Magnification

This enlargement is quantified by a size ratio called optical magnification.

When this number is less than one, it refers to a reduction in size, sometimes called de-magnification.

Typically, magnification is related to scaling up visuals or images to be able to see more detail, increasing resolution, using microscope, printing techniques, or digital processing.

Some optical instruments provide visual aid by magnifying small or distant subjects.

For optical instruments with an eyepiece, the linear dimension of the image seen in the eyepiece (virtual image at infinite distance) cannot be given, thus size means the angle subtended by the object at the focal point (angular size).

is the angle subtended by the image at the rear focal point of the eyepiece.

For example, the mean angular size of the Moon's disk as viewed from Earth's surface is about 0.52°.

Thus, through binoculars with 10× magnification, the Moon appears to subtend an angle of about 5.2°.

as the distance of the object with respect to the front focal point.

Note again that a negative magnification implies an inverted image.

The longitudinal magnification is always negative, means that, the object and the image move toward the same direction along the optical axis.

However, the traditional sign convention used in photography is "real is positive, virtual is negative".

[1] Therefore, in photography: Object height and distance are always real and positive.

The maximum angular magnification (compared to the naked eye) of a magnifying glass depends on how the glass and the object are held, relative to the eye.

If the lens is held at a distance from the object such that its front focal point is on the object being viewed, the relaxed eye (focused to infinity) can view the image with angular magnification

In this case the angular magnification is independent from the distance kept between the eye and the magnifying glass.

If instead the lens is held very close to the eye and the object is placed closer to the lens than its focal point so that the observer focuses on the near point, a larger angular magnification can be obtained, approaching

A different interpretation of the working of the latter case is that the magnifying glass changes the diopter of the eye (making it myopic) so that the object can be placed closer to the eye resulting in a larger angular magnification.

Note that both astronomical telescopes as well as simple microscopes produce an inverted image, thus the equation for the magnification of a telescope or microscope is often given with a minus sign.

[citation needed] The angular magnification of an optical telescope is given by

is the focal length of the objective lens in a refractor or of the primary mirror in a reflector, and

The diameter of this may be measured using an instrument known as a Ramsden dynameter which consists of a Ramsden eyepiece with micrometer hairs in the back focal plane.

This is mounted in front of the telescope eyepiece and used to evaluate the diameter of the exit pupil.

With any telescope, microscope or lens, a maximum magnification exists beyond which the image looks bigger but shows no more detail.

[citation needed] 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×.

Small, cheap telescopes and microscopes are sometimes supplied with the eyepieces that give magnification far higher than is usable.

The maximum relative to the minimum magnification of an optical system is known as zoom ratio.

Magnification figures on pictures displayed in print or online can be misleading.

Editors of journals and magazines routinely resize images to fit the page, making any magnification number provided in the figure legend incorrect.

If a picture has a scale bar, the actual magnification can easily be calculated.