There are three ways to control exposure in an image:

• Shutter Speed regulates how long the shutter is open and the sensor is exposed to light. Longer shutter speeds let more light in and produce a brighter image.

• Focal Ratio adjusts the aperture of a camera lens. In a telescope the aperture is fixed. For a fixed focal length, larger apertures collect more light and produce a brighter image.

• ISO - adjusts the gain in the camera. Gain defines how many photoelectrons are mapped to each digital number in the output of the digital image file. Using a higher ISO produces a brighter, but noisier, image.

For video mode, the main exposure settings of shutter speed, aperture and ISO are set automatically by the camera as the default. This is not good for planetary photography. It is better to control the exposure by manually adjusting the shutter speed and ISO.

For High-Definition Video:

• Set the mode dial on the top of the camera to Movie Mode.

• Set the Movie Exposure Setting option to Manual in the menus.

For Live View Video:

• Set the mode dial on the top of the camera to Manual Mode.

To control the exposure manually in both high-definition video and in Live View video, adjust the ISO and shutter speed.

Note that you will only be able to increase the shutter speed up to a certain point when shooting in the camera's native video modes. When shooting at 60 frames per second in Movie Crop Mode, the longest shutter speed available will be 1/60th of a second. When shooting at 24 or 30 frames per second in normal high-definition video mode, the longest shutter speed available will be 1/30th of a second. If the image is not bright enough at these shutter speeds, increase the ISO to brighten the image.

For Live View, the image is what Canon calls "simulated". Adjusting the shutter speed and ISO will change the brightness of the image, but the shutter speeds are not accurate. While in Live View, you can change the brightness of the image by changing the shutter speed all the way down to the bulb setting.

Image Simulation

Exposure for a normal still image is determined by focal ratio, ISO and shutter speed.

If an object is moving in a normal daytime exposure, the shutter speed will be the main determinant in freezing the motion of the object. At long shutter speeds, a moving object will be blurred because it has moved during the exposure. The faster the object's motion, the shorter the exposure needed to stop its motion.

For planetary photography, shutter speed comes into consideration for seeing. Along with lucky imaging, using higher shutter speeds with more sensitive cameras is one of the main reasons that we can take better planetary images with digital cameras than with film cameras.

In the days of film, shutter speeds of several seconds were required. That long of an exposure will blur any shorter moments of good seeing. Shorter exposures have a better chance of capturing moments of better seeing.

The power of using video for planetary photography came from two main factors - shooting shorter exposures and those shorter exposures allowing a greater total number of frames to be shot in a given time before planetary rotation smeared detail.

Using a DSLR's built-in high-definition video mode, the longest exposures are limited to 1/30th or 1/60th of a second by the framing rate as discussed above. For most planets, at the high focal ratios needed for sufficient sampling, we will be using shutter speeds in this range. However for really bright objects such as the Sun, sometimes much shorter exposures can be used. These shorter exposures can help freeze the seeing even more, up to a point, depending on the quality of the seeing.

Live View, however, uses a different system than still images or high-definition video.

Although Live View seems to present us with the same system of changing exposure by adjusting focal ratio, ISO and shutter speed, the latter two are not really accurate. In Live View, the shutter speed is fixed by the framing rate of the Live View video signal.

The image shown in Live View is "simulated". This means that the shutter speed is not necessarily accurate. The Live View video is displayed at 30 frames per second. This means the exposure is about 1/30th of a second. Yet the "simulated" view will allow you to change the shutter speed all the way down to 30 seconds and the image will continue to get brighter. Obviously a single frame in Live View running at 30fps cannot be 30 seconds long.

This means the brightness adjustment is done in the camera by adjusting the gain, which is normally set by the ISO. So the ISO setting is not accurate either when recording Live View.

