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Aperture and Shutter speeds

Less is often more: correct exposures for digiscoping

The aperture, the shutter speed, and the combination of the two are key parameters in photography. However, in digiscoping the aperture is restricted in one direction. The selected level of magnification determines the largest possible aperture. As a result, Dr. Jörg Kretzschmar believes that varying the shutter speed is the most crucial aspect of achieving perfect shots.

© Tormod Amundsen

Dr. Jörg Kretzschmar is a biologist and discovered digiscoping as a result of his work. He is one of the best-known digiscopers in the German-speaking countries and his shots always arouse a great deal of interest and admiration. For his pictures, he uses an STX 25-60x85 spotting scope with a TLS APO lens system from SWAROVSKI OPTIK.

The general rule for shutter speeds is that they should be at least the inverse value of the focal length. However, where the focal length is equivalent to 2000 mm, a shutter speed of 1/2000 second is more of an exception in digiscoping. But, depending on the type of camera, digiscopers have one big advantage. By choosing a compact or system camera, we can avoid the blurring caused by the shutter release of a DSLR camera. The ISO number can also be set to 800 or even more than 1000 without problems. Depending on the sensor dynamics, current camera models allow for higher ISO numbers, which in turn permit faster shutter speeds. Using a self-timer or remote shutter release it is possible to produce impressive results at shutter speeds up to 1/400 second.

Digiscoping – A special case

The aperture is quite a different consideration in digiscoping. The spotting scope is our lens and this has a fixed aperture which is generally on the far side of 9 (the lowest zoom level of the SWAROVSKI OPTIK STX 85 is 25x, giving an aperture of 8.8). This cannot be changed. Digiscopers take photographs of the exit pupil of the spotting scope using a camera which also has a lens and an aperture. In this case our choice of aperture has no impact on the depth of focus. This is determined by the spotting scope and its eyepiece. However, it does make sense to stop down compact cameras by 1 or 2 stops, because the lenses of most types of camera take poorer shots with an open aperture than with one which is slightly stopped down.

Stopping down improves sharpness

Stopping down on a compact camera does not increase the depth of focus, but it does slightly improve the sharpness. Ultimately, for digiscoping the aperture of a compact camera rarely needs to be set above 2.8 or 3.5. The only exception is if a higher aperture value is used not to improve the image quality, but to reduce the shutter speed in order to create deliberate blurring effects, for example using movement.

© Tormod Amundsen

Exposure compensation is important for birders

The third approach alongside shutter speeds and/or aperture choice is exposure compensation. As a general rule, when digiscoping birds, you should be brave and underexpose your images. The reasons for this are (a) pure white areas of plumage which will otherwise be overexposed and (b) the fact that slightly underexposing a subject gives it more life.

We can make use of this trick because the human brain has fewer problems with poorly defined shadows than with undefined lights, which it finds very disturbing. In addition, it is generally easier to brighten or define shadows using the simpler image editing packages.

Try things out to gain experience

Modern wide-angle zoom eyepieces underexpose images in the lower magnification range from 20x to around 35x by between 2/3 and 11/3 stops. However, anyone who wants to exploit the potential of a zoom eyepiece for digiscoping using 40x or even 50x magnification will find it easy to overexpose images. Otherwise the digiscoping shots would simply be too dark and lifeless, because of the lack of light.

The figures given here should, of course, be tried out in each individual case, depending on the spotting scope (lens diameter), eyepiece, and camera combination. With the Leica D-Lux 4 on the STX 25-60x65 spotting scope from SWAROVSKI OPTIK plus a wide-angle zoom that I prefer to use, I tend to increase the stops slightly by 1/3, regardless of the magnification. I use the same setting on the STX 25-60x85, but only from a zoom magnification of around 40x.

Small setting with a big impact

Anyone who has not yet looked into exposure correction for digiscoping should do so now. Either by trying out the correction values in the different observation conditions and with different subjects (the extent of the contrast is decisive here) or by allowing the bracketing function available on most compact cameras to do so. In a similar way to sharpness, small differences in the exposure correction settings can have a major impact.
Anyone who knows a little about photography will understand the importance of the combination of shutter speed and aperture for the resulting picture. In the case of digiscoping, the basic photographic rules apply, but also the special factors mentioned above.