why correct vision to only 20 feet?

Why don't vision professionals write prescriptions for glasses for people with presbyopia to allow them to focus at distances greater than 6 meters (20 feet)?  The difference between focusing at 6 meters and 100 meters is about 0.2 diopters.  My vision is corrected to 6 meters with -4.75 diopters.   Road signs, especially at night, don't come into sharp focus for me until I'm maybe 30 feet from the sign.  I would like lenses with -5.00 diopters but all the professionals say they only test and prescribe for correction to 20 feet.  Why is that?  

[Michael J Kutryb, MD]

Ask your doctor to slightly overminus you and state the case you just gave to us.  I don't see why your prescription couldn't be overminused by 0.25 diopter. The only downside is that it might lessen your near vision a little and could possible cause eyestrain.  Your request is not unusual and not unreasonable.  In return  you would probably give up just a slight bit of vison up close though and if you are young you could slightly encourage more myopia in the near future.

MJK MD

[mountain_top]

Thank you very much for your response.  I (63 years old) wear trifocal lenses so near vision is not a problem.  With the -4.75 and the trifocal I do have a zone at about 1 meter where none of the lenses bring things in focus but that doesn't seem to bother me like having things in the distance out of focus.  I will take a copy of your response with me next time in the hopes that it will make a discussion about my prescription a little easier.  Thanks again!