digital pathology holds great promise for the future of anatomic pathology and many pathology laboratories today are starting to adopt this technology for a variety of applications , including telepathology ( teleconsultation ) , quantitative image analysis and digitization of lab workflow . slide scanning systems are one component of the technology that will enable the digitization of the analog glass workflow to a digital workflow . as more pathologists adopt digital pathology , one key question that is often asked by new users is  why is the image size not the same as my microscope ?  more specifically , when a glass slide is manually viewed with a conventional light microscope using a 20 objective and then viewed as a digital image that was created using the same 20 objective lens , the perceived size of the image may not be the same . many pathologists are surprised to see that the images acquired with the same objective lens scale so differently when compared with the monitor , which is a common phenomenon in all digital pathology imaging systems . the reason this occurs is because the size of a whole slide image ( wsi ) is dependent on ( 1 ) the objective lens magnification being employed by the scanner , ( 2 ) size and number of individual pixels within the digital camera 's sensor and ( 3 ) size and amount of individual pixels within the monitor . the images from the vendor 1 , captured with a 5.5 micron pixel sensor , appear on the screen 1.7 larger than the image on the right , captured with a 10 micron pixel size sensor . the image from vendor 1 's scanner contains more information within the image due to its pixel resolution . keeping the objective lens constant and varying the pixel size in the sensor will yield an image of different perceived size when viewed at full digital resolution . depending on the digital camera and numerical aperture ( na ) of the objective lenses within the wsi system , , magnification is dictated by the microscope objective and should not be confused with scale . images of an enterobius parasite in an appendix are shown using a similar 20 objective , but with different sensor pixel sizes . images a and c are sampled with a 5.5 micron pixel size , whereas b and d were sampled with a 10 micron pixel size . the apparent magnification differs by 1.7 , the ratio of the pixel sizes graphic depicting the added boost in magnification that comes from the workstation monitor before this can be appreciated , one must understand the differences between digital resolution and optical resolution . ultimately all definitions of resolution describe the minimum distance two objects can be separated by and still are distinguished as separate features . for example , using any viewing system , if the smallest distance between two objects is 1 micron before they get  blurred  together and appear as one object , we would state that the resolution is 1 micron ( i.e. , the system can resolve two objects that are 1 micron apart or a single object that is 1 micron in size ) . for a purely optical system , which has no sensor , the resolving power is dictated by a complex interplay of light and glass optics , which is often described in microscopy as the rayleigh resolution limit ( r ) . we can calculate r as follows :   provided that we know the color of light (  is around 0.53 microns for the green light ) and the property of the glass optics known as the na . the na describes the number of different angles from which the glass optics will collect light , such a funnel for photons . thus , we see that based upon glass optics , the aperture size ( of what ? ) matters and the wavelength of light used to observe ( observe what ? ) matters , but that magnification does not play a direct role in resolving power . however , an unfortunate confusion occurs with light microscopy because when a feature of interest requires higher resolution , an objective with higher na is used and very often that objective also has higher magnification . thus , changing the na and magnification often occur simultaneously . for example , when switching from a 20 with 0.5 na objectives to one that is 40 with 0.9 na , the smallest resolvable feature drops from 0.64 microns to 0.36 microns ( for the green light ) . with the 0.9 na objectives , some of these principles change , when a digital camera sensor and monitor is involved , as the resolving power is no longer dictated solely by the optical lens . this is because the size of a pixel and the amount of pixels will also dictate if two objects will appear as separate features . using the same example as above , if two objects are 1 micron apart when projected onto a sensor ( after being  magnified by an objective ) and the pixel size is 10 microns , the two objects are likely to appear as one . this means that the sensor can not resolve a 1 micron size image feature [ figure 2 ] . in this example , the sensor would require a 0.5 micron pixel size in order to insure that two objects appear as two distinct features . according to the nyquist resolution requirement for pixel size ( p ) : optical resolution is solely dependent on the objective lenses whereas , digital resolution is dependent on the objective lens , digital camera sensor and monitor and are closely tied together in system performance . the objective lenses must have the optical resolution required to  see  features of interest ; digital resolution can not make up for any short comings in the objective . however , the sensor can fail the glass lenses if the digital resolution is not fine enough . magnification is a change in the apparent size of an object , performed so that the object can more easily be seen . in traditional light microscopes used for pathology , a wide range of magnifications are available to ensure that the image observed through the oculars is large enough for easy viewing . in this case , magnification is not necessarily used to increase resolution , but utilized to make viewing easier . in the wsi system , with the introduction of a digital camera sensor and monitor , magnification plays the role of matching digital and optical resolution . with the prior example , we can not see 1 micron sized features with a 10 micron sensor , unless we magnify the image by another factor of 20 . this yields features that are 