Parfocal length

The “Front Lens” of the microscope is the convergent (double convex) lens. The focal length is the distance between the front lens and the object at F. Usually, it is imprinted with f=xxx on the lens’ bezel.

Another factor is that, within a fixed space, higher focal lengths further constrain the size of the image sensor. Inversely, some manufacturers have recently been able to improve image quality by switching to a lens with a slightly shorter focal length but a much larger sensor.[3] For example, the Vivo X90 Pro+'s 90mm lens still has a 60% larger effective sensor area when digitally cropped to match the field of view of the X80 Pro's 125mm lens.[a]

This is a list of smartphones with a telephoto lens that offers a focal length (35mm equivalent) of at least 100mm or "4× optical zoom" with an imaging area equivalent to a 1/3.5″ or larger sensor.

Microscopyu

The working distance can then be determined. For example, if the front lens has a focal length of f=200, the object will not focus at less than 200 mm working distance. In addition, using a 300 mm instrument would not be advised.

As such, the following list excludes lenses using sensors smaller than 1/3.5″, which seems to be a lower bound for acceptable image quality (for comparison, most compact travel zoom cameras use 1/2.3″ sensors, which are over twice[b] as large).

Focal length and working distance are critical. You need enough space under your microscope to accommodate your instruments. Surgical microscope focal lengths vary in two types: fixed focus and variable focus.

Also, remember that with increased focal length (working distance), more of the objective will be in view. This is called Field of View (magnification aspect discussed later).

Field of viewmicroscope

Note that this is not a list of the "best" smartphone telephoto lenses, since there are many factors that affect image quality. For example, the Samsung Galaxy S22 Ultra's telephoto lens outperforms the S21 Ultra's, due to hardware and software improvements despite the S22 Ultra's smaller sensor size and shorter focal length.[2]

Fixed focal length convergent lenses, usually f=175 or f=200, are used for ophthalmic procedures. Some operating microscopes for ENT procedures may also use a fixed front lens.

Smartphone lenses are often marketed in terms of "optical zoom"[1] relative to the phone's main camera. For example, 120mm is usually referred to as "5× optical zoom", because the main camera's focal length is commonly around 24mm. However, only a few smartphones, such as the Sony Xperia 1 IV and V, include a true zoom lens that offers continuous optical zoom, whereas nearly all smartphones feature a collection of prime lenses interpolated by digital zoom.

The focal length is variable for neurosurgical, spine, and ENT procedures. For example, the objective lens combination can be configured so the focal length is from f=200 to f=450. So, any object within f=200 to f=450 will be in focus. Plus, any object outside f=200 to f=450 will never be in focus. The minimum working distance from the surgical site objective is 200 mm. So, if an instrument is 300 mm long, you would not want the microscope in the 300 to 450 mm range.