Focus change with brightness?

[Marine6]

I think I’m missing an obvious solution but can’t put my finger on it.
Using a March HM 10-60x56 scope (an couple of years old, not the newer one with the thermal compensation) for field target which is an air rifle discipline that uses the parallax of a scope to range targets from 10-55 yds.
During overcast days the POI is where I ranged it since I set up the scope during overcast weather, so when the scope ranges 50 yds I set the elevation to hit there and it does.
When the sun comes out the scope ranges targets way differently...on the farther targets, say 50 yds, it ranges more like 40 yds so if I set the elevation to correspond with the parallax range the POI is low. I can compensate for this on my home course as I know the distances of my targets but it’s more difficult at a match.
This is irrespective of temperature...the ranged distance doesn’t seem to change with temp, just with brightness although I know thermal expansion affects the scope as well (which is why I have a new HM on order), not sure if there’s a way to compensate for it since the change in POI differs somewhat with intensity of light and I’d like to be able to use hard data not just guess and use Kentucky windage, so to speak.
Any help appreciated.

 

[ma smith]

squinting might link brightness with focus shift, but that seems like a stetch....

 

[brianf]

Only person I trust with this question @koshkin

 

[Marine6]

Paging @koshkin!

 

[Denys]

I think I’m missing an obvious solution but can’t put my finger on it.
Using a March HM 10-60x56 scope (an couple of years old, not the newer one with the thermal compensation) for field target which is an air rifle discipline that uses the parallax of a scope to range targets from 10-55 yds.
During overcast days the POI is where I ranged it since I set up the scope during overcast weather, so when the scope ranges 50 yds I set the elevation to hit there and it does.
When the sun comes out the scope ranges targets way differently...on the farther targets, say 50 yds, it ranges more like 40 yds so if I set the elevation to correspond with the parallax range the POI is low. I can compensate for this on my home course as I know the distances of my targets but it’s more difficult at a match.
This is irrespective of temperature...the ranged distance doesn’t seem to change with temp, just with brightness although I know thermal expansion affects the scope as well (which is why I have a new HM on order), not sure if there’s a way to compensate for it since the change in POI differs somewhat with intensity of light and I’d like to be able to use hard data not just guess and use Kentucky windage, so to speak.
Any help appreciated.

I have the same scope on my F-TR rifle albeit the model with the temperature anti drift lenses. I do not believe that your issue has anything to do with the lenses you have now. The way it was explained to me by Deon is that the anti-drift lenses were addressing a drastic change in temperature over a short period of time, due to the higher fluorite content of the Super ED lenses used.

What you are describing is a rather benign change in ambient temperature. Now, it could well be that the newer model will address your issue.

That said, I know that the March-X series, like the 8-80X56 and the 10-60X56 are very popular in FT comps, especially outfitted with the large focus wheel. I have the same wheel on my March-X scopes but not for ranging like you do, it's for my arthritis and the fact I like to jump quickly from 1000 to 600, check mirage, and pop back to 1000 to take the shot.

I'm at a loss to understand why brightness would make a difference in focus.

 

[larry meyer]

Bright light can effect the depth of field (focus close range) of your eye (s). Will this effect what you see w/parallax adj? I don't know, can you try an other scope w/same conditions? good luck!

 

[Denys]

Bright light can effect the depth of field (focus close range) of your eye (s). Will this effect what you see w/parallax adj? I don't know, can you try an other scope w/same conditions? good luck!

The image with its depth of field, is formed at the first focal plane in the riflescope. The eye will not change the DOF of the image coming from the riflescope anymore that it would do that for a photograph.

 

[Marine6]

Thinking about what Larry said...everyone who uses reading glasses (which I do) knows it is a lot easier to read in bright light as the pupil is constricted so maybe that has something to do with it...my depth of field is greater as the brightness increases?
Will try focusing with reading glasses on when it’s cloudy and compare that to the focus without reading glasses when it’s bright.
If there aren’t already enough things to factor in??

 

[koshkin]

Only person I trust with this question @koshkin

Let's hope your confidence is not entirely misplaced....

I'd be surprised if it has anything to do with atmospheric effect.

