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FFP reticle architecture accuracy/tolerance


Bukshot

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Throwing this out to the UKV community for discussion:

Recently got a S&B 1-8x24 Dual CC and the floating dot in the reticle appeared slightly off centre. Initially thought maybe it's parallax and head/eye position but not convinced so raised it with S&B and waiting for them to come back to me. 

In the meantime decided to do a very non scientific exercise and compare the reticle etching tolerance in the 1-8x24 to an almost identical reticle in my S&B 3-20x50 US. Took a photo of the reticle in both scopes as accurately as I could and in AutoCAD superimposed a 0.5 mil grid over the top to check the accuracy/tolerance of the reticle markings. The width of the 'thick' open bars is 1 mil so that was my benchmark for aligning the photo with the grid. See pictures below.

There's obviously an etching tolerance that reticles are produced to, plus allowing for some distortion perhaps as I took the photos on my iphone. However the reticle in the 3-20 looks a lot more precise and true. The one in the 1-8 looks far less so. 

Both scopes are FFP and in both reticles:

Distance of central opening between the ends of the horizontal 'thick' bars should be 10 mils

Distance between crosses in the horizontal 'thick' bar should be 5 mils. 

Width of 'thick' bars should be 1 mil

Distance between mil dots & hashes on the cross hairs should be 0.5 mils and 1 mil. 

Opinions please.

 

1-8 superimposed.jpg

3-20 superimposed.jpg

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An easy way to check in a shop etc if the reticle is centred - if it is ffp and the reticle side bars extend to the edge of the tube) is to see if the side bars or whatever other suubtensions there are on the horizontal of the reticle appear / disappear at the same time if you zoom in and out, or if a particular marking is the same distance in from the tube on either side of the reticle.  I’ve noticed slight differences many times. Tried to keep my head centred behind the scope.

There are differences in how well reticle are centred. In ffp scopes I don’t think it makes any practical performance  difference but annoying when you have paid 3k for a scope.

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What lens did you use on your camera?  It looks like there's optical distortion in the image (for example a fisheye lens is an extreme example).  I would be very surprised if your camera rendered an exactly proportioned image.

Surely the test would be to create a grid and set it at a known distance down range, just look through the scope and line up the subtensions.

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Just now, Bukshot said:

Took the picture with my iphone in clamp stand. Appreciate there will be distortion to an extent. 

barrel distortion:  https://nocamerabag.com/blog/fix-perspective-distortions-iphone

 

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Buckshot, another way of doing it is to use an optical collimator. I have a USO one which I think is good. It just really gives you a grid square…….. an argument could then be made how precise is the grid square in the collimator.

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Thanks for all your replies. 

Appreciate that there will be accumulative tolerances/errors to take into account. Was curious why the reticle on the 3-20 appeared 'truer' than the 1-8 using the same technique to take the photographs in both scopes. 

Will look at all your suggestions. 

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