Figure 1. Digital camera adapter assembly for the rear port of the Topcon TRC-50EX.
Richard J Kinch
http://www.truetex.com
This document describes how to install and operate the digital camera upgrade kit for the rear port of the Topcon TRC-50EX and TRC-50DX series retinal cameras.
Other components you must obtain separately:
Optional components you may wish to obtain separately:
Familiarize yourself with the digital camera: If the digital SLR camera is new to you, be sure the learn about 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, viewing photos on the camera, transferring photos to a computer.
Attach the adapter to the Canon camera: Identify the Canon bayonet side of the adapter, which the all-aluminum side. This is mechanically a standard Canon EF mount bayonet, such as is used on Canon EF lenses for the EOS camera series, except it has no electrical contacts for the Canon side. The Canon bayonet shape on the adapter matches that of any Canon lens you might have. Via the adapter, the Topcon instrument will appear to the Canon camera to be an elaborate, manual-focus EF lens. Attach the Canon camera to the EF lens mount in the usual way, that is, by inserting the bayonet while aligning the red dots and turning the adapter. Take care not to pull on the attached cable.
It is normal for the bayonet engagement to be somewhat stiff, which makes locking the adapter difficult since there is no lens body to hold onto. To ease the locking twist on the adapter into the Canon bayonet receptacle, insert a small screwdriver or other shaft across the slot on the Topcon side of the adapter, and use it as a handle to turn the adapter clockwise, as if attaching a normal Canon lens.
Connect the interface cable to the Canon digital camera: The interface cable has a split end for the Canon digital camera. The split end terminates in a remote shutter control connector (Canon N3 connector) and a PC-sync plug. Insert these two connectors into the remote shutter release socket and PC sync socket on the Canon camera. Dress the cable around the bottom of the camera body.
Remove any old film camera and the bayonet arm from the Topcon instrument: Lift the Topcon bayonet arm to remove the old MT-10 film camera or other accessory from the rear port. Use a Phillips screwdriver to remove the screws holding on the arm from the Topcon bayonet receptacle ring. The arm and its handle must be removed, because they will interfere with the digital camera. Retain the arm should you wish to later revert the digital upgrade.
Attach the camera adapter to the Topcon instrument: The digital camera with adapter attaches upside-down to the rear port of the Topcon instrument, This inverted orientation avoids mechanical interference with the instrument, and also yields photos that are upright, since Topcon used an inverted method on their instrumentation film camera bodies. This attachment works in the usual way described in the Topcon documentation for attaching a camera to the rear port: Insert the adapted digital camera into the bayonet receptacle, routing the cable between the Topcon viewfinder eyepiece and the bottom (facing upwards) of the camera body, and turn the Topcon bayonet ring down to lock the attachment. Since you previously removed the arm normally used to actuate the Topcon bayonet ring, use a thin screwdriver or other probe to reach in between the camera body and the instrument to the left of the Topcon viewfinder eyepiece, pushing down on the handle-mounting notch of the ring to rotate it to the locked position.
Adjust instrument for a fixed test subject: Turn on the retinal camera and set the instrument controls to select the rear port as if using a 35mm camera attachment there. Tape a business card or other flat, high-contrast, detailed subject to the patient headrest to serve as a fixed test target. 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 "+" refraction compensation, as for anterior segment views. Set the retinal camera flash intensity to the lowest possible setting (one or one-half step above "NF" (no-flash)).
Adjust Canon camera settings: Configure the Canon camera as follows:
Take test subject exposures: Focus and align on the test subject using the instrument 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 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. Use the Canon menu buttons to review exposures. Zoom in when reviewing images to verify that image is sharply in focus when the viewfinder was also 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, well-dilated colleague or patient for your first live retinal digital image tests. Collimate and focus on the retina in the usual way with the viewfinder, using the joystick button to take exposures. Check the images for exposure brightness and adjust the flash intensity to compensate; the digital camera may be more sensitive than the usual film and lower flash intensity is usually indicated.
Extended framing: The digital camera does not apply a field mask to the retinal image, and thus shows a slightly wider angle of view than the film camera, which the film camera masked off. This extra area of view is normal in the digital upgrade. Should you wish to have a masked look to your photos, such as for publication, you may apply it via a photo post-processing application on a computer
The instrument is effectively a manual lens: As noted above, the Topcon instrument in effect becomes a manual lens for the digital camera. The digital camera will thus have no focus or aperture settings, as these are controlled by the Topcon instrument. The exposure adjustment will depend only on the Topcon flash energy and the ISO speed setting of the digital camera. Changing the exposure time will not affect the exposure intensity, as this is a function of the flash energy and not the time the shutter was opened. Using a shorter exposure time setting than recommended above will possibly miss the Topcon flash altogether and result in a dark photo. Using a longer exposure time will have no effect other than to possibly delay the next photo.
Ignore the film-winding error: The digital camera adapter does not return a signal to the Topcon instrument verifying that the film winding succeeded. The Topcon console will thus flash an "F-Err" code after each exposure as if the camera were out of film. This is normal and may be ignored. The code message imposes a minimum interval between exposures of 3 seconds. See the next section for a workaround.
Workaround for film-winding error: If the 3-second delay between exposures is too long, you may wish to apply the following workaround, where you switch the upper and rear-port solenoid connectors inside the Topcon upper unit. Thereafter, when you set the Topcon console to the upper-port mode, the instrument actually operates the rear-port light path. Since the upper-port mode does not expect film-winding messages, this avoids the error and the delay between exposures.
To apply the workaround:
To operate the digital camera on the adapted rear port, you must now set the Topcon console to the UPPER setting. This setting allows you to shoot at the maximum rate of the instrument, which is somewhat more frequently than once per second. You may wish to cover the original UPPER label with a new label to indicate it now selects the rear-port digital camera. Cover the LOWER label as well, since this is now disconnected.
This modification does not permit use of the upper port, only the rear port. It also does not illuminate the LED data indicators and timer in the rear-port image, so these numbers will no longer appear in the digital camera images (timing information is however available in the JPEG photos EXIF data). Should you wish to revert the Topcon instrument to its original UPPER/LOWER functioning, you can simply reverse the above steps.
Adjusting the ISO speed of the digital camera: The Canon digital cameras offer a range of ISO speeds, from 64 or 100 up to 800 or 1600. This provides you another variable affecting the exposure of the digital photograph, along with your ability to vary the adapter aperture (explained above) and the flash energy on the Topcon instrument. Higher speeds of ISO 200 or 400 will enhance the sensitivity of the camera while not introducing much noise. Speeds of 800 or more will typically introduce a degree of speckled noise into the pixels of the photograph, especially considering that the retinal image is by nature of very low contrast. Use the histogram feature of the digital camera to analyze your retinal images for proper exposure level and to observe the degree of contrast. 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 of the retinal image elements.
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.
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.
Alternate Canon camera models: Only full-frame Canon digital SLR cameras (such as the model 5D or 5D Mark II, and future models based on the EF lens mount and N3 remote shutter control) work properly with this adapter. Smaller format cameras such as the Canon Rebel series, the Canon 40D, or the Canon 50D crop the frame by an 0.6X factor, so much of the retinal image will be cropped out. These cameras are however compatible with our upper-port adapter for this Topcon instrument.