope of a scope they sell for $699.

So the prices are good, but how well do these generic scopes perform? Amscope says things like "This microscope offers high resolution and good depth within a broad field of view. It provides crystal clear sharp stereo images" but neither Amscope nor MicroscopeNet offer specifications, such as line pairs per millimeter for resolution or numerical aperture for the objectives.

Both offer a 7-day evaluation period, but shipping cost is on you, both ways, if you decide to return it.

After exchanging emails with MicroscopeNet and getting good impressions, and talking around town, I decided to give MicroscopeNet a try; I like that they are in Canada (as am I), eliminating potential customs/border issues which remain despite a free trade agreement (NAFTA) with the US. I ordered a V434B trinocular stereomicroscope, pictured in figure 1, priced at about $450 (C$ ≅ US$). Two days later, it arrived in a well-packaged box.

Mechanical

Generally the V434B was as described. It's not a multi-thousand-dollar instrument so I expected some 'rough edges', and I found a few, including:

• The zoom control has resistance at a certain point (at about 3.5x) when zooming in, easily overcome by backing off a bit and then resuming travel toward full zoom, or just applying more pressure. It's not a problem, but it can be confused with being at the end of the zoom range.
• The one focus knob is a bit stiff to operate. It needs to lift the weight of the scope and any camera mounted, which is probably why it is so stiff. There is no fine focus knob. With just the single, stiff focus control, fine focusing is a bit of a challenge.
• The diopter adjustment rings on each eyepiece are so large diameter that they begin to intrude on space between the eyepieces for one's nose.
• The phototube adjusts focus by turning the threaded tube (the black portion in the photo), which is a bit wobbly when near its full extension (stabilized by the locking screw, visible at the back in figure 1).

The lighting controls are smooth and work well, though of course there is a colour shift as the halogen bulb brightness is adjusted.

The stand and components are solid and aligned. I like that the post mount allows for considerable vertical range, and that the optical unit rests in a ring, thus it can be rotated or removed and used independent of the stand.

Optical

Subjectively, the V434B delivers sharp images of quality comparable to an Olympus SZ61, Meiji EMZ-TR, and Acuter 700, and better than a Wild M4A. Even at 4.5x zoom with 20x eyepieces, there is less aberration than observed in more expensive equipment. Alignment seems fine; I haven't noticed any eye strain, etc.

Resolution (based on the Rayleigh Criterion) of the microscope with three different cameras was measured using three different methods (see appendix, below) at selected zoom settings, obtaining the following results:

* Resolution at 0.7x predicted by extrapolation, not measured

The table below shows the field of view as observed through the provided 20x eyepiece at various zooms, and also through the phototube with a 10x Wild eyepiece (not provided with the package), as photographed with Canon A570. The column 'Min pixels wide' is a calculation based on the measured resolution (from the table above) and the width of an inset no-vignetting camera frame (4:3 aspect ratio), with a pixel sampling factor of four (for details, see How many pixels does a microscope camera need?). The fields of view are of course affected by the eyepiece used. If the camera has insufficient pixels, it will not be able to capture all the detail provided by the microscope (the image will be fuzzier than when viewed via the eyepiece), especially at lower magnifications (higher field of view).

Adding a 2x Barlow lens from MicroscopeNet created significant chromatic aberration (blue fringes).

Conclusion

The V434B is good value, ie., good performance and utility per dollar. In that ratio, the main benefit is in the denominator, ie., the price is great. The quality is good, seemingly comparable to some name brand models.

Perhaps we're benefiting from large numbers of microscopes being manufactured in China for their industrial assembly lines, research, etc, with mass production driving quality and price improvements. If so, presumably this will put pressure on the prices of name-brand equipment. Or maybe this is a 'manufacturing second' with some flaw I haven't yet noticed or consider significant.

Caveat: I'm not a professional microscopist. If you have suggestions or information that would improve this page, I'd be pleased to receive comments.

Related pages

Measuring the resolution of a microscope

Characteristics of the Canon A75 and Canon A570 IS digital cameras

Adapters for afocal optical coupling (mounting a camera on a microscope).

Appendix: How microscope resolution was measured

Three methods were used to measure resolution of the V434B microscope: Airy pattern, FFT of images, and MTF of a slanted edge.

Airy pattern

See Measuring resolution for information about the procedure used here.

The numerical aperture of a microscope can be obtained by measuring the Airy pattern produced. In the photomicrograph below (figure 1) of an integrated circuit, a few of the surfaces happen to strongly reflect light into the stereomicroscope objective, creating strong diffraction patterns that are visible where they spill into adjacent dark areas. The inset image in figure 1 shows the diffraction pattern at the pixel level. Green lines were placed by eye to mark the minima.

Fig 1. Integrated circuit, photographed with Canon A75 on a V434B stereomicroscope at 4.5x

Figure 1 was obtained with the V434B at 4.5x zoom via the phototube with a 10x Wild M12 eyepiece and a Canon A75 camera focused at infinity at maximum zoom (3x, or 16.2 mm). The camera produces JPEG images with in-camera image processing (eg., sharpening).

In this configuration, each sensor pixel corresponds to 1.030 μm in the subject plane (determined by photographing a 1 mm scale with the same microscope set-up). Assuming an average wavelength of λ = 550 nm (yellow-green) for the observing light, we get:

NA = 550 nm / (2π 1.40) =  0.063 

Fig 2. Airy rings, using a Canon A570
on a V434B stereomicroscope at 4.5x

To check this result, I repeated the procedure with an Canon A570 IS, which has a pixel pitch of 1.87 μm. Figure 2, at the right, shows an Airy pattern photographed with the A570 at full 4x zoom (23.2 mm) (only the green channel intensity is shown). Based on those rings, using the same method as above, the estimated NA is  0.058 .

