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Dumb Question of the Day-FOV calculation |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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Posted: December/08/2014 at 21:54 |
Tried to research this a bit but came up empty handed. May sound like a dumb question, but I'm not sure of the answer.
Scopes advertise a specific FOV at 100 yards on a given power, usually highest and lowest. If I want to know the FOV at 200 and 300 yards, is it as simple as multiplying the 100 yard figure by 2 or 3? Or is it not a linear ratio like that? Seems like the FOV would grow by more than double or triple, but maybe not?? |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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It is a simple linear ratio. ILya |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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Thanks!
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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...on second thought, are you sure it's linear??? Not challenging your knowledge, but.....well.....I guess maybe I am! But not in an argumentative way.
Let's use the example of a Zeiss 5-25x Conquest HD. Stats show FOV at 100 yards on 25x to be 4.2'. So if it's linear, then that means FOV at 200 yards = 8.4', and 300 yards = 12.6'. This seems reasonable until you start thinking that the difference between each 100 yard increment is only 4.2', whether it's between 100 and 200 yards, or between 900-1000 yards. Hard for me to believe that the FOV increase from 900 yards to 1000 yards would only be another 4.2'. Maybe I'm wrong...but that just doesn't seem right??? The entire reason I'm asking this question is because I'm considering buying that scope with the RZ-1000 reticle. Without getting into a debate about SFP ballistic reticles, my concern with this scope is that my particular load comes in at about 24x (nearly full power) for the reticle to be "calibrated" (according to the Zeiss calculator). It seems to me that's a whole lot of power for the closer ranges, and target acquisition would be tough without constant power changes. I realize I would still have to make power adjustments even with a FFP reticle, but at least with that setup I wouldn't have to make sure I was on a specific power in order to be accurate. And then of course there is the whole debate about how accurate the ballistic reticle will be in the real world. Maybe I'm just trying to talk myself out of it that scope...but I love Zeiss glass. I have a Vortex Viper PST on order. It's already been 5 months, and they're saying another 1-3 months. My biggest concern with the Vortex is that the one I looked through got pretty hazy at the higher powers. The Zeiss is nice and crisp, but I just don't know about the SFP ballistic reticle. I would love to try it, but once I mount it, it's mine. Can't return it. |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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The FOV is twice wider for a distance that is twice longer. That is what linear means. At 200 yards, it is twice wider than at 100 yards. At 400 yards, it is twice wider than at 200 yards. At 300 yards, the distance is 1.5 times longer than at 200 yards, so the FOV is 1.5 times wider than at 200 yards. This is pretty simple geometry. ILya
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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You didn't even mention string theory...
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Opinion,untempered by fact,is ignorance.
There are some who do not fear death... for they are more afraid of not really living |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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Yes...that's the same thing I said. It grows by only 4.2 feet for each 100 yards. The math works out the same. The increase between each 100 yard increment is only 4.2 feet, whether between 100 and 200 yards, or between 900 and 1,000 yards. Still doesn't seem quite right, but perhaps it is. I understand what "linear" means....that's why I asked if it was linear! The question wasn't what linear means...the question was IS it's linear. I'll take your word for it that you're right, but I may have to throw a challenge flag! The FOV from the scope to whatever the distance in question is represents an isosceles triangle. There can be only one isosceles triangle at which the altitude (in this case the distance to target) would equal the width of the base (in the case the FOV), and that is where the peak of the triangle is at 90 degrees. So I guess my question is...is the FOV ALWAYS at 90 degrees on every scope, every time? |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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...To correct myself, I meant to say is the base is twice that of the altitude, not the same as.
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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O.K., I did some more thinking, and I believed I answered my own question.
It is NOT linear. This can be proven by looking at the stats of different scopes that have the same zoom range and objective lens size. Again, the only way it could be linear is if the FOV is a right isosceles triangle, and if this was always the case, then every scope with the same objective size would have the same FOV at 100 yards as every other scope if they were on the same power. I think the way to calculate the FOV at various ranges would be to take each individual scopes FOV stats for the maximum and minimum power, and use that to figure out the angle at the peak of the triangle. Given that angle, you could then calculate the base of the triangle (FOV) at each given altitude (range). ....pretty simple geometry, really. |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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...but then again...the scope length (distance between the glass) could be the reason for the different specs, but I don't think so. Still don't think it's linear.
