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The aperture is responsible in photography for opening the objective lens, thus controlling the amount of light reaching the camera’s sensor. It is, along with exposure time and ISO light sensitivity, one of the key parameters controlling an image’s exposure to light. There are some special features relating to the aperture in digiscoping.

© Tormod Amundsen

Photographers use “f”-numbers when talking about apertures: f/1.4, f/8, etc. “f/” stands for the aperture and the number describes the size of its opening and the focal ratio between the focal length and aperture. The higher the number after f/, the smaller the aperture. So, if we’re talking about a small aperture, we refer to f/8, f/16, f/22, etc. But if we’re talking about a large aperture, there will be smaller numbers (e.g. f/1.4, f/2, f/2.8).

The special aspect with digiscoping is that the aperture cannot be varied. It is preset by the field of view and objective lens diameter of the long-range optical devices (spotting scope or binoculars). However, a lack of light can be compensated for using state-of-the-art technologies to sensibly increase the ISO light sensitivity (a higher ISO value means brighter pictures at the same shutter speed). Many camera models (e.g. Nikon D300) allow you to input an f-number manually as a menu option. The camera uses this f-number to calculate the shutter speed – as soon as there is no lens available – allowing you to continue using the aperture priority/value (A/Av) function.

Many camera models (e.g. Canon 7D) now have an automatic ISO setting, with the relevant ISO sensitivity being automatically controlled by the camera. However, you must ensure that the exposure times don’t get too long as the cameras are usually not “aware” that a spotting scope or one with very long focal lengths is being used. You are therefore recommended to stick to the range permitted for the automatic setting.

If a camera objective lens is used (in combination with the DCB II digital camera base), the aperture can actually be varied, but it will barely have any impact on the formation or brightness of the image.

Aperture table for spotting scopes

Aperture at
Min. zoomMax. zoom
ATX 6511.5 (25x)27.7 (60x)
ATX 85  8.8 (25x)21.2 (60x)
ATX 95  9.5 (30x)22.1 (70x)

Apertures for 25-50x W eyepieceApertures for 20-60x eyepiece
Min. zoomMax. zoomMin. zoomMax. zoom
ATS 6511.5 (25x)23.1 (50x)9.2 (20x)27.7 (60x)
ATS 80  9.4 (25x)18.8 (50x)7.5 (20x)22.5 (60x)