according to the invention , the position of the focusing knob or mechanism on the fundus camera is recorded , which position reflects the optical error or glass refraction of the eye . the glass refraction of the eye is used to calculate the object to image size ratio or magnification . this focus mechanism position is preferably automatically incorporated into a means to calculate the eye - camera magnification to arrive at a good estimate of the true or absolute measurements of retinal structures on the fundus photograph , such as the optic nerve , its components including the neuroretinal rim area and the cup . all of these measurements are important in the diagnosis of glaucoma . the applicant of the present application has noted that the observation and photography systems of the fundus camera rely on the principle of indirect ophthalmoscopy (“ some essential optical features of the zeiss fundus camera ”, bengtsson b and krakau c . e . t . acta ophthalmologica vol 55 , 1977 : 123 - 131 ). the location of the intermediate real image of the fundus as created by the front lens of the camera depends on the optical power of the eye being photographed . the setting or position of the focusing mechanism on the camera to best see the intermediate image also reflects the glass refractive error of the eye . this measure of the glass refraction of the eye can then be used to calculate the eye - camera magnification factor produced on a fundus image . if , for example , the size of the optic disc is desired , this eye - camera magnification factor can be used along with the area of the disc occupied in the image ( e . g . the area can be measured in pixels using image processing software , such as adobe photoshop ™) to arrive at an area measurement of the disc . this area measurement can then be corrected to yield a good approximation of the true disc area with “ correction factors ” ( see rudnicka et al . “ magnification characteristics of fundus imaging systems ” ophthalmology vol 105 , number 12 , december 1998 ; 2186 - 2192 ) or deriving “ correction factors ” from a standard group of subjects whose disc area has been determined by techniques as described above . in tests performed , it has been found that the relation between focus mechanism position and glass refraction is highly correlated . two telecentric fundus cameras — the topcon trc - 50f50ft and the topcon trc - 50x were used to perform twenty degree red free photographs of the optic nerve in twenty subjects ( n = 11 with the topcon trc - 50f50ft and n = 9 with the topcon trc - 50x ). a vernier scale was attached to the focussing knob which permitted a measurement of the knob position to be recorded . a correlation of this measurement with the eye refractive error was performed . the position of the focusing knob on both cameras correlated highly with the refractive error of the eye being photographed r = 0 . 97 for the trc - 50f50ft and r = 0 . 99 with the trc50x . it should be noted that the photographs must be done in such a fashion so as to minimize the effect that the photographer &# 39 ; s own lens accommodation may have on the focussing of the image . this can be done by having a photographer of sufficient age so his own diminished accommodative powers will not interfere with the focussing of the camera , or by ensuring that focussing the camera in done in such a way as to not employ one &# 39 ; s own focussing ability . this problem will be not be important in cameras that employ an automatic or semi - automatic electronic focussing mechanism . the position of the focussing mechanism reflects the optical refractive error of the eye being photographed which in turn can be used to calculate the eye - camera magnification . this position measurement can be incorporated into a simple method to calculate optic disc or other retinal object size . although the above example was performed with telecentric cameras , some changes in this simple technique for retinal object size determination will also permit magnification factors for non - telecentric cameras to be calculated ( rudnicka et al . magnification characteristics of fundus imaging systems . ophthalmology vol 105 , number 12 , december 1998 ; 2186 - 2192 ). currently , the measurement of fundus ( retinal ) objects from a photographic image captured with a fundus camera is done by calculating manually the eye - camera magnification system with the formulas which employ either the length of the eye , the glasses strength of the eye , or the glasses strength and corneal curvature as mentioned above . also known from the literature is that the known value of the glasses strength of the eye can be used to set the position of the focusing knob to bring the image of the retina into clear focus when taking the photograph of the retina , without adverse affect resulting from the photographer &# 39 ; s own accommodation ( see the mentioned bengtsson and krakau ( 1997 ), p . 131 ). according to the present invention , the position of the focussing mechanism on the retinal camera is used directly to calculate the eye - camera magnification . this magnification factor can in turn also be used along with a simple software measuring tool whose scale changes according to the eye - camera magnification to calculate retinal object size ( e . g . the optic nerve and the optic cup which are indices for diagnosing glaucoma , vessel caliber or tumor diameter ). this is useful for rapid screening of large numbers of people especially with automation of photograph reading . as will be appreciated , the method according to the invention may comprise the following steps as described herein below with reference to the appended drawing illustrating schematically a fundus optical imaging system . the patient &# 39 ; s eye is positioned for retinal photograph ( or optic nerve ). the aerial image of the object is focused on the film or ccd ( image plane ) with the focusing mechanism . the position of the focusing mechanism is recorded with the image . this can also be done by automatic means , such as a position sensor measuring the position of the focusing mechanism and a digital signal can be obtained for providing a focusing position measurement . the recorded position is then calibrated to the eye optical error ( glass refraction ) and is used to calculate the eye - camera magnification . an accurate scale of the fundus image can be determined using the eye - camera magnification calculated in the previous step . when measuring objects found in the image using a software image analysis tool , the scale determined in the previous step is used to measure linear objects ( e . g . vessel widths ) or two - dimensional objects ( e . g . the optic nerve head ) in absolute units ( e . g . in mm or mm 2 ). the position of the focusing mechanism recorded previously can be used in future photographs of the same patient to help control for inter - session eye - camera magnification variability by helping place the camera in front of the eye . however , once the diagnosis of an ocular condition necessitating a fundus photograph is made ( e . g . glaucoma ) in a given patient , the eye - camera magnification is constant ( unless of course the patient has had surgery to correct for myopia or cataract surgery or develops a condition such as a cataract which could change the glass refraction of the eye ). as mentioned , the position of the focusing mechanism on the camera can be used to help minimize variability of magnification between photo sessions . for each session , the patient sits in front of the camera and the camera is manually moved to a fixed distance ( say 10 cm ) from the patient which is determined by the photographer . there is a certain error in this positioning owing to its manual input . this error can cause a change in the position of the focusing mechanism and hence a change in eye - camera magnification . knowing the position of the focusing mechanism of the previous photographic session will permit a decrease in inter - session image magnification variability .
the determination of magnification in an ophthalmic fundus camera image is achieved by detecting the focusing mechanism position , and using a calibration function to calculate the magnification from the focus position .