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Old July 3, 2020, 04:43 PM   #1
Bart B.
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Scope Image Quality

'Tis interesting to me that scope sights don't have an image resolution quality standard like camera lenses do. They focus target image in the reticle focal plane much like SLR cameras do in their 2 focal planes. Such a standard might want to use the second/rear focal plane because that's where the eyepiece lens focuses on. Like camera lenses, scope resolution at different powers will vary.

Cameras use lines per millimeter in their image plane as a standard. Their test subject is placed a fixed distance from the camera for each focal length. Scopes could use similar test targets at 100 yards. I'm working on how to use one with scopes.

Suggestions welcomed.
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Old July 3, 2020, 05:48 PM   #2
LeverGunFan
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John Barsness has performed optical testing using a test target of alternating black and white lines, starting at 1 inch wide and decreasing to 1/16 inch wide. He places the test target at 25 yards and illuminates it with a 100 watt light bulb. You can read about it in his book "Modern Hunting Optics" available at this link.

The USAF 1951 test target is a common test standard. Amateur astronomers at the Texas Star Party have adapted it to test telescopes, as described here.

The selection of the range to the test target is an interesting problem - at what range will atmospheric effects overly influence the test results?
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Old July 3, 2020, 07:13 PM   #3
Bart B.
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I think the 100 yard test target is a good compromise for use in good weather conditions. Its alternating line width should start at 1/100 inch so resolution could be expressed as lines per USSF MOA definition. Trigonometry perfectionists could place the test target at appropriate shorter range using this conversion base for corrected line spacing:

1.04719753642832854694747069666400334739860873986429
830552235157457471965151538005004775737357536725837... inches per hundred yards.

I think test targets might need to be scope magnification specific. One for each 5X increments from 5 to 35 power. This gets complicated the more I look into the details.

Regarding what range will atmospheric effects overly influence the test results. The more atmosphere there is between scope and target the more resolution will be degraded. The best time of day to test is at the break of dawn before sunrise and no wind. I can see 30 caliber bullet holes in the black bullseye at 1000 yards with cheap Weaver 20X scopes then. If there's any mirage (heat waves) visible in the scope, don't test.

Last edited by Bart B.; July 4, 2020 at 07:43 AM.
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Old July 4, 2020, 08:42 AM   #4
reynolds357
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Quote:
Originally Posted by Bart B. View Post
'Tis interesting to me that scope sights don't have an image resolution quality standard like camera lenses do. They focus target image in the reticle focal plane much like SLR cameras do in their 2 focal planes. Such a standard might want to use the second/rear focal plane because that's where the eyepiece lens focuses on. Like camera lenses, scope resolution at different powers will vary.

Cameras use lines per millimeter in their image plane as a standard. Their test subject is placed a fixed distance from the camera for each focal length. Scopes could use similar test targets at 100 yards. I'm working on how to use one with scopes.

Suggestions welcomed.
If you did that, it would prove a Bushnell Banner is not "as good" as a S&B.
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Old July 4, 2020, 11:06 AM   #5
Bart B.
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In my opinion, image quality and visual resolution is a distant second on the important things in achieving best accuracy using a scope.

Most important is the line of sight direction and angle from the scope axis. Any non-repeatable positions of adjustable parts will change the LOS direction and angle. That ends up moving the line of fire to different places about the desired point of impact.

Good metallic sights are 99.7% repeatable. Easily measured with dial indicators

You need an opto-mechanical collimator that can be shocked in the bore axis to see how repeatable its mounted scope is mechanically repeatable. I built one.

Last edited by Bart B.; July 4, 2020 at 11:13 AM.
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Old July 4, 2020, 02:45 PM   #6
reynolds357
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Quote:
Originally Posted by Bart B. View Post
In my opinion, image quality and visual resolution is a distant second on the important things in achieving best accuracy using a scope.

Most important is the line of sight direction and angle from the scope axis. Any non-repeatable positions of adjustable parts will change the LOS direction and angle. That ends up moving the line of fire to different places about the desired point of impact.

Good metallic sights are 99.7% repeatable. Easily measured with dial indicators

You need an opto-mechanical collimator that can be shocked in the bore axis to see how repeatable its mounted scope is mechanically repeatable. I built one.
I have never gotten that precise. I just check them with a regular collimator.
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Old July 4, 2020, 05:30 PM   #7
LeverGunFan
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A few other ideas.... What about a control scope to check atmospheric conditions? It would be used every test session to see if conditions had changed from one session to another that would affect results. Probably would not have to be a rifle scope, could be a good spotting scope. Record the resolution it achieves each session and see how it varies from day to day.

In industry it is common to test 30 samples to get valid statistics on performance variation. Not possible for scope tests, but if you get a scope that performs much worse or much better than it's peers, test another sample to verify results.

Camera lenses are part of an image forming system, so the resulting image (film or digital) can be analyzed to determine the lens resolution. To eliminate the influence of visual acuity and eye defects, can a high resolution image be taken through the scope? Then the image can be looked at under magnification to determine resolution.
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Old July 5, 2020, 03:40 PM   #8
Bart B.
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You can use a monocular, binocular half or another scope to magnify the rifle scope image.

You'll have to adjust the scope eyepiece to make the reticle and target images appear at some distant range.

Quote:
In industry it is common to test 30 samples to get valid statistics on performance variation.
Remember this every time you base accuracy on one 3 shot group.

Last edited by Bart B.; July 5, 2020 at 04:42 PM.
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