TargetTalk

This arises really out of an incidental remark by Tim S on the importance of the foresight aperture and its affect on grouping in the recent adjacent discussion of Parker Hale/Gehmann rearsight adaptors. I thought it best to start a new topic.

My question is: what foresight aperture should one ideally be thinking of? I realise of course that this will vary with eyesight and personal preference, but are there good optical reasons for a certain range or even something more precise in terms of the inner aperture and, separately, the thickness of the ring? I have a Centra Duo Vario foresight (22mm version) that gives a wide range of adjustment but a minimum ring thickness of 1.8mm, whereas I have read that 1.4 is usually the most popular thickness and 1.1mm is also offered for the Centra High End inserts, for example. In fact, without knowing anything about the subject I have been using a considerably thicker ring than the minimum available and found it comfortable, but could this be to my unwitting disadvantage? And I have opted for 5.00mm for the inner aperture because, with fairly old eyes and not shooting to ISSF standards, I use a 0.5 eagle eye in the foresight as well. Should I be experimenting more, and, if so, any suggestions as to what combination of dimensions I might usefully try out? Grateful for any enlightenment and help that might be forthcoming.

Is this on a standard length barrel, no tube? If so, 5.0mm doesn't sound unreasonable with an eagle eye. Unless the lens is behind the element, it will only magnify the target, so you need a larger foresight aperture to compensate. Your lens + 5.0mm aperture, would translate to a 4.0-ish one without. As for the thickness of the foresight, yes some folks do prefer a very thin ring, and others like a thick ring. Personally I find the 1.4mm hi-ends OK, but I'd buy a thicker ring if Centra made one.

So long as you keep notes on your current setting, since you have an adjustable foresight why not experiment? I would give each new setting a good trial though.

The amount of white and ring size greatly depends on the shooter. Don't get the white too small or your brain will fill in what is not there. Don't assume that what looks best will give the best results; a slightly out of focus target giving better results for example. Narrow white looks good to my eye but my scores suffer. It takes a lot of time to find the right combination, and about the time you do your eyes will change.

Mark

Cumbrian,
Try this article, it has a lot of good info. It should give you an idea on which size aperature to start with.
Linky>>>>>http://www.usashooting.org/library/Inst ... b_2011.pdf


Scott

Tim S wrote:... with an eagle eye. Unless the lens is behind the element, it will only magnify the target...

Tim,
I used to think this until I got an Eagle Eye to try.

To me, it seems like it doesn't matter which side of the element the lens is on, the target is magnified but the foresight element is not - OR the magnification of the element is miniscule because the lens is so close to it.
I can't do the optical maths but I'm sure that whichever side of the element the lens is on, you'll need a larger foresight aperture.

Regards,
Ken.

Thanks Ken,

Many thanks to all who have replied. I'm grateful for all the extremely useful facts and suggestions. It would appear that my actual aperture is probably about right, though I will experiment with one a bit wider still, but the bigger experiment will have to be with a thinner ring.

Tim - my barrel is the standard Anschutz 1800 of 27 inches, though distance from rearsight iris to foresight ring is 33 I/2 inches. I am, however, in the process of making an 8 inch extension tube, which will, as I understand these matters, require an enlarged rear aperture.

Scott - your reference leads to a fascinating article that I, and probably others, would not have known about previously. Just leaves me puzzling over how to achieve a thin ring, however, without buying a new Centra foresight and a selection of High End inserts to experiment with.

Another question, leading on from my last sentence: is there any advantage in the Centra Crystal foresight and the accompanying Crystal inserts? I would have thought that you could have too much light coming in at all angles, instead of a focused beam from the target end, but I daresay Centra know what they are doing.

Roger

Roger,

Yes you should use a larger foresight aperture if adding an 8in extension tube. I'd hazard a guess that you'd want a 6mm+ aperture; why not use the handy excel calculator that Sam Huish put on the Stirton site a few weeks back?

As for the Centra Crystal tunnel and inserts, I would not think that you would need these at 25yds indoors. I see their use being on outdoor ranges; but then I know a few folks who use Jim Duguid's Panoramic rearsight indoors.

Tim,

Many thanks again for the advice and reference. Quite relieved that I won't need the even more expensive Centra Crystal set up.

Roger