In some microscopic images, there is a bar scale at one of the corners or near a specific object. How are they added to the images? Does the slide have some markers, or are they digitally added? If so, how are they computed? I am planning to buy a microscope and wondering is there something I should look for. Thanks for any help!
There are two common ways to generate a scale bar:
Measure it. You can take a picture of an object with known spacing such as a grid target and then measure the size of each pixel.
Compute it. If you know the magnification of your objective, and the magnification of any relay optics after the object but before your camera, you can compute the total optical magnification. Then using the pixel size on your camera, you can get the actual pixel size. For example, suppose you have a camera with 7.4 micron pixels, a 20x objective, and a 0.5x adapter lens before your camera, then you would have a pixel size in your image of 7.4/20/0.5 = 0.74 microns.
It depends on the type of microscope you want to buy. Most microscopes come with a slide with precise measurement bars on them and you select the objective and if it is connected to some sort of camera and comes with a software then you can calibrate the scale bar for a particular objective using the software it comes with. Otherwise if you know the pixel per micron for each objective, then once you acquired your images, you can draw a scale bar using a software like Image-J. In terms of what microscope to buy and what to look for, it depends on the application so you need to clarify the question. I have answered a question similar to this here (What to look for when buying a light microscope?).