Adjust objective lense on a side focus scope to reduce minimum distance.

[Burga]

Hi Guys,

I have an Sightron s3 6-24x50 FFP and am pretty happy with the scope overall, one of my few gripes are that it's minimum focus distance is I think 40m.

Ideally I would like to be able to use it as close as 25m parallax free

On a couple of scopes previously I have adjusted the objective lense to reduce parallax for close range pest control and that's great.

Would anyone know what if any adverse effects I would encounter if I did the same to my side focus scope?
Would it still track true 0.1mil per click?
Would it throw the reticle out if calibration?

Anyone out there with a good understanding of the internal workings of scopes and calibration?

Thanks in advance,

Regards

Burga

 

[Stephen]

Kind of confused here. Adjust the objective lense when there is a side parallax adjustment? I must be missing something. I thought a scope either had an adjustable objective or side focus (parallax) adjustment "knob".

 

[dcali]

I'm not sure I follow either. The minimum focus isn't something you can adjust - it's just the result of the mechanics of the optics as a whole.

 

[Ned Ludd]

As Stephen noted, there are scopes designed to have the parallax (or focus, as it is perceived) adjustment in the front at the objective lens (Adjustable Objective - AO), or on the side (side parallax knob). From a usage and practicality viewpoint, they are quite different, although the end effect is the same. High power scopes also typically have a minimum parallax setting. That minimum setting could be anywhere from 9 or 10 yards to over 50 yards, depending on the scope brand/model.

Your Sightron S3 6-24x50 FFP has a minimum parallax adjustment of 40 yd. That is a property that is built into the scope; you cannot change it. If you wish to use a scope at much closer ranges, there are ways to do it using additional lenses over the objective, but that is not usually very practical or desirable. The other (better) choice is to buy a different scope with something like a 25 yd, or 10 yd minimum parallax setting. For example, the Sightron S-TAC4 4-20x50 scope has a 10 yd to infinity parallax setting.

Your Sightron scope also has a feature known as a "Fast Focus Eyebell" in addition to the side parallax knob. Sightron does not provide much information on exactly what this feature is, but it looks to be a simple limited front focus adjustment. I do not believe it will have any effect on the minimum parallax setting.

I have encountered the same issue it sounds like you're having with a couple of Nightforce scopes I have in QD mounts. There is a local indoor range that goes out to 50 yd, which is supposed to be the minimum parallax setting of these scopes. However, the true value is probably closer to 75 or 80 yards. You cannot even come close to focusing them at the indoor 50 yd range. It is maddening when you first encounter it, but as I mentioned, the best way around it is to get a different scope with something like a 10 yd minimum parallax setting.

 

[dolek]

That is an interesting question .I have no clue what ratios come into play with parallax and magnification . Following with interest.

 

[Burga]

Thanks for your input guys.

I emailed sightron directly and the response was "The design only allows for a 40 yard setting. Changing the objective will affect the long range focus quite a bit and is not recommended."

That doesn't really tell me how it will be effected but I guess it's probably more a question for some sort of optical engineer.

Ned I believe the fast focus eyebell is just the diopter adjustment to focus on the reticle. Its not lockable like the nightforce nxs.
That would be frustrating to not even be able to get to the stated minimum parallax. I have heard that adjusting the diopter effects the parallax also which is one reason why sightron don't have distance markings on the side focus knob.

I think ultimately I will need another scope.
But hey I am always keen to learn more about optics, how they work and how the top models achieve all the desirable specs ie. Extreme mag range and high field of view.

Cheers,

Burga

 

[Justin Credible]

I've adjusted the objective on many scopes that did not have an adjustable objective to set the focus at anywhere from 50' to ~300 yards. But I have never done this on a scope with an FFP reticle or a side focus. And they've always been inexpensive scopes that I wouldn't cry about if I broke. So keep those things in mind! I am certainly making no recommendations here, it's your scope so you take all the risk.

