A high power or compound microscope achieves higher levels of magnification than a stereo or low power microscope. It is used to view smaller specimens such as cell structures which cannot be seen at lower levels of magnification.

Focaldistancevs focal length

While easy to calculate, remembering the calculation is not always easy. Keep this page in your physical or mental bookmarks for the next time you need to calculate lens working distance.

Focus distance

While lens working distance matters little to most of my shooting, there are situations where knowing the distance in front of the lens matters for lens selection. Macro photography at MFD (Minimum Focus Distance) is typically the scenario where working distance matters the most, and along with physical obstruction (such as a lens hood bumping into part of the subject), frightening away the subject can be a problem. You would run too if it seemed that something 5x or 10x your height was going to bump into you.

C-Mount is a 1" diameter threaded port with a specific focal depth to the camera sensor. The port is female on the camera side and male on the microscope attachment. A C-Mount adapter is often necessary to attach a dedicated digital microscope camera to the trinocular output of a microscope, which is often a 23mm smooth-wall trinocular tube (DIN standard) or sometimes a proprietary mount that is unique to the optics of a particular microscope. Many C-Mount adapters will include some type of adjustment to assist in bringing the camera and the primary ocular view into par-focus with one another.

This distance from the sensor to the rear of the lens mount is called the flange focal distance (FFD). The distance from the back of the Canon EF and EF-S lens mount to the contacts is .3" (7.6mm), giving it an FFD of 1.7" (44mm). The distance from the back of the Nikon F lens mount to the contacts is .24" (6mm), giving it an FFD of 1.8" (46.5mm). Here is a table of flange focal distances.

FOV to focal length

Minimumfocus distancehuman eye

A low power or stereo microscope typically employs objective lenses of 50x or less. It is used to view specimens that are visible to the naked eye such as insects, crystals, circuit boards and coins. A stereo microscope has three key parts:

Using this calculation shows that the Sigma 105 OS lens's working distance is about 5.6" (141.6mm). Install the hood and the minimal working distance goes down to 3.7" (93.6mm).

Focal lengthcamera

Minimum focus distance is an important lens specification, but this number does not indicate the working distance, the distance from the end of the lens (or the end of the lens hood if in place) to the subject. This is the amount of space you have to work in.

C-Mount is the industry standard attachment for digital imaging devices that are dedicated to microscopes. Choosing the appropriate adapter for your camera and microscope has an enormous impact on the quality of the image and the size of the field of view that you will be able to capture. Different relay lens values are appropriate to different camera sensor sizes and will dramatically affect the camera's field of view. C-Mount is a 1" diameter threaded port with a specific focal depth to the camera sensor. The port is female on the camera side and male on the microscope attachment. A C-Mount adapter is often necessary to attach a dedicated digital microscope camera to the trinocular output of a microscope, which is often a 23mm smooth-wall trinocular tube (DIN standard) or sometimes a proprietary mount that is unique to the optics of a particular microscope. Many C-Mount adapters will include some type of adjustment to assist in bringing the camera and the primary ocular view into par-focus with one another.

The manufacturer provided MFD (Minimum Focus Distance) or MOD (Minimum Object Distance) spec is the distance from the imaging sensor to the subject. This is no problem, as data from the site's Lens Specifications and Measurements tool, along with a simple calculation, will provide the needed working distance.

Focal length

To use the measured lens length in the above formula, the electrical contact protrusion length from the lens mount must be subtracted from the flange focal distance to get the imaging sensor-to-contact distance (ISCD) shown in the new formula below. Simply take the "measured" LL (Lens Length – with or without hood) and add 1.4" (36.4mm) to account for the Canon EF and EF-S lens mount imaging sensor-to-electrical-contact distance (ISCD). The Canon RF mount ISCD is about 0.51" (13mm). The Nikon F lens mount ISCD value is 1.6" (40.5mm). The Sont E mount ISCD is about 0.57" (14.5mm).

To determine the working distance at a focus distance other than the MFD, simply plug your focus distance into the MFD value in the formula.

Minimumfocus distancecalculator

Before you start building your slides, make sure you have everything you will need, including slides, cover slips, droppers or pipets and any chemicals or stains you plan to use.

Just prior to creating this page, I completed the Sigma 105mm f/2.8 EX DG OS HSM Macro Lens review, so I'll use that Canon-mount-equipped lens for an example. From this lens's specs, we see that the MFD is 12.3" (312mm), the total measured lens length is 5.3" (134mm) and 7.2" (182mm) with the hood installed.

If you would like to rely on our measured lens length, the formula must be adjusted slightly. Most lens manufacturers (including Canon, Nikon, Sigma, Sony, Tamron, and Zeiss) provide lens length specs that exclude the distance from the rear of the lens mount to the protruding electrical contacts. This usually explains the discrepancy that you see between the manufacturer specs and the actual measurements shown in the specs and measurements tool.

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