Mr. KG, Thanks for starting a new thread!
Generally, and with variations within each, there are four main types of optical microscopes. Biological, dissecting, metallurgical and USB. But what do we need for blade edge microscopy? I’ll note some pros/cons for each type in consideration of blade edge microscopy.
Biological microscopes:
This is the type of microscope most folks think of when they think of a microscope. This type is NOT good for blade imaging.
Pros:
Good price/performance ration.
Very high magnification.
Stable frames and accurate focusing.
X/Y stage available.
Cons:
Biological scopes are designed for glass slide mounted specimen observation with the light source below the microscope stage. Light passes through the mounted specimen to the lens. Viewing a blade only produces a black silhouette of the edge. It does not work, and is not designed for using other than transmitted light through the specimen.
Extremely short working distance. The working distance is the space between the objective (lens) and the stage, in other words, where you would slide the blade under the lens. This can be as short as a few millimeters (+/-) in biological scopes. Lenses are available to increase working distance, but they are very expensive and the scope is not designed for incident illumination (light reflected off the surface of the subject).
High power = short Depth of Field/Focus (DOF). High power biological scopes have extremely shallow DOF. Depending on the magnification and aperture size this can be measured in um.
Dissecting microscopes:
Dissecting microscopes are good for looking at blades. They are designed for viewing the surface of 3D objects using incident lighting. Great for dissecting and general inspection of parts including live view on a computer screen.
Pros:
Extremely large working distance. We’re talking 100 mm or more. You can put your hand under the lens.
Sturdy frames and accurate focusing.
Very long DOF.
Large field of view.
Usually zoomable over a large magnification range.
Many have stereo eyepieces.
Halogen lighting.
Good for attaching a camera to.
Cons:
Low magnification. Generally 50X or less, some 80X with 4X eyepiece. Still very useful, but in many instances will leave you wanting for more magnification.
Metallurgical microscopes:
If you have $1,200.00+ US to spend on a microscope, this is probably what you want. They are professional instruments with high quality optics, designed or looking at metallic surfaces. Many different versions available. Top/bottom viewing, lighting options including through lens, good choice of high quality glass. Available in bright-field and dark-field versions. I won’t go into the difference here, but dark-field is probably ideal. as it scatters the light source to reduce reflection and increase contrast.
Pros:
High magnification. 800X not uncommon.
High quality optics.
Incident halogen lighting.
Large working distances.
Good DOF.
Sturdy stands and accurate focusing
X/Y stage available.
Great for camera work. Excellent image quality.
Available in brightfield/darkfield versions, some with darkfield adapters so you can have both. These usually are more expensive.
Cons:
Prices range from expensive to a whole lot more expensive.
USB microscopes:
This is what we all end up with because they range in price from almost free to only a couple of hundred dollars. They include software for image viewing/saving on a computer. The major downsides are that they are very difficult or next to impossible to get high quality imagery from, and they are difficult to use.
That said, they are actually useful, and I have posted many examples of USB microscope images here on the Exchange. Sadly, it’s what I use. I’d be lost without it. I consider a microscope, even a USB microscope, and an edge sharpness tester absolutely essential. Anyone interesting in edges needs both.
Pros:
Extremely inexpensive.
Very long working distances. 3 mm – infinity.
Portable.
Computer ready.
Great fun for ages 4 and up.
Cons:
Feeble, crappy, junk for frames if they have one at all. Most are worthless.
Very fussy focusing. Can be a frustrating hair pulling freak show to focus.
Crap for optics. Junky glass/plastic lenses, low resolution, fuzzy images.
Poor light transmission due to low quality glass or plastic lenses.
Cheapo crap for lenses causes lots of bizarre artifacts and reflections in the images.
Top LED lighting surrounding the lens. Produces lots of reflection and rainbow effects off steel.
Top LED lighting is probably the worst kind of lighting for reflective surfaces.
Optical magnification limited to about 200X. Much above that and focusing and DOF become a nightmare.
Requires you to build a frame to make focusing slightly less of a nightmare.
Many have a nightmare rotating knob on the barrel for focusing. Impossible to focus without moving the scope.