Tips on setting aperture and focus

It is very important to set the aperture and focus correctly for your measurement volume. Incorrect settings might lead to sub-optimal marker detection by the cameras.

Aperture and focus are set on the lens of the camera. The way aperture and focus are set depends on the camera and lens combination.

• For cameras with motorized lenses (Arqus A12 and Oqus 7+ with standard lens option), the focus and aperture are controlled from QTM via the Lens Control interface in the Camera Settings sidebar in the 2D View window, see chapter Camera settings sidebar.

• For cameras with manual lenses, you will need to extend the strobe mechanics to get access to the focus and aperture rings on the lens. For information on how to extend the strobe mechanics for the available camera types, see the information on setting aperture and focus for the respective cameras in chapter Connecting the system.

It is recommended to use the following procedure for setting the aperture and focus.

1. Select the camera in the 2D view window in QTM using the Camera selection bar or by double clicking on the image area.

2. Place a marker in the measurement volume at the position which needs to be in focus. Use the same marker size as used for the actual measurements.

3. Set the focus. The focus can be best set in Marker or in Video mode. It is best to set the aperture f-value as low as possible (e.g. f/2) when focusing since this makes differences in focus more visible.

   1. In Marker mode, make sure that you can only see the marker that you have placed in the volume. Then open a Data info window and plot the x-size of the marker. Change the focus until the x-size is as small as possible.

   2. In Video mode, change the focus until the marker is as small and as sharp as possible. Make sure that the Flash time is long enough in the Camera settings sidebar. You can also zoom in and translate the 2D view for better detail.

4. Set the aperture. The recommended range of aperture is between f/2 and f/8. The optimal setting depends on the following considerations.

   • For more incident light, for example in large capture volumes, use lower f-values from f/2 to f/2.8.

   • For more focal depth, use higher f-values from f/4 to f/8. Use of higher f-values can be helpful in smaller volumes where the relative distance between markers and camera can vary, and there is enough light reflected from the markers.

   • Depending on the lens, use of the lowest f-values may lead to vignetting, which may become visible as a deformation of the markers in the vicinity of the edges and corners on the sensor.

   • Note that by increasing the aperture by one f-stop (e.g., from f/2.8 to f/4) the intensity of the incident light is reduced by half.
     

5. When you have set the focus and aperture correctly, switch to Marker intensity mode in QTM to optimize the marker settings Exposure & Flash time and Marker Threshold. For more information, see Tips on marker settings in QTM.

6. For cameras with manual lenses, do not forget to return the strobe mechanics to the original position.