Question: Why is the practical magnification limit ~1500x for typical magnification in a light microscope?
About the article "Microscope", section "Optical".

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The most common type of microscope (and the first invented) is the optical microscope.
This is an optical instrument containing one or more lenses producing an enlarged image of a sample placed in the focal plane.
Optical microscopes have refractive glass and occasionally of plastic or quartz, to focus light into the eye or another light detector.
Mirror-based optical microscopes operate in the same manner.
Typical magnification of a light microscope, assuming visible range light, is up to 1250x with a theoretical resolution limit of around 0.250 micrometres or 250 nanometres.
This limits the practical magnification limit to ~1500x.
Specialized techniques (e.g., scanning confocal microscopy, Vertico SMI) may exceed this magnification but the resolution is diffraction limited.
The use of shorter wavelengths of light, such as the ultraviolet, is one way to improve the spatial resolution of the optical microscope, as are devices such as the near-field scanning optical microscope.
Sarfus, a recent optical technique increases the sensitivity of standard optical microscope to a point it becomes possible to directly visualize nanometric films (down to 0.3 nanometre) and isolated nano-objects (down to 2 nm-diameter).
The technique is based on the use of non-reflecting substrates for cross-polarized reflected light microscopy.
Ultraviolet light enables the resolution of microscopic features, as well as to image samples that are transparent to the eye.
Near infrared light can be used to visualize circuitry embedded in bonded silicon devices, since silicon is transparent in this region of wavelengths.
In fluorescence microscopy, many wavelengths of light, ranging from the ultraviolet to the visible can be used to cause samples to fluoresce to allow viewing by eye or with the use of specifically sensitive cameras.
Phase contrast microscopy is an optical microscopy illumination technique in which small phase shifts in the light passing through a transparent specimen are converted into amplitude or contrast changes in the image.
The use of phase contrast does not require staining to view the slide.
This microscope technique made it possible to study the cell cycle in live cells.
The traditional optical microscope has more recently evolved into the digital microscope.
In addition to, or instead of, directly viewing the object through the eyepieces, a type of sensor similar to those used in a digital camera is used to obtain an image, which is then displayed on a computer monitor.
These sensors may use CMOS or charge-coupled device (CCD) technology, depending on the application.
A primary difference between a stereo microscope and a digital microscope is the magnification.
With a stereo microscope the magnification is found by multiplying the lens magnification by the eyepiece magnification.
Since the digital microscope does not have an eyepiece, the magnification cannot be found using this method.
Instead the magnification for a digital microscope is found by how many times larger the sample is reproduced on the monitor.
Therefore the magnification will depend on the size of the monitor.
The average digital microscope system has a 15" monitor, would result in an average difference in magnification between an optical microscope and a digital microscope of about 40%.
Thus the magnification number of a stereo microscope is usually 40% less than the magnification number of a digital microscope.
Since the digital microscope has the image projected directly on to the CCD camera, it is possible to have higher quality recorded images than with an stereo microscope.
With the stereo microscope, the lenses are made for the optics of the eye.
Attaching a CCD camera to an stereo microscope will result in an image that has compromises made for the eyepiece.
Although the monitor image and recorded image may be of higher quality with the digital microscope, the application for the microscope may dictate which microscope is preferred. 
The Hirox Digital Microscope System supports magnifications of up to 7000×.
A primary difference between an optical and a digital microscope is the magnification.
With an optical microscope the magnification is the lens magnification multiplied by the eyepiece magnification.
The magnification for a digital microscope is defined as the ratio of the size of image on the monitor to the subject size.
The Hirox Digital Microscope System has a 15" monitor. 
The magnification through a telescope magnifies a viewing object while limiting the FOV.
Magnification is often misleading as the optical power of the telescope, its characteristic is the most misunderstood term used to describe the observable world.
At higher magnifications the image quality significantly reduces, usage of a Barlow lens—which increases the effective focal length of an optical system—multiplies image quality reduction.
Similar minor effects may be present when using star diagonals, as light travels through a multitude of lenses that increase or decrease effective focal length.
The quality of the image generally depends on the quality of the optics (lenses) and viewing conditions—not on magnification.
Magnification itself is limited by optical characteristics.
With any telescope or microscope, beyond a practical maximum magnification, the image looks bigger but shows no more detail.
It occurs when the finest detail the instrument can resolve is magnified to match the finest detail the eye can see.
Magnification beyond this maximum is sometimes called "empty magnification".
To get the most detail out of a telescope, it is critical to choose the right magnification for the object being observed.
Some objects appear best at low power, some at high power, and many at a moderate magnification.
There are two values for magnification, a minimum and maximum.
A wider field of view eyepiece may be used to keep the same eyepiece focal length whilst providing the same magnification through the telescope.
For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by diffraction. 
With any telescope or microscope, a maximum magnification exists beyond which the image looks bigger but shows no more detail.
It occurs when the finest detail the instrument can resolve is magnified to match the finest detail the eye can see.
Magnification beyond this maximum is sometimes called "empty magnification".
For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by diffraction.
In practice it is considered to be 2× the aperture in millimetres or 50× the aperture in inches; so, a 60mm diameter telescope has a maximum usable magnification of 120×.
With an optical microscope having a high numerical aperture and using oil immersion, the best possible resolution is 200 nm corresponding to a magnification of around 1200×.
Without oil immersion, the maximum usable magnification is around 800×.
For details, see limitations of optical microscopes.
Small, cheap telescopes and microscopes are sometimes supplied with the eyepieces that give magnification far higher than is usable.