Preliminary setup

Use PixelScope's Hardware dialog to enter these values.

• Collimator focal length
• Filter
• F-number
• Nominal focal length
• Microscope calibration value

The Optical Layout window provides a concise graphic summary of the values you have entered:

The astute reader may question why so many values are required. After all, the EFL calculation only requires the first and last items in the list above. Nevertheless, we suggest entering the correct values anyway. If you do this the Optical Layout window provides concise documentation of the setup.

Pick a target

The EFL wizard requires that you use a reticle with precisely known feature spacing. Two options are possible:

• You may use the USAF target supplied with Optic Studio. In this case you must specify which Group / Target to use.

• You may use some other target with precisely know feature size.

The small button in the upper left opens the USAF dialog, which is used to specify Group / Target values.

Making the actual measurement

• Drag a selection rectangle around the target

• The EFL is automatically computed and displayed:

Take special care to avoid Setup errors:

• The measured EFL depends directly on the calibration values you enter. Take a moment to verify you've entered the values correctly. The Layout window is a convenient way to do this.
• If you are using the USAF target, make sure the target you measure is the same as the target you've specified!

Some comments on Accuracy:

• PixelScope is extremely accurate at measuring feature separation. However, the measured result will only be as accurate as image formed by the test lens. If the lens has poor image quality then there simply may not be a "true" focal length.
• A lens with spherical aberration will appear to have different focal lengths for different apertures. Stopping down the lens will permit you to measure the paraxial focal length.
• In general, larger targets will give slightly more accurate values.
• Be sure to focus carefully.