Objective size and Mirage reduction

[Tim Singleton]

Hard to believe this thread got started by a few comments on a March aperture reducer.... From a shooter's point of view, the aperture reducer is good thing because it increases the depth of field. That allows a shooter to see mirage over hundreds of yards instead of only a few yards, see pickups/let offs and velocity changes quicker and more accurately. Most shooters spend zero time worrying about a darker image or any of the rest, it's all about seeing what's going on so we can make better decisions. When mirage is thick and you're using an aperture reducer, the image in your scope is much closer to the targets actual position! We've all seen the black center of a target look like a bouncing ball during thick mirage. An aperture reducer will minimize the amount of movement, also make it easier to see the lines on the scoring rings.
Dan P.

This has been my experience as well


It seems like others are basically sayings its still bouncing around we just can't see it with the reducer on
I'm not sure I follow that

 

[Fred Bohl]

Diffraction Effects on Target Images

The telescopic sight must do three primary tasks to provide a useful image to the eye/brain for it to perceive the needed target detail. First, it must preserve enough contrast (ratio of relative light levels). Second, it must provide sufficient resolution (clarity or sharpness). And third, it must provide sufficient magnification (apparent image size) so that the required details are detectable at the user’s visual acuity level.

Fortunately almost all modern target scopes have progressed to the point that all internal optical aberrations (deficiencies) have been reduced to a level less than that caused by diffraction of the incoming wave front by the limited size of the objective clear aperture.

The effect of diffraction on the perceived image is to blur the edges of details of a size at or above the resolution limit by widening the edge due to diverting part of the incoming energy into the blur tail. For details of a size below the resolution limit the contrast is lowered as the energy is spread even further until the even the peak value will have too little contrast to be perceived. This is shown in the following illustration:



To put the sizes of the details and resolution limits being discussed into perspective, for the range of target scopes we tested the resolution limits were from 0.035 MOA (0.036 inch at 100 yards) for the Nightforce 12-42x 56mm to 0.048 MOA (0.051 inch at 100 yards) for the Weaver T36. It is also helpful to realize that these values are about 0.12% of the whole field of view.

From the illustration it should be obvious that as the width of the line printed on the target gets smaller, the edge blur becomes a larger portion of the apparent width as seen through the scope. Also note that apparent width is also dependent on the contrast threshold so that at the resolution limit the edge blur can make the line appear to be from 1.5 to 2 times actual width at the resolution limit. When the width of the line printed on the target is less than the resolution limit, the contrast ratio drops significantly so that even seeing the broad blur becomes difficult.

An additional limiting complication to using the scope for extremely fine measurement or even extremely precise aiming is the eyepiece. The eyepiece is itself a magnifier and usually an adjustable magnifier (the "diopter" corrections for adjusting focus for a particular user). Since the eyepiece is behind the reticle even in a fixed scope, adjustment of the eyepiece modifies the apparent size and clarity of the cross hair and/or dot. Also, there is a minimal blur of the edges of the cross hair and/or dot itself even at "best focus" but fortunately in high quality target scopes this blur is smaller than that on target details (about 0.25 to 0.5 that of the target blur on the test scopes).

 

[Fred Bohl]

...
It seems like others are basically sayings its still bouncing around we just can't see it with the reducer on
I'm not sure I follow that

When the resolution of the scope is reduced, the edges of the target features are "blurred" more than normal so that small displacements and distortions due to mirage tend to blend together to form an apparently larger single image with less distinct edges. Small target details (less than the resolution limit) will also blend together but may loose enough contrast to not be seen in the changing view.

 

[BoydAllen]

There are a couple of other variables that deserve mentioning. One is optical quality. All of the other stuff can be negated by differences in the quality of the lenses,how accurately they are assembled, and the quality of baffling and coatings. We have all seen this, scopes of identical magnification and objective size that have obvious differences in sharpness and/or contrast, some looking quite good to their maximum magnification while others start to degrade before they get to the same magnification. The other is that when we are looking through an optical system rather than looking at an enlargement the requirements for sharpness are different. If a picture is being viewed at a size that is many times that of the negative or sensor, then the sharpness of the taking lens (and enlarging lens in the case of a photograph)has to be proportionately greater in order for the image to be seen as being sharp. Viewing distance gets into this as well. My point here is that since you will be using YOUR eyes to look through the scope, it is perfectly valid to come conclusions based on what you actually see. Don't let anyone tell you what you are seeing. Some years ago I did a rather simple test at a rifle range that involved a very crudely made aperture reducing device. It was a piece of tablet backing pasteboard with a hole cut through it with scissors, using a quarter for a marking guide. I walked up and down the line and asked each rifle shooter to tell me whether the target image was sharper when the aperture was in place. Of course it is a given that the image was much darker, but it was a bright day so the darker image was still usable. In every case except on the shooter said that the image was sharper with the disk in place. The one case was my 4200 series B&L 36x. When we look at camera lens resolution tests it is common for the greatest sharpness to occur a couple of stops down from the maximum, that would be where the diameter of the opening was halved. The quality of the lens is also a factor, as well as the maximum f stop (which is the focal length divided by the aperture). Diffraction does not become an issue until the lens is stopped well below where any direct viewing system would be usable.

 

[Fred Bohl]

Boyd and all others following this post -- as I noted in the Scope Brightness – Perception vs Measured thread, our little group of 3 OPTIC-NUTs is currently engaged in a project the intent of which is to find a useable protocol to do comparison testing of high-power target scope using human visual observers.

The project is going very well and when completed I will report our results in a new thread.