This describes the focus settings that I use for the Sigma 8mm f/3.5 fisheye lens used for GBV virtual tours.
1. To focus the lens:
· Set the aperture to full open f/3.5 to get smallest depth of field (DOF)
· Set up a target at exactly 1m from the nodal point
· Use Live View and magnify as much as possible
· Focus manually – never use autofocus for this lens
· Leave the focus control on Manual
· Tape down the focus ring so that it can’t move
· Check this regularly in case it has moved
2. To check the focus:
· Do this regularly, especially for large critical jobs
· Set up a ruler and targets like this example
· Capture an image with the lens wide open so that you can make sure that the point of focus is at 1m
Zoom in and check the 1m point
· Capture another image with the lens stopped down to f/8 so that you can check the DOF (Depth Of Field)
Zoom in and check the 1m point, notice the huge increase in DOF
We have reached the limits of the lens/ sensor resolution, but you can see how the DOF has increased by stopping down.
3. Why use the 1m focus setting? Mainly because this is the closest that we will get to an object in GBV and we want this to be the sharpest point of focus.
4. Calculate the DOF yourself:
· Install an APP on your smartphone: DoF Calc for Android, Dofmaster for Apple.
· With an APSC sensor, 8mm f/8 lens Hyperfocal distance is ~43cm. Everything from ~21cm to Infinity is acceptably sharp
· If you focus at 100cm then everything from ~30cm to infinity is in sharp focus but the sharpest point of focus is 1m