Prone -- distance from the "eye" to the rear apper

[Eric U]

You are missing the boat if you are trying to adjust for light conditions with only filters. I do use different filters for different conditions, but that is in conjunction with rear iris adjustments. There are very few conditions that I can just make a filter change without a corresponding iris adjustment.

Eric U

[Martin Catley]

Eric thanks for your input into this group, can you comment on the idea that you control the amount of light coming through by changing foresight diameters.
A prominent Coach once suggested this, leave your Rear Peep alone and change your foresight I have found this a practical solution especially if you have an Iris up front.
This avoids in theory, altering your focus on the Foresight.

Martin C

[Eric U]

I guess we're going off on a tangent from the OP's question, but I digress...

I may be one of a minority, but I have found no reason to change front aperture diameter due to differing light conditions.

I'm a simple former rocket scientist who has returned to my shooting bum roots, but I don't see how you can possibly change the amount of light reaching your eye by adjusting your front sight aperture diameter. Sure, different light conditions can make your ring of white around the bull look different, but with rear aperture adjustments I haven't felt the need to change the front.

A question: you started your match under bright sunny conditions and a big cloud makes it overcast. Are you going to break position to reach up and adjust the front iris? I'm not, I've already got another 4-5 shots down range after a quick rear iris/filter adjustment. Repeat now that the cloud is gone after 2-3 minutes, and every light change thereafter. You have to take into account that I'm a fast rhythm/flow shooter in prone. If you are a wait for a condition shooter, then you aren't adjusting much during the match anyway.

I do know some prominent prone shooters who change their front aperture diameter for conditions and/or different ranges. But, they also adjust the rear.

This is just my experience. Your mileage may vary.

Eric U

[RobStubbs]

You are missing the boat if you are trying to adjust for light conditions with only filters. I do use different filters for different conditions, but that is in conjunction with rear iris adjustments. There are very few conditions that I can just make a filter change without a corresponding iris adjustment.

Eric U

Let me explain my train of thought. Changing the rear sight iris changes the sight picture (maybe just a small amount agreed). It does also change the depth of field, so too small and everything is in focus - that's not good. Closing down the same iris in very bright conditions merely concentrates that 'beam' of light in a smaller part of your retina. That part of the retina sees just as much light which is what you were trying to avoid in the first place. Changing light with a neutral filter 'merely' reduces all the light to the larger area of the retina, making it easier and less of a strain on the eye.

Rob.

[Waisted]

Closing down the same iris in very bright conditions merely concentrates that 'beam' of light in a smaller part of your retina.

Sorry, Rob that is incorrect. Let's look at the camera to try to illustrate what's actually happening.

The camera has an iris, just like your rear sight. The iris is used for exposure, ie, the wider the iris, the more light. But if you choke down the iris, to very dark, do you get an image which only prints in the centre of the picture? Nope, you get the full size picture, but darker.

So your belief that if you reduce the iris diameter, only the centre of the eye's iris gets light, is actually not correct.

I know, it's counter-intuitive, but that's the fact of the matter.

[peterz]

I'm a physicist. Waisted has it exactly right.

[justadude]

To get a better appreciation for the behavior of the rear iris search on "pin hole cameras". These are cameras that have no lenses just a simple pinhole (usually in a thin piece of metal) as the sole optical element.

A small hole in a thin piece of metal should sound familar as this is exactly what the rear aperature or iris is. As pointed out already, that hole can be small, you still get a full image. Make the hole smaller, the exposure takes longer but you still get a full image.

Due to some fun things with optics, as the hole gets smaller a greater range of distances will appear to be in focus, to some degree that is the advantage of an adjustable, it allows you to increase the range of focus until the image becomes so dim that the eye cannot discern elements of the sight picture clearly. Assuming no other filters and "normal" eyes the crossover between sharper image and too dim an image often occurs around 1.1-1.3 mm or so. (No one flame me everyones eyes and home range is a little different OK I know that.)

I will state the basics, that have already been mentioned here a bit differently: Image clarity and brightness is controlled by opening and closing the rear iris.

Image field of view and precision of alignment is controlled by eye relief. Close eye relief=big field of view but harder to make sure everything is aligned. Long eye relief=narrow field of view but easy to see front to rear sight misalignment.

My 2 cents
'Dude

[Guest]

With Eric U being the 2nd ranked prone shooter in the world (as of 30 July 2010), I think I will take his advice.

[Waisted]

With Eric U being the 2nd ranked prone shooter in the world (as of 30 July 2010), I think I will take his advice.

Sensible chap. Eric U has it right, of course, regarding adjustment of rear sight for light level, and maybe adjusting front sight for aesthetics (my words). However…

From Eric U:The closer you get to your rear iris, the more things you can see around your front sight...like wind flags and your number board. But, it is harder to keep your sights aligned.

