Richard J Kinch
This document describes how to install and operate the digital camera upgrade kit for the rear port of the Topcon TRC-NW3 series retinal camera.
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 Adobe Photoshop or Corel PhotoPaint.
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 50mm 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.
Attach the sleeve, neutral-density filter, and Canon Extension Tube EF25 II to the Canon lens and camera: The sleeve should be delivered to you with a neutral-density filter attached. If not, observe that the sleeve has one end threaded for the attachment of a filter. Remove the front lens cap from the Canon 50mm lens, and screw on the sleeve and neutral-density filter to the front of the lens. Engage and screw the threaded components carefully, as the threads are very fine and easily damaged if mis-threaded. Tighten the threaded connections firmly but do not overtighten. Remove the rear lens cap from the Canon lens, and both caps from the Canon Extension Tube EF25 II, and attach the extension to the rear of the lens. Finally, attach this sleeve-filter-lens-extension assembly to the digital SLR camera body in the normal way. This assembly will now attach to the Topcon adapter.
Attach the digital camera mechanically to the adapter: Loosen the white thumbscrew on the sleeve so that the sleeve slips over the rear cylinder of the adapter. Slide the sleeve over the cylinder as far as it will go, rotate the camera body so that it is horizontal with respect to the instrument attachment, and lock the sleeve onto the adapter by tightening the thumbscrew. Do not overtighten the thumbscrew, which only requires a moderate turn to hold the sleeve firmly in place.
Attach the digital camera electronically to the adapter: Two cables exit the adapter: a hot-shoe connector and an Canon E3 remote plug (2.5mm subminiature type). Connect the hot-shoe connector to the hot shoe on top of the camera, and the remote plug to the remote connector on the left side of the digital camera body.
If you have a Canon digital camera other than a Rebel model, use the N3-E3 remote connector adapter we will have optionally provided to connect the E3 plug to the N3 socket on your camera body.
Your adapter and Canon camera should now be assembled, but before attaching them to the Topcon instrument, the Canon lens focus must first be calibrated as follows.
Set the Topcon TRC-NW3 flash intensity selector switch to LOW: Locate the flash intensity selector switch on the Topcon instrument base. This is 2-position HIGH/LOW switch, found by moving the carriage all the way forward, exposing the top of the base to the right of the joystick. This is item 33 on page 4 of the original Topcon instruction manual. (This is not the flash selector switch to the left of the joystick, although they both affect flash intensity.) Set the flash intensity selector switch to the LOW setting.
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 circular 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:
Attach the digital camera with adapter to the Topcon instrument: Attach the adapter to the rear port of the Topcon instrument in the usual way, as was done with the original Polaroid attachment. That is, match the bayonet connector and electrical connector to their receptacles, press them together, and turn the locking lever counterclockwise.
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. Focus on this test target with the Topcon video display (use the "+" diopter compensation lens setting on the instrument to focus on the near-infinity object). Once the test target is focused using the video display, raise the mirror in the adapter with the mirror-up knob to see the same image now 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, lock this focus with a sticker on the lens, and lower the mirror. 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.
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 video display 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 instrument video display 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 rear 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 selector 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. 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 any 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.
No-flash test kit:: A test kit is available on loan to diagnose a no-flash condition in your Topcon TRC-NW3. See the instructions at http://www.truetex.com/topcon_trc-nw3_diagnosis.htm.
Relative exposure factors for film versus digital cameras:: You should understand the relative exposure intensity of the digital camera versus the 35mm or Polaroid film formerly used. The normal operation of the adapter requires that you insert a neutral-density filter on the front of the lens. This is due to the difference in size between film and digital images, given that the Topcon flash illumination is calibrated to a fixed amount of energy to expose a 35mm film frame with ISO 100 sensitivity. The Topcon TRC-NW3 provides only a very limited adjustment to flash energy via the Flash Selector switch (HIGH/M/LOW). While the Canon digital camera provides a lowest sensitivity setting of ISO 100, the digital sensor size is only about 38 percent (or 1/2.6) the area of a 35mm film frame. Since the adapter captures all the light energy of the original exposure, this results in a digital exposure intensity that is 2.6 times brighter than the film exposure. Inserting a neutral-density filter compensates for this effective difference in intensity. A standard ND2 neutral-density filter removes 1/2 the intensity, and an ND4 filter 1/4 the intensity. Thus an ND2 filter yields a 2.6/2 = 1.3 extra exposure factor, while an ND4 filter yields 2.6/4 = 0.65 exposure factor, for the digital sensor relative to 35mm film. You may also set the Topcon flash intensity selector to HIGH or LOW to slightly adjust these exposure factors. Choose the filter (ND2 versus ND4) to accommodate the retinal reflex characteristic most frequently encountered; usually the ND2 filter will be appropriate since you can use the Topcon LOW intensity setting and spare the patient some of the flash dazzling. 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.
Tethering software: 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.