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> Rifles general discussion
> Objective diameter
Objective diameter
@ 11:19 am (GMT) |
Scott StruifI have a question that can by answered by science rather than experience:I get the impression that scope manufacturers want us to believe that the bigger the objective lens, the more light it can gather and deliver to the eye. Is this a marketing ploy to get us to purchase their more expensive models with large objective lenses, >= 50mm. Is there a formula, or, even better, a chart, that shows the relationship between objective size and light delivery to the eye at given magnifications? If you want maximum light gathering at full-power, does that necessarily mean a bigger objective? If so, is a scope that can deliver the most light at, say, 25X better because you can take a 600 yard shot at dusk? There’s no point shooting an animal at twilight at that distance if you can’t see the result with the naked eye. If you limit your twilight hunting to what you can shoot at your scope’s lowest power, is a large objective an advantage? I assume the tube diameter has little to do with it. From what I can tell, the advantage of a 34mm tube over a 25mm tube is range of internal adjustment, not light delivery capability. |
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@ 05:50 am (GMT) |
Scott StruifRe: Objective diameterI found this article:https://www.targettamers.com/guides/rifle-scope-objective-lens-diameter/ It says that the bigger the objective, the more light is delivered via the exit pupil. However, if the exit pupil of the scope is larger than the diameter of the shooter’s eye pupil, the extra light is wasted. The exit pupil diameter of any scope can be calculated by dividing the objective diameter by the magnification setting. For example, if a 50mm scope is set to 9X, the exit pupil size is 5.55mm. If the shooter’s eye pupil is dilated to 5.55mm or larger, then all the light captured and delivered via the exit pupil is usable by the shooter. However, if the 50mm scope is set at 6X, the exit pupil is 8.33. If the shooter’s eye pupil is dilated to 5mm, the extra light is wasted. The article further states that young people’s eyes are capable of dilating to 7-8mm, but older people’s eyes average 5-6mm. The article also discusses glass quality, weight, mounting considerations, etc., but I’m only interested in whether the “science” is accurate. Take the S-TAC 2-10x32, for example. Most people would say the objective is too small to be useful in low light conditions. However, applying the author’s logic, at 6X the exit pupil would be 5.33 . . . fine for use by an older shooter at dusk. |
@ 05:32 pm (GMT) |
VinceRe: Objective diameterHi Scott,Interesting question so I looked it up too https://riflescopescenter.com/exit-pupil/ different website but seems to be supplying the same answers, essentially the maximum dilation range of a young healthy eye is 2mm - 9mm, they seem to be suggesting that the maximum dilation in "shooting light" would be 7mm, presumably less for old buggers like me. My 280ai wears a sightron stac 4-20x50 at the moment so using the formula 50/20 on max zoom gives an exit pupil of 2.5mm, fine for daylight but no good early and late. If I assume maximum dilation of 6mm for my eye 8 power is the maximum zoom in marginal light I will get any benefit from, if the scope was a 6-20x42, 7 power would be the maximum I would get any benefit from. Assuming these calculations are roughly correct the benefits of the bigger front objectives come down to longer range shots in marginal light. Not sure if there is a calculation for yards / zoom for effective shooting but even if it's 100 yards per power setting on average the minutes or seconds gained can't be much Cheers Vince |
@ 08:43 pm (GMT) |
Mike DavisRe: Objective diameterthere is a reason the most popular and enduring fixed power scopes and binoculars are in some basic sizes...4x32 is slightly more than needed 6x42 is perfect10x50 binos....good compromise. with my 4.5x14x40mm leupold right on dust...6-7x is far and away the best,any higher its just too dark. with my basic 3x9 scopes somewhere in 4-5 range works best. shooting in morning is better,anything seen is only going to be easier to see LOL. dusk is better as animals coming out to feed...... this time of the year its early start and very late finish. on Friday we had wallaby 50ish yards away right on first light.my buddy just couldnt quite make it out through damp scope...but I had no such worry with my crude /course open sights and fluked a headshot.... |
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@ 07:27 am (GMT) |
Scott StruifRe: Objective diameterThanks for the input. I’ve got a Weaver K4 on a woods rifle. It’s 4x38, great in low light, & fits perfectly in Talley extra-low rings. I can shoulder the rifle with my eyes closed, and know I’m spot-on when I open my shooting eye. However, I need more power for practicing at ranges >100 yards. I doubt a 40mm scope would work with those rings - the bell of the K4 is that close to the barrel. I was trying to figure out if a 32mm would be sufficient at 3 or 4X in the woods at dawn or dusk. Obviously it is. |
