comparable to that of conventional lenses (e.g., a 100 mm 2.8 lens has a 100 mm/2.8 = 35.7 mm entrance-pupil diameter). Because they focus at close subjects, the cone of light from a subject point to the entrance pupil is relatively obtuse (a relatively high subject numerical aperture, to use a microscopy term), making the depth of field extraordinarily small. This makes it essential to focus critically on the most important part of the subject, as elements that are even a millimetre closer or farther from the focal plane might be noticeably blurred. Due to this, the use of a microscope stage is highly recommended for precise focus with large magnification, for example when photographing skin cells. Alternatively, more shots of the same subject can be made with slightly different focusing lengths and joined afterwards with specialized focus stacking software which picks out the sharpest parts of every image, artificially increasing the perceived depth of field of the resulting image. Lighting The problem of sufficiently and evenly lighting the subject can be difficult to overcome. Some cameras can focus on subjects so close that they touch the front of the lens. It is difficult to place a light between the camera and a subject that close, making extreme close-up photography impractical. A normal-focal-length macro lens (50 mm on a 35 mm camera) can focus so close that lighting remains difficult. To avoid this problem, many photographers use telephoto macro lenses, typically with focal lengths from about 100 to 200 mm. These are popular as they permit sufficient distance for lighting between the camera and the subject. Ring flashes, with flash tubes arranged in a circle around the front of the lens, can be helpful in lighting at close distances. Ring lights have emerged, using white LEDs to provide a continuous light source for macro photography, however they are not as bright as a ring flash and the white balance is very cool. Good results can also be obtained by using a flash diffuser. Homemade flash diffusers made out of white Styrofoam or plastic attached to a camera's built-in flash can also yield surprisingly good results by diffusing and softening the light, eliminating specular reflections and providing more even lighting. Chromatic aberration Many macro lenses are characterised by a high amount of chromatic aberration, especially when using reversed-lens, extension tube or close-up lens. Some macro lenses, called apochromatic lenses, are designed