Image of the day

Captured by
Herb Bubert

Sun on June 20th

My Account

New to Astromart?

Register an account...

Need Help?

Visual Acuity - What’s Yours?

Started by EdZ, 05/18/2003 04:26PM
Posted 05/18/2003 04:26PM Opening Post
Visual Acuity is a limiting factor in the eye’s ability to achieve resolution and magnification is employed to make an image large enough for the eye to perceive. If your visual acuity is 200 arcseconds, you need 12x mag to see a 17” double or 20x mag to see a 10” double.

Acuity is the apparent separation results for a range of varying doubles measured with one or more pieces of equipment. The results will fall into a fairly narrow range. This would be the observer's acuity range.

“The resolution of the eye is 3 to 5 minutes of arc; therefore a minimum magnification must be utilized to enlarge the image sufficiently to exceed the eye’s resolution.” C. R. Kitchin, “Telescopes and Techniques” Springer-Verlag, 1995. Other sources will quote lower limits. My personal records of many doubles puts me right in the middle of this range. Possibly some observers can achieve better results.

In my best binoculars I can rarely achieve apparent separations of less than 200 arc-seconds. I’ve recorded many observations between 210 and 240 arc-sec. Some telescope results are slightly better, possibly attributed to the better resolution of telescopes. The best results I’ve achieved with very good optics was about 160 arc-sec, equivalent to a 2” double at 80x. The closer the doubles or the wider the mag delta, the harder it becomes to achieve these results.

So my usual visual acuity is 3 to 4 arc minutes. I am not capable of seeing separation in a 4 arc-min pair naked eye; I cannot see e1-e2 Lyra (207") as separated. Optics improve the point source resolution and contrast making it easier to see separation.

I've read one binocular observer was able to split doubles of 4 seconds magnified 20x = to apparent separation 80 arcsec. My results do not indicate this cannot be accomplished, but they indicate it’s clear if we wish to compare our ability to perceive resolution with our instruments, we must first establish the visual acuity of the individuals. Anyone ever split Castor 4” or Algeiba 4.4” with 20x or 25x binoculars?

We could test acuity with a list of doubles. Some that might get better success with binoculars are 17CrB at 6” or 7CrB at 6.3”, E(sigma)953 in Mon at 7.1”, Mesartim y Ari at 7.8” or y Del at 9.6”. A wide range of binocs might be able to see these close matched doubles. Smaller binocs try 100Herc at 14.2”, or E2470 and E2474, the Double’s double in Lyra at 13.4” and 16.2”, or a multitude of doubles in the 20” range.

Some may achieve acuity of 2.5 arc minutes or 150”, or less, requiring a magnification of only 150”/9.6” or 16x to see separation in yDel.

I’m very interested in collecting results from binocular users that might lead to some realistic conclusions concerning acuity. Having determined acuity, we should be able to compare results of resolution tests from various individuals, as long as all other measurement criteria are held constant.

Please Email results to
Posted 05/23/2003 06:33PM #1
The viewer who originally stated he was able to split 75Her 4.1" with his 20x60 Pentax has since corresponded with me to let me know he was not able to do this upon retry. I have since used my TV85 and determined the lowest magnification I could split 75Her 4.1" was at 43x. The results (for me) is an apparent separation of 176 arc seconds.

Milt, yDel is listed as 9.6" in Skycataloge 2000.0, my bible. You split it with 18x50s and that results in an apparent separation of 173 arcseconds. Inge has sent me some results that reflect significantly greater acuity. Once he has seen a split at a higher power, he is able to go to lower power. He is getting down into the 120-130 arcsecond range on the best occasions, although these are the exception and not the rule.

No one, with any binocular has yet reported that they are able to split Castor 4” or Algeiba 4.4”. 75Her at 4.1" fits perfectly in with this group. I'd like to hear from someone with 25x or 30x binoculars that would attempt these three splits. Also refer back to the original post for a few easier doubles in the 6" to 14" range.

Posted 05/23/2003 11:36PM #2
Every time I see a new patient, I glance at their visual acuity, which we check for virtually every new patient.

Most of course do 20/20 or slightly worse. BUT, an occasional patient makes 20/15, and rarely, we'll see someone with 20/13. Supposedly, there are some people with exquisite acuity, all the way down to 20/10. Reportedly, the late Ted Williams, the last man to bat .400 (actually it was .406), had vision in that range, which didn't hurt while flying fighter planes for the Marines.

Perhaps there are ophthalmologists or optometrists reading the board who can comment further.