EXIT PUPIL

DRINK­ING OUT OF A FIREHOSE

Gun World - - Scope-Eye Chronicles -

When drink­ing out of a gush­ing firehose, you can con­sume just so much. The rest spills out of your mouth and is wasted. The same prin­ci­ple ap­plies when look­ing through an exit pupil: You can con­sume just so much. The rest “spills out” of your eye and is wasted. The exit pupil of an op­tic is the col­umn of light trans­mit­ted by the oc­u­lar lens, ap­pear­ing as a cir­cle of light vis­i­ble in the cen­ter of the eye­piece when viewed from half an arm’s length away from your face. This exit pupil is mea­sured in mil­lime­ters.

The for­mula to de­ter­mine exit pupil of an op­tic is sim­ple:

Ob­jec­tive lens di­am­e­ter in mil­lime­ters di­vided by mag­ni­fi­ca­tion

Ex­am­ple: With a 3.5-10x50 scope set at 10 power, di­vide the 50mm ob­jec­tive lens size by the mag­ni­fi­ca­tion of 10; the an­swer is 5mm. Now, lower the mag­ni­fi­ca­tion to 5x. Us­ing the same for­mula, the exit pupil is now 10 (50mm ob­jec­tive lens di­vided by 5x = 10mm).

To con­tinue the metaphor, If you can match the amount of water ex­it­ing from a firehose to the max­i­mum amount you can drink, this is the most ef­fi­cient, and noth­ing is wasted. You also don’t have to man­han­dle the im­pres­sive power that a fully charged firehose wields.

EXIT PUPIL IS SIM­PLY A MATH EQUA­TION, THOUGH. IT DOES NOT TAKE INTO AC­COUNT LENS QUAL­ITY OR COATINGS.

Like­wise, if you can match the amount of light ex­it­ing a ri­fle­scope or binoc­u­lar eye­piece to the max­i­mum amount your eye can “swal­low,” this is the most ef­fi­cient, and noth­ing is wasted. You also don’t have to man­han­dle the ex­tra weight, size and ex­pense that an op­tic with a gi­ant exit pupil might wield.

Young pupils di­late (open) a lot larger than older pupils. Var­i­ous sci­en­tific stud­ies re­flect that an av­er­age max­i­mum pupil size when dark-ad­justed is about 7mm for peo­ple in their 20s to a max­i­mum of 2mm for some­one in their 80s.

YOUNG PUPILS VS. OLD PUPILS

I cor­re­sponded with a Ph.D. from MIT who was in­volved in such pub­lished stud­ies. He cau­tioned me to point out that the vari­a­tions of max­i­mum di­la­tion among in­di­vid­u­als is larger than the ef­fect of age degra­da­tion of di­la­tion from 10 to 80 years old and that the gen­eral trend is that young eyes can ex­pand ap­prox­i­mately 40 per­cent more than very old eyes. Keep this in mind when view­ing any charts show­ing the pupil size for a given age. Max­i­mum pupil di­la­tion is very much de­pen­dent on the in­di­vid­ual. Other fac­tors—in­clud­ing stim­uli such as sex­ual ex­cite­ment, a math prob­lem, buck fever, medicines and fightor-flight re­sponses un­der stress—also af­fect di­la­tion size. Exit pupil is one rea­son the 7x50 binoc­u­lar is preva­lent for marine use. Di­vid­ing the 50mm ob­jec­tive lens size by the 7x mag­ni­fi­ca­tion re­sults in a 7.1mm exit pupil—enough to match the di­la­tion of most users and en­abling a view that is as bright as an eye al­lows. The hu­man eye ad­justs to dark­ness in a few min­utes but takes hours to fully ac­cli­mate to no light, gain­ing pupil di­am­e­ter slowly with time. That’s why fighter pi­lots on an air­craft car­rier wear dark gog­gles long be­fore night op­er­a­tions.

How­ever, an exit pupil a bit larger than the amount of your pupil di­la­tion is use­able—just not any brighter. It gives you a bit more room to move your head around be­hind the op­tic and still have a full field of view. You move your head po­si­tion more than you might think with both binoc­u­lars and ri­fle­scopes and quite a lot with spot­ting scopes.

