Calibrate shutter close lag

The DSLR shutter close lag can be determined by using DSLR Trigger Intervalometer to photograph the Exposure Test application, which displays a moving block within a grid. The shutter open lag must be calibrated before starting this procedure.

Camera Settings:

• Use RAW with no jpg (less in camera processing).

• Use Mirror Lock (if camera supports it)

• Use Bulb mode

• Use the aperture you will use in the field (fully open is recommended for satellite trails)

• Set ISO so that grid lines will appear white and the trail of moving blocks is easy to see.

• Manual focus.

DSLR Trigger Settings:

• Use Bulb mode

• Use Mirror Lock (if camera supports it)

• Set Exposure to 1.0 seconds

• Set Mirror Lock to the value you will use in the field.

• Use a sufficiently big 'Fire interval' so the camera does not buffer images. This can affect shutter lag.

• Set 'Shots' to about 9. We will need to average the results.

'Settings' dialog

• Make sure the shutter open delay is set to the correct value

• Set the shutter close delay to zero

• Tick 'Log all events'

Now press 'Start'.

Analysing the results:

This shows a 1 second exposure of the 'Exposure Test' application. Each column represents 10 ms.

There are 'holes' in the block's trail. This is partly because the screen could not keep up - its refresh rate is 16 ms. The Windows OS is also multitasking.

The first block (exposure start) is located in the third row, 950 ms column. It is not in the zero column because there is a delay before the screen displays an image (screen / graphics card). This is not important because it affects the start and end positions equally. The trail finishes in the fourth row, 970 ms column.

Our objective is to measure the actual exposure length. In a one second exposure, the first block should appear in the same column as the last block. So, in this image, the exposure was 20 ms (two columns) too long. This indicates that we need to set the shutter close lag to 20 ms.

I took 9 images and measured:

50 ms, 30 ms, 40 ms, 40 ms, 40 ms, 40 ms, 50 ms, 30 ms, 20 ms

Giving an average of about 38 ms.

In DSLR Trigger, set the shutter close lag to the averaged value (Settings dialog). Then repeat the test.

This is an image taken with a shutter close lag set to 38 ms. Note that in this image the trail started on the last row and the trail wraps around to the top row. The first and last blocks are in the same column, as required for a one second exposure.

I took 9 images and measured:

10 ms, 10 ms, 0 ms, 0 ms, -20 ms, -10 ms, -20 ms, 10 ms, 10 ms

Giving an average of about -1 ms, which is more than close enough. You should aim to get within 10 ms.

The DSLR Trigger log file will include the calculated exposure time, based on the measured shutter button press and release times, and the user supplied shutter open and close lag values. The exposure time may turn out to be a few milliseconds from the requested 1000 ms because the program cannot always press the shutter button at exactly the specified time (multitasking OS). However, it does record the time it finally managed to press / release the shutter button. Check the exposure times to make sure they were all about 1000 ms.