@ 12:55 pm (GMT) |
Martin TaylorRe: Objective diameterMy deer and general use 06 has a Lup 3-9x40 VXR Firedot, in the dark southern gullies when the deer bail up under the ferns it really works brilliantly.I always leave it on 3X and lightly illuminated centre dot in low light. When l want to push out to 100+ etc during the day it has the 9x and maintains great eye relief. I think its a combo of good glass and the 30mm tube. Also have same in a 2-7x32mm and it also performs extremely well. I will say my 50mm extended range scopes perform very differently in lower light set at the same mag. The ability to line up the eye box changes dramatically. Colour, definition and ability to focus all vary. Glass quality and tube size seem to effect the outcome. |
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@ 06:35 am (GMT) |
Scott StruifRe: Objective diameterThanks, Marty. If the “science” that Vince and I found on the Internet is correct, a 50mm objective at 4X would deliver a 12.5mm exit pupil. If the shooter’s pupil is dilated to 6mm, almost half of the light would be wasted. However, in broad daylight, the same 50mm scope set at 20X would deliver an exit pupil of 2.5 mm, roughly corresponding to the average shooter’s contracted eye pupil. So a big objective makes sense for long-range, daylight shooting. Your experience with your 3-9x40 and 2-7x32 scopes in low light is helpful.You mentioned “eye box.” I hadn’t heard the term, so I did some more digging. If the scope’s exit pupil exceeds the shooter’s eye pupil diameter, he can position his eye anywhere within the exit pupil and see the image. This sounds like a good thing, but it seems like it could cause a parallax problem if the eye isn’t dead-centered on the exit pupil. I found an video that touches on eye box size, but doesn’t discuss parallax. They use a Swarovski Z8i 0.75-6x20 to demonstrate. They say that the exit pupil should be 20mm at 1X magnification, but the scope internally limits it to 10mm because the shooter’s pupil can only dilate to 8mm. They suggest that this is done to block the excess, unusable light. But it seems more likely it’s done to avoid parallax issues that might arise if the exit pupil were overly large. https://www.optics-trade.eu/blog/minimal-and-maximal-exit-pupil/ |
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@ 06:08 pm (GMT) |
VinceRe: Objective diameterHi GuysThis caused further reading and a more complicated equation to answer but if I understand this correctly http://rimfirebenchrest.com/articles/parallax.html then the larger front objectives which gather increased (often wasted) light and a bigger eye box also allow a broader range of movement by the shooter behind the scope without parallax error. The irony is that like most of you I predominantly hunt close range in the bush with a 3-9x40 that lives on 3 power permanently. I have seriously considered scopes in the 1-5 or 2-7 range as well as something like an aimpoint for this sort of shooting, the only reason for not trying a red dot is that I like the safety aspect of having a bit of magnification. I would maybe consider a longer shot on big game in marginal light if it was morning and I had the daylight to follow up but I think the main benefit is probably with varmint shooting at long range in marginal light Cheers Vince |
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@ 10:49 pm (GMT) |
Martin TaylorRe: Objective diameteryeah not sure on the science just looked threw plenty of scopes.And wasted $ on some crap that had good reviews etc. One of Nathan & Steph's pet hates, garbage scopes. You will hear people say things like "that scope just lines up easily". Your eye will want to line up in the centre of the light naturally, just like a peep sight ring. So l don't think a bit more is wasted as such. We all notice that most variable scopes work better in the low power settings. You can see this in daylight easily on many 20 plus power variable scopes. Things just get a bit darker and lining up constantly is harder. Moving side to side & back n forth to find a good picture. Try the torch shinning in front objective and a piece of paper at the exit. Move it back n forth to find the clear ring of light (eye relief & exit size diameter). Now crank up the magnification and find the clear ring of light again. It will be smaller & tighter to find, along with shorter relief. All governed by quality, design & glass size. A great topic with heaps of twists and turns! |
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@ 07:35 am (GMT) |
