front lens ring

[Link]

Why ? The ring on front lens.

ty Don

 

[Deleted member fkimble@charter.net]

Not sure what your asking? Front ring holds the lens in?

Frank

 

[Link]

No I have seen pictures of a ring that blocks off maybe 1/4'' around the front lens glass. Maybe for light restriction?

Don

 

[Martin52]

http://marchscopes.com.au/accessories

Scroll down - see the MD disc - purpose is to reduce light. This what you're talking about ?

 

[Link]

http://marchscopes.com.au/accessories

Scroll down - see the MD disc - purpose is to reduce light. This what you're talking about ?

Do those work? If so who sells them for Siii x 50

TY Don

 

[tazzman]

Do those work? If so who sells them for Siii x 50

TY Don

Aluminium dust cover with hole milled out and painted?

 

[Link]

My question is how well does the mirage disc really help?

Me too. ? Mirage has been a real problem this summer.

TY Don

 

[powderbrake]

I find it helps sharpness of the image, and it forces you to hold your eye in the center of the image because the exit pupil is smaller.

 

[F Class John]

They work but to varying degrees I find depending on how and where the sun is in relation to the mirage. But I always have a couple in my bag just in case. I’ve actually used one between the scope and my sunshade and then another one on the outside of the sunshade if it’s really bad out.

Dan Pohlabel sells them for all kinds of scopes.

 

[Turbulent Turtle]

Me too. ? Mirage has been a real problem this summer.

TY Don

"Mirage" is always a problem in South Texas. But let's talk Modifier Disks, as March calls them.

One of the big differences between a riflescope/spotting scope/binoculars and a camera lens is that the spots optics (the first bunch) do not have adjustable apertures; they are at their maximum. A camera lens has a built-in adjustable aperture and that is used as one of the three legs of the exposure stool to get a properly exposed picture. The other 2 legs are shutter speed and film/sensor sensitivity. Pre-digital photography, you picked a film according to the ASA (what we now call ISO) value, dialed that into the camera and then you adjusted your aperture in concert with the shutter speed to properly expose the picture.

It's all about balancing the incoming light. If you wanted to use a faster shutter speed to freeze the action, you increased the size of the aperture to bring in more light. Larger aperture + faster shutter = Smaller aperture + slower shutter. To change the exposure you can change the size of the aperture or change the shutter speed or both. There are several rules of thumb to selecting shutter speed and then matching the aperture size to achieve the proper exposure. But usually one wants the sharpest picture possible so one uses a faster shutter speed to "freeze" the action. Unless you're shooting in very bright sunlight and/or have a sensitive film/ISO value, you will be using as bigger aperture to bring in more light because of the shorter time of exposure.

However, using a larger aperture means that you are reducing what is known as Depth Of Field. That is the area in front of and behind the subject on which the lens is focused, that still appear in focus well enough to properly resolve the objects in front of and behind the subject. In photography, sometimes you want to reduce the DOF for artistic effect and so you open the aperture as wide as you can and you crank up the shutter speed. Portrait photography is a good example of such artistic effects. You capture the face of the person and blur the background and even the foreground, leaving only the face of your subject in sharp focus, we call that a bokeh effect.

A couple of optics rules here, DOF is a function of the size of the aperture, the distances involved and the magnification of the riflescope. A 56mm riflescope will have a smaller DOF than a 40mm riflescope at the same magnification and distance. However, a 56mm riflescope will have a sharper picture of the target compared to the 40mm riflescope, everything else being equal. Reducing the aperture of a lens will degrade the resolution of the image. This does not happen right away, and camera lenses are usually tested to find at which point the resolution is affected. A picture taken at f/2.8 will blow up much nicer than the same picture taken at f/16. However, the picture taken at f/2.8 will have a much narrower DOF compared to the picture taken at f/16.

So, smaller aperture, wider DOF but maybe decreased resolution. Let's carry on.

When you focus on the target at 100 yards at 20X, you will notice that the area in front of the target is out of focus, the closer it is to you. On the other hand, the area behind the target seems in focus a lot further out. That's the depth of field and it's always smaller in front of the subject compared to behind it.

Now, when we are shooting at a target, we will encounter a phenomenon that we incorrectly call "mirage." This mirage is showing us the conditions between the firing line and the target. Usually we have a berm behind the target line so we don't see the mirage behind the target line and indeed, we don't really care what's the conditions are behind the target. What we want to see are the conditions between us and the target, and that's the short area of the DOF. We have optics that magnify greatly so DOF is already minimized if we are focused on the target.

The modifier disk is used to reduce the amount of light coming into the riflescope thus creating a smaller objective, acting like an aperture in a camera lens. Camera lenses usually have an aperture mechanism that will cut the light in half for every step. So full aperture, then 1/2, 1/4, 1/8, 1/16, 1/32, etc... Each of these half (or doubling if going the other way) is the equivalent on an F-stop mentioned earlier.

In order to cut the light in half for a 56mm lens, you create a disk that has a hole in it with one half the area of the 56mm lens. The area of a circle is equal to Pi * Radius squared. If my calculations are correct, the area of a 56mm lens is 2,463 square mm. Half of that is 1231.5 square mm. That gives us 39.60mm as the diameter of a lens that's half the size of a 56mm lens. (Why do you think I compared the 56mm lens to a 40mm lens earlier on?) So if you created a disk with a hole with a radius of 20mm, and screwed it in front of your lens, you would cut the light coming in by half.

For a 50mm lens, the lens that's half the area is a 35.4mm lens. That's why you see a lot of 36mm objectives. The lens that would be half the size of the 40mm lens mentioned earlier would be a 28mm lens.

So the disk only seems to have about 1/4 inch of metal has in fact .31inch of metal. Very close to the 1/4 inch guesstimate.

I spent a few minutes with a DOF calculator trying to compare the DOF for a 1000 yard target and the impact of a 1-stop modifier disk. Without the MD, the front DOF starts at about 600 yards downrange. With a 1-stop MD, the DOF starts at 400 yards downrange. So the 1-stop MD buys you 200 yards of DOF but reduces the light and the exit pupil size by half. Also, since a riflescope is a blend of camera lens and magnifying glass, the resolution of the target will be reduced. Everything is a trade-off in optics.

So the MD, in theory, will allow you to see more mirage in the riflescope. If that's what you're looking for.