What an objective lens is
The objective lens is the lens closest to the specimen on a microscope. It collects light coming from the specimen and forms the first magnified image, which the eyepiece then enlarges further for your eye.
It’s fair to call the objective the most important optical part of the microscope. The eyepiece can make the image bigger, but only the objective decides how much fine detail is in that image to begin with. That ability depends mainly on its numerical aperture (NA), a number printed on the barrel that describes how wide a cone of light the lens can accept. Higher NA means finer resolvable detail and a brighter image, but also a shorter working distance and shallower depth of field.
Most compound microscopes carry three to five objectives on a revolving nosepiece. A common set is 4x, 10x, 40x and 100x. For a plain-language comparison of the first three, see the difference between 4x, 10x and 40x. For where the objective sits among all the other parts, see our guide to the parts of a compound microscope.
How objectives are designed
Despite the name, an objective is rarely one lens. It’s a stack of glass elements inside a metal barrel, designed together to do two things: magnify, and correct the optical errors (aberrations) that any simple lens produces.
The main errors designers fight are:
- Chromatic aberration: different colors focus at slightly different points, giving colored fringes around edges.
- Spherical aberration: rays through the edge of a lens focus differently from rays through the center, softening the image.
- Field curvature: the image is sharp in the center but goes soft toward the edges (or the reverse), because the lens naturally focuses onto a curved surface rather than a flat one.
Correcting these takes more elements and more specialized glass. A basic low-power objective may use only a few elements, while high-end high-magnification objectives can use well over ten. That’s a large part of why objective prices range so widely.
High-power objectives are also designed around the glass between them and the specimen. Most biological objectives assume a coverslip of 0.17 mm thickness (a No. 1.5 coverslip is the usual match). Objectives for metallurgy and electronics are often designed for uncovered specimens instead.
Reading the markings on the barrel
A typical biological objective might be engraved something like this:
Plan 40x / 0.65 ∞ / 0.17
Here’s what each part means:
- Plan: the correction class (see below). Other common labels are Achro, Fluor/FL, Semi-Plan, and Apo.
- 40x: magnification. On its own, this is the magnification of the objective only; total magnification depends on the eyepiece too.
- 0.65: numerical aperture.
- ∞: the objective is infinity-corrected and needs a matching tube lens. A number such as 160 instead means it’s designed for a finite 160 mm tube length. The two are not freely interchangeable; see finite vs infinity-corrected objectives.
- 0.17: the coverslip thickness, in millimeters, that the objective is corrected for. A dash (–) means it works with or without a coverslip; 0 means uncovered specimens only.
You may also see Oil, Oel, W or Water for immersion objectives, Ph plus a number for phase contrast objectives, WD followed by a working distance, or an R or S suffix on some brands to indicate a spring-loaded front. If you want to understand the working distance figure, see our explainer on objective working distance.
Correction classes: achromat, fluorite and apochromat
Objectives are graded mainly by how well they correct color errors:
| Class | Color correction | Typical use |
|---|---|---|
| Achromat | Brings two colors (red and blue) to a common focus | Student, routine and most amateur work |
| Semi-apochromat / fluorite | Better correction, closer to three colors | Higher-grade work, fluorescence, photography |
| Apochromat | Corrects three colors, with the highest overall correction | Demanding research and photomicrography |
Achromats are the standard on most microscopes and are perfectly good for visual work. Their residual color error is usually smallest in the yellow-green part of the spectrum, which is one reason a green filter can sharpen the image a little; see our guide to microscope filters.
Fluorite and apochromat objectives cost considerably more. The improvement is most visible at high magnification, in photographs, and with brightly colored stains.
Plan objectives: what “plan” means and when it matters
“Plan” refers to correction for field curvature. A plan objective gives an image that is sharp across most of the field at the same focus setting. A non-plan achromat is typically sharp in the middle, with the edges going soft unless you refocus.
Any correction class can be made in a plan version, so you’ll see plan achromat, plan fluorite and plan apochromat. “Semi-plan” lenses sit in between, flattening the field over a larger central area than a basic achromat.
Does it matter for an amateur? It depends on what you do:
- For casual viewing, most people center what they’re interested in, so a curved field matters little.