Think you could get a brighter pic­ture from an EOTech or Sure­Fire red dot than an is­sue 4x32 ACOG? Think again. The exit

pupil on that ACOG is a whop­ping 8mm—more than just about any eye can open. And it’s al­most never com­plete dark­ness, ei­ther, so rarely are your pupils di­lated as much as pos­si­ble. For sol­diers, there might be lighted rooms, street lights and head­lamps, and for an early-morn­ing tree stand hunter, per­haps there is a flash­light, an auto dash­board, or the moon and stars.

In day­light, the game changes. The view from my lit­tle 8x20 Swarovski pocket binoc­u­lar has only a 2.5mm exit pupil, but be­cause con­di­tions are rel­a­tively bright when I use the binoc­u­lar, and my pupil is not di­lated, I have all the light I need. It’s the same with a high-mag­ni­fi­ca­tion scope at the range: When set at 25x, the small, 2mm exit pupil of my 8.5-25x50 Le­upold Mark 4 is just fine in the day­time. But in dark­ness, I can turn it down to a lower power, in­creas­ing the size of my scope’s exit pupil, thereby al­low­ing my di­lated pupil to ac­cept more light.

Exit pupil is sim­ply a math equa­tion, though. It does not take into ac­count lens qual­ity or coatings. So, in some cases, a slightly smaller exit pupil from a qual­ity op­tic might re­sult in a brighter over­all pic­ture than a larger exit pupil from a lesser-qual­ity prod­uct.

ENOUGH SIZE AND LIGHT

Match­ing your op­tic’s exit pupil to the max­i­mum amount your pupil can di­late is ex­tremely use­ful and the most ef­fi­cient in low-light sit­u­a­tions—par­tic­u­larly with binoc­u­lars, where the exit pupil is not ad­justable by vary­ing the mag­ni­fi­ca­tion. In day­light, the exit pupil has much smaller im­por­tance in re­gard to light and more im­por­tance in giv­ing com­fort and ease of use in view­ing. Most mid-range vari­able-power ri­fle­scopes such as a 3-9x40 set at medium mag­ni­fi­ca­tion will of­fer an exit pupil with enough size and light for just about any con­di­tions and any eye. gW

MOST MID-RANGE VARI­ABLE-POWER RI­FLE­SCOPES SUCH AS A 3-9X40 SET AT MEDIUM MAG­NI­FI­CA­TION WILL OF­FER AN EXIT PUPIL WITH ENOUGH SIZE AND LIGHT FOR JUST ABOUT ANY CON­DI­TIONS AND ANY EYE.

This Vor­tex Ra­zor HD Gen II, set to 3x, has a fire­hose­like 16.6mm exit pupil.

When set at 2x, this VX-6 2-12x42 de­liv­ers a whop­ping exit pupil of 21! But, as when drink­ing out of a firehose, most of it is wasted.

Choos­ing a binoc­u­lar with an exit pupil that matches your max­i­mum

di­la­tion is the most ef­fi­cient.

The RV1 with the 3XP and #78 A.R.M.S. mount are

per­fect mates. This 36x44 BSA tar­get scope only de­liv­ers a 1.2mm exit pupil, but it’s use­able in day­light con­di­tions. Head place­ment is crit­i­cal with such a tiny exit pupil.

The large 56mm ob­jec­tive on this 5-25 Steiner T-5Xi al­lows for a 2.2mm exit pupil at 25x.

Any 3-9x40 scope such as this Hi-Lux USMC M40 copy will de­liver an exit pupil from 13.3 to 4.4. (U.S. Marine Corps sniper Car­los Hath­cock ad­justed the large avail­able exit pupil to his full ad­van­tage in Viet­nam.)

SIG clearly marks par­al­lax yardages on its new Tango 6 5-30x56 scope, shown here in the Ki­netic Devel­op­ment Group QR mod­u­lar SIDS-141 34mm mount. As of this writ­ing, a cus­tom el­e­va­tion tur­ret with your bal­lis­tic in­for­ma­tion is or­der­able with all Tango ri­fle­scopes, price in­cluded. Although it is very compact, the 8mm exit pupil on this Le­upold 2.5x shot­gun scope is more than most pupils can di­late.

This tiny 8x20 monoc­u­lar de­liv­ers a day­light-use­able, 2.5mm exit pupil.

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