Magnification versus Resolution

The below table is about a photograph's resolution capacity, which relates both to pixels and output image size. If a camera's resolution is insufficient, an image cannot be sharp. The other factors affecting image quality are the microscope's performance, preparation or suitability of specimen and the resolution of the unaided human eye (able to distinguish two points 0.1mm apart). 

DPI - Dots Per Inch

Modern quality lenses should never be a limiting factor here. However, using a compound microscope under oil immersion, or with the 'high dry' lens, the image could be enlarged past 1000 x, and past the limit of resolution imposed by the wave-length of 'white' light. This would exceed the useful magnification which is possible, and the image cannot be sharp. 

Other reasons for an unsharp image are bad focus, thick specimens (using a compound microscope 2µm sections give best resolution) or specimen movement. It must be clearly understood that microscope or specimen related problems are totally divorced from the performance of a camera.

How many MP do I require?

The above table is a guide for selecting a camera size (pixels) which suits your requirements. These cameras mostly differ in resolution - number of pixels. Note that magnifications always compare lengths; with cameras we consider the number of pixels along the longer axis divided by dpi. Since computer screens (they measure the diagonal) and pixels are given in inches we have retained that measure here; to convert inches to mm multiply by 25.4. The column '72' relates to common computer screen and Internet resolution. So if the requirement is to display images on the screen or post these on the Internet, they may be enlarged to the indicated inches.

Requirements for printing are more stringent and many publications require 300 dpi, which curtails 'enlargeability'. 

Additionally, in many photos the subject, does not fill the frame and when printing, part of the original is cropped. Sometimes this is deliberate as under a dissecting microscope, depth-of-field (or apparent focus) is most effectively increased by taking the photograph at a lower power and then enlarging by printing the centre part of the image only. The last column '300 dpi/4' assumes that only a quarter of the photograph is printed, which would greatly increase depth-of-field, but lower resolution markedly - unless the print is small.

Further Information

• en.wikipedia.org/wiki/Dots_per_inch
• en.wikipedia.org/wiki/Pixels_per_inch
• en.wikipedia.org/wiki/Image_resolution
• en.wikipedia.org/wiki/Display_resolution
• Image Resolution Calculator