Richard J Kinch, PhD
This document describes how to install and operate the digital camera upgrade kit for the upper port of the Topcon TRC-50EX or TRC-50IX retinal cameras.
Identify the upgrade kit contents:
Other components you must obtain separately:
The adapter kit works by design with ordinary, standard Canon retail camera bodies and lenses, and with the constantly improving standard computer software supporting digital photography. No special version or modifications of hardware or software are required. New Canon camera models in the future should also be compatible, as should advances in Canon and third-party computer software support. You may process the standard RAW or JPEG images using industry standard software such as ImageMagick or Adobe Photoshop.
Optional components you may wish to obtain separately:
Familiarize yourself with the digital camera and lens: If the digital SLR camera is new to you, study at least the following features which will be used for retinal photography: On/off switch, mode switch "M" setting for manual operation, setting manual exposure time, attaching and removing lenses, remote shutter release connector, hot shoe, viewing photos on the camera, transferring photos to a computer.
Also read the instruction manual for the Canon 60mm lens to learn about the following features of the lens: filter mounting thread, focusing ring, focus mode switch, and distance scale. You must understand these features to apply the manual focus calibration procedure below, which is mandatory for proper operation of the adapter.
Solder resistor(s) to Topcon upper-port connector: The upper-port adapter does not make any connection to the electronic connector (the "pogo-lands" connector just in front of the upper-port receptacle) associated with the upper port. Thus to indicate the presence of an adapter to the Topcon unit, it is necessary to modify this connector as follows.
NOTEWe recommend you remove and send the pogo-lands connector to us for this modification, for which there is no additional charge. If you wish to install the modification yourself, use the following instructions, referring also to Figure 8 below.
Remove the upper port pogo-lands connector located just in front of the upper-port receptacle by removing the two flat-head Phillips screws. These screws are typically tight and thread-locked, so use a well-fitting screwdriver and firm technique. Two new flat-head screws and 2mm hex key are provided to replace these screws, should they become marred during removal. Once unscrewed, free the pogo-lands connector from the yellow-wire connector by pulling to separate.
All Topcon models: Solder a resistor to the connector as follows. Identify pins 12 and 19 on the solder side of the pogo-lands connector. Solder one 1K ohm resistor between these two pins.
Topcon model TRC-50IX only: Also identify pins 4 and 13, and solder a 1K ohm resistor between those two pins. Locate this second resistor so as not to mechanically interfere with the reassembly of the connector into the TRC-50IX upper unit.
Reassemble the pogo-lands connector to the instrument by reversing the steps in the previous paragraph.
Choose an adapter orientation: The adapter orients on the upper port in one of two ways, "forward" or "right-hand". See Figure 1 (forward) versus Figure 6 (right-hand) below. You should choose one of these orientations to initially install the adapter. For operation when tethered to a computer, choose the forward orientation, which places the digital camera upside down and directly over the top of the Topcon upper unit. This is the most compact orientation. The camera body is also located in away from the operator, making it awkward to review photos on the camera display, and requiring an external HDTV display or tethered PC. The "right-hand" orientation places the camera out to the right of the upper port, with the top of the camera body facing the operator. Photos are upright in the camera, and the camera is somewhat more accessible to the operator (although still on its side), but the camera is somewhat awkwardly suspended over the side of the Topcon upper unit.
If you are shooting stand-alone (that is, not tethered to computer, using the digital camera only to review photos), then you may optionally orient the camera to the rear and upright. This is convenient for instantly reviewing photos on the camera display as you are shooting, but the camera is somewhat close to the operator's head when looking into the instrument viewfinder eyepiece.
Attach the Canon lens to the adapter: Observe that the adapter provides a Topcon upper-port bayonet fitting, and a 52mm threaded lens fitting. Take care especially with the Topcon fitting, since the aperture is thin and fragile.
Observe that three setscrews attach and lock the Canon lens fitting to the diagonal, and a sleeve with single setscrew locks onto the Topcon bayonet adapter. See Figure 4 below. The lens fitting and the bayonet attachment sleeve rotate when their respective setscrews are loosened, and adjustment of this rotation is a part of assembling the lens and installing the adapter on the upper port. You may loosen and tighten these setscrews with the provided 1.3mm and 2mm hex keys. By carefully adjusting the rotation and locking the adjustment with the setscrews, you perform several critical adjustments: the forward versus right-hand orientation of the adapter, the alignment of the adapter squarely to the instrument, and the alignment of the camera squarely to the image. See Figure 3 versus Figure 7 below.
The side faces of the diagonal each provide a setscrew, and the end face a third setscrew. Note that the bayonet fitting partly covers the end setscrew. This setscrew (which holes the threaded lens fitting) should have already been not quite tightened, so that it allows the rotation of the threaded lens fitting when you loosen the side setscrews. The notch on the bayonet fitting should also be already oriented for forward orientation.