Fig 3. Airy rings, Canon Rebel XT
on a V434B stereomicroscope at 4.5x

To determine whether the camera lenses or built-in imaging processing or the phototube eyepiece were influencing the result, I tried projecting the image directly onto the sensor of a Canon Rebel XT with no camera lens or phototube eyepiece, just a plano skylight filter on the body for dust protection. The pixel pitch of the XT is 6.42 μm. Raw images were used, with no sharpening or other post-processing in their conversion to TIFF. To obtain magnification sufficiently high to resolve the diffraction bands, the camera body was mounted on a tripod with approximately 20 cm of space between the top of the phototube and the flange of the camera. Figure 3, left, shows an Airy pattern (green channel intensity). Based on those rings, using the same method as above, the estimated NA is  0.064 .

The resolution as measured by the Rayleigh Criterion is the Airy radius (the distance to the first minimum), then using the figure 1 result:

r_Airy = 0.61 λ / NA = 0.61 λ / 0.063 =  5.25 μm_subjPlane (at 4.5x zoom)

By the Rayleigh Criterion, two points of light on the subject plane can be resolved if separated by more than the Airy distance, r_Airy. But information is still available from subject plane objects that are smaller than r_Airy, especially if they are isolated, or too dim to generate visible rings and a broad central peak, as can be assessed by looking at figure 1; note the 5 μm scale bar in the lower-left corner.

Figure 1 is a 100% crop. Figure 1's inset and figures 2 and 3 were created by zooming the camera-produced images in Photoshop to make individual pixels visible (outlined by thin white lines in figure 1 inset), then doing a screen capture and analysing the captured image.

FFT

See Measuring resolution for information about the procedure used here.

Two targets were used: a straight edge razor blade coated with carbon (this was the MTF target, discussed in the next section), and a piece of metal with 'random' scratches and stains (see figure 4). Both targets are expected to have higher spatial frequency content than can be passed by the microscope. The targets were photographed at four zoom levels: 1.5x, 2.5x, 3.5x, and 4.5x. The results presented are from a Canon A75, and were checked with a Canon A570 and Rebel XT with no lens, both of which yielded similar results.

A 512 x 512 pixel well-focused area of each photo was selected and then processed with ImageJ. First contrast was maximized (to normalize the FFT) using Process->Enhance Contrast, and then a FFT generated (Process->FFT->FFT).

The length of the perpendicular line in the FFT, caused by the edge in the subject, reflects the maximum spatial frequency passed by the microscope and camera. Having the subject edge slanted separates it visually from the energy at the axes that are artifacts of the FFT process.

Similarly, the radius of the fuzzy ball created by the random scratches on metal reflects the maximum spatial frequency passed.

Fig 4. Straight edge (left) and scratched metal (right), with FFT's below
Zoom 1.5x
Zoom 2.5x
Zoom 3.5x
Zoom 4.5x

The maximum spatial frequency declines with zoom strength. The estimated FFT values are given in the spreadsheet fragment in figure 5, below. Of course the estimates are qualitative, somewhat arbitrarily based on where the 'fuzzy white area' (which is probably dependent on display factors) seems to fade out. MTF (discussed in the next section) is based on FFT analysis and provides more quantitative results.

Fig 5. Resolution, derived from maximum spatial frequency in FFT

The values in columns B and C are from ImageJ. The scale values in column E were obtained by photographing a millimeter scale. Column G is simply column B / column E; similarly for H. The resolution figures are thus in subject plane units of length. The NA figures are calculated as NA = 0.61 λ / resolution, using λ = 550 nm.

The results in figure 4 also tell us that Canon A75 is a reasonable match for the microscope in this zoom range. If the 'FFT cloud' were to extend to the edges of the plane, as it nearly does at 1.5x zoom, there is a risk of aliasing or, if it goes beyond 2 px/cycle (the Nyquist frequency), losing information. But likely at 0.7x zoom there would be too much spatial frequency information for the A75 to capture.

MTF

See Measuring resolution for information about the procedure used here.

The MTF (Modulation Transfer Function) of an optical system is a measure of how well the system transmits contrast, as a function of spatial frequency. An MTF of 1 is perfect transmission, and of 0 is no transmission. Typically MTF varies inversely with frequency. Points of interest are MTF(50), the frequency at which contrast has been reduced by half (50%), MTF(30), where contrast has been reduced to 30%, and MTF(9), which is the frequency at which contrast is as at the Rayleigh Criterion of resolution.

MTF is computed from an image of a contrast edge slanted about 5% with respect to the camera sensor rows. This was done at four zoom levels of the V434B stereomicroscope, once using a Canon A75 and again with a Canon A570, mounted on the trinocular port via a 10x eyepiece. The cameras were zoomed to their maximum (3x and 4x, respectively), focused at infinity, and with maximum aperture. The results were processed with two MTF software packages, Imatest and QuickMtf. The results are shown below.

Fig 6. Resolution, derived from MTF(9), at various zoom level. Canon A75

All figures with micron units refer to length on the subject plane. Scale was determined by photographing a millimeter scale. Resolution results for MTF(9) are highlighted, as those are the Rayleigh criterion resolutions.

Here are the results using a Canon A570:

Fig 7. Resolution, derived from MTF(9), at various zoom level. Canon A570

Ideally, a sharp edge slanted at 5 degrees would make the transition from low to high ('rise distance', from 10% to 90%) on the camera's sensor over a distance of 1.25 pixels (not zero pixels, due to the slant). Non-ideal images, blurred by the loss of high frequency components, have a longer transition. Thus the rise distance is a measure of resolution. These measurements are recorded in columns P and Q in the tables above, both in sensor pixels and subject plane microns.