Ophthalmologists give lens for near and distant vision. But during viewing beyond these two areas like desktop computer none of these lens power can focus the monitor. Usually the desktop computer is viewed at 24 to 26 inch distance. Though varilux lens is appropriate in this situation but it is costly. So, another fixed lens may be allowed to keep on the computer table to serve the purpose. I (using +3.25 for near work) personally use +2.5 lens during computer work comfort-fully. What is the mathematical formula to adjust the lens power without doing refraction test?
If there is no accommodation capability (3.25D for near work), you can use the formula: Addition= 1 / [distance of work (in meters)].
This way, if you want to see correctly the laptop at a distance of 66cm (0.66m), you will need an addition power of 1.5D over the distance correction.
I don't think there is a standard mathematical formulae for that. Even though there is one, it gives a tentative value to start with as it depends upon the person's amplitude of accommodation and accommodative reserve. Subjective refraction is probably the best method to determine the appropriate power. For an absolute presbyope, the add is bascically the reciprocal of viewing distance as mentioned by Francisco. Various options are available: single vision lenses, Trifocals or PALs. Also, I think optometrists are the best persons to consult regarding these matters but it is also wise to go to ophthalmologists where there are no optometrists (esp in developing countries). I hope that helps.
If you do not want to do the refraction test, you can calculate the lens power through the vergence of the object (in your case, the object is the screen of the computer).
Vergence is a concept in ophthalmic optics in which distances are converted to a dioptric power. Vergence is defined as
Vergence= n/d.
where d is the absolute distance between an object and a plane of interest, and n is the refractive index of the space. In your case, n=1 and d is de distance between your eyes and the screen. This means, that your eyes have to make an additional effort equal to "vergence" to focus the screen.
Finally, the appropriate lens could be (in agree with Francisco Alba-Bueno)
Power= R + vergence
where R is the refraction of your eyes in distance vision.
However, ophthalmic optics takes in account the person's amplitude of accommodation and accommodative reserve to prescribe near or intermediate glasses, in order to obtain a comfortable vision, and normally the final equation is:
Power= R + vergence -1/2AA (or 2/3 AA)
Being AA the amplitude of accommodation.
So, in agree with Nabin Paudel, subjective refraction is probably the best method to determine the appropriate power.
However, there are tables that estimate the amplitude of accommodation as a function of age. For example, in the book Clinical Procedures in Primary Eye Care
by David B. Elliott you can find it.
hi..discussed this query with our optometrist. There is as such no standard formula to calculate power for computer work. Either refraction is done my taking object at computer distance as used by distance in a way similar to done for near correction or subtract 1 to 1.25 D from near correction . Hope this helps.
I do not consider it ethical to simply calculate a possible lens power. You need to consider the workstation - do they use desktop, lap top, ipad, iphone - do they use multiple screens and the flexibility of their visual system. Varifocals can cause postural problems, bifocals more so. Degressive lenses can be particularly useful. For more information look at my paper http://www.optometry.co.uk/uploads/articles/August%2021%2009%20Clinical.pdf
As indicated by Nabin, it would be good to go for subjective refraction. The baseline for starting subjective refraction is the addition required @ 40 cm (usual reading distance) according to age. A person will require a little bit less addition at computer distance (about 0.5 DS less than the addition @ 40 cm.
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