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What is the resolution of my spotting scope?

Resolution is a measurement of a spotting scope's ability to distinguish fine detail and produce a sharp image. Better resolution also delivers more intense color and often higher contrast.

Resolution varies in relation to the size of the scope's objective lenses. Generally, a larger objective lens will deliver more detail to the eye than a smaller objective lens, regardless of the magnification. Actual resolution is determined by the quality of the optical components, the type and quality of the optical coatings, atmospheric conditions, collimation (proper optical alignment), and the visual acuity of the user.
What does near focus mean for spotting scopes?

Near focus is simply the distance between the scope and the nearest object you can focus on. In general, you can focus closer with lower magnification and smaller aperture spotting scopes.

I want to use my spotting scope to take pictures – what’s its f-number?

The f-number or focal ratio is just the focal length of the spotting scope divided by the aperture (diameter of the lens). It’s almost universally given as a quantity like f/5, f/2.8, etc., called the f-stop or photographic speed. These terms are used interchangeably.

Example: an 80mm spotting scope with a 480mm focal length has a f-number of 6 and its photographic speed is f/6.

Lower numbers mean the lens is delivering light more quickly and consequently shorter exposures can be used when taking a picture through the scope.
What is the eye relief for spotting scopes?

Eye relief is the distance (in millimeters) between your eye and the spotting scope eyepiece that allows the full field of view to be comfortably observed. It measures the spacing from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. Your eye should be positioned here to see the full field of view of the eyepiece.

Eye relief should be at least 10mm, 15mm for the best comfort, maybe more if you wear eyeglasses.

Put your eye so it's just behind the eyepiece to take advantage of its eye relief. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Getting too close will prevent you from blinking and may also cause a black ring to appear around the field of view.
What is the eye relief for spotting scopes?

Eye relief is the distance (in millimeters) between your eye and the spotting scope eyepiece that allows the full field of view to be comfortably observed. It measures the spacing from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. Your eye should be positioned here to see the full field of view of the eyepiece.

Eye relief should be at least 10mm, 15mm for the best comfort, maybe more if you wear eyeglasses.

Put your eye so it's just behind the eyepiece to take advantage of its eye relief. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Getting too close will prevent you from blinking and may also cause a black ring to appear around the field of view.
Spotting Scopes
Will my spotting scope let me see bullet holes in targets on a shooting range?

Whether or not your spotter will be able to clearly see or resolve a bullet hole in a distant target depends on the size of the bullet, the size of hole it makes and the range to the target. These numbers are used to calculate the angular size of the bullet hole. Then you can compare the angular size to the resolving power of your scope to see if it’s adequate.

First, we’ll assume the bullet hole is the same size as the bullet. The holes are often larger, so this is a conservative assumption.

If necessary, convert hole size and range to inches.

To calculate the angular size of the bullet hole, divide the size of the bullet hole by the range. Multiply the result by 206,265. The number you get will be the angular size in arc-seconds of the hole as seen from the shooting line.

Compare this number to the following chart to find out which scope is best for your target spotting.

.

Scope diameter
 (mm)
cLEAR Resolution
(ARC-SEC)
50 3.7
60 3.1
65 2.8
70 2.6
80 2.3
90 2.1
100 1.9
130 1.4





Example: a 50-caliber bullet hole in a target at 1000 yards. What size scope is the smallest I can use to spot this hole?

1000 yards equals 1000x3x12 or 36,000 inches. 0.50 divided by 36000 is 0.0000138.

0.0000138 x 206265 = 2.8 arc-seconds

A 65mm scope will clearly resolve 2.8 arc-seconds. So you would need at least a 65mm objective size spotting scope to see your shot.

Other factors affecting your ability to see the bullet hole are air quality, steadiness of the air and lighting conditions.
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