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SEMO Shooter
Optics Apprentice Joined: January/06/2013 Location: SE Missouri Status: Offline Points: 199 |
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I had a Conquest 4.5-14x50 with the Rapid Z 1000 reticle. I seldom trade or sell my scopes. But that was one I traded. The scope itself was fine, but the Rapid Z reticle did not match my loads very well. The reticle was ok for plinking steel, but not for serious longer range accuracy.
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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What did you replace it with?
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SEMO Shooter
Optics Apprentice Joined: January/06/2013 Location: SE Missouri Status: Offline Points: 199 |
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I ended up trading it and some cash for a Nighforce. I would have been happy with that Zeiss if it had a plex reticle. I did not find the Rapid Z 1000 reticle useful for my type of shooting which is 100 to 700 yard target shooting. I still have 5 Conquest scopes so I'm not a Zeiss hater. I have a 3-9, 3.5-10, two 4.5-14, and a 6.5-20. All have plex reticle except the 3.5-10. It has a #58 ZRF reticle.
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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Did you play with the magnification to try to dial the reticle in?
How far off would you say the reticle was in the 400+ yard range? Within 4" or so? Very curious about this because I wouldn't be using it for precise target shooing. I would be using it for shooting steel, and possibly medium range hunting. The 14x wouldn't be so bad, but the RZ1000 only comes in the 5-25x HD now. Can't get a lower power range. |
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SEMO Shooter
Optics Apprentice Joined: January/06/2013 Location: SE Missouri Status: Offline Points: 199 |
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I played with the magnification ring. It was within 4" at 400 yards with Federal 168 grain match. It is supposed to be calibrated to work with 168 & 175 grain match ammo. Who's match ammo, and what velocity does it match? If you load your own will your loads match the ballistics needed?
Your hunting loads will have quite different ballistics that will not match up as well. Do you think you would actually take a longer shot than 300 or 400 yards? The center line of the reticle is for 500 yards with 100-400 all above center. Lots of lines and a very busy reticle. Have you considered a Rapid Z 600? It is much simpler. I am not saying the Rapid Z 1000 won't do what you want, but it is not a magic answer to making an acceptable hit at longer distances. |
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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All of the points you make are reasons I'm second guessing it. The rifle is mainly used for having fun plinking steel at various ranges, all while practicing ranging and trying to get shots off quickly.
For the hunting aspect, I realistically wouldn't shoot past 450 yards, and even that may never happen. I know my limitations and don't want to wound an animal. The only reason I would even consider it is because my practice range and the field I would be hunting in are the same field, so I will have practiced at every distance within that area. As far as the RZ-600 is concerned, I have a Conquest 3-9 with the RZ 600 on top of my -.06. That reticle doesn't match any of my .308 loads very well, at least according to the calculator. The optimum power setting isn't high enough for longer range shots (even on the higher power scopes). It's exactly the opposite problem as with the RZ-1000. I think I'm just going to cancel the order and wait for the Vortex to come in. |
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SEMO Shooter
Optics Apprentice Joined: January/06/2013 Location: SE Missouri Status: Offline Points: 199 |
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I think you will be quite happy with he Vortex. One of my buddies has a Viper PST mounted on his Savage 260 and I like his scope. He is a good shooter and consistently makes head shots on steel targets at 600 to 800 yards.
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tucansam
Optics Apprentice Joined: February/05/2007 Status: Offline Points: 140 |
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On a side note, what determines a scope's FOV? I had my USO SN3 3.2-17x44 at the range today, alongside my friend's Razor HD with a 50mm objective. He has a 35mm main tube, my USO's is 34mm.
At 20x power, his scope had a wider FOV than my USO at 17x. What determines that?
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Trailblazer
Optics GrassHopper Joined: July/15/2014 Status: Offline Points: 34 |
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Well, I would say his 50mm objective versus your 44mm would be the biggest reason, but there must be other factors as well. To a lesser degree, I would have to think that the geometry of the internal components would play a small factor as well.
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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Objective lens diameter has no direct relationship to the field of view. If anything, it is easier to make a wide field of view scope with a smaller objective lens. The FOV is determined by the lens presription, i.e. the curvatures of different lenses used, not by their diameter. Go make a 30mm aperture and put it right up to your 50mm scope objective. FOV will stay the same. ILya
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