Focal length at 50 metres

[Cumbrian]

Apologies for the rather cryptic and perhaps unhelpful title, but I wanted it to be succinct. I have started to shoot at 50 metres (outdoors) for the first time. My previous experience has been confined to 25 yards indoors. Even after a settling in period, my scores per 100 are 2 or 3 points adrift of what I can usually achieve at 25 yards. I have shot on fairly windless days, so I can't blame the weather. So far as I can judge, my main problem is getting the black into focus before my eye loses it all, a problem that I don't generally have at 25 yards. Indeed, I don't seem to see the black at 50 metres as clearly as I do at 25 yards however much time I give it. Is that simply to be expected? I know that focusing clearly on the black is never going to be - and should not be - as precise as the focus on the foresight but the difference in focus I experience at the two distances is appreciable. I shoot with a 0.5 eagle eye in the foresight and a focal length correction lens in the rearsight, which is permitted under NSRA rules in the UK, though obviously not by the ISSF. Is it possible that I need a different lens in the rearsight that would put the focal length further out because the target is further away? If I don't, is there any thing else I might try to help get the black into (relative) focus quicker? Would an extension tube help, for example? I would be very grateful for advice and suggestions.

[Tim S]

Roger,

I may be corrected on this, but I don't think there is any need to have separate corrective lenses for 25 yards and 50m. Certainly my optician, himself a smallbore shooter, has never suggested this. As I understand it a lens prescribed for shooting (i.e. distance +0.5 dioptres) will focus your eye about 3ft/1m ahead of the foresight, to give a sharp foresight and a less blurred target. As such, the distance to the target doesn't make much difference.

What you are experiencing is common. Partly it can be attributed to the target, which at 50m is a little less forgiving that at 25 yards; partly to the ammunition, which groups less tightly than at 25 yards; and also due to light, which will be brighter than on your indoor range, but fluctuating.

What you may need to do is play with the rearsight aperture and even the eye relief to get a better sight picture.

As extension tube will put the foresight further out, which can help with focus (that bit closer to infinity helps older eyes), but may necessitate a change in your prescription. If you have a good hold I think a tube can help your aim, but if your hold isn't so great the increased sight base will make any wobble more apparent. This can lead to hesitation over triggering. It will also had 200-300g+ to the end of the barrel, which may affect your hold and the barrel harmonics.

[Cumbrian]

Tim, that's a really helpful reply - as usual. Thank you. I'm glad to learn that I don't need a new rear lens and that part of the problem is just the shift to a longer distance. I have a home made extension tube that I will try out. And I will certainly experiment with the rear aperture and eye relief as you suggest. One move I did find that helped was to wear a cap, which I don't use indoors. It seemed to concentrate the whole picture and centre it on the rearsight.

Roger

[RossM]

Have you tried shooting with no lenses at all? My test for students is to ask if they can see nice round black dots on the target from either 25yds or 50m. If they can then for a first attempt try shooting "as is". My starting front sight size is 3.8 to 3.9. Other will differ I am certain.
One very important thing to consider if your balck is a little grey is the fact that with a eagle in the front and a lense in the back there are 4 glass surfaces that you are looking through. Every surface will drop 4% of the light and the contrast will be decreased. That's my tip. Have fun.

[Cumbrian]

That's a good point, Ross - thank you. I'll try that as well. I suppose it's rather like zero budgeting that was once suggested by the bursar in the school where I taught. Neither he (for many reasons) nor the idea were popular but I can see the logic.

[KennyB]

I think Tim made a good point stating that you may have to play with eye relief - the level of light outdoors is way higher than indoors which results in your eye's iris being more contracted. One upshot of this is that the peephole appears smaller - i.e. there is less perceived gap between the inner edge of the iris and the outer edge of the foresight tunnel. The distortion free "sweet spot" in the center of the peephole is also smaller. I find I have to move the rearsight back by around 8mm when outdoors.

2-3 points off your indoor scores (per 100) isn't unusual.

I've seen Art (Shooting Sight) suggest that with an Eagle Eye you need less correction (+0.25 rather than +0.5) but that's a small shift. 1/8 dioptre is supposed to be the smallest noticeable change - if you can find lenses that go to that resolution.
Optically speaking, I've always thought that "infinity" starts at 6 meters so the target at 25y and the target at 50m are both at infinity and your corrective lens should still only need to focus at twice the foresight distance.

Those are my thoughts anyway..

Ken.