Ive heard a few discussions about how the two differ, and which is better. What I really wanna know is, how do the two designs differ mechanically? Whats doing what inside these two configurations?
Side focus Vs. Adjustable objective
- Thread starter Canadian bushman
- Start date
Coyotefurharvester
Silver $$ Contributor
Truth or myth, I have always heard that side parallax must be adjusted from infinity down. The objective lens can be adjusted in any direction. I have both and prefer the objective type for hunting, when shots are under 400 yards. Because parallax isn't a large factor,especially inside 300 yards, and the focus is not as "touchy". Some of the side adjustments have a small travel movement range, so the focus range is tight. I'm not a long range steel or target shooter.
Turbulent Turtle
F-TR competitor
I'll try to keep this as simple as I can.
In a riflescope, the objective lens group transmits the image to the first focal plane in the riflescope. This first focal plane is located at the front of the inner tube, somewhere very near where the adjustment knobs are located. As we know, the image transmitted to the first focal plane is perfectly in focus for a certain portion of said image. The portion that is in perfect focus will be located at a specific distance from the objective lens of the riflescope. The portions in front of that distance or beyond that distance will be increasingly out of focus, the further from that focus distance they are located. This is much more visible for the objects located between the objective lens and the focus distance, than for the objects located behind the focus distance. Riflescopes without any means of focus adjustments have their focus set at a distance where most people would be aiming and the maker relies on the optical effect called depth of field to make most the image appear in focus, even if not perfectly in focus. That fixed distance is usually around 100 to 150yards or thereabouts.
This fixed focus creates a problem for precision shooters who ply their trade at distances other than the one used by the riflescope maker. Of course, the further away the target is from the set distance, the worse the problem known as parallax is. And to further complicate things, the higher the magnification used to focus on the target is, the worse the parallax is.
We all know what parallax is and how it manifests itself, here we are only concerned about differentiating the two most common methods of setting the proper focus.
In a riflescope, the objective lens is not a single lens, it consists of a group of lenses, singles and doublets. In an AO (Adjustable Objective,) rotating the front part of the bell actually adjusts the distance between the lenses in the objective group and thus adjusts the focus on the first focal plane. It is a rather simple system that works extremely well and has been in similar use in camera lenses for eons. It can be very precise and requires little additional hardware to work. The problem is that from position, it's a bear to use. For prone, known distance shooting, it's great; for varying distances, it becomes cumbersome to use.
The Side Focus method was introduced to address this last issue above. However, instead of directly adjusting the lenses in the objective lens group like the AO, the SF (Side Focus) system introduces an extra lens between the objective lens group and the first focal plane. This lens goes back and forth, adjusting the focus of the image on the first focal plane. This back and forth movement is effected by the use of the side focus knob. This system is more complicated, adds hardware and another lens in the scope and is more expensive and heavier. It is also not as precise as the AO can be; the movement of the lens is small compared to the rotation that can be found in a good AO design. This is the main reason I outfitted my March-X scope with that 4 inch side focus wheel; that provides me with much better focus control. That and it makes the side focus knobs easier for the arthritic left hand.
I hope this is the information you were looking for.
In a riflescope, the objective lens group transmits the image to the first focal plane in the riflescope. This first focal plane is located at the front of the inner tube, somewhere very near where the adjustment knobs are located. As we know, the image transmitted to the first focal plane is perfectly in focus for a certain portion of said image. The portion that is in perfect focus will be located at a specific distance from the objective lens of the riflescope. The portions in front of that distance or beyond that distance will be increasingly out of focus, the further from that focus distance they are located. This is much more visible for the objects located between the objective lens and the focus distance, than for the objects located behind the focus distance. Riflescopes without any means of focus adjustments have their focus set at a distance where most people would be aiming and the maker relies on the optical effect called depth of field to make most the image appear in focus, even if not perfectly in focus. That fixed distance is usually around 100 to 150yards or thereabouts.