- For photography or scanning, plan objectives help a lot, because the camera records the whole field and edge softness becomes obvious.
- At low magnification with wide-field eyepieces, field curvature is more noticeable, because you’re looking at a wider area at once.
If you photograph your slides regularly, plan achromats are often the most cost-effective upgrade.
Objective color codes
Many objectives have a colored ring around the barrel that identifies the magnification at a glance. The scheme comes from an international standard (ISO 8578), and most reputable makers follow it closely. Some budget objectives use the colors loosely or not at all, so treat the engraved number as the final word.
| Ring color | Magnification |
|---|---|
| Black | 1x to 1.5x |
| Brown | 2x to 2.5x |
| Red | 4x to 5x |
| Yellow | 10x |
| Green | 16x to 20x |
| Turquoise | 25x to 32x |
| Light blue | 40x to 50x |
| Cobalt (dark) blue | 60x to 63x |
| White | 100x and above |
Immersion objectives and their color rings
High-NA objectives often need a liquid between the front lens and the coverslip. A second, usually narrower ring near the tip identifies the medium:
| Ring | Immersion medium |
|---|---|
| Black | Oil |
| White | Water |
| Orange | Glycerin |
| Red | Special or multi-immersion (varies by maker) |
What is immersion oil used for? A 100x oil objective is designed to look through a continuous layer of oil and glass, which have nearly the same refractive index. Without oil, light leaving the coverslip bends away at steep angles and never reaches the lens, so the image is dim and blurry. With oil, those rays are collected, which is what allows NA values above 1.0 (commonly around 1.25 for an achromat).
Use only immersion oil made for microscopy, and only on objectives marked for oil. Our step-by-step guide to the 100x oil immersion objective walks through the full routine. Wipe the 100x lens clean with lens tissue after each session; dried oil is a common cause of hazy images. Avoid swinging a dry 40x objective through an oiled slide, since it can pick up oil on its front lens.
Parfocal and parcentric: what they mean
When objectives are parfocal, a specimen in focus with one objective stays in focus, or very nearly, when you rotate to another. You should only need a small turn of the fine focus. This works because the objectives are built so their in-focus positions all fall at the same distance from the nosepiece. That distance (the parfocal distance, often 45 mm on standard objectives) is set by the design.
When objectives are parcentric, or parcentered, whatever is in the center of the field with one objective stays near the center with the next.
In practice, parfocality is best between neighboring objectives. Mixing objectives from different makers or series can break it, because their parfocal distances may differ.
Parfocal objectives also make the standard focusing routine safer: focus at low power, then switch up and use only the fine focus, which avoids driving a high-power lens into the slide.
Spring-loaded (retractable) objectives
Many 40x and 100x objectives have a spring-loaded front section. If the lens touches the slide, the front pushes back into the barrel instead of cracking the slide or the lens. It’s a useful safety feature, especially on student microscopes, but it isn’t a reason to be careless with coarse focus. Some 100x oil objectives also let you push in and twist the front to lock it up, which helps when cleaning or changing slides.
Threads, parfocal distance and compatibility
If you plan to buy objectives separately, check:
- Thread: most traditional objectives use the RMS thread (0.8 inch diameter, 36 threads per inch). Some modern systems use larger metric threads, such as M25 on certain Nikon objectives.
- Optical system: finite (160 mm) or infinity, and for infinity objectives, which maker’s tube lens they’re designed for.
- Parfocal distance: commonly 45 mm on standard objectives, with some modern systems longer. Mixing distances means objectives won’t be parfocal with each other.
- Coverslip correction: 0.17 for most biology work.
Choosing objectives as an amateur
A few practical suggestions:
- For general biology, a 4x, 10x and 40x achromat set covers most work. Add a 100x oil objective if you want to look at bacteria or blood cells in detail.
- Prioritize NA over magnification. A 40x/0.65 shows more detail than a 40x/0.55.
- Consider plan objectives if you take photographs.
- Stay within one optical system and, ideally, one series, so objectives remain parfocal and parcentric.
- Used objectives from major makers can be good value, but inspect the front lens for scratches, haze or old oil, and confirm the system markings before buying. Our used microscope checklist covers what else to check.