01 Resolution

First, let's talk about resolution.

The term "resolution" carries different meanings in various contexts.

For example, the resolution of an image refers to the number of pixels it contains:

For a camera sensor, this refers to the number of discrete samples it takes and records details with. It constitutes a digital pixel grid that determines the "sampling density."

For example, the resolution of the OV7670 shown in the figure below refers to the size of its effective photosensitive array, specifically 640×480.

For lenses, since they have no direct correlation with electronic parameters like pixels, their resolution is more aligned with optical characteristics—their ability to project the details of the subject onto the imaging plane (the photosensitive element).

In other words, lens detail is an analog continuous signal that measures the lens's ability to capture closely spaced details.

The unit is LP/mm (logarithmic pairs per millimeter), indicating the number of distinct black-and-white line pairs that can be distinguished within 1 mm; a higher value is preferable.

Generally, a dedicated test card is commonly used to measure a lens's resolution.

For example, the ISO 12233 test card shown below features lines of varying densities; the imaging characteristics of these lines can serve as a basis for determining resolution.

Simply put, lens resolution is an objective metric that cannot be significantly improved through post-processing. With high-resolution lenses (or optical systems), even the finest details remain sharp and distinct.

The sensor's resolution is also an objective metric, after all, the pixel count is clearly indicated.

02 Sharpness

Sharpness refers to the contrast and steepness of edges in an image (such as boundaries between black and white / light and dark areas, etc.).

Generally speaking, when a lens with high sharpness forms a clear image, the boundaries between black and white (or areas with large color differences) will be steeper, as shown in the figure below:

We can observe that images with higher sharpness exhibit faster edge transition speeds (i.e., greater clarity).

The phrase "as sharp as a blade within the focus" refers to exceptional sharpness.

However, unlike resolution, sharpness can be optimized to some extent through post-processing (when hardware is insufficient, software steps in). For example, post-processing software like LightRoom can enhance image sharpness to a certain degree.

03 Clarity

Finally, let's talk about clarity.

Unlike sharpness and resolution, clarity is a more subjective measure.

Clarity refers to the overall visual perception of a image as "clear" or not; it cannot be quantified through data but results from the combined effects of multiple physical factors.

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The so-called 'high-definition' photos pursued during shooting refer to images that are exceptionally sharp, clear, and rich in detail—a comprehensive manifestation of high resolution, high contrast, high sharpness, low noise, and minimal color distortion.

For example, in this photo, I used a Canon RP (full-frame) camera with a Xiaoxiaobai lens at its telephoto end (the MTF of telephoto lenses is generally superior), combined with ample ambient light (low ISO for reduced noise and shorter exposure time):

Even when we enlarge the image, we can still clearly see the details of the cat's face.

In fact, clarity can also be improved to some extent during post-production.

For example, in addition to adjusting sharpness on the LR setting, we can also enhance the local mid-tone contrast of the image by moving the clarity slider.

In short, capturing a 'clear' photo requires the coordinated effort of multiple factors—including the lens, camera body, and post-processing—all of which are indispensable.