Monday, April 13, 2009

How to test/ calibrate a manual SLR camera's shutter using a digital camera


Before loading a newly acquired second hand camera with film, it is often a good idea to check if the shutter is working correctly. This is particularly true for well used, older cameras.

While slow shutter speeds with exposures greater than 1 second can be verified using a stopwatch, it becomes much harder to check for fast shutter speeds.

This document presents a technique to verify the accuracy of a manual camera’s shutter speed by using a digital camera as the standard shutter and the recording device.

Equipment set up:

Figure 1: Shutter calibration equipment set up.
Red: digital sensor, blue: shutters, magenta: light rays, green: lens

1. The manual SLR’s lens is removed, and the film back opened to expose the shutter from both sides.

2. The digital camera is pushed as close to the manual SLR’s shutter as reasonably safe.
Caution: Ensure that nothing touches the shutter, as interrupting its movement will risk damage.
3. Arrange a subject beyond the manual SLR for the digital camera. Ideally, this subject should be of uniform colour and not throw bright reflections. The uniform colour will make brightness comparisons much easier.
Tip: Use a lens with a long focal length on the digital camera, and set it’s aperture to a small opening (5.6 or smaller). This will minimise vignetting caused by the SLR’s frame partially blocking the view of the digital camera.

4.Open the manual SLR’s shutter (using bulb mode or a long exposure), and focus the digital camera on the subject.

5. Illuminate the subject with a bright light source, and shade the digital camera from the light source. Ensure that minimal ambient light reaches the area between the digital camera and the manual SLR.

Set up notes:

When properly set up, the only light that reaches the digital camera’s sensor will be light from the illuminated subject. For the light to reach the digital camera’s sensor, it needs to pass through two shutters – the digital camera’s and the SLR cameras.

Light from the subject will reach the digital camera’s sensor if and only if both shutters are open.

Testing procedure:

6. With the manual SLR’s shutter open (using a long exposure), meter the scene and adjust the digital camera’s aperture and ISO so that the shutter speed matches the value to be tested.

7. With the SLR’s shutter open (using a long exposure or bulb mode), take a photo of the scene using the preset ISO, aperture and shutter speeds. This is the standard image, because the exposure time is that of the correct shutter speed. The SLR’s shutter was open throughout the experiment, and the exposure occurred only during the time the digital camera’s shutter was open.

8. Set the digital camera’s shutter speed to bulb (or a long exposure), retaining the same aperture and ISO settings in the previous step. Set the manual SLR’s shutter speed to the tested speed.

9. Open the digital camera’s shutter, then release the manual SLR’s shutter. Close the digital camera’s shutter (by releasing the shutter button if in bulb mode, or allowing it to close after the preset time). This is the tested image, because the exposure time is that of the tested camera’s shutter speed. The digital camera’s shutter was open throughout the experiment, and the exposure occurred only during the time the SLR’s shutter was open.

Comparing the results:

10. Both the calibrated and the tested images are opened, and their histograms observed.

11. Brightness peaks associated with the illuminated image are identified and their locations along the histogram compared.

If both shutters have identical exposure times, the peaks of both images would lie in the same locations.

Error estimation:

In the unfortunate event that the shutter speeds do not match, and the digital camera’s shutter speed is trusted, the error of the manual SLR’s shutter can be estimated.

12. Repeat step 7 using several exposure times.

13. Compare the images from the various exposure times with the tested image obtained from step 4. Find the closest match between the tested image and one of the standard images.

14. Calculate the manual SLR’s shutter error in terms of number of stops.

15. The error estimation needs to be repeated for every shutter speed available on the manual SLR to establish the real exposure that each shutter setting will give.

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