And I just have to disagree there, about it being hard to keep your sights aligned if you gwt closer to your rear aperture, and sorry If I have misunderstood that point (which is pretty likely!).

Yesyes, I'm a noob at shooting and Eric U is a God, but science is, actually, science, and not even God can change that. (OOPS! Incoming…)

The point I'm making is about keeping the sights aligned. I think that Eric U's concern is about the shortness of the eye relief. He worries about getting his pupil off-line (the target bull, foresight and rear sight are surely already perfectly positioned!), since there is much more room for manoueuvre at close quarters, which might introduce parallax and send his shots into oblivion.

But parallax with an aperture sight, closed down below your pupil size, does not exist. Maybe (probably) he means something else…

[Waisted]

I'm a physicist. Waisted has it exactly right.

Thank you :)

Same here. Physics and Maths. They call it "Math" over here…

Doesn't make me any better at shooting :( I once read a book about the scientific analysis of the golf swing. Fascinating. But the author was a crap golfer, even though he described the swing perfectly! Sucks.

So Eric U may not have all the science at his fingertips, but he shoots real good! Sucks (for us scientists, not for him! :).

[Eric U]

I simply mean that if you are closer to your rear sight you will see a bigger ring of white around your front sight. With that bigger ring of white it is easier to have the bull perfectly centered in the front aperture, but inadvertently not have the front sight perfectly aligned with the rear sight. This is incorrect sight alignment. Ask a pistol shooter how important that is. If this is your meaning of parallax, then yes, it is quite possible (probable with newer shooters) to have parallax with iron sights. Nothing that can't be trained for with a short eye relief, but much easier to detect with a longer eye relief.

Eric U

[Eric U]

"easy" not "easier"

[Roodaddy600]

Is it weird to anyone else that there are more posts on opening and closing iris's and front sights than on the guy's actual question? Maybe its just me...

Oh and as for Eric being a god what backward pagan religion is that??


Shane

[RobStubbs]

Closing down the same iris in very bright conditions merely concentrates that 'beam' of light in a smaller part of your retina.

Sorry, Rob that is incorrect. Let's look at the camera to try to illustrate what's actually happening.

The camera has an iris, just like your rear sight. The iris is used for exposure, ie, the wider the iris, the more light. But if you choke down the iris, to very dark, do you get an image which only prints in the centre of the picture? Nope, you get the full size picture, but darker.

So your belief that if you reduce the iris diameter, only the centre of the eye's iris gets light, is actually not correct.

I know, it's counter-intuitive, but that's the fact of the matter.

Agreed, I was getting myself all confused. I know a pin hole affect only works below a certain size and of course cameras nowadays don't only use the pin hole affect, they use a sequence of lenses to acheive the same thing. Not sure it helps explain things in any more detail, but there's a good article on pinhole cameras here; http://www.wesjones.com/pinhole.htm.

Rob.

[Guest]

Is it weird to anyone else that there are more posts on opening and closing iris's and front sights than on the guy's actual question? Maybe its just me...

That may be because the two things are very closely linked.

Oh and as for Eric being a god what backward pagan religion is that??

Name me a religion that isn't backward. And what is your definition of "pagan"?.

Oops.

[RobinC]

A very intersting thread, BUT, please Eric as per the original post, for us non world level shooters, with average rear apertures what actually is the optimum eye relief?

Robin

[RobStubbs]

A very intersting thread, BUT, please Eric as per the original post, for us non world level shooters, with average rear apertures what actually is the optimum eye relief?

Robin

The trail of posts above should tell you there's no such magic answer. It's partly a personal thing, and there are recommendations as to what range of distances it should be within, to get within the technically optimal window.

Rob.

[Leo]

From the guy that asked the original question -- thank you! Great discussion, I know I benefitted from the various opinions... almost like "on line coaching". I did move closer and I can get a much larger field of view at the front post -- paralax (sp) is a concern as some of my shots ended up in "6" -- but that's a learning process.

Thanks to all for taking the time to offer an opinon. :)

[Eric U]

There really isn't a one-size-fits-all optimum. The "average" eye relief that I see on good shooters is 1.5-2". Like I said earlier, I shoot with eye relief that is a little closer. The functional range of eye relief is from where you are back so far that you only have a faint ring of gray around the front sight (not recommended), to as close as your eye or glasses can get to the sight (also not recommended, esp if it is a big-bore rifle). My suggestion is to try a few different sight positions in between to see what you like and what works for you. As stated by another poster, there is no one size fits all answer to the question.

Eric U

[RobinC]

I realise there is no magic answer Rob, but like Leo I have found this thead truly facinating and Eric's comments very useful on a subject that many shooters just let "happen" rather than analyse. With a shooter of his experience we should take note, what I wanted and I suspect the original poster also was an average to start experimenting from, and 1.5 - 2 inches seems to fit.
Thanks Eric all this has been most helpfull.
Robin