Using the 1.3mm hex key, tighten the side setscrews for the threaded lens fitting. Screw the 60mm Canon lens just slightly hand-tight onto the threaded lens fitting on the adapter, until the lens seats against the adapter body. Take care with these threads as they are very fine and easily damaged if misthreaded. Once the lens seats, loosen the side setscrews, attach the Canon camera body to the lens, and rotate the camera and lens on the adapter so the camera body is at the proper angle for the forward- or right-hand orientation you have chosen. In the forward orientation, the Canon camera should be upside down, which will then yield an upright image. Tighten the side setscrews firmly.
Attach the adapter with Canon camera and lens, to the Topcon upper port. Loosen the single setscrew with the 2mm hex key to rotate the adapter so that it aligns with the optical axis of the instrument.
Calibrate the focus of the Canon camera to the Topcon instrument: Focus calibration is a quick and easy, one-time procedure, but it is absolutely critical to the proper performance of the adapter. You must set the digital camera lens to a fixed, manual focus on the adapter's field stop, which permits you later to focus the retinal image in the normal way. If the focus calibration is off, then all digital photos will be out of focus even though the instrument display indicates a correct focus on the patient eye. Proceed as follows:
Alternative focus calibration methods: You may also calibrate the adapter focus by focusing an image in the instrument and then focusing the digital camera to the same image, as follows. Set up a distant focusing target in the center of the instrument view, such as a small, bright light across the room or down a hallway. A distant, luminous, pinpoint test target will greatly simplify the focusing image and improve the accuracy of the calibration. A lighted ophthalmoscope, transillumination lamp, or penlight are excellent for this purpose. Perform this procedure with the Topcon power off. Focus on the test target with the Topcon viewfinder (use the "+" diopter compensation lens setting on the instrument to focus on the near-infinity object). Take care to compensate for any refraction error in your observing eye by rotating diopter correction in the viewfinder eyepiece, so that the viewfinder crosshairs appear in sharp focus. Once the test target is focused using the Topcon viewfinder, flip the upper-port mirror in the Topcon TRC-50EX/TRC-50IX upper unit by removing the rubber plug on the left side and inserting a probe. This should redirect the image to the upper port, so you can now see the same image in the digital camera viewfinder, being careful not to alter the Topcon focus knob or carriage position. While watching the Canon viewfinder image, focus the Canon lens in the Canon viewfinder and lock this focus with a sticker on the lens. If your Canon model provides a live view feature, you can focus even more accurately than with the viewfinder, by using the 10X live view magnification. If you don't have the live view feature, you can still take test photos on the digital camera and enlarge them on the digital camera display, to set the focus accurately by trial and error.
Attach the digital camera with adapter to the Topcon instrument: Attach the adapter to the upper port of the Topcon instrument in the usual way. That is, match the bayonet connector to the receptacle, insert fully, and turn the locking lever counterclockwise.
Connect the electronic interface cable between the base of the Topcon instrument and the digital camera: Connect the 9-pin end of the cable to the CONTROL receptacle on the left side of the base of the unit. There are several possible ways to run the cable up to the digital camera, but it is convenient to run the cable underneath the center of the base and back up with the other cables running to the upper unit on the right side of the instrument. The clip on the lower front of the upper unit is intended as a cable retainer for upper-port accessories, so you may use that to also hold the new cable. Connect the two camera-end connectors to the camera, namely the hot-shoe connector to the flash hot shoe, and the remote-shutter connector to the remote connector on the left side of the digital camera body.
Connect the optional AC power supply to the digital camera: An external AC power supply, which replaces the battery in the digital camera, is a useful option to install, to avoid depending on a battery needing to be charged. Refer to the digital camera instruction manual to connect this power supply. Route the power cable along the same path as the electronic interface cable.
Connect the optional USB cable to the tethered computer: If you will using a tethered computer with the digital camera, connect the USB cable at this time. Route the USB cable along the same path as the electronic interface cable.
Dress the cables the manage them on the instrument: Once all the cables to the digital camera are installed, you may wish to dress them with cable ties so that they run along the right side of the instrument and the original grey Topcon cable to the base, and underneath the instrument base to the left side of the instrument. These cables will consist of the electronic interface cable, optionally the USB interface cable to a computer, and optionally the AC power supply instead of using battery power. From there you may connect the cables to the Topcon instrument, the AC power source, and the USB computer receptacle. Make sure that the upper unit of the Topcon instrument is able to travel and swing across its full range of motion without binding or chafing the cables.
Configure the Canon camera for retinal photography:
Configure the Topcon instrument for digital photography:
Adjust the instrument for a fixed test subject: Tape a business card or other flat, high-contrast, detailed subject to the patient headrest to serve as a fixed test target. A long strip of clear packing tape stuck across the headrest poles at the canthus marks will serve well. Stick the tape to the operator's side of the poles, and stick the card slightly to one side, roughly offset to where a patient eye would be position. Frame and focus a view of this target using the Topcon viewfinder in the normal fashion, perhaps pulling away from the subject and using the "+" diopter compensation lens, as for anterior segment views.
