For C-mount, photo port, and eyetube applications
for microscopy and other intrumentation

Richard J Kinch
August, 2011

Description:The SLR focal digital camera adapter mechanically and optically couples instruments using standard focal interfaces to digital SLR cameras. The adapter assembly consists of a cylindrical, custom-machined adapter which provides a T-mount threaded camera attachment on one end, and a C-mount or tubular fitting to the optical instrument on the other. Inside the adapter, a multielement lens relays the instrument's focal image to another image plane on the camera sensor. The T-mount threaded end is further adapted to a camera lens bayonet (Canon, Nikon, Olympus Four-Thirds, etc) using a stock T-mount adapter.

Attaching the adapter: Attach the adapter to your camera in the usual way, inserting and locking the bayonet as with an ordinary lens.

If the relay lens cell on the adapter is projecting out too far from the adapter bayonet fitting and interfering with your camera's reflex mirror, loosen the white Nylon setscrew and push the cell further into the adapter until it clears when reassembled. Generally, the further out you can adjust the cell, the better, but it must not be so far out that it does not clear the internal components of your camera bayonet receptacle.

Attach the other end of the adapter to your instrument in the normal way, either carefully screwing onto a C-mount threaded interface, or slipping into a photo port or eyetube.

Setting aperture and focus: The microscope or other adapted instrument effectively becomes the lens for the camera. Operate your camera in the manual mode. Aperture and focus are determined by the attached instrument, and not by the camera itself as with a conventional lens. Set your camera's ISO sensitivity and exposure time to obtain correctly exposed photographs. Under high magnifications, using the camera's self-timer or a remote cable release, and/or the "mirror lockup" mode of the SLR mechanism, will minimize image blurring from vibrations.

Setting rotational alignment: If the camera body does not properly align rotationally with your instrument, loosen the three tiny setscrews on the camera bayonet mount, rotate the camera body into alignment, and re-tighten the screws.

Calibrating parfocality: The adapter is designed for "parfocal" operation. That is, once the adapter focus is calibrated to your instrument, one need not be concerned about adjusting a different focus for photographing with the camera versus the normal visual view of the instrument. In practice, this means that the camera will take a well-focused photograph whenever a trained operator sees a properly calibrated and focused instrument view through the normal eyepiece view. The adapter camera should be in focus when your instrument eyepiece view is in focus.

To check parfocality, first check that the diopter settings on the eyepieces are set to properly compensate for any refraction error in your own eyes (assuming your eyepieces provide such a setting). Then focus a bright, medium-magnification image using the camera viewfinder. View the same image using the instrument eyepiece, and verify that the image is in good focus. If not, the instrument and camera are not parfocal, and you either have to (1) focus using the camera viewfinder (not the eyepieces) to take photographs in focus, (2) find an eyepiece diopter setting that brings the eyepieces into focus when the camera is in focus, or (3) recalibrate the adapter to be parfocal as follows.

You can recalibrate this parfocality by removing the adapter from your camera and instrument, loosening the setscrew in the adapter body, pushing the relay lens slightly in or out, checking and repeating until you establish the parfocal condition, and finally re-tightening the setscrew.

Note that there are two positions for the lens cell which will achieve parfocal focus, one with the cell forward, towards the instrument, and another with the cell rearward, closer to the digital camera. The forward focus yields a larger cropping factor (the image is more magnified in the camera (you see less of the periphery of the image), and conversely the rearward focus yields a smaller cropping factor (the image is less magnified in the camera, you see more of the periphery of the image). The forward focus yields the better optical quality image, because the optical design is corrected for this image conjugate ratio, and the field of view is narrower; this is the configuration supplied from our laboratory.

Magnification and cropping: In relaying the intermediate image of your microscope to the digital camera's focal plane, the relay lens adapter slightly magnifies or minifies the relayed image projected onto the digital camera's image sensor. To understand the cropping of the microscope's circular field of view onto the digital camera's rectangular image field, you must understand the size of these fields and the magnification imposed by the adapter.

The size of the microscope intermediate image accepted by the adapter varies depending on the type of photoport. An eyetube or trinocular phototube will typically deliver a circular image of 18mm diameter, or up to 22mm depending on the wide-field design quality. This is the same image you examine by eye when viewing with ordinary eyepieces (in which case the "field number" of the eyepiece indicates the diameter in millimeters). This image will actually extend larger than 18mm, but the image outside that diameter will be of poor quality, as the optical design corrects the image only for the specified field number. Hence an eyepiece contains a field stop, which is the sharp circular edge you see in the eyepiece view, to mask out the outer parts of the image which are not meant to be seen.

If you are using an existing C-mount trinocular port on the instrument, then the existing C-mount adapter will typically contain lenses to impose an additional magnification of perhaps 0.5X (which is actually a "minification"), yielding an image of 9mm to 11mm diameter. Some C-mount adapters have no lenses, imposing a 1X (unity) magnification factor (which is to say, no magnification).

Most current digital SLR cameras provide an image sensor that is a rectangle 22mm wide by 15mm high (the so-called "APS-C" format, or what Nikon terms "DX"). A few more expensive models provide so-called full-frame sensors, equal in size to the old 35mm film frame, which is 36mm wide by 24mm high.

To understand the cropping of the microscope image to this photo sensor frame, consider how a circular image from the microscope will overlay the rectangle camera frame. The SLR adapter imposes a magnification of 1.5X with the parfocal cell adjustment away from the camera. The typical 18mm intermediate microscope image will then be projected at 1.5 times 18mm, or a 27mm diameter, onto the 22mm x 15mm rectangle. Thus the photographs will show almost all of the eyepiece field of view, with just a bit of the top and bottom cropped out. The right and left sides of the rectangular photograph will be dark, or will show an unfocused extra outside area of the image not normally seen in the eyepieces. This is typically the most desirable cropping, and one can apply a black field around the edges to match the aesthetics of the eyepiece view, using digital post-processing, if desired.