This fixed focus creates a problem for precision shooters who ply their trade at distances other than the one used by the riflescope maker. Of course, the further away the target is from the set distance, the worse the problem known as parallax is. And to further complicate things, the higher the magnification used to focus on the target is, the worse the parallax is.
We all know what parallax is and how it manifests itself, here we are only concerned about differentiating the two most common methods of setting the proper focus.
In a riflescope, the objective lens is not a single lens, it consists of a group of lenses, singles and doublets. In an AO (Adjustable Objective,) rotating the front part of the bell actually adjusts the distance between the lenses in the objective group and thus adjusts the focus on the first focal plane. It is a rather simple system that works extremely well and has been in similar use in camera lenses for eons. It can be very precise and requires little additional hardware to work. The problem is that from position, it's a bear to use. For prone, known distance shooting, it's great; for varying distances, it becomes cumbersome to use.
The Side Focus method was introduced to address this last issue above. However, instead of directly adjusting the lenses in the objective lens group like the AO, the SF (Side Focus) system introduces an extra lens between the objective lens group and the first focal plane. This lens goes back and forth, adjusting the focus of the image on the first focal plane. This back and forth movement is effected by the use of the side focus knob. This system is more complicated, adds hardware and another lens in the scope and is more expensive and heavier. It is also not as precise as the AO can be; the movement of the lens is small compared to the rotation that can be found in a good AO design. This is the main reason I outfitted my March-X scope with that 4 inch side focus wheel; that provides me with much better focus control. That and it makes the side focus knobs easier for the arthritic left hand.
I hope this is the information you were looking for.
I like the front AO - simplicity, precision & cost - how much light transmission would be sacrificed with the introduction of that extra lens? I have never had any problems with SF or AO. I do most of my shooting at fixed ranges except for the rodents & they usually hang around long enough for me to rotate the AO with my rifle jammed into its rest. Going along with the simplicity I prefer fixed power scopes of not more than 20X but these are now history. I can see the advantage of a big wheel SF attachment but they sure look funny. I regret selling off my 2 Leupold 12X40 AO FX3's - despite their modest magnification I was able to make hits on small targets & rodents up to 500 yards and had no concerns about 2nd FP reticle changes.
Are their any optical advantages, besides light transmission, to the lens configuration of an AO style optic, versus that of the SF?
Maybe, better recoil resistance, or a larger adjustment range?
Maybe, better recoil resistance, or a larger adjustment range?
Turbulent Turtle
F-TR competitor
A riflescope, especially a variable one, has many lenses in it and the addition of one more for the side focus is not going to make any appreciable difference in over light transmission, especially if the lens is fully multicoated. I seriously doubt the difference could even be detected by the mark 1 eyeball.
That extra lens is surely as well (or as badly) attached inside the scope as any of the other lenses, so recoil tolerance or resistance should not be affected, all things being equal. There is one more knob to catch on stuff, but then again, a good whack on the objective could mess up the mechanism of the AA and jam the lens at one spot. That would require quite the hit and at the proper spot.
As I explained in my previous post, the SF is more expensive, heavier and has more hardware than an AO. But if I had to pick one thing that it doesn't do as well as an AO is very fine focus. The issue of course, only really manifests itself at closer range because optics and how they work. You will no doubt have noticed that the focusing scale is logarithmic, where it takes as much or more travel to change the focus from 50 yards to 75 yards as it does to change it from 100 yards to infinity. The AOs usually have more granularity in adjustment range than SFs, but that's why I have that big wheel on my focusing knob.
Another trick for you with the big wheel. Sometimes I like to focus at around the 600 yard line to see what the mirage is doing there and then focus back to 1000 yard to take the shot. With a regular knob, that's difficult to do, but I attach a small zip line on my wheel to indicate where it needs to be for 600 yards (does not need to be super precise) and then I can pop it back to 1000 in a jiffy.