Take test subject exposures: Focus and align on the test subject using the viewfinder in the usual way. Press the joystick button to trigger an exposure. At the moment of exposure, listen for the Canon camera mirror flip and shutter mechanisms operating, along with the Topcon mirror mechanism flipping. Observe the light of the retinal camera flash on the subject. Immediately after the exposure, the Canon camera should momentarily display a view of the image on the camera display.
Review test subject exposures: If you are using a tethered PC to capture photos, refer to the tethering software user manual for image review. If you are using the stand-alone camera to shoot onto the camera's memory card (with no tethered PC), use the Canon menu buttons to manually review exposures on the camera display. You may lengthen the automatic review time with the "Review time" menu setting on the digital camera. Zoom in when reviewing images to verify that the captured image appears sharply in focus when the video viewfinder also appeared in focus. Note that the full resolution of the digital camera is much finer than the instrument resolution, so the highest-resolution images will not appear in focus when zoomed in completely.
Take live subject exposures: It is most convenient to obtain a cooperative colleague or patient for your first live retinal digital image tests. Collimate and focus on the retina in the usual way with the video display, using the joystick button to take exposures. Check the images for exposure brightness and adjust the Topcon flash energy to compensate.
Analyze exposures for optimal exposure: Viewing the image exposure histogram in the digital camera is the only sure way to evaluate a proper exposure. The histogram provides a quantitative analysis of your retinal images for proper exposure level and degree of contrast. The goal is to have a histogram spread roughly in the middle third of the dynamic range. For color retinal photography, observe the green histogram rather than the white, since for a retinal image most of the diagnostic information is represented in the green portion of the color spectrum. The position of the hump in the histogram indicates the exposure level, which should be around the middle of the range, and the width of the hump indicates the contrast. If the histograms indicate an exposure off to one end or the other of the camera's dynamic range, you may shift the exposure range by switching to a darker or light neutral-density filter on the adapter sleeve. (With careful assembly, it should not be necessary to recalibrate the focus when switching filters.)
Correcting exposures and improving contrast with digital post-processing: Simple post-processing of your digital images on a computer will correct many mis-exposures and improve correct exposures. Retinal images are by nature limited in contrast, and by digitally stretching the contrast, the visual impression of the photographic is much improved. Filtering enhancements such as red-free images are available via digital post-processing, even though the original instrument lacks the physical filters.
Establish your practice routine: After completing the above, you will have a powerful digital facility for retinal photography. You must now handle digital data where you used to handle 35mm film slides or Polaroid prints. It most regards the digital methods are quicker, easier, and cheaper than film, but do require an initial investment in careful design and training. Your digital system is based on standard digital cameras, and so is compatible with ordinary software for capturing, post-processing, and cataloging digital photographs. Your upgraded digital retinal camera should, with occasional maintenance, provide decades of reliable service.
Relative exposure factors for film versus digital cameras: You should understand the relative exposure intensity of the digital camera versus the 35mm film formerly used. The digital SLR sensor frame is smaller than the area of the original 35mm film frame by a factor of 1:2.6, and consequently the same flash energy yields 2.6 times more light intensity on the sensor. Thus 2.6 times less flash energy will produce an equivalent exposure to ISO 100 35mm film when shooting with a ISO 100 digital camera sensitivity setting. Thus where one have used, say, 50 watt-seconds for an exposure, now one would use 50/2.6 = 19 watt-seconds (or 18 watt-seconds being the closest available setting). The retinal reflex typically yields an exposure histogram of only 25 percent of the digital dynamic range, so fortunately there is enough room to compensate for over- or under-exposures, if you are willing to apply digital post-processing.
The instrument is effectively a manual lens: The Topcon instrument in effect becomes a manual lens for the digital camera. The Canon lens is set at a fixed focus and acts only as a relay lens. Camera modes such as aperture priority (Av) and shutter priority (Tv) are unavailable, since these factors are controlled outside the digital camera.
Using an AC adapter on the Canon camera: To avoid having to worry about having a charged Canon camera battery, you may wish to purchase the Canon optional item that powers the camera from an AC adapter.
Lowering resolution on the camera: Since the highest resolution images of the camera are finer than the optical resolution of the instrument, you may wish to set the camera to record lower-resolution images. This will decrease storage space and improve tethered transfer times. These settings are found on the camera's "Quality" menu.
Live view and review via tethering software on a PC: If you want to have live previewing and capture of photos on a computer while shooting with the Canon camera, consider using "tethering" software. This type of application uses a "tether" from the camera to the computer via a USB cable. A simple version is included with the Canon camera software support disc. DSLR Remote Pro (http://www.breezesys.com/DSLRRemotePro/) is one popular aftermarket package sold for this application. Tethering to a computer with a large display is the best way to instantly review your photos for proper focus and exposure while you still have a chance to retake a missed shot. The small display on the camera does not adequately reveal the quality of the image focus.
Live view and review via an HDTV: Certain Canon camera models such as the T1i, T2i, 50D, and 7D provide a live view and photo review capability, which you can connect directly to an HDTV via the HDMI connector on the camera. Using this feature does not require a computer, although it does require directly accessing the camera controls.