The following settings are applied to the image preview and recorded image:

• Picture Style

  • Sharpness
  • Contrast
  • Color Saturation
  • Color Tone

• White Balance

• Exposure

• Auto Lighting Optimizer

• Peripheral Illumination Correction

• Highlight Tone Priority

Note that these custom settings for Picture style in Movie Mode are separate and distinct from the Picture Style settings in single-frame, still-image mode.

Exposure Simulation

In addition to things like Picture Style, contrast and white balance being applied to Live View and Movie recordings, Exposure Simulation is also applied to Live View.

In consumer model Canon cameras, such as the EOS T3i (600D), T2i (550D), XS (1000D), and T3 (1100D), Exposure Simulation is automatically applied to the Live View. This means if the ISO, aperture value and shutter speed are incorrect, the Live View display will reflect this by showing an overexposed or underexposed image.

In "prosumer" and professional Canon models such as the EOS 60D, 50D, 5D Mark II, and 1D Mark III, you can turn this feature off with a menu setting. If Exposure Simulation is set to "Disable", then the image displayed in Live View will look correct, even if the actual exposure is wrong. You have to set Exposure Simulation to "Enable" to be able to determine the correct exposure for Live View for planetary photography.

LCD Screens

Laptop screens are rarely calibrated for brightness, so if you are trying to judge the correct exposure on your laptop, it may not actually be the optimum exposure. The brightness of an image on a laptop screen can also vary depending on your viewing angle. Red plexiglass used over the screen, and night-vision mode, both commonly used to protect your dark-adapted night vision, can also cause unreliable judgments as to the exact correct exposure needed for a given object.

If you are not using a computer and are just recording videos to the memory card in your camera, also note that the back-of-camera LCD screen can vary in brightness and turning down the LCD brightness for night work can also lead to unreliable judgments about exposure.

Test Exposures

We will talk a little more about exposure for each planet in their respective sections, but in general, the best way to determine correct exposure is by shooting a test.

Because of the problem of variations in LCD screen brightness, it's best to first figure out how your particular LCD screen looks in relation to a correct exposure. We can do this by shooting a couple of test videos at different exposures and examining them on a calibrated desktop monitor so you can record or remember your "fudge" factor.

Let's say a test video of Jupiter looked great on your laptop or camera LCD screen when shot at 1/125th of a second exposure at ISO 800 at f/30. But when you look at the other test videos on your calibrated desktop monitor, you see that the best exposure is really 1/60th of a second at ISO 800 at f/30. This will tell you that you need one stop more exposure than what looks best on the laptop or camera LCD at the scope. A "stop" is equivalent to doubling the exposure or cutting it in half.

The correct exposure is usually just short of the point where you start losing highlight details due to overexposure. This exposure provides the best signal-to-noise ratio in the image. If the image looks dark and grainy or noisy in the video, it is underexposed.

The next time out, you can adjust the exposure so the image looks good on the same laptop or camera LCD, and then add or subtract the fudge factor that you determined.

Determining the best exposure by shooting a test every time you go out and applying your fudge factor will mitigate the problem of brightness variations on laptop and camera LCD screens, and also take into account factors such as transparency, extinction and focal ratio.

Other Considerations

The correct exposure is determined by planetary surface brightness, not the planet's magnitude. A larger planet at the same magnitude as a smaller planet will require more exposure because the light is spread out over a larger area. Using a test exposure will take care of this problem.

Exposures must be longer if the object is closer to the horizon because of extinction. Exposures must be longer if the transparency is poor. Again, using a test exposure will solve these problems.

Note that exposure is dependent of the simulation settings in videos, which can be different than single still frames.

Nikon DSLRs

In addition to there not being much software besides Images Plus that will record Live View from Nikon DSLRs, there is an additional problem.

Many Nikon DSLRs do not allow adjustment of the brightness, and therefore the exposure, in Live View by changing the ISO and shutter speed. In these cameras, you will have to experiment with exposure compensation to see if you have enough control to achieve the correct exposure.

Nikon's more recent models, such as the D7000 do seem to be able to adjust the brightness of the Live View display by changing the ISO and shutter speed.