Fred Bohl
Gold $$ Contributor
As long as you remember to tighten the lock ring on the AO, it will be more resistant to change from recoil. Few if any SF designs are lockable and depend on friction alone to hold position (another possible use for a scope checker?).
To be fair, it should be noted that early SF mechanisms were prone to recoil shift and backlash (the origin of "adjust from infinity in" method to avoid backlash issues). The latest SF mechanisms are virtually free of backlash and recoil shift problems in high end scopes.
Turbulent Turtle
That makes a lot of sense and answers quite a few questions I had. Thank you for taking the time to walk me through it.
That makes a lot of sense and answers quite a few questions I had. Thank you for taking the time to walk me through it.
Turbulent Turtle
F-TR competitor
Hello Fred,
Those are great comments. It's important to understand that the first iterations of something are always prone to improvements. That's the way most technology works; you produce something, it gets used, people find issues with it, you make it better; rinse, repeat.
At some point, your something becomes quite solid and dependable and most people forget the travails of the early models. You hope.
Then there are some low price SWFA SS scopes that have rear focus, (the ring thingy that turns just in front of the fixed ocular, not the diopter for reticle focus). Focusing these things is somewhat touchy when approaching max ranges ("logarithmic") but work fine when used for stuff like .22LR silhouette shooting up to 100 meters.
I have noticed that I cannot tell the difference between a scope with 92% or 98% light transmission and would assume high quality glass & lens coatings are uniform for all lenses; there appears to be a lens/coating race among scope makers. Glass and coating quality appears to be improving every year.
I have noticed that Leupold makes very few AO type scopes now. Previous Leupold AO scopes had friction type AO and I never have any problems with inadvertent focus shifting and I could change focus quickly. My much used cheap Weaver T10 & T24 aluminum tube scopes have a lock ring for their AO and if I don't use the lock ring I can also change focus quickly. Would the mechanics of the AO have less integrity than SF for prevention of fogging (leaky scopes)? How much better is argon/krypton (sounds hi-tech) than nitrogen for preventing fogging? Is "thermal shock" a big deal? Do they still dunk scopes in warm water and look for bubbles, especially in this situation from AO or SF?
I would have much use for a fixed power, 18X50, 30mm AO mil/mil scope (regressive?) with equivalent glass/coatings as the higher end scopes - but few others would, so I guess I will never have this.
I have noticed that I cannot tell the difference between a scope with 92% or 98% light transmission and would assume high quality glass & lens coatings are uniform for all lenses; there appears to be a lens/coating race among scope makers. Glass and coating quality appears to be improving every year.
I have noticed that Leupold makes very few AO type scopes now. Previous Leupold AO scopes had friction type AO and I never have any problems with inadvertent focus shifting and I could change focus quickly. My much used cheap Weaver T10 & T24 aluminum tube scopes have a lock ring for their AO and if I don't use the lock ring I can also change focus quickly. Would the mechanics of the AO have less integrity than SF for prevention of fogging (leaky scopes)? How much better is argon/krypton (sounds hi-tech) than nitrogen for preventing fogging? Is "thermal shock" a big deal? Do they still dunk scopes in warm water and look for bubbles, especially in this situation from AO or SF?
I would have much use for a fixed power, 18X50, 30mm AO mil/mil scope (regressive?) with equivalent glass/coatings as the higher end scopes - but few others would, so I guess I will never have this.
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Perhaps this will help.
https://en.wikipedia.org/wiki/Parallax
Just trying for coincidence. I can remember how to spell parallax because of the two parallel "l's".
https://en.wikipedia.org/wiki/Parallax
Just trying for coincidence. I can remember how to spell parallax because of the two parallel "l's".
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Parallax is caused by the focal plane of the object not being in the same location as the reticle. It is eliminated by proper focusing. Parallax and focus are different things, but focus is the fix for parallax. The AO and side parallax/focus dials are focusing mechanisms.
Justin
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