With the other parfocal cell adjustment toward the camera, the adapter imposes a minification of 0.75X, yielding a 0.75 times 18mm, or 13.5mm diameter image being projected onto a frame that is 15mm high. So you will photograph the entire eyepiece field of view, with some extra image visible, and the right and left sides of the photograph dark.

If the existing C-mount adapter imposes something like an 0.5X minification, then the camera image will be quite small inside the frame, and one would typically want additional magnification in the adapter. There are several methods to achieve this additional magnification, such as adding a macro extension tube to the adapter, or attaching a teleconverter to the camera lens mount. The items and their resulting magnification factors are detailed in a table appended to the end of this document.

Other cropping factors are possible as you adjust the lens cell in and out, although the adapter will not be precisely parfocal to the instrument. If you are willing to focus the camera using the camera viewfinder and your instrument's focusing knob, you can adjust the lens cell position for various cropping factors in between.

Changing the T-mount adapter: The stock T-mount fitting provides the specific bayonet attachment for the camera used. The adapter body provides a T-mount thread (M42x0.75) to accept any stock T-mount fitting. You may change the type of SLR camera bayonet (Canon, Nikon, Olympus (Four-Thirds), Sony, etc) provided by simply switching to a different stock T-mount fitting. These fittings are inexpensively available from us directly, or from camera dealers.

Blackening: The structural components of the adapter exposed to the instrument light path are made from black materials or blackened with a coating. If the black coating is damaged or rubbed off, you can touch it up with a black Sharpie permanent marker. The blackening helps prevent stray light from causing lens flare or reduced contrast in the final image.

Cleaning the optics: Cleaning the optics should not normally be necessary, as they are protected within the adapter. The exposed lens face nearest the camera can be cleaned without disassembly.

If cleaning of the internal optical surfaces should become necessary, the relay lens cell may be removed and disassembled as follows. First, measure and record the depth of the lens cell from the shoulder on the camera end, so that you may reinsert it at this same depth to maintain parfocality; a set of calipers with a depth gage feature will best measure and restore this precise distance. Loosen the body setscrew and remove the lens cell. If the cell does not freely fall from the body, push it out using a probe inserted into the instrument end. Separate the relay lens cell body end from the eyetube end, unscrewing these two mating parts of the cell cylinder from each other. The outside of the cell may be shimmed and the threaded halves secured secured with a piece of ordinary Scotch brand transparent tape, which you can remove and replace with new tape during reassembly. Remove the two lenses and one spacer ring. Take care to note the orientation of the cell in the tube, and the lenses and spacer ring within the cell, so that you do not reassemble them backwards, which would degrade image quality. Carefully clean the lenses and remove any dust, as you would for a microscopic instrument. Take special care with canned-air dusters, since they can spit bits of liquid refrigerant, which will "frostbite" a lens coating and thus permanantly mar the lens.

Warranty: We manufacture these adapters to high mechanical and optical standards, and guarantee the performance will be in accord with the specifications. Please contact me via email ( should you have any difficulties we can correct.

Increasing adapter magnification: You may attach certain ordinary camera lens accessories to increase the magnification in the adapter, if you desire a more tightly cropped view of the central portion of the microscopic field of view. With these relatively inexpensive items, one can extend the magnification in the adapter up to 4.8X. The lens cell may be adjusted to be parfocal with the lens cell away from the camera and towards the instrument. Adding the import-manufacture macro extension tube set to the adapter is a very inexpensive method to gain additional magnification, such as from eBay seller fotodiox_gear for Canon SLRs or for Nikon SLRs. Purchasing links for other Canon items at the online camera retailer are given below. Similar accessories are available for the Nikon lens mount. The 1.4x or 2.0x teleconverter attachments are much more expensive but will also work with your SLR lenses for conventional photography.

Adapter as Supplied
With No Extender
(Eyepiece View Area in Photo)
Lens cell adjusted parfocal,
toward camera
0.75X (100%)
Lens cell adjusted parfocal,
away from camera
1.5X (70%)
With Added Extender
(Eyepiece View Area in Photo)
1.4X teleconverter
(Adorama CA14XAF2U)
2.0X (53%)
Canon EF 12 II extender
(Adorama CAETEF122)
2.2X (48%)
Macro extension 0 mm
(lens mount fittings only)
2.4X (44%)
Macro extension 7 mm 2.8X (38%)
2.0X teleconverter
(Adorama CA2XAF2U)
2.9X (36%)
Canon EF 25 II extender
(Adorama CAETEF252)
3.0X (35%)
Macro extension 14 mm 3.2X (33%)
Macro extension 7+14 mm 3.4X (31%)
Macro extension 28 mm 3.7X (28%)
Macro extension 7+28 mm 4.0X (26%)
Macro extension 14+28 mm 4.4X (24%)
Macro extension 7+14+28 mm 4.8X (22%)

Specifications (March, 2010)
Input interface optionsC-mount coupling (1"-32 thread)
ISO standard eyetube (23mm or 30mm) or phototube (38mm)
Zeiss OPMI eyetube (25mm)
Relay image size ratioAs supplied: 0.75x or 1.5x
With optional attachments: Up to 4.8X
WeightApproximately 225 grams (8 ounces)
(Varies with camera bayonet options)
Overall lengthApproximately 85mm (C-mount shoulder to tip of camera bayonet; varies with camera model)
Camera compatibilityAny SLR type camera with T-mount adapter
(Canon, Nikon, Olympus, Sony, etc)
Optical elements3 elements in 2 groups, fully coated
ManufactureCustom made and assembled in USA