Squash preparation of onion root tip cells stained to reveal chromosomes, Bright field illumination, light microscopy. This image was taken with the X40 objective lens. Depth of focus is extremely low at this magnification. Smaller apertures give greater depth of focus, but image quality deteriorates and resolution is greatly reduced. The aperture in this image was optimised for contrast and resolution. Chromatic abberation is inevitable in light microscopy, but has been kept to a minimum in this image.