Microscope Objective FAQs

What is numerical aperture (NA), and why is it important in microscope objectives?

Numerical aperture is a measure of an objective’s ability to gather light and resolve fine details. A higher NA means the lens can capture more light from the specimen and distinguish smaller features, leading to brighter images and higher resolution. In practice, objectives with larger NA values enable you to see finer structural details than lower-NA lenses.

What is chromatic aberration, and how do apochromat objectives correct it?

Chromatic aberration is a focusing error where different wavelengths of light do not converge at the same point, causing color fringing in the image. Standard achromatic objectives correct this for two colors (typically red and blue), but some color blur can remain. Apochromatic objectives are engineered to bring three colors (red, green, blue) into a common focus, virtually eliminating chromatic aberration for a color-accurate, sharp image. Olympus plan apochromat lenses leverage this high degree of correction to produce images free of color halos even in multicolor fluorescence experiments.

What’s the difference between achromat, fluorite (semi-apochromat), and apochromat objectives?

These terms refer to the level of optical correction in objective lenses.

• Achromat lenses are the most common and are corrected for chromatic aberration at two wavelengths (usually red and blue) and for spherical aberration in one color.
• Fluorite (semi-apochromat) objectives use special glass (fluorite) to achieve better correction—typically bringing more colors into focus and improving spherical aberration, resulting in higher contrast and often a higher NA than achromats.
• Apochromat objectives are the most highly corrected, bringing three or more wavelengths into focus and correcting spherical aberrations in multiple colors. They offer the best image quality and usually have the highest numerical apertures.

In short, achromats are cost-effective for routine use, fluorites offer a good balance of performance and cost, and apochromats deliver the best resolution and color fidelity for demanding applications.

What does “plan” mean in objective names (such as plan achromat or plan apochromat)?

Plan designates a flat-field objective, meaning it is corrected to produce a flat, in-focus image across the entire field of view. Without this correction, the image periphery would be out of focus when the center is in focus. Plan objectives eliminate curvature of field, so even the edges of the view are sharper. For example, the Olympus MPLN series (plan achromat) objectives are designed to provide optimal flatness throughout the field for uniform clarity. This is especially important for digital imaging or widefield eyepieces where a non-plan lens would show blurry edges.

What are Olympus MPLAPON and MPLAPON-Oil objectives, and when should I use each one?

The Olympus MPLAPON series comprises high-end plan apochromat objectives designed for use in air. These dry objectives are known for their excellent color correction and resolution. They correct chromatic aberrations across a broad range of applications and have very high image fidelity, making them ideal for observing critical samples in brightfield, fluorescence, or DIC observation.

MPLAPON-Oil is the oil immersion version of this series. It has the same advanced apochromatic optics but uses immersion oil to achieve an even higher numerical aperture (NA up to 1.45) for enhanced resolution.

Use an MPLAPON dry objective lens when you need a lower magnification and avoiding immersion oil is important. Switch to an MPLAPON-Oil objective lens when you need a higher resolution and enhanced brightness (typically at 60X–100X magnification) and your sample can be used with immersion oil.

Why does using immersion oil increase an objective’s resolution compared to a dry objective?

Immersion oil has a higher refractive index (~1.51) than air (1.0), which enables the objective to capture light at larger angles. With air as the medium, the theoretical maximum NA is 1.0 (since sin 90° = 1 and n=1.0). In practice, most high-end dry objectives top out around NA 0.95. By using oil between the coverslip and lens, the refractive index increases to ~1.51, enabling NA values around 1.3–1.4. This higher NA dramatically improves resolution because the lens gathers more diffracted light from fine details. In short, oil immersion lenses can resolve finer details (and produce brighter images) than dry lenses of the same magnification.

Can I use an oil immersion objective without oil?

No, using an oil immersion objective without the specified medium degrades performance. Oil immersion objectives are designed so that oil (n≈1.51) fills the gap between the coverslip and lens. If you use air (with air, n=1.0) or water when an objective is meant for oil, the optics will suffer from spherical aberration and loss of resolution because the light rays will focus incorrectly. Manufacturers strongly advise against using oil objectives with water or other mismatched media. Always use the intended medium (oil, water, or glycerin, as specified on the objective). Otherwise, the image may be blurred and lack the objective’s stated resolving power.

What is a correction collar on a microscope objective, and when should I use it?

A correction collar is an adjustable ring on certain high-performance objectives that compensates for cover glass thickness or immersion medium differences. Many objectives are designed for a standard 0.17 mm coverslip thickness (#1.5 cover glass). If your coverslip is thicker or thinner than this, or if you are imaging into a fluid of different refractive index, it can introduce spherical aberration and blur.

