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  • Writer's pictureMaria Chernetska

How f-numbers work?

The real f numbers themselves indicate the number of times the effective diameter of the aperture divides into the focal length of the lens. So f/2 means define an opening diameter of half the focal distance; f/4 is a quarter, and so on.

The system operates because each f-number takes into account two major factors that control the brightness of an image:

1 Distance from objective to the image.

For remote subjects (objective centered on infinity), the image is formed at a focal distance from the objective. The inverse square law of light shows that doubling the distance of a surface from a light source quarters the light it receives. This is why a lens of (say) 100mm focal length forms an image only a quarter as bright as a 50mm lens.

2 Diameter of the bright beam.

Doubling the diameter of a ring increases its surface four times. Now you find that both lenses operate at relative apertures of f/8 (100 12, and 50 6), which is correct since their images correspond in brightness. So: f -number = focal distance effective aperture diameter In practice, the ratio f-number to brightness deteriorates when working very closely because the

Lens-to-image distance will then be very different from a focal length.

The f-number parameters are also commonly called "shutdowns". At the start of the photograph, long before the iris diaphragms, each stopper was a thin piece of perforated metal with a hole of the required size, which you slipped into a slot in the lens barrel. Hence photographers speak of ‘stopping down. The opposite action is ‘opening up. In practice, you will notice that the upper and lower limits of the f-number scale vary according to the objectives.

Most low-profile camera lenses stop at f/16 or f/22.

The bigger targets are designed to go down to f/32 or f/45.

Smaller openings are useful for additional depth of field, but if taken to extremes, diffraction begins to destroy image details. For this reason, no target will stop at the size of a pinhole.

The f-number of the maximum aperture of your lens, as well as its focal length, name, and individual part number, are engraved on the edge of the lens. You can find the same hat, two lenses in the brand, and focal length, one is almost twice the cost of the other because it has a maximum Aperture one stop wider. This can be a high price to pay for the ability to take photos in low light or use faster shutter speeds, particularly when you can buy an excellent ultra-fast film. However, the "faster" lens can have a higher overall optical quality.

As always, it is a question of performance against price, but note that digital SLR particularly benefits from high-quality optics.

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