There are a couple of things here that could be happening. One is the obvious: if the scope is in the sun, it can warm up very quickly and change temperature when the sun is out. However, since the OP says that this scope is not terribly susceptible to temperature variations, this is probably not it. Also, unless I am misinterpreting things, the focus shift would be in the opposite direction.

What is more likely is that this is caused by the change in the eye pupil size. In bright light your eye pupil shrinks and has greater depth of field. In low light, your eye pupil dilates and has shallower depth of field. For a lot of people it also introduces a slight change in focus. That means that in low light and in bright light, your eyepiece settings are not the same, which, in turn, will make your parallax settings to be different.

It is an interesting topic and something I should probably dig into at some point. I generally set up the eyepieces on my scopes in low-ish light, since the setting optimal with a dilated pupil is also perfectly reasonable for me when the pupil is smaller. The reverse is not necessarily true.

Since I do not shoot F/T (thinking about it though), the OP is likely looking at finer variations than I usually do.

ILya

 

[Denys]

ILya, you have to help me out here. The OP says that his riflescope "(w)hen the sun comes out the scope ranges targets way differently...on the farther targets, say 50 yds, it ranges more like 40 yds." I took that to mean that he is adjusting the side-focus to the clearest picture (just as one would focus a camera lens for maximum crispness) and he reads the distance of the dial or whatever mechanism he's using to measure that. The 10-60X56 does not have distance on the side focus, but I can see that using the large wheel, it would be a snap to set up the various distances, especially since the granularity is still rather large at short distances. I do that trick on my 10-60X56 with its large wheel, but I just have a small tie-warp at the 600 yard distance to look at the mirage there and then bounce back to 1000 to take the shot.

So, if my understanding is correct, the OP is saying that when it's brighter, the adjustment for maximum crispness of the target at 50 yards actually shows up as 40 yards on the wheel. The image focus is all done at the objective lens by the time the image gets to the FFP. I simply do not understand how a shrinking pupil would change the focus of the image at the FFP. I look forward to enlightenment.

I would also add that introducing reading glasses into the mix is a quick way to reach the great state of Confusion.

 

[koshkin]

ILya, you have to help me out here. The OP says that his riflescope "(w)hen the sun comes out the scope ranges targets way differently...on the farther targets, say 50 yds, it ranges more like 40 yds." I took that to mean that he is adjusting the side-focus to the clearest picture (just as one would focus a camera lens for maximum crispness) and he reads the distance of the dial or whatever mechanism he's using to measure that. The 10-60X56 does not have distance on the side focus, but I can see that using the large wheel, it would be a snap to set up the various distances, especially since the granularity is still rather large at short distances. I do that trick on my 10-60X56 with its large wheel, but I just have a small tie-warp at the 600 yard distance to look at the mirage there and then bounce back to 1000 to take the shot.

So, if my understanding is correct, the OP is saying that when it's brighter, the adjustment for maximum crispness of the target at 50 yards actually shows up as 40 yards on the wheel. The image focus is all done at the objective lens by the time the image gets to the FFP. I simply do not understand how a shrinking pupil would change the focus of the image at the FFP. I look forward to enlightenment.

I would also add that introducing reading glasses into the mix is a quick way to reach the great state of Confusion.

Imagine a situation where you have set up your side focus for a particular distance. Let's say 50 yards. Image is sharp and there is no parallax.

Then, you go and adjust the eyepiece by a little bit. Is the image still parallax free at 50 yards?

ILya

 

[brianf]

Lol told you just ask koshkin...

 

[Denys]

Imagine a situation where you have set up your side focus for a particular distance. Let's say 50 yards. Image is sharp and there is no parallax.

Then, you go and adjust the eyepiece by a little bit. Is the image still parallax free at 50 yards?

ILya

In a decent scope, it should be. The image is formed at the first focal plane or in this case, the second focal plane. The eyepiece transmits the SFP image to the eye. I fail to see how adjusting the eyepiece would change the focus adjustment of the SFP. Perhaps I'm missing something which is why I remain an optics student.

 

[koshkin]

In a decent scope, it should be. The image is formed at the first focal plane or in this case, the second focal plane. The eyepiece transmits the SFP image to the eye. I fail to see how adjusting the eyepiece would change the focus adjustment of the SFP. Perhaps I'm missing something which is why I remain an optics student.

