Exit-pupil diameter--isn't it calculated as the objective-lens diameter divided...

[jeffreybehr]

...by the power? So a 50mm-objective-lens scope that's 10 power has an exit pupil of 5mm?

'My' NightForce has a 56mm OL and a power range of 12 to 42. So at 12-power the exit pupil is 4.7mm while at 42-power it's only 1.3mm?

How does this jibe with the 'Maximum' data of the prospective March 10-60X52 published here... http://www.6mmbr.com/bulletin.html ? The 0.86mm number for 'Minimum' appears correct as 52mm divided by 60, but the Max ought to be,52/10 =) 5.2mm and not 3.06mm. Is there some other restriction in the tube?

 

[rstreich]

You're right. Good catch. A 3.06mm exit pupil would make it close to 17x on the low end. That also fits better since a 6x range on the magnification is pretty steep.

 

[Forum Boss]

Just listing the numbers Lou Murdica posted here, and I'm sure he's just listing the specs he got from Japan.

But yep, you've got the exit pupil calculation correct. In reality, I don't think it makes much difference as I don't think the guys who buy a "10 to 60" will spend much time at the low-power end of the spectrum, even if it's 17-power as opposed to 10.

 

[rstreich]

Maybe Fred will jump in here with some of his optics wisdom, but I'd feel better if it were a 17-60x than I would if it were a 10-60x.

 

[jeffreybehr]

...I don't think the guys who buy a "10 to 60" will spend much time at the low-power end of the spectrum, even if it's 17-power as opposed to 10.

Yes, but I imagine they'll spend more time at 40 - 60 power, where that smaller-than-NightForce's objective lens creates a smaller exit pupil at the same power.

I guess we'll soon see just how good they really are, huh!?!?

 

[tenring]

If memory serves me correctly, I beleive that years ago, I read that the human eye cannot absorb light from an exit pupil greater than 5mm.

 

[Spotcheck_Billy]

If memory serves me correctly, I beleive that years ago, I read that the human eye cannot absorb light from an exit pupil greater than 5mm.

The figures that I have show that the pupil of healthy young adults eyes will measure approximately 4mm in bright daylight and 7mm in poor light. I believe that the ability to absorb light from the exit pupil of a scope parallels those figures.

But I'm certainly no optics expert, and I've been wrong before

 

[jeffreybehr]

Tenring and S...Billy, I'm thinking you're both right about the approximate maximum-diameter exit pupil usable by the eye, but I'm thinking more about the smaller-at-the-same-power EP diameters of the March. I've shot 1000Y. targets only twice, with the same Leopold 6.5-20X50 scope. At 20 power, its EP is 2.5mm, and it's plenty difficult enough to use. I think I don't want to have to use the March's 0.87mm EP; My NF's 1.3mm EP will be plenty small enough for me.

 

[Fred Bohl]

Robert

I agree - I would be more comfortable with a lower zoom ratio. That 10 to 60 is really stretching the envelope 6 to 1 when others are just intro introducing 5 to 1 and 4 to 1 is common. I hope this is not marketing ploy as such marketing trumps engineering schemes in the past have given us such optical disasters as the Redfield "Wide Field" and the Leupold VX-L,"what's a little added image distortion - it will sell just fine").

Re exit pupil size ---

SIZE OF MAX. IRIS OPENING vs. AGE

AGE MAXIMUM IRIS OPENING

<25 7.0 mm
30 6.5
35 6.0
45 5.5
60 5.0
80 4.5

Bottom Line: Gets harder to see in twilight as you get old.

Note also that we generally will have smaller actual Iris openings in bright daylight particularly with both eyes open.
For small exit pupils it would be helpful to to close the other eye and use stray light shield such as the Dvorak "Soft-Eye",Click Here) to screen ambient light.

Also note that the actual perceived image brightness is proportional to the area of the exit pupil which is in turn proportional to the exit pupil diameter squared. Therefore, for a given objective diameter, higher magnification rapidly decreases apparent image brightness.

For the very small exit pupil of high power scopes, an added burden is that you have to have evermore precise alignment of your eye behind the scope to direct this small diameter light bundle on the most sensitive area of your retina.

 

[rstreich]

Fred,

Do you have a pointer to something or is there a quick explanation of the mechanism in a variable power scope? I know I saw one somewhere once upon a time, but I can't remember where.

robert

 

[Fred Bohl]

Fred,

Do you have a pointer to something or is there a quick explanation of the mechanism in a variable power scope? I know I saw one somewhere once upon a time, but I can't remember where.

Robert

For more complete explanation of the exit pupil thing the place I pulled the table from is a friends article on our club site that may help. It is long but you can skip the parts your not interested in. Click Here

 

[rstreich]

Thanks, Fred. That's a useful document to have laying around--lot's of good info. It just didn't have the bit I was looking for unfortunately. I was trying to remember what the mechanism was that provides variable magnification.

I can only remember parts of it, but I think there's an additional three or four surfaces involved and some increased error at both the upper and lower ends of the magnification range. Am I way off-target here?

robert

 

[Fred Bohl]

Robert,

Sorry, the images are low resolution so it is not obvious and hard to keep track of the mechanical links.

Almost all designs for variables use the same concept. A greatly simplified explanation follows:

Within the erector tube the rear lens element is allowed to slide front to back slightly which changes the magnification,it takes very little movement at that point in the optical chain to get large changes in apparent magnification). A link,typically a pin) affixed to this sliding rear element passes through a slot in the erector tube and via a complex seal also through the outer tube. The inside surface of the power,zoom) ring has a cam grove that slides the pin front to back as the ring is turned which then slides the rear lens element in the erector tube thus varying the magnification.

It is obviously a mechanical nightmare to design a stable,image stays aligned on the reticule) and sealed mechanism to accomplish this task on a one-off let alone production basis. However the real challenge is to design an optical system with an internal depth of focus so that the image stays in reasonable focus and color corrected over the variable magnification range.

The last is why I'm concerned about the zoom range. But the overall design and manufacturing issues of a zoom is why I do not use variables for serious target work.

 

[Clark]

This chart helps me.