ED glass, CA and mirage

[Turbulent Turtle]

Another fun weekend match filled with oodles of mirage. The Super ED glass in my scope lived up to its billing and I remained at 50X all day long. The IQ is affected, this is not magical glass after all, but the image is still usable, and I was able to place my shots very surgically. Unfortunately, I'm not a great surgeon and I got bit by fast switches and several elevation issues.

@PracticalTactical Thanks for reminding me of the fast focus ocular lens, I had forgotten about that one. I actually like that item very much. The way March has implemented it, you lock it in after you have found your setting. This way, there's no danger of it moving around by mistake. Kinda like the way some lockable turrets work.

Another newer neat feature is the sunshade. Mine is on all the time.

 

[PracticalTactical]

@PracticalTactical Thanks for reminding me of the fast focus ocular lens, I had forgotten about that one. I actually like that item very much. The way March has implemented it, you lock it in after you have found your setting. This way, there's no danger of it moving around by mistake. Kinda like the way some lockable turrets work.

Another newer neat feature is the sunshade. Mine is on all the time.

I have no issue with a "locking ocular lens", but abhor the wiggle junk and instability produced by all but the most expensive scopes with fast focus eye piece.

The only conceivable practical application of a fast focus eye piece is for a classroom loaner rifle where 25 people might use it the same day... or ~ possibly ~ in a military setting where a rifle is shared.

If you want to shoot small groups like for bench rest or F Class... fast focus eye piece is absolutely not something I recommend.

 

[Fred Bohl]

TT an update:

Finally managed to do a side by side comparison of my 50x52 with ED and a friends 48X52 HM with Super ED on a day with serious "shimmer" during the afternoon. There is a definite improvement in IQ with the Super ED over the ED and detectable by both of us scope owners/users.

However, two other shooters who we invited to do a comparison of the two did not report any IQ difference. My suspicion is that neither one was used to using such high IQ scopes particularly in serious "shimmer" conditions. Both were of the "turn down the zoom in bad mirage" school and where surprised at their ability to see a nearly clear target image. They're both good shooters that are very good at reading wind flags but tend to treat mirage/shimmer as a problem not an aid.

 

[Turbulent Turtle]

Thanks for the update, Fred. Very interesting.
It underlines the importance of experience when comparing riflescopes. The two people who know riflescopes and who deal with mirage and try to look through it, notice a "definite improvement in IQ" between the two riflescopes. I have always stated that mirage (shimmer) will always degrade the IQ, but it degrades less rapidly with Super ED compared to ED and especially traditional glass.

This is a further reinforcement that one should never rely on comparisons that cannot be clearly stated when reading other people's opinions.

 

[Delfuego]

You guys are blaming the "user" for not knowing how to see?

 

[Fred Bohl]

No, I'm not blaming anyone for "not knowing how to see".

Over my 63 years of using telescopic sights (including 50 years of SRB competition) I've helped a lot of newcomers. And ever since scope power rose above the mid twenties it has been necessary to spend a lot of time convincing new to high power shooters that they need to practice using higher powers first to train their brains how to deal with empty magnification (no more detail just closer view) and also how to handle and use the now visible mirage/shimmer.

For all of the changes in equipment and methods I've been thru there has been one constant - Success requires practice and more practice to achieve comfortable familiarity with the equipment and techniques.

 

[Fred Bohl]

For those that have emailed or messaged me asking me to clarify post #106 above.

All telescopes and telescopic sights are diffraction limited by the objective aperture being finite as opposed to infinite. Please take note of the following in particular the italicized paragraph:

Objective Diameter	40 mm​	45 mm​	52 mm​	56 mm​
Dawes Resolution Limit	0.048 MOA​	0.043 MOA​	0.039 MOA​	0.035 MOA​
Match Magnification	20.7​	23.3​	26.9​	29.0​

Dawes Resolution Limit - (smaller is better)

Smallest separation between features and minimum POA (point of aim) error (confusion circle)!

Dawes Resolution Limit (MOA) = 1.933 ÷ objective lens diameter (mm)

Match Magnification - (larger is better)

Magnified normal visual acuity of the human eye (1MOA) is equal to