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Mic Objective Guide

by GT Vision Ltd

Please find detailed below explanations about the terminology used to describe GT Vision’s range of microscope objectives within our product listings.

Biological and Materials objectives are used on upright and inverted compound microscopes (and can be added to some portable and monozoom microscopes and they can be used in photography).  Each objective has a number of designations printed upon the barrel of the objective, these are described below.  A typical example is shown in the diagram below:

When you select a product you will see the following table, here is a summary of the meanings of the specification attributes where necessary

SPECIFICATIONS 

PARAMETER EXAMPLE VALUES DEFINITIONS
Product Code  0307 A unique product identifier
Magnification 4X the meaning of this is self evident
Features Brightfield Any special features such as phase contrast, fluorescence etc.  Brightfield is the most common biological type.
Designed For Biological Upright Microscopes Which type of microscope this objective is most commonly used on
Objective Grade Grade 5 – Plan Achromatic GRADE NUMBER:-
1 – lowest possible grade, cheap, found mainly on toys, not even achromatic
2 – basic glass objective with steel barrel, reasonable quality found on schools microscopes, usually a narrow barrel, all achromatic
3 – education grade, normal width barrel, better quality found on many secondary level and college teaching and amateur microscopes, typically achromatic
4 – routine grade, suitable for most uses, good quality, typically semi-plan achromatic
5 – suitable for research and routine use, high quality
6 – research grade optics
7 – very high quality, exrtemely good image resolution
8 – about as good as it gets

FIELD FLATNESS & COLOUR CORRECTION (improving quality as you descend the list):-

ACHROMATIC – the most basic class of microscope objective but they can still give good quality images. Achromats are corrected for chromatic aberration of two colours , red and blue.
SEMI PLAN ACHROMATIC – Slightly better than Achromatics with a flatter image at the edges.
PLAN ACHROMATIC – Better still with a substantially flat image
SEMI-PLAN APOCHROMATIC (also PLAN FLUORITE) – A superior class of objective giving a high order of correction for all three colours with higher resolution
APOCHROMATIC – Corrected both chromatically and spherically for all three colours combined with special additional correction for field curvature

Numerical Aperture (N.A.) 0.1 The Numerical Aperture of a microscope objective is the measure of its ability to gather light and to resolve fine specimen detail   It is influenced by the media through which light travels in front of the objective (oil immersion objectives give a higher NA for instance).
Coverglass Correction 0.17 mm Most higher magnification biological microscope objectives are corrected for the effect of the refractive index of the coverslip placed over a specimen, usually the thickness is 0.17mm.  With inverted micrscope you may be looking through the bottom of flasks hence a larger correction thickness of, for example, 1.2mm.
Materials microscope specimens are rarely covered and require no correction, hence the value  or 0 or –
Thread Size RMS RMS is the most common thread size (20.32mm in diameter) for a microscope objective, the second-most common is M26 (26mm).  Other sizes are also used.  It is essential to match the thread size with your microscope objective turret in which the fit.
Working Distance (W.D.) 21.0 mm The general rule is that the lower the magnification the longer is the working distance.  However, to maintain ‘parfocality’ on a turret of objectives, the working distance will often tailored so the sum of the barrel length and working distance for all objectives of the same type will give precisely the same value.  Special longer working distance objectives usually have an extra ‘L’ written on the barrel.
Tube Length Infinity Mechanical Tube Length.  This must match on all objectives on a turret of objectives and it must match the specification of the microscope. This is the length of the microscope body tube between the nosepiece thread mounting hole, where the objective is attached, and the top edge of the eyepiece tubes where the eyepieces are inserted.  160 and Infinity are the most common.  This value is not necessarily an indication of quality.
Parfocal Distance 45 mm Designed to reduce the need for refocussing when different objectives are selected on a turret of objectives.  Mosty it is 45mm.  It is the distance from the top of the objective barrel when inserted to the point on the subject that is in focus.
Focal Length Unspecified Rarely used as focal lengths normal pertain to a single lens system and objectives have several lenses
Barrel Colour / Size Silver / Standard This has no significance.  There are colour bands corresponding to magnifications however.
Brand GX Microscopes
Condition New

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