Picking a Test Lens

 

Picking a Test Lens

Spend some time and money, it is worth investing in a good test lens.  The degree of ease and accuracy of Autofocus adjustments depend on it.  


Here are some tips for picking a test lens:

• You will need a high quality prime lens with a flat focus field and an aperture of F/ 1.8 - F 2.8.

 

• Lens Recommendations:  EF 50mm F/2.5 Macro, EF 100mm F/2.0, or the EF 100mm Macro F/2.8.  The EF 100mm Macro F/2.8 is available in normal and L versions, has a flat field, a USM Ring motor, and produces images with high sharpness and contrast.  There are also other lens you can use.

 

• Invest in a lens with a good focusing motor and a flat focus field.  Avoid EF-S lenses since they won't fit full-frame cameras.  Also avoid zoom lenses and those with non-USM motors since their focus can drift due to play in the lens cams.

 

SPT software eliminates the requirement for an expensive test lens made especially for Auto Focus adjustment (costing $1,000 and more) by creating a compensation file (newer software) that corrects for discrepancies in the lens.  Older software creates the compensation by creating a custom 3D target for your test lens.

Each software has complete help files explaining in detail the method, tools and procedures you need to calibrate a test lens.

 

A short peak:
To determine the sharpness of a lens.  Use the software Focus Tool and Focus Bell graph.  The Bell Curve for a lens that has sharp focus will show a good peak and a good bell curve.  The sharp peak should show focus down to 1mm.

 

Notice the focus Bell Graph is definitive and you can define point of focus down to 1mm

 

Notes

It is not necessary that the peak focus be on the zero line.  Just that the lens is sharp enough to have a single point of focus.  In the extreme example below, the test lens has a depth of focus of 10mm.  You would never be able to use this as a test lens.

 

• Here are examples of soft lenses  There is a Bell Graph of sorts but it's so slight that anybody would be hard to define the point of focus.

 

 

 

• If the lens focus is inconsistent, sometimes focusing at zero and other times focusing at plus or minus 2mm, autofocus adjustment would not possible with this lens.

 

 

 

 

If focus inconsistencies are due to the lens, then the most likely problem is play in the lens or element guides.

 

• Do not use the old 'L' series lenses.  They are not sharp enough for the newer HD sensors on newer cameras.