20 microns apart and therefore can be accurately digitized by the 10 micron pixel sensor . once the image has been digitized with good fidelity by the sensor , there is no longer much need for additional magnification . the role of providing an image that is sized appropriately and easy to review will now depend on the workstation monitor . the appearance of an image on a digital display can be very different than the traditional light microscope view . the traditional microscope provides a pair of 10 oculars for viewing , thereby providing an extra boost of magnification for looking at the material on glass slides . similarly , the presentation of the digital image on a monitor provides a boost of extra magnification that is often overlooked [ figure 3 ] . continuing on with this example with the 10 micron pixel , when that pixel value is moved to a typical workstation monitor , it will generally be displayed as a 270 micron pixel . that translates to a magnification factor of 27 , more than was formerly achievable using the oculars . if we consider a sensor with a more typical pixel size ( e.g. , 6.45 microns ) , the magnification factor becomes ~42 . hence , it is apparent that the scale of the image will certainly be different , dependent upon the sensor used , regardless of the objective lens magnification . graph depicting the effect of the camera sensor size and monitor display on apparent magnification . in this example , the images are acquired with a 20 microscope objective lens , shown as a blue horizontal line . the x - input axis is the sensor pixel size and y - output axis is the apparent magnification , as viewed on the screen at a distance of 60 cm . the apparent magnification on the screen increases as the sensor pixel size is decreased , as shown by the family of curves . the red curve is apparent magnification using a standard 96 dpi monitor , the purple curve using a 220 dpi mac retina display  these two representing the current extremes in monitor resolution the magnification of an image on a computer screen is further influenced by one more element : how far the person viewing the image is sitting from the screen . to illustrate , the graphs in figure 3 bring together the complex relationship between sensor pixel size , objective lens magnification and the type of workstation monitor used to view the image . the bright blue , horizontal line represents the magnification due to the objective lens used in the scanner , which in this example is 20 . the family of curves represents the apparent magnification of the image as seen with different types of monitors , when seated 60 cm from the screen . the red curve is for a standard digital display with 96 dpi and the green curve shows the result for a higher density medical grade barco 120 dpi computer monitor . finally , the blue curve is the apparent magnification of an image when using the most advanced  retinal  display monitor with 220 dpi , as is now available with certain apple macintosh computers . with a standard 96 dpi display , a system with an 11 micron pixel will have the same apparent magnification as the standard light microscope . an advanced wsi scanner , such as vendor 1 with a 5.5 micron pixel sensor , provides an equivalent 40 magnification . . there are differences between digital pathology image viewing and traditional microscopy , which can lead to confusion by both vendors and pathologists . therefore , it is important to remember that wsi differs from traditional microscopy because of the extra sensor and monitor aspects . perhaps it should be preferable to refer to image quality by other labels that can be compared across different systems . as digital pathology systems continue to evolve , microns / pixel will likely be more widely and appropriately used to denote characteristic features within a digital image . microns / pixel may be a better , vendor - neutral descriptor for  magnification  or image resolution quality for digital pathology systems and should be considered a standard for future digital pathology systems and should be considered by standards - creation efforts ( e.g. , digital imaging and communications in medicine ) . microns / pixel , shown in the rightmost column of table 1 and in figure 4 , relates directly to how the glass slide is digitized and will unambiguously represent increases in optical magnification or smaller pixels sizes . microns / pixel is expected to become the standard of annotating pathology images and will most likely be used for diagnostic and prognostic considerations . the number of mitosis per high power field or mm is an example of a diagnostic consideration . another area that the display of micro per pixel can impact is automated diagnostics , which is the use of computer aided algorithms ( cad ) to detect and quantify cellular processes and disease . the resolution between the various digital pathology systems on the market can vary and the display of micron / pixel can help standardize the cad results , since it indicates the resolution . finally , wsi technologies that offer lower micron / pixel values allow pathologists to get more information from the image , which could lead to considerable time savings if pathologists are able to render a faster diagnosis since less image review time is needed . a standard monitor resolution of 96 dpi was used in the calculations image viewing software that includes a display of micron / pixel in upper right corner micron / pixel are expected to become the standard of annotating pathology images and will most likely be used for diagnostic and prognostic considerations
many pathology laboratories are implementing digital pathology systems . the image resolution and 
 scanning ( digitization ) magnification can vary greatly between these digital pathology systems . 
 in addition , when digital images are compared with viewing images using a microscope , the cellular features can vary in size . 
 this article highlights differences in magnification and resolution between the conventional microscopes and the digital pathology systems . as more pathologists 
 adopt digital pathology , it is important that they understand these differences and how they ultimately translate into what the pathologist can see and how this may impact their overall viewing experience .