Optical or digital zoom – which should I use?
Many digital cameras feature both an optical and digital zoom.
The optical zoom lens on the camera alters the focal length so that the subject appears closer or further away. At higher zoom levels, the subject and the entire scene appear closer without any loss of detail or quality. However, the objective lens gathers less light.
When using the digital zoom, by contrast, the camera’s internal software simply recalculates the image data already captured to produce a zoom effect. This results in a loss of image resolution. Most digiscopers disable or turn off the digital zoom because it leads to a loss of resolution and causes images to break up or pixelate.
Focal length calculation – how does it work?
Focal length affects the magnification, field of view and depth of field. Although a long focal length means high magnification, it also means a smaller field of view and a shallower depth of field.
To determine the equivalent focal length for a digital camera compared to a 35 mm camera, you need to carry out a few calculations. Each digital camera has a 35 mm equivalent figure. For the Nikon P-6000, for example, the 35 mm equivalent figure is 28 – 112 mm. Since the Nikon p-6000 has a digital focal length of 6 – 24 mm, this results in a factor of 4.66 (28 mm divided by 6 mm). This factor or the 35mm equivalent focal length is usually stated in the camera’s user manual.
In this case, the equivalent focal length is the adjusted focal length of the digital camera multiplied by the factor 4.66.
Example:
The Nikon P-6000 is set to a focal length of 14 mm.
14 mm x 4.66 = 65.2 mm focal length equivalent for a 35 mm camera.
To obtain the entire 35 mm equivalent focal length from your digiscoping equipment, you need to multiply that focal length by the magnification of your scope.
Example:
Your telescope is set at 20x magnification.
65.2 mm x 20 = 1,304 mm total 35 mm equivalent focal length
To determine the total digital focal length of your digiscoping equipment, you simply need to multiply the focal length of your digital camera by the magnification settings of your telescope.
Example:
Your digital camera is set to 14 mm focal length; the telescope is at 20x magnification:
14 mm x 20 = 280 mm total digital focal length
Aperture - what effect does it have?
The aperture is the opening through which light passes before it reaches the CCD.
Selecting a lower aperture number or ’f-number‘ (i.e. f2.4) increases the size of the aperture and allows more light into the camera.
The aperture also controls the depth of field of the image:
- Low f-number (larger aperture) = shallow depth of field
- High f-number (smaller aperture) = deep depth of field
A shallow depth of field focuses on the subject and blurs the areas in front of and behind the subject. This happens if you use a high magnification with your scope.
Using a high f-number, which provides a deep depth of field, means that the whole image should be in sharp focus. This is often used when digiscoping flocks of birds or when the landscape is important for capturing a certain species. However, a deeper depth of field may only be possible when using your telescope at low magnifications.
If your camera has an ’Aperture Priority‘ mode, you can set the aperture to the desired f-number manually and the camera will then calculate the best shutter speed automatically.
What are scene programmes?
Scene programmes are a series of preset parameters for photographing different subjects or scenes. They have intuitive names such as ’Sport‘, ’Landscape‘ and ’Portrait’. Selecting the ‘Sport‘ programme, for example, indicates that the photographer wants to take a picture of a fast-moving subject and the camera uses a fast shutter speed.
The ’Portrait‘ programme automatically selects a larger aperture to ensure that the background is out of focus (short depth of field) while the subject remains sharp. These scene programmes can save a lot of time and effort and usually produce excellent results.
If you use a preset scene programme for digiscoping you should set the camera to ’Landscape‘ for taking photographs of motionless animals; set it to ’Sport‘ for flying birds, and so on. And make sure you disable the flash!
What does light sensitivity mean?
The CCD sensor in a digital camera can be adjusted to become more, or less, sensitive to light. In analogue photography, you would need to use different films for the different light conditions:
ISO 100 – bright sunlight
ISO 200 – most conditions
ISO 400 – Poor lighting conditions
ISO 800 – Very poor lighting conditions
You can usually also increase the amount of light for the shot by enlarging the aperture or increasing the exposure time. However, if you are limited by the available f-numbers and shutter speeds, increasing the ISO value will get more light into the camera.
Some camera models also adjust the light sensitivity automatically. If it is too dark for the camera to work with its current exposure settings, it selects a higher level of sensitivity for the CCD to obtain clearer results. The only downside is that higher sensitivity tends to result in increased noise, meaning reduced image quality.
What is white balance?
Different types of light from different sources have different light temperatures, which means the captured image can show the same object in different colours depending on the light source. For instance, there is a difference between light from the sun in a clear sky and light from a fluorescent tube.
Therefore, in addition to measuring the light intensity, the camera also needs to know the colour temperature of the ambient light to ensure that colours are reproduced correctly.
Today’s digital cameras include an automatic white balance function. The camera automatically analyses the surrounding light, determines a precise colour temperature range and corrects any colour casts in order to capture images with natural colours.
Many digital cameras also allow the white balance to be adjusted manually and usually offer preset parameters for matching the light temperatures of sunlight, cloudy days, artificial light or fluorescent light.
Some camera models even offer a ’one touch‘ function, which lets the user adjust the white balance according to the current light source. For this, the photographer simply needs to train the lens on something white (a sheet of paper is usually sufficient) and then press the one-touch button to save the values.
What is noise reduction?
Noise is visible interference on the CCD sensor which results in unwanted colour spots on the captured image. The amount of noise tends to increase in line with the ISO value.
In noise reduction mode, the camera takes two pictures: the normal shot and a second one with the same exposure time but with the shutter closed. The camera compares the two images to identify areas showing unwanted noise dots, and then compensates accordingly.
Many digiscopers use a noise reduction programme such as Neat Image.
What does shutter speed do?
The main purpose of the shutter speed setting is to control exposure time. However, it also influences the way in which movement is displayed. Using fast exposure speeds can ’freeze‘ the motion of flying birds or running deer.
Slow shutter speeds, by comparison, provide the photographer with a blurred image, which may be desirable if you want to give the impression of speed. The main benefit of slow shutter speeds, however, is that they enable photographs to be taken in poor light conditions.
If the camera has a ’Shutter Priority‘ mode, the photographer selects the shutter speed manually and the camera chooses the most suitable aperture setting to achieve optimum exposure.