The parfocal distance is the distance between the position where the objective is mounted and the focal plane of the specimen. Theoretically, objectives which have the same parfocal distance but different magnifications can keep the same focus, even if the magnification is changed from one level to the next.
The majority of microscope companies such as Olympus and Zeiss produce objectives which have a 45 mm parfocal distance. The new CF160 objectives from Nikon have a parfocal distance which is significantly higher, at 60 mm.
The manufacturers’ aim is always to get the same parfocal distance across varying magnifications, but perfect parfocality is a challenge. One reason is the low depth of field (below 5 microns) with objectives that have high magnification over 40X, which means that the parfocal distance must be accurate to within 5 microns.
This hinders the achievement of perfect parfocality across a range of objectives. Objectives must be produced to very exacting standards of precision, and this makes deviations in parfocality with different objectives a routine phenomenon.
The use of automated microscopes with Z-stages that are motorized allows for the automated adjustment of parfocality. Logos Biosystems has brought out the CELENA S, a digital imaging system which allows automatic correction to compensate for the parfocality differences that are common when different objectives are used.
Its motorized Z-stage is equipped with a very accurate ball screw and stepper motor, fitted to allow highly precise positions to be attained consistently. It also has a user-friendly interface that allows rapid parfocal corrections through quick and easy settings.
OBJECTIVES panels in the CELENA® S user interface.
How to Set up the System for Parfocality Correction
The CELENA S comes with objectives that are already pre-calibrated, but additional or substituted objectives should be calibrated.
This involves the following steps:
- Click ‘LIVE’
- Go to the ‘OBJECTIVES’ panel and turn the Parfocality correction off
- Put a slide on the objective stage
- Select an objective
- Adjust the focus to be as sharp as possible and then click ‘Set’
- Repeat the fourth and fifth steps with other objectives as desired
How to Use Parfocality Correction
This is carried out as follows:
- Switch on Parfocality connection
- Put a sample onto the stage
- Focus on the sample
- A different objective can then be selected, and after waiting for one second, the Z-stage will correct to keep the focus automatically.
- The parfocality correction feature calculates the distance from one objective to another, and corrects for this. This allows the user to keep the focus unchanged when a different objective is used. If the objective being used is not in the right focal position, the focus cannot be corrected simply by using another objective. That is, the objective selected must first be focused properly before another one is substituted.
- The parfocality correction feature is suitable for objectives that are adjusted for the same thickness of the vessel bottom.
The technical specifications of the CELENA S are given in the following table:
About Logos Biosystems, Inc.
Dedicated to the development and commercialization of innovative technologies to support life science research community. Founded at year 2008, Logos Bio is developing a series of lab automation systems and imaging instruments. The primary commercial target of Logos Biosystems is cell biology and molecular biology.
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