Adjusting the objective should not affect the click value, reticle, or the magnification of the scope. Moving the objective simply moves the location of the focal plane inside the scope, just like an AO scope does, it has no effect on the focal length which is what would affect magnification. So I see no reason why you couldn't tweak the objective position to "recalibrate" your side focus to the range you want. It actually sounds like a great idea! As the manufacturer said though, you may not be able to dial it out to as long of a range. I'm really not sure what the limit would be though. I'd be willing to wager that when they install the objective they set its position by using a collimator that produces a target that appears to be at a certain distance and set the side focus to that value. Then they turn in the objective until the target is in focus and lock it down. I've seen fixed focus scopes done this way (except without setting a side focus of course!). That is how I have tweaked my scopes except I don't have a collimator so I use a target set at the distance I want to set the focus.

That all assumes that the entire objective moves as a single unit. I have no idea how that scope is assembled so I'm just assuming that. If that is not the case, say there is a doublet up front that moves with the adjustment but a single lens behind it that is mounted separately, then there will be a small focal length change which would affect magnification/click values/reticle calibration. This might be so small as to not be a big deal though, I just don't know.

One thing you will likely see by adjusting the objective, which you may be aware of since it sounds like you've done this before, is that the zero will shift. This is due to the lens having some wedge in its construction and/or mounting so the image gets shifted around as it rotates. That's why an AO scope is designed so the objective moves in and out without the lens actually rotating as you rotate the collar. So I wouldn't plan on moving things back and forth. There may also be an issue with the gas seal that prevents fogging, but I don't know that for certain.

I hope that all made sense!

Justin

 

[ronemus]

You'll probably have better luck finding an accessory lens that is put in front of the objective, preferably screwed into the sunshade threads. They're rare for rifle scopes (Leupold sells/sold them for some scopes), but are common in the photography world (billed as macro lenses). If the lens isn't perfectly centered and perpendicular to the scope axis there will be a POI shift, so you'll want something that mounts rigidly and reproducibly. Since optics are never perfect, you'll probably want to leave it permanently mounted though witness marks on the scope and a screw-in lens so you can return it to the same position would probably be OK.

 

[Burga]

That's some good thoughts Justin and Ronemus,

I had no idea there was such a thing as a collimator for that purpose.
I am tempted to have a go just to satisfy my curiosity. I feel like there is room to play because going from minimum parallax to infinity seems to be only 90ish degrees turn of the knob, and yet there is still another 90ish degrees left on the knob.
although I have noticed to change focus at closer ranges takes more travel than further distances so I'm sceptical of wether I could get down as low as 20yards.

This might seem a bit backwards but what I'm thinking I might do is turn the knob out to the infinity end of its adjustment and then adjust the objective lens to the point where the hills I can see 2000m away start to be affected and then check how close I can focus,
and finally see if the diopter adjustment changes it very obviously or not.
Ofcoarse I'll record how much I have moved things by, so I can put it back if it's a huge fail.

At the end of all of this 6x power may be too high power for what I want to do but gotta try eh.

Cheers,

Burga

 

[jamesdmock]

Thanks for your input guys.

I emailed sightron directly and the response was "The design only allows for a 40 yard setting. Changing the objective will affect the long range focus quite a bit and is not recommended."

That doesn't really tell me how it will be effected but I guess it's probably more a question for some sort of optical engineer.

Ned I believe the fast focus eyebell is just the diopter adjustment to focus on the reticle. Its not lockable like the nightforce nxs.
That would be frustrating to not even be able to get to the stated minimum parallax. I have heard that adjusting the diopter effects the parallax also which is one reason why sightron don't have distance markings on the side focus knob.

I think ultimately I will need another scope.
But hey I am always keen to learn more about optics, how they work and how the top models achieve all the desirable specs ie. Extreme mag range and high field of view.

Cheers,

Burga

There were several .22 RF competitors in our area who did just what you are proposing. They carefully removed the lock ring on the objective and used a small suction cup to screw the objective to obtain a close focus. It probably affects the long range focus, but these were dedicated RF shooters.

 

[Burga]

That's good to know Jamesdmock

There is hope.

Cheers