It is not. In any scope, decent or otherwise, unless your eye has accomodated to the eyepiece change.

If it was, you would not need an adjustable eyepiece at all.

ILya

 

[Marine6]

I calibrate the parallax side wheel (6” in diameter) by setting out targets at 2 yard intervals, focusing the scope, and marking the side wheel.
This particular scope was calibrated in overcast weather during the winter.
I assume if I had calibrated it during sunny bright weather the effect would have been the opposite.
As far as I can tell the image remains parallax free since I have adjusted the eyepiece that way and haven’t changed it.
Seems that Occam’s Razor would indicate it’s my eyes that are the issue, not the scope. By narrowing it down to that I don’t have to worry about adjusting the scope, just need to shoot enough in bright conditions to know how many elevation clicks to add to the indicated yardage when the sun comes out. A sliding scale with more or less clicks depending on distance to the target.
Or something like that.

 

[koshkin]

I calibrate the parallax side wheel (6” in diameter) by setting out targets at 2 yard intervals, focusing the scope, and marking the side wheel.
This particular scope was calibrated in overcast weather during the winter.
I assume if I had calibrated it during sunny bright weather the effect would have been the opposite.
As far as I can tell the image remains parallax free since I have adjusted the eyepiece that way and haven’t changed it.
Seems that Occam’s Razor would indicate it’s my eyes that are the issue, not the scope. By narrowing it down to that I don’t have to worry about adjusting the scope, just need to shoot enough in bright conditions to know how many elevation clicks to add to the indicated yardage when the sun comes out. A sliding scale with more or less clicks depending on distance to the target.
Or something like that.

Or figure out how much adjustment of the ocular you need to compensate for the change in the conditions and mark that.

ILya

 

[Denys]

Thanks for taking the time to explain that, ILya. I have been navigating under the misapprehension that once the diopter was set for your eye, it should locked down and left alone unless and until your vision changes. I always considered the diopter setting on a scope to be the equivalent function as prescription glasses. Both handle the afocal image coming to your eye and cause it to be focused a little in front or behind the retina to accommodate an imperfect eyeball.

Now you are correcting me by explaining that you need to adjust your diopter setting (glasses or eyepiece ) depending on how bright it is. It will take me some time to comprehend that one because I'm old and slow. They say you can't teach an old dog new tricks, let’s hope they are wrong. We shall see.

I sure hope I'm understanding you correctly here. If I'm not please correct me.

 

[Marine6]

Or figure out how much adjustment of the ocular you need to compensate for the change in the conditions and mark that.

ILya

I’m going to have to experiment with that...if it works it seems like it would make things easier. Will give it a try tomorrow...thanks!

 

[Marine6]

I’m going to have to experiment with that...if it works it seems like it would make things easier. Will give it a try tomorrow...thanks!

So...adjusting the ocular did not work. Once the sun came out I focused the scope at a known distance and as usual the range shown on the focus wheel was several yards short of the actual distance. I put the focus wheel at the known distance and of course it was blurry...I then adjusted the ocular in both directions and could not attain focus.
So I put it back to where it was before.
BLUF is that this thread has helped me out a lot...I now realize it’s my eyes, not the scope, so I don’t have to go tilting at windmills trying to get the scope to do something that it can’t do. It’s pretty easy to add a few clicks to the elevation and as a result I’m hitting where I’m aiming which was the whole objective.
Appreciate the help.

 

[Denys]

I didn't think it would apply. So I went looking back in my notes of my time in Service Rifle competition because something was tickling my memory about bright Vs cloudy days. I shot my last SR match about 16 years, so that's my excuse for not remembering right away.

In my SR days, it was axiomatic that on bright days the target actually appeared smaller, and on dark days it was the reverse. I have no clue why, but the maxim was "bright skies, aim high." My problem with your specifications is that I have zero experience in your discipline. I don't even know what "rules" transfer to it. Since 2006, I've been an 1000 yard F-class guy, almost exclusively. What I had learned from my SR days helped me with wind calling but high magnification scopes are not the same thing as NM sights on an AR-15. The sight picture and how you use it is so completely different.

