Numerical Aperture, Resolution & Magnification Explained -

What Do Numerical Aperture, Resolution, and Magnification Mean in Light Microscopy?

Three terms underpin how much detail a light microscope can show: numerical aperture (NA), resolution, and magnification. They are related yet distinct. Understanding each—along with their limits and trade-offs—prevents common pitfalls like chasing “more power” that adds size but not detail, or closing an aperture for contrast only to sacrifice fine structure. This section defines the terms and previews how they interact throughout the optical system.

Numerical aperture (NA) is a measure of light-gathering ability and resolving power of an objective (and condenser). It depends on the refractive index of the medium and the acceptance angle of light.
Resolution is the smallest spacing between two features that can be distinguished as separate. It is limited by diffraction and improved by higher NA and shorter wavelengths.
Magnification is the enlargement of the image. It does not create new detail by itself; it only makes existing detail bigger. Past a certain point, additional magnification is “empty.”

\"Cross — *Cross section of a microscope objective: Achromatic objective with a numerical aperture of 0.65 and a 40-times magnification*
Artist: Ice Boy Tell

In practical terms, NA determines the finest details the optics can transfer, while magnification determines how large those details appear to your eye or camera. Crucially, the specimen must be adequately illuminated to support that resolution, which brings the condenser NA and illumination geometry into play. We’ll unpack how NA controls the optical transfer of detail in How Numerical Aperture Sets Optical Resolution and Contrast, why magnification must be matched to both NA and sensor pixels in Magnification, Camera Sampling, and Empty Magnification, and how the condenser and wavelength influence contrast and limit in Wavelength, Illumination, and the Role of the Condenser.

A few key relationships, stated up front, guide everything that follows:

NA = n · sin(α), where n is the refractive index of the immersion medium and α is the half-angle of the objective’s acceptance cone.
Lateral (XY) diffraction-limited resolution for point objects (Rayleigh criterion): d ≈ 0.61 · λ / NA.
Axial (Z) resolution scales as ∝ λ · n / NA² in widefield imaging, showing a stronger dependence on NA than lateral resolution.
Sampling at the sensor should meet the Nyquist criterion: the specimen-plane sampling interval should be no larger than roughly half the optical resolution.

If you remember one thing: you cannot out-magnify the diffraction limit set by NA and wavelength. Proper illumination and sampling ensure you approach that limit rather than leaving performance on the table.

How Numerical Aperture Sets Optical Resolution and Contrast

The numerical aperture of an objective lens is defined by NA = n · sin(α). Here, n is the refractive index of the medium between the front lens and the specimen (for example, air ≈ 1.00, water ≈ 1.33, typical immersion oil ≈ 1.515, glycerol ≈ 1.47, silicone oil ≈ 1.40–1.41), and α is the half-angle of the widest cone of light that the objective can accept. Two aspects are immediately clear:

For a given objective design, using a higher-index immersion medium increases NA.
Wider acceptance angles (larger front lens, closer working distance) increase NA.

Why does NA matter? Diffraction spreads each point of light from the specimen into a finite-sized blur known as the point spread function (PSF). High NA objectives collect steeper angles of diffracted light, producing a tighter PSF (smaller blur circle). When two points are close together, their PSFs overlap. If the overlap is too great, the peaks merge and the points become indistinguishable.

The classical Rayleigh criterion for the minimum resolvable distance between two point sources is:

d ≈ 0.61 · λ / NA

where λ is the wavelength of light in the medium. Another widely cited expression, particularly for periodic structures (like gratings), is the Abbe limit:

d ≈ λ / (2 · NA)

\"Two — Two airy disks at various spacings: (top) twice the distance to the first minimum, (middle) exactly the distance to the first minimum (the Rayleigh criterion), and (bottom) half the distance. This image uses a nonlinear color scale (specifically, the fourth root) in order to better show the minima and maxima.
Artist: Spencer Bliven

The precise constant depends on the definition of “just resolved” and the imaging scenario, but both formulas capture the same scaling: smaller wavelengths and larger NA yield finer resolution. This is a fundamental limit for conventional (diffraction-limited) optical systems.

