Re: Calling S&B PMII 3-12 owners
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: toovira</div><div class="ubbcode-body">My guess is that they do that to limit amount of light coming in. As FOV goes wider, more lights are coming in....which contrary to what other people think, too much light can be bad.</div></div>
This is not "done" intentionally, it is just a compromise because of a technical problem that many manufacturers do not seem to be able to work around.
What you can see when you turn down the power is simply the edge of the reticle that acts as a field stop and limits your FOV once you decrase the magnification beyond a certain value. The reason for this is that the reticle is physically too small to give full FOV on low power. This is especially easy to understand on FFP scopes: As you decrease magnification, you can see more and more of your reticle, until at a certain point, you have reached the edge of the reticle and you just see the black circle that is your field stop. Now if the scope allows you to decrease magnification further, you will just see more of that field stop while your image shrinks and seems to "move away" from you. There is nothing to be gained by this effect, you just loose FOV on low power. The only potential "advantage" is that you cannot see the poor image quality around the edges of the FOV of non-premium optics, because those areas are simply masked by the field stop.
Now why don't all manufacturers use bigger reticles to avoid this phenomenon? As said, in order to do this, the physical diameter of the reticle has to be increased, which means that your erector tube gets bigger. A bigger erector tube inside a given main tube diameter means less adjustment range, something that is not exactly desirable in long range optics. Additionally, potential weaknesses of the optical system will be exposed mercilessly on low power. Many scopes suffer from heavy distortion on low power, somethig that will jump at you once you've seen a scope where this is corrected properly. Take a FFP scope of your choice, put it on lowest power and move your head/eye up and down inside the exit pupil. You will see the reticle and image warp and make all kinds of funny movements that increase towards the edges. Feel free to compare directly to a Premier scope.
If you cannot or don't want to increase the diameter of the reticle, you can either use a lower zoom ratio (3x instead of 4x or 5x, that's why many cheaper scopes come in 3-9x for example) or limit the FOV throughout the whole power range, which will give a smaller FOV on high power, but allow the FOV to increase all the way to low power.
The bottom line is that the non-increasing FOV is a compromise that is a result of cutting one or more of several corners. Avoiding this is mechanically and optically demanding and you have to design the scope from the ground up with this in mind. That's why we are proud that Premier can offer a true 5x zoom ratio with full FOV throughout the power range <span style="font-style: italic">and</span> at the same time provide a huge reticle adjustment range.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: toovira</div><div class="ubbcode-body">Before you say that doesn't make sense, think about when you walk out in absolute white out condition (sunlight/snow or desert). What do you do naturally? Squint your eyes. Doing that, is same as aperture stop aka f/#, etc.</div></div>
As mentioned above, we are talking about a <span style="font-style: italic">field stop</span> here, as opposed to an <span style="font-style: italic">aperture stop</span>. A field stop reduces the FOV, while an aperture stop limits the amount of light that passes the system. Given the power ranges we are talking about, the exit pupil of the scope set at low power will always be smaller than the eye pupil of the user, so the eye acts as the limiting aperture stop of the system. Of course a smaller FOV will limit the absolute amount of light entering the eye, but it will not limit the amount of light per area on the retina, which is what bothers the eye after all and makes it squint.
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