In the first focal plane the reticle size will change with the magnification. At say 6x the reticle will be smaller than say at 20x. This is good when using mildots because you can use any magnification to range something. Second focal plane as you know the reticle stays the same size. I really like the first focal plane on my Springfield Armory scope. If I was ordering an IOR scope in mildots and had the choice of first or second focal plane I would go for first.
Sakofan, I agree with HBar. I had this exact decision to make, either the IOR 2.5-10x with illuminated MP8 reticle in the 1st plane or the non illum version with the MP8 in the 2nd plane. I went with the illuminated version only because it is in the 1st plane but have found other good reasons for having the illum reticle since.
To answer your question:
1st focal plane reticle appears to enlarge, looking only at the reticle, as you increase the scope magnification. Advantage is it maintains it's size in relation to the tgt so that the mil marks can be referenced at any magnification. Disadvantage is at 2.5x the reticle is fairly thin and can be lost against a dark tgt in low light, but the illum feature prevents this. Also worth noting the is IOR illum scope does not have the heavy bars at the top of the horizontal stadia and the sides of the vertical stadia so it does not overwhelm the eye with light when it is turned on.
2d focal plane reticle appears to remain the same size as the magnification is changed. On some scopes the mil spacing is correct at the max magnification and at 10x on others. Advantage on a 6.5-20x variable with the mil spacing correct at 10x, the reticle covers a smaller portion of a small long-range tgt with the scope magnification on 20x. Disadvantage is the mils can be referenced with the scope not precisely on the correct magnification, resulting in unsat results. With the IOR 2.5-10x the mil spacing is correct at 10x so there is no real advantage to having the reticle in the 2nd focal plane other than it is still easy to see in low light at the lowest magnification without requiring illum and it is just what Americans are accustomed to seeing in a variable magnification scope.
SakoFan, good to see you found your way here. There is a lot more knowledge than at AR and hunters here are aware that their bullets travel further than 200 yds.
Yes, your primary advantage with the MP8 in the 2nd focal plane is in low light. At close range you would be able to place your shot into a whitetail's vitals as long as you can see the heavy bars.
I wish I could say that the IOR's 3.75" eye relief has always been enough for me but I did a snap shoot offhand on a possum that was at my dog's food bowl outside at about 20yd one night without obtaining proper cheek contact. The lights in the house were on so I might have been too close to the scope to block the light from striking the ocular. I did not feel contact but obviously there was since my nose was cut by the horn on the red lever on the Butler Creek cap. I think I lost almost as much blood as the possum.
The eye relief of the IOR should always be enough ifyou have enough cheek on the stock comb or cheekrest and have it there firmly enough, either on your .300WM or my .300WSM. You might add a cheek rest to the comb of your stock if the scope is mounted using the lowest rings possible. Also, ensure your scope is as far forward as possible while still having a full field of view.
Reticles: Front or Rear Decisions
Introduction and History There are two planes of focus in the common rifle scope (lensmatic), for the placement of the reticle. They are commonly called the front focal plane and rear focal plane models. One exception is the Shepherd scope, which has both. Artillery rangefinders have always had at least two reticular focal planes, and sometimes three or four. Optical collimating scopes have always had two focal plane reticles, or aiming points. Interferometers need more than one focal plane aiming point to function.
Why front or rear focal plane placement? Question: What are focal planes and what is the difference between putting the reticle in the front or rear focal plane? Answer: Only in a variable power scope is the reticle placement a major problem. In the rear focal plane, or behind the power changing lens system (erector tube), was the first solution that occurred to optical engineers, and most American scopes are still being built that way. Unfortunately, this apparently ideal solution has a very serious flaw.
Any tolerance change in the centration of the lens system and their spherical/longitudinal movement with the power change, will shift the point of impact. A variation of one thousandth of an inch will move the zero point approximately one inch at 100 yards. Since the mechanical parts that hold the power changing lens system slide inside each other, (some allowances are made for temperature changes, manufacturing tolerances and wear), there must be some movement made to accommodate this. Consequently this lateral and vertical movement will often shift zero by as much as several inches as power is changed.
A better solution is to place the reticle in the front focal plane, or ahead of the power changing lens system. The movement of the erector system will, optically, have no effect on the point of aim here. So why don’t all scope manufacturers build them this way? The downside of this method is that Americans typically do not like reticles that grow in size when the power is turned up. There is no actual growth in the reticle size. As the magnification increases, so does the reticle along with the objects in the field of view. A one inch dot reticle will still be one inch, at any power, be it low or high. It is only the appearance that is altered. If the power is turned from 2x to 4x, or doubled, the size of the objective image is doubled, and so is the reticle along with it.
Since the front focal plane reticle is a superior aiming device but aesthetically not very popular, there is only that problem to overcome.
That problem has been solved by U.S. Optics engineers in the form of creating a series of front focal plane reticles that do not appear to change in apparent size as the power is changed. These reticles all have the same effect when sighting with them. U.S. Optics designs these reticles to not only diminish the negative idea of apparent change, but uses that concept to create an exclusively positive concept change. In other words, we use the single image concept of a reticle magnified to an almost unusable thick, heavy image at high power to create another entirely different and very usable, highly magnified reticle, without the normal disadvantages. We call this system of reticles our High-Low Imaging System, or High-Low Reticle. It is a completely different picture at high power, thus usage is dual purpose and increases the versatility of the scope tremendously.
With this system, the variable power scope no longer has any disadvantages, and many decidedly great advantages over a fixed power scope.
Randy, that was a rather convincing piece on what I have always seen as the absolute major and winning advantage of a first focal plane reticle:
Your zero cannot wander as you change power setting.
-makes the choice easy to me!