Objectives with correction collars enable you to tune the lens for these variations—the collar mechanically shifts internal lens groups to reoptimize focus. By adjusting the correction collar to match your exact coverslip thickness, you can prevent image blur and aberrations caused by non-standard cover thickness. In practice, you simply turn the collar until the image (especially at high focus depth) becomes sharpest. Use the correction collar whenever your coverslip or sample deviates from the standard 0.17 mm, such as in live-cell chambers with thicker bottoms, to maintain the best image quality.

Why is the 0.17 mm cover glass thickness so important for objectives?

High-power objectives are calibrated assuming a 0.17 mm coverslip (the typical thickness of a #1.5 cover glass). If you use a significantly thicker or thinner glass between the objective and sample, the light rays will not converge as intended, leading to spherical and even chromatic aberrations. This results in blurry or distorted images. Many advanced objectives include a correction collar to compensate for these aberrations. For objectives without one, it is critical to use the recommended coverslip thickness. Using the standard 0.17 mm cover glass helps ensure the objective’s optical design performs optimally, yielding a sharp image and correct focus across the field.

Are Evident/Olympus objectives compatible with microscopes from other brands?

Compatibility is not guaranteed, especially for modern infinity-corrected systems. Microscope objectives often use common threads (for example, RMS thread is common on many Olympus objectives), so you might be able to screw an Olympus objective into another brand’s nosepiece. However, the optical design can differ: infinity-corrected objectives must be used with the proper tube lens focal length and corrections. For example, Olympus infinity objectives are designed for a 180 mm tube lens, whereas another brand may use a 200 mm tube lens; mixing them can change the effective magnification and introduce aberrations. Also, not all manufacturers use the same thread pitch or parfocal distance for high-end objectives. In summary, Evident or Olympus objectives should be used on Evident/Olympus microscopes (or those specifically designed for them) to achieve the specified performance. Always check thread type, tube length, and optical compatibility before swapping objectives of different manufacturers.

What factors should I consider when choosing an objective for confocal microscopy?

Confocal microscopy demands high resolution and efficient light gathering, so the primary considerations are high NA and excellent optical correction. Choose a high-NA objective (e.g., 60X/1.40 oil or 60X/1.20 water) to collect as much fluorescence signal and detail as possible. In confocal and other high-magnification techniques, we push to the limits of NA to achieve enhanced resolution. Plan apochromat objectives are preferred for confocal because they offer superior chromatic correction (important when imaging multiple fluorescent channels) and a flat field of view. Our Olympus X Line™ series objectives, for example, combine improved NA, flatness, and chromatic aberration correction across 400–1000 nm, which is ideal for multicolor confocal imaging. Also consider the working distance and immersion medium. For thick specimens or live cells, a water or silicone immersion lens can be advantageous to reduce refractive index mismatches. For fixed samples on slides, an oil immersion lens provides a higher NA and resolution.

Do I need special objectives for differential interference contrast (DIC) microscopy?

DIC uses polarized light and Nomarski/Wollaston prisms to create contrast. As a result, objectives for DIC should be largely strain-free (free of internal birefringence) so they do not disturb the polarized light. Unlike phase contrast objectives, DIC objectives usually lack built-in rings or special modifications—the main requirement is high optical quality and low strain in the glass. Our plan objectives (e.g., Olympus UPlan or MPLN series) are manufactured to be compatible with DIC. Many are labeled for DIC use or simply perform well due to minimal internal stress. Historically, “POL” or strain-free versions were sold for polarization microscopy, but many modern Olympus apochromat and fluorite objectives are sufficiently strain-free for DIC. So you do not need a dedicated DIC objective, as you can use a high-quality plan achromat/fluorite/apochromat that is known to work with DIC (and make sure to use the proper DIC prism slider set for your objective magnification).

Which objectives are recommended for live-cell imaging (such as long time-lapse microscopy of living cells or tissues)?

Live-cell imaging often benefits from specialized immersion objectives, particularly water or silicone immersion lenses. These objectives have immersion media with refractive indices closer to that of live cells (water ~1.33 or silicone oil ~1.40) to minimize spherical aberration when imaging into aqueous samples.

Our silicone immersion objectives are highly suited for live-cell work. Silicone immersion objectives (e.g., Olympus UPLSAPO 60XS silicone) have a high NA (around 1.30) and a relatively long working distance (~0.3 mm) to image into thick specimens. The main advantage of silicone oil is that it does not evaporate or dry out at 37 °C (98.6 °F), making long-term time-lapse imaging more stable and less maintenance-intensive than water immersion. Water immersion objectives (NA ~1.1–1.2) are also used for live cells, especially for short-duration imaging, as they naturally match cell media. However, water can evaporate or cause focus drift over time.

For extended live-cell experiments, silicone immersion objectives are often the top choice, providing high resolution and brightness in native biological conditions without the drawbacks of drying or spherical aberration. As a rule, always confirm that your chosen objective is compatible with your microscope and that you have the appropriate immersion oil or water delivery system for long-term use.