So, I think that you are totally correct and that it is your vision that is deceiving you and in bright days, you are aiming lower than you do on overcast days, It has nothing to do with the scope. In F-class, I act as scorer as much as I shoot and I always try to learn from the shooter for whom I am scoring. I have noticed over the years, that sudden light changes during a string will cause the shooter to have elevation issues. Over a 1000 yard range, you can be in overcast conditions at the firing line and have the target line if bright sunlight. Or have changes from shot to shot even. It's insidious when you are talking about very high precision. I am sure its similar in your discipline.

At the rate primers are being converted to use unobtanium, I may be forced into converting to pellet rifle competition. If that occurs, I shall keep this episode in mind.

 

[Marine6]

I didn't think it would apply. So I went looking back in my notes of my time in Service Rifle competition because something was tickling my memory about bright Vs cloudy days. I shot my last SR match about 16 years, so that's my excuse for not remembering right away.

In my SR days, it was axiomatic that on bright days the target actually appeared smaller, and on dark days it was the reverse. I have no clue why, but the maxim was "bright skies, aim high." My problem with your specifications is that I have zero experience in your discipline. I don't even know what "rules" transfer to it. Since 2006, I've been an 1000 yard F-class guy, almost exclusively. What I had learned from my SR days helped me with wind calling but high magnification scopes are not the same thing as NM sights on an AR-15. The sight picture and how you use it is so completely different.

So, I think that you are totally correct and that it is your vision that is deceiving you and in bright days, you are aiming lower than you do on overcast days, It has nothing to do with the scope. In F-class, I act as scorer as much as I shoot and I always try to learn from the shooter for whom I am scoring. I have noticed over the years, that sudden light changes during a string will cause the shooter to have elevation issues. Over a 1000 yard range, you can be in overcast conditions at the firing line and have the target line if bright sunlight. Or have changes from shot to shot even. It's insidious when you are talking about very high precision. I am sure its similar in your discipline.

At the rate primers are being converted to use unobtanium, I may be forced into converting to pellet rifle competition. If that occurs, I shall keep this episode in mind.

I didn't think it would apply. So I went looking back in my notes of my time in Service Rifle competition because something was tickling my memory about bright Vs cloudy days. I shot my last SR match about 16 years, so that's my excuse for not remembering right away.

In my SR days, it was axiomatic that on bright days the target actually appeared smaller, and on dark days it was the reverse. I have no clue why, but the maxim was "bright skies, aim high." My problem with your specifications is that I have zero experience in your discipline. I don't even know what "rules" transfer to it. Since 2006, I've been an 1000 yard F-class guy, almost exclusively. What I had learned from my SR days helped me with wind calling but high magnification scopes are not the same thing as NM sights on an AR-15. The sight picture and how you use it is so completely different.

So, I think that you are totally correct and that it is your vision that is deceiving you and in bright days, you are aiming lower than you do on overcast days, It has nothing to do with the scope. In F-class, I act as scorer as much as I shoot and I always try to learn from the shooter for whom I am scoring. I have noticed over the years, that sudden light changes during a string will cause the shooter to have elevation issues. Over a 1000 yard range, you can be in overcast conditions at the firing line and have the target line if bright sunlight. Or have changes from shot to shot even. It's insidious when you are talking about very high precision. I am sure its similar in your discipline.

At the rate primers are being converted to use unobtanium, I may be forced into converting to pellet rifle competition. If that occurs, I shall keep this episode in mind.

I was a bit skeptical about the ocular making a dif but I don’t have the scope expertise of a lot of y’all so I figured why not give it a try.
I never used scoped rifles in the Marines, just an M16 with iron sights.
Re pellet rifles, I just bought 6,400 pellets for $200 so the cost-benefit is pretty good.
The really fun thing about pellet guns is the ability to go out into your yard and shoot. I live on several acres on the NC coast and have numerous FT lanes set up.
It’s not the same as long distance firearm shooting, but is a great sport nonetheless.
Here’s a link to the latest AAFTA manual which explains a lot about the sport if you’re interested.

http://www.aafta.org/uploads/1/2/7/6/127666620/aafta_handbook.pdf