UPDATE - Too much light through my scope

[Jose_Luis_G]

Ok that looks pretty cool. Does the shutter shift from the recoil?

No, not at the moment.
Please note that I have used it for 22LR only.
Once I test it with my 6.5x55, I will drop the results in this same thread.

 

[Mozella]

............ snip...........However, a reminder, as you reduce the objective diameter you also reduce the scope resolution (increase the diffraction limit) and with that reduce your ability to see very small detail clearly or to aim precisely.
......... snip.............

I admit that I know little about rifle scope optics. I just buy one I can't quite afford and strap it on.

However, I have done quite a bit of photography over many many years. When you say reducing the objective diameter will have a negative effect on rifle scope resolution, you may be correct; however, the recommendations given in this thread keep the objective lens the same. In other words, nobody is talking about removing the objective lens and replacing it with a smaller one.

The advice is to use a ND filter, which will simply cut down the amount of light, or to fit a cap with a reduced aperture which effectively "stops down" the scope similar to stopping down a camera lens. That has the effect of reducing the transmitted light while increasing resolution and increasing depth of field.

Reducing the aperture of a camera lens nearly always increases the ability of the lens to resolve fine detail. Two stops from wide open is a good thumb rule. Past that center resolution normally doesn't increase and may actually decrease, but edge sharpness will continue to increase until eventually, somewhere around F-16 or so for common lenses, resolution will start to decrease.

My Vortex Eagle came with a disc with a center hole in it. I know that when I put the disc on, apparent resolution increases as does depth of field. Based on my photography background, that's what I would expect to observe.

Am I missing something here?

 

[Fred Bohl]

It would be more correct for me to have said reducing the clear aperture of the objective rather than reducing the diameter of the objective.

 

[Mozella]

It would be more correct for me to have said reducing the clear aperture of the objective rather than reducing the diameter of the objective.

OK, but when you do that with a camera lens, the resolution increases, at least up to a point, and the light reaching the film/sensor decreases.

As I said, I'm not a rifle scope optics expert, but when I install the special cap with a reduced size aperture on my Vortex scope, the apparent resolution increases and the brightness decreases just like with a camera lens.

I'll ask again. Is there something I'm missing here? My experience is just the opposite from your earlier post.

It seems to me that a cap with a hole in it would solve the OP's problem, unless he didn't want increased depth of field in which case a ND filter might be more appropriate.

 

[Fred Bohl]

Mozella,

Let me add a few more thoughts here.

My earlier answer was about the use of ND filters and that will do what you are looking at doing, without changing the optics of your riflescope. They will just make the image progressively darker, the higher you go in opaqueness. Just make sure you buy quality filters.

But more to your question, the aperture reduction device you are talking about will also do the trick, but they will changes the optics of your scope. Starting with your objective lens at 65mm, reducing it by half (one stop) means a disk with 46mm diameter hole. One more stop will mean a hole of 32.5mm diameter and the next stop will be 23mm diameter. Those numbers will give you -1 EV (half), -2 EV (quarter) and -3 EV(eighth) in the amount of light coming through.

The issue will be that as you reduce the aperture, you will increase the depth of field in your riflescope. What this means is that it will become much easier to focus, but that will also mean that you can increase parallax without realizing it, with its attendant issues and this is what I mean by changing the optics of your riflescope. It's not a bad thing, but you need to keep it in mind when focusing, just need to be very picky about the focus. You could always focus first and then add the aperture-reduction disk.

TT got it right the best solution to "too much light" would be quality ND filters that only reduce the amount of light through the scope.

When you "stop down" by reducing the clear aperture of the objective by using the OP's adjustable diaphragm or different sizes of "modifier disk", you in fact increase the apparent depth of focus but you definitely decrease the actual resolution. These are interrelated optical consequences.

I think part of the problem is that we have different definitions of resolution. In simple terms for our purposes I define resolution as the ability to determine if two bullet holes in a target at a particular distance and of a particular size that overlap by 50% are in fact two holes and not just one. If you reduce the objective aperture size you will have to either move the target closer or increase the bullet diameter to be able to make that determination.

 

[Turbulent Turtle]

OK, but when you do that with a camera lens, the resolution increases, at least up to a point, and the light reaching the film/sensor decreases.

That's not really correct. Many camera lenses are known to produce better IQ when stopped down a couple of steps but that's for the overall image. The center of the image does not gain anything in resolution or clarity by stopping down, it's the edges that may improve.

On the other hand it is well-known in photographic circles that as you increase the depth of field, you actually lose resolution. Yes, the overall image may look more in focus, but the target, arguably the object in the middle of your overall image is not as sharp.

The main difference between a camera lens and a riflescope is the inner tube and it may well be pointed elsewhere than the exact middle of your objective lens when zeroed at 1000 yards.

Before someone comes squawking that moving the inner tube 30MOA to get to 1000 yards just placed the whole thing off-center from the objective lens into bad lens territory, consider this: I estimate the distance between the inner tube face and the objective lens to be about 5.5 inches (140mm) on my scope (March-X 5-50X56,) according to the dimensions from the website. A half degree (30MOA) represents a minuscule displacement of 1.2mm (.048inch) from the exact center of the 56mm objective.

 

[geezerhood]

A great solution! It is like an adjustable iris used by bullseye pistol shooters adapted to the scope. Used for light reduction rather than for increasing the depth of field. I am definitely going to make one for myself. Now to figure out how to add a sun shade to the contraption.

 

[Turbulent Turtle]

A great solution! It is like an adjustable iris used by bullseye pistol shooters adapted to the scope. Used for light reduction rather than for increasing the depth of field. I am definitely going to make one for myself. Now to figure out how to add a sun shade to the contraption.

Adding a sunshade is not complicated. You can make one using the ever-popular handy-dandy tool know as "duct tape" or you can buy one to fit your scope. What brand and model do you have? All optics have a pretty standard thread at the front to add filters of sunshade. Usually you add 6mm to the size of your objective and look for something in that size. So for a 56mm objective, you will have a 62mm filter size for filters and sunshades. Usually.

 

[1lnbrdg]

Let me add a few more thoughts here.

My earlier answer was about the use of ND filters and that will do what you are looking at doing, without changing the optics of your riflescope. They will just make the image progressively darker, the higher you go in opaqueness. Just make sure you buy quality filters.

But more to your question, the aperture reduction device you are talking about will also do the trick, but they will changes the optics of your scope. Starting with your objective lens at 65mm, reducing it by half (one stop) means a disk with 46mm diameter hole. One more stop will mean a hole of 32.5mm diameter and the next stop will be 23mm diameter. Those numbers will give you -1 EV (half), -2 EV (quarter) and -3 EV(eighth) in the amount of light coming through.

The issue will be that as you reduce the aperture, you will increase the depth of field in your riflescope. What this means is that it will become much easier to focus, but that will also mean that you can increase parallax without realizing it, with its attendant issues and this is what I mean by changing the optics of your riflescope. It's not a bad thing, but you need to keep it in mind when focusing, just need to be very picky about the focus. You could always focus first and then add the aperture-reduction disk.

Adding a rear ocular(?) lens to to keep the cross-hairs in the center of the scope will help with the parallax as the XTC shooters are doing.