F number cameracanon

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Some objectives offer only relatively large f-number values because image aberrations could not be properly compensated for lower values. Unfortunately, that limits their light gathering power, which for distant objects is determined by the f-number. Particularly for close objects, the light gathering power can be reduced substantially. That aspect is relevant for macro photography, where exposure times have to be increased accordingly.

f-number calculator

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Most photographic objectives contain a diaphragm (an optical aperture) of variable diameter. It is common not to directly specify the used aperture diameter, but instead the f-number. This is defined as the ratio of focal length and the diameter of the entrance pupil. Specifications are often done in the format f/N, where N is the f-number. For example, f/5.6 means that the entrance pupil diameter is the focal length divided by 5.6. The notation f/# is also common.

The entrance pupil is the aperture stop as seen from the object side. It may not be identical to the physical aperture if there are lenses between the entrance and the aperture.

F-stop vs aperture

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F number camerafor beginners

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Fstop symbol

The f-number of a lens is directly related to the maximum angles of output rays as obtained for parallel input rays: the tangent of that maximum angle is half the inverse f-number.

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These explain quite comprehensively a wide range of aspects, not only physical principles of operation, but also various practical issues.

F-number lens

For photography of small objects over short distances with a substantial magnification (macro photography), the image brightness is substantially lower than one might expect from the f-number. It depends on the working f-number, which is larger (see above).

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Normally, the f-number of a photographic objective can be changed in certain steps, with typical values like 2, 2.8, 4, 5.6, 8, 11, 16 and 22, progressing roughly such that each step (“going up one stop”) reduces the aperture area by a factor of 2, which has two consequences:

F-stop chart

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When imaging an object which is not at infinity, the output ray angles are smaller. One can define the working f-number based on that for given imaging conditions; it is correspondingly larger than the f-number.

f-number formula

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Generally, the term lens speed is often used in the context of photographic objectives. Lenses with low f-number, which therefore allow for relatively short exposure times, are often called fast lenses, while those with high f-number are slow. The lens speed usually refers to the minimum possible f-number of an objective.

The products described in this website constitute only the standard items we manufacture for research and industrial market. One of our major activity is in OEM market where we develop and supply low volume opto-mechanical devices & positioning as per custom design. We also undertake contract manufacturing of opto-mechanical products in standard and custom specifications.