Scott StruifRe: Objective diameterThe math in the article was over my head. I agree that the bigger the objective, the bigger the eye box . . . which means more room for off-center eye alignment. However, I don’t get how the bigger exit pupil could, in and of itself, be more forgiving, parallax-wise. Wouldn’t you need a parallax adjustable scope for that? I don’t understand exactly how they work, but I assume they help.Here’s what Nathan had to say in The Practical Guide To Long Range Shooting, page 88: “Besides shoulder position and forend control, parallax is a major issue among those who come to me with difficulties shooting accurately. Your rifle scope may well have an adjustable parallax, but this cannot compensate for poor eye alignment. In essence, if your eye is not properly aligned with the scope and you inadvertently look through the scope at a slight angle (varying from shot to shot), the point of impact (POI) will differ greatly for each shot. Errors can be quite profound and this error often shows as a double group, if the shooter is jostling technique and is not aware of eye alignment issues.” |
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@ 09:53 am (GMT) |
Scott StruifRe: Objective diameterThis is a good explanation of how parallax adjustment works. But he’s not a hunter. He mentions the method of purposefully mounting your scope to get a shadow ring around the image. Nathan also mentions the technique in his book. Seems like a reasonable approach for hunters, unless you have the luxury of adjusting your parallax dial on the fly.https://youtu.be/NRfWpuFTeqI |
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@ 11:24 pm (GMT) |
Martin TaylorRe: Objective diameterParallax within the scope and poor eye alignment are different things, same result though.Set the para adjustment so the site picture is as stable as possible on the cross hairs. Now good alignment with picture & outer edge of scope (even on all sides) is still needed because you can add more era between your eye and scope. More light just makes it easier for our eyes to work. |
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@ 02:31 pm (GMT) |
Scott StruifRe: Objective diameterThanks for all the input. My understanding of the “science” is that if the parallax of the scope is adjusted perfectly, and the lenses are perfect, then your eye could be slightly off center. But that’s all theoretical BS. It has to do with the lenses bending light. If you’re hunting, you set the parallax as close as possible, if you have time. It’s no substitute for proper eye alignment, which, if done right, makes parallax a non-issue.That S-TAC 2-10x32 I mentioned has been discontinued. Only Leupold makes a 9X scope with an objective <40mm . . . a 3-9x33, which they market for rim fire enthusiasts. The rest are all 40mm and above. The reason can’t be that 33mm is too small for 9X. It has to be the perception that anything smaller than 40mm is no good for low light hunting. |
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@ 08:29 am (GMT) |
Scott StruifRe: Objective diameterWhat if you mount a scope on an untrue barreled action that requires the windage of the scope to be adjusted to at or near its limit to zero it? The erector lenses will never be geometrically aligned with the axis of the objective and ocular lenses. |
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@ 06:37 pm (GMT) |
VinceRe: Objective diameterHi Scott,regarding my previous post I am using eye alignment and parallax interchangeably which isn't correct although as Martin said the result is effectively the same. In terms of the untrued barrel that isn't aligned with the scope wouldn't this cause a situation where you would have a distance where the scope was zeroed (horizontally instead of vertically) and outside of that you would have to dial horizontally based on distance to correct the zero? Cheers Vince |
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@ 05:26 am (GMT) |
Scott StruifRe: Objective diameterHi Vince. After I googled “rifle scope parallax” the other day, YouTube started suggesting videos to me. One was a Vortex podcast about budget rifles, <$1000. Scopes wasn’t the topic. However, they have an arsenal of cheap rifles they use for testing. They said they’ve noticed that rifles that use a barrel nut to set headspace often require a considerable amount of windage adjustment, one way or the other, to zero. From this they concluded that barrel nuts cant barrels out of alignment with actions. They said many of these rifles shoot fine, once zeroed. However, if the erector tube is near the end of its travel laterally, it limits the amount of possible elevation adjustment. They didn’t mention parallax.The engineer designing the scope assumes all the lenses are perfectly aligned. If they aren’t, there’s going to be some aberrations in the way the light is bent. I’m sure they know this. They have to account for some margin of error. My question is, What effect does this have on parallax? Another video YouTube suggested was by a guy whose company is contracted by the military to do training. He said a big issue they have to deal with is parallax. (This is consistent with what Nathan said in his book.) He said young eyes have the ability to focus what is actually out-of-focus. He said they used to teach that the sight-picture should be clear edge-to-edge. Now they encourage trainees to use the shadow-ring around the sight-picture for eye alignment. He said the problem is, you can teach a perfect, consistent cheek-weld from the prone position, but it’s going to change in other positions. |
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