Objective NA and condenser NA work together

In transmitted-light brightfield, the condenser focuses illumination onto the specimen, and its numerical aperture (NA_cond) shapes the angular spectrum of light entering the sample. An adequately high NA_cond is essential to generate and collect the high spatial frequencies that encode fine detail. If the condenser aperture is stopped down too far, contrast for fine features is lost—even if the objective NA is high. Matching the condenser to the objective is a cornerstone of proper illumination.

Opening the condenser aperture increases resolution and brightness but may reduce contrast in low-absorption specimens.
Closing it increases contrast and depth of field but reduces the highest resolvable spatial frequencies.

NA, signal-to-noise, and contrast transfer

Beyond diffraction, image quality depends on how well different spatial frequencies (details of different sizes) are transferred with contrast from specimen to image. This behavior is described by the optical transfer function (OTF) and its magnitude, the modulation transfer function (MTF). High NA systems support a higher cutoff frequency and tend to transfer mid-to-high spatial frequencies with better contrast, all else equal. They also collect more photons, improving the signal-to-noise ratio (SNR) per unit time under the same irradiance at the specimen.

Practically, this means that a higher NA objective does not merely make features “sharper” by tightening the PSF; it also raises the image’s photon budget and makes faint details easier to detect at a given exposure. There are limits, of course—sample photodamage and fluorophore bleaching in fluorescence microscopy, or heat and drift in transmitted modalities—but the directional benefit remains: higher NA generally improves both resolution and SNR within the constraints of the sample and optics.

Immersion media and refractive index matching

Immersion objectives use media whose refractive index more closely matches glass (and in some cases tissue) to increase NA and reduce refractive index mismatch at interfaces. Typical values: water ≈ 1.33, glycerol ≈ 1.47, silicone oil ≈ 1.40–1.41, immersion oil ≈ 1.515. Matching the immersion medium to the sample’s environment helps reduce spherical aberration, particularly when imaging deeper into aqueous samples. We expand on spherical aberration and cover glass effects in Aberrations, Cover Glass Effects, and Objective Corrections.

A final reminder: NA is a property of the objective (and condenser) and does not change with additional magnifying optics such as eyepieces or cameras. Magnifying more cannot improve d once NA and wavelength are set; it can only change the sampling of that detail on a display or sensor. We return to this crucial point in Magnification, Camera Sampling, and Empty Magnification.

Magnification, Camera Sampling, and Empty Magnification

Magnification describes how large an image appears relative to the specimen. In microscope systems, total magnification to the camera is determined by the objective, tube lens, and any relay optics; to the eye, it also includes the eyepiece power. The key is that magnification should be chosen to sample the optical resolution appropriately. Too little magnification (undersampling) throws away resolvable detail. Too much magnification (oversampling) wastes sensor pixels and dims the image or exacerbates noise without revealing finer features.

Sampling mathematics in one paragraph

Let the camera pixel size be p (e.g., micrometers). The specimen-plane sampling interval is s = p / M, where M is the total magnification to the sensor. To satisfy the Nyquist sampling criterion, choose s ≤ d/2, where d is the system’s optical resolution (e.g., d ≈ 0.61·λ/NA for point objects under incoherent widefield conditions). Rearranged: M ≥ 2p / d. This expression connects magnification to NA, wavelength, and pixel size, ensuring you record the optical detail your lens can deliver.

Illustrating undersampling and oversampling

Undersampling: Fine details blur together on the sensor because each pixel averages over too much specimen area (s > d/2). Even though the objective may resolve the features optically, the digital image cannot reconstruct them.
Oversampling: The same optical detail spreads across many pixels (s ≪ d/2), which may help with interpolation but does not reveal new information. The image is larger and often noisier per pixel (for a fixed exposure) without added resolution.

In practice, many microscopists aim for approximately two to three pixels across the smallest resolvable feature width to balance resolution capture and SNR. The exact choice depends on the modality and the goals of the measurement or visualization. The principle remains constant: match magnification to the camera so you neither throw away nor waste optical resolution.

Empty magnification explained

Empty magnification occurs when you increase magnification beyond what the optics can resolve. The smallest resolvable spacing d is fixed by NA and wavelength. Enlarging the image further only magnifies the diffraction blur and noise. For visual observation with eyepieces, the apparent sharpness ceases to improve once the eye’s acuity and the objective’s resolution are both satisfied. For cameras, the Nyquist expression above provides an objective guide. If your M is already high enough that s ≤ d/2, additional magnification is typically empty.

Field number, field of view, and magnification

For visual observation, microscope eyepieces are often specified by a field number (FN) in millimeters, which approximates the diameter of the intermediate image the eyepiece admits. The specimen-plane field of view (FOV) is then roughly FOV ≈ FN / M_objective (for finite systems) or otherwise depends on the objective and tube lens in infinity-corrected systems. Higher magnification reduces FOV. Cameras are limited by sensor size and the microscope’s relay optics; total magnification to the sensor should capture the desired FOV while also meeting the sampling requirement.

These considerations show why magnification is never a standalone performance claim. It must be understood in the context of NA, wavelength, and pixel size. We will return to camera and tube lens choices in Digital Imaging Considerations.

Wavelength, Illumination, and the Role of the Condenser

Wavelength directly enters the resolution limit through the diffraction formulas. Shorter wavelengths yield smaller d. However, illumination is not only a matter of color; it also includes how light is delivered to the specimen spatially (angles) and temporally (coherence). In transmitted brightfield, the condenser sets the angular distribution of illumination, which governs the excitation of spatial frequencies in the sample and affects contrast transfer.

Shorter wavelength improves resolution—but with trade-offs

Blue light improves resolution relative to red light because d ∝ λ. The improvement is real but must be balanced against sample absorption, photobleaching (in fluorescence), and detector sensitivity.
Ultraviolet (UV) further improves potential resolution, but many optical materials and biological samples absorb strongly in the UV. Specialized optics and safety considerations apply. For visible-light microscopy, most practical work balances contrast and SNR in the blue–green region.

Köhler illumination and condenser aperture

In brightfield, Köhler illumination uses an extended light source imaged at the condenser aperture, not at the specimen plane. This produces uniform, quasi-incoherent illumination, helping the microscope transfer spatial frequencies faithfully. Two controls matter:

Condenser aperture diaphragm: Adjusts NA_cond. Opening it approaches the objective’s resolving capability; closing it boosts contrast and depth of field but limits high-frequency transfer.
Field diaphragm: Confines illumination to the observed field, reducing stray light and improving contrast without directly altering resolution.

Proper condenser adjustment is often the difference between observing diffraction-limited detail and missing it. If a high-NA objective appears soft on fine structures, check the condenser aperture before assuming focus or lens issues. The condenser’s role is so critical that misadjustment can emulate “low NA” behavior. For more on balancing these trade-offs, see How Numerical Aperture Sets Optical Resolution and Contrast and Depth of Field, Depth of Focus, and Axial Resolution.

Coherence and contrast techniques

Illumination coherence (how uniform the phase and frequency are) affects contrast transfer. Extended-source Köhler illumination provides low coherence, which supports the classic 0.61·λ/NA lateral resolution behavior. Highly coherent illumination, such as a laser, modifies the transfer characteristics and interference effects in ways that can change contrast for certain spatial frequencies. Specialized contrast methods—phase contrast, differential interference contrast (DIC), darkfield, and polarization—do not alter the diffraction limit, but they redistribute contrast so that transparent or anisotropic structures become visible. These techniques complement the fundamental limits set by NA and wavelength.

Depth of Field, Depth of Focus, and Axial Resolution

The concepts of depth of field (DOF), depth of focus, and axial resolution are related but apply to different spaces in the imaging chain and are controlled by NA in different ways:

Axial resolution (Z) defines how closely two features can be separated along the optical axis and still be distinguished. It is fundamentally limited by diffraction, with a dependence that scales as ∝ λ · n / NA² in widefield imaging.
Depth of field is the range in object space over which the image remains acceptably sharp. In microscopy, DOF decreases rapidly as NA increases. It is influenced by diffraction and, in digital imaging, by the acceptable blur relative to pixel sampling.
Depth of focus is the tolerance in the image plane (e.g., at the camera sensor) over which the image remains acceptably sharp as the sensor is moved. For a given NA, it depends on magnification and the system’s acceptable blur criterion.

Two intuitions help keep these straight:

Higher NA tightens the PSF not only laterally but also axially; therefore, axial resolution improves (smaller Z extent), and the DOF becomes thinner.
At higher magnification, the same amount of defocus spreads over more pixels at the sensor, affecting the depth of focus in image space. Nevertheless, the dominating trend for object-space DOF is its inverse square dependence on NA.

Why does this matter? If your specimen is thick compared to the DOF at the chosen NA, only a thin slice will be in focus at once. Strategies to address this include optical sectioning methods (such as confocal or structured illumination), mechanical focus stacking for extended depth images, or using a lower NA objective to increase DOF. Each approach has trade-offs for resolution, SNR, and imaging speed.

For everyday brightfield work, it’s common to slightly close the condenser aperture to boost DOF and contrast on thick or low-contrast specimens. As emphasized in Wavelength, Illumination, and the Role of the Condenser, this simultaneously sacrifices the finest resolution. The optimal setting depends on whether the goal is to see the broad structure at once or to resolve the finest details.

Working Distance, Field of View, and Image Brightness Trade-offs

Design constraints link working distance (WD), NA, and magnification. For a given magnification class, pushing NA higher typically requires a larger front lens and a shorter WD to accept steeper rays. Meanwhile, the field of view (FOV) shrinks with higher magnification unless the system optics and sensor/eyepiece are sized accordingly.

Working distance and NA

High-NA objectives often have shorter WDs because they must physically get closer to the coverslip to collect high-angle rays. This limits maneuvering room and makes them more sensitive to cover glass thickness and immersion conditions. Long-working-distance objectives exist for specialized tasks, but they usually have a lower NA at a given magnification than their short-WD counterparts due to geometric constraints.

Brightness and NA

In transmitted brightfield under Köhler illumination, opening the condenser aperture increases irradiance at the image and supports finer spatial frequency transfer. The light-gathering ability in such systems scales with the square of NA in the small-angle approximation. In epi-illumination (e.g., reflected-light brightfield or fluorescence detection), a higher NA objective typically collects a larger fraction of emitted or reflected light from a given point, improving SNR at a fixed exposure time. These relationships are nuanced and depend on the modality, but the general direction holds: higher NA tends to increase image brightness and SNR, within the limits of the source and sample.

Field of view constraints

The usable FOV is set by the microscope’s relay optics, the objective’s field correction, and the sensor or eyepiece aperture. Wide, flat fields require objectives corrected for field curvature (e.g., “Plan” types) and sufficient image-circle coverage by the tube lens and camera. Higher magnification narrows the specimen FOV for a given sensor size, which may require stitching for large samples. The FOV–magnification trade-off often dictates practical decisions: if the task is to survey, use a lower magnification and NA; if it is to resolve the smallest structure, use a high NA and accept a smaller field.

Aberrations, Cover Glass Effects, and Objective Corrections

Real optical systems deviate from the ideal due to aberrations. Even if the diffraction limit suggests a particular d, uncorrected aberrations can broaden the PSF, reduce MTF, and degrade contrast. High-NA objectives are especially sensitive to manufacturing precision, immersion conditions, and cover glass properties.

Common aberrations in microscopy

Spherical aberration: Rays at different heights in the aperture focus at different axial positions. It is particularly sensitive to refractive index mismatch and cover glass thickness errors.
Chromatic aberration: Different wavelengths focus at different axial positions (longitudinal) or different lateral positions (lateral). Objectives are corrected to varying degrees (achromat, fluorite/semi-apochromat, apochromat).
Coma and astigmatism: Off-axis aberrations that degrade image sharpness away from the field center.
Field curvature: The best focus lies on a curved surface rather than a flat plane. “Plan” objectives largely correct this, producing flatter fields.

Cover glass thickness and correction collars

Many high-NA objectives are designed for a standard cover glass thickness around 0.17 mm (commonly labeled as #1.5 or 1.5H). Deviations in thickness or refractive index introduce spherical aberration that broadens the PSF, lowering resolution and contrast even though the nominal NA is unchanged. Correction collar objectives allow you to compensate for some of this error by adjusting internal lens spacing to restore focus convergence for the actual cover glass and sample conditions.

Practical guidance for minimizing aberrations:

Use the immersion medium specified by the objective. Refractive index mismatches are a common source of spherical aberration, especially at high NA.
Use the cover glass thickness printed on the objective (often 0.17 mm). If your slides vary, a collar can help; fine adjustments can produce noticeably sharper images.
Keep optics clean and avoid air gaps or bubbles in immersion layers, which act like mismatched interfaces.
Center and align the condenser. Off-axis illumination exacerbates coma-like effects and reduces contrast.

Objective correction classes

Objectives are offered with different degrees of chromatic and field correction:

Achromat: Corrects chromatic aberration for two wavelengths and spherical aberration for one. Adequate for many brightfield tasks.
Fluorite/Semi-apochromat: Improved correction for spherical and chromatic aberrations; often higher NA than achromats at the same magnification. Common in fluorescence and demanding transmitted-light work.
Apochromat: High-level chromatic and spherical corrections across multiple wavelengths, with flat fields in “Plan Apo” types. Typically support the highest NAs at a given magnification with excellent color fidelity.

These nomenclatures are about aberration correction, not NA per se. Yet higher-correction objectives often have designs that support higher NA and better MTF, making them the preferred choice when pushing resolution and color accuracy.

Choosing Objectives by Numerical Aperture and Use Case

With the physical relationships in place, selecting an objective becomes a matter of matching NA, immersion, working distance, and corrections to the specimen and task. The goal is not simply to pick the “highest NA,” but to choose an optic that delivers usable resolution and contrast for your sample while accommodating practical constraints such as thickness, refractive index, and FOV.

Air, water, glycerol, silicone, and oil immersion

Air objectives: Convenient and versatile. Practical NA is limited by air’s refractive index (~1.00). Suitable for dry-mounted or thick samples where immersion is impractical.
Water immersion: Closer refractive index to aqueous specimens reduces spherical aberration when imaging into water-based samples. Beneficial for live, hydrated preparations.
Glycerol immersion: Intermediary refractive index (~1.47) can better match certain clearing media or thick specimens where oil would cause mismatch.
Silicone oil immersion: Index near 1.40–1.41 with mechanical stability over time; useful for live-cell imaging where temperature fluctuations and long time courses are common.
Oil immersion: Highest practical NA in the visible range with standard cover glasses. Excellent for thin, mounted specimens on glass where index matching to glass is desired.

NA and magnification combinations

Objectives of the same nominal magnification may have very different NAs and corrections. For instance, two 40× objectives could have NAs ranging from modest to very high depending on their design and immersion medium. The NA—not the magnification—primarily sets resolution. Choose the objective whose NA and correction suit the level of detail and color accuracy required. Then set magnification and camera sampling to capture it (see Magnification, Camera Sampling, and Empty Magnification).

Specimen thickness and working distance

Thick or three-dimensional specimens challenge high-NA, short-working-distance objectives. If your subject cannot be placed near the coverslip, a long-working-distance objective may be necessary, accepting a lower NA at a given magnification. Alternatively, re-preparing the sample for a thinner geometry or using an immersion medium matched to the specimen can recover performance.

Compatibility with contrast methods

Many objectives are marked for compatibility with phase contrast rings, DIC prisms, polarization, or fluorescence. Such markings indicate mechanical and optical provisions (e.g., phase annuli) and coatings. Using a method with an incompatible objective can degrade contrast. While contrast techniques do not beat the diffraction limit, they profoundly affect visibility of features near that limit by redistributing image contrast.

Cover glass and mounting media

When your objective is designed for a 0.17 mm cover glass, ensure your preparations use the correct thickness and that the mounting medium’s refractive index is appropriate. For high-NA oil objectives, a glass-like refractive index in the mounting medium reduces spherical aberration. For water-immersion objectives imaging into aqueous samples, avoid oil-like media that would exacerbate mismatches.

Digital Imaging Considerations: Pixel Size, Tube Lenses, and Relay Optics

Digital imaging makes sampling considerations explicit. The camera’s pixel size and the microscope’s total magnification set the specimen-plane sampling interval. Choosing a camera and configuring the tube lens and any relay optics should begin with the optical resolution and the FOV you need.

Pixel size and total magnification

The fundamental relationship is straightforward: s = p / M, where s is the specimen-plane sampling interval, p is the camera pixel pitch, and M is the total optical magnification from the specimen to the sensor. Combine this with s ≤ d/2 to establish the minimum M that meets Nyquist for the objective’s resolution. If your imaging is noise-limited, you might accept mild undersampling to gain SNR or speed; if your priority is to extract the finest details and perform deconvolution, you may choose to slightly oversample.

Infinity-corrected systems and tube lenses

In modern infinity-corrected microscopes, the objective forms a collimated beam that the tube lens refocuses to create the intermediate image. The system’s nominal objective magnification assumes a particular tube lens focal length specified by the manufacturer. Using a different tube lens changes the effective magnification at the camera while not changing the objective’s NA. This is a powerful way to tune sampling and FOV without changing objectives, provided the optics maintain image quality over the altered field and angular spread.

Relay optics and camera adaptors

Microscope camera ports often incorporate relay lenses (e.g., 0.5×, 1×, 1.6×) that scale the image onto the sensor. Adding a reducing relay increases the FOV on a given sensor but reduces sampling (larger s); adding an enlarging relay decreases FOV but increases sampling density (smaller s). Choose relays to meet both Nyquist and FOV goals. Be mindful that extreme reductions can vignette or push optics beyond their corrected image circle, degrading corners.

Sensor size, field flatness, and telecentricity

Large sensors can capture wide fields, but only if the objective and tube lens supply a flat, well-corrected image across that area. Objectives labeled “Plan” are designed for flat fields, but the relay path must also be up to the task. Additionally, some imaging tasks benefit from telecentricity (chief rays parallel to the optical axis in object or image space), which improves measurement accuracy and uniformity across the field; this is a property of system design and not an afterthought.

Bit depth, dynamic range, and noise

Pixel sampling is only half the story. The camera’s bit depth and noise characteristics (read noise, dark noise, and photon shot noise) affect how much of the transferred optical detail is visible and quantifiable. Higher NA helps by collecting more photons in a given exposure, raising SNR. But maximizing SNR also depends on exposure time, illumination stability, and the camera’s conversion gain. In low-light fluorescence, for instance, pairing a high-NA objective with a sensitive, low-noise sensor can make the difference between detecting a feature and missing it.

Frequently Asked Questions

Is a higher numerical aperture always better?

Higher NA generally improves resolution and photon collection, which boosts SNR and contrast transfer for fine details. However, it also reduces depth of field, often shortens working distance, and can increase sensitivity to cover glass thickness and immersion conditions. For thin specimens where maximum detail is the goal, high NA is advantageous. For thick, uneven, or delicate samples where you need more DOF or working distance, a slightly lower NA may be more practical. The optimal choice balances these trade-offs, as discussed in Depth of Field, Depth of Focus, and Axial Resolution and Working Distance, Field of View, and Image Brightness Trade-offs.

Why doesn’t my 100× objective look sharper than 40×?

Two common reasons:

Empty magnification: If the 100× objective has similar or only modestly higher NA than the 40×, the theoretical resolution (d) may be similar. More magnification simply enlarges the same level of detail. See Magnification, Camera Sampling, and Empty Magnification.
Illumination and aberrations: If the condenser aperture is not opened sufficiently to match the 100× objective, or if immersion/cover glass conditions are not ideal, spherical aberration and reduced high-frequency transfer can make the higher-power view look soft. Review Wavelength, Illumination, and the Role of the Condenser and Aberrations, Cover Glass Effects, and Objective Corrections.

Additionally, ensure camera sampling matches the higher-resolution potential of the 100× lens. If s = p / M remains too large for the new d, you may be undersampling and not capturing the available detail.

Final Thoughts on Understanding NA, Resolution, and Magnification

At the heart of light microscopy, numerical aperture and wavelength set the maximum detail that can be transferred from specimen to image. Magnification determines how that detail is sampled and displayed. The condenser aperture and illumination geometry shape contrast and enable high-frequency transfer. Meanwhile, aberrations, cover glass thickness, and immersion conditions decide whether your system performs near its physical limits or falls short.

The practical workflow is clear:

Choose an objective with the NA and immersion suitable for your specimen’s thickness, medium, and the level of detail you need.
Set up illumination (condenser aperture and field stop) to support that resolution while balancing contrast and DOF.
Match magnification and camera sampling to the optical resolution so you neither undersample nor indulge in empty magnification.
Control aberrations by using the correct cover glass thickness, the specified immersion medium, and—when available—adjusting correction collars.

With these principles, students, educators, and hobbyists can diagnose soft images, design better imaging setups, and extract more information from their specimens—without chasing specifications that don’t translate to real performance. If you found this deep dive helpful, explore related topics on optics and contrast methods, and subscribe to our newsletter for future articles on microscope fundamentals and practical, physics-grounded advice.

Numerical Aperture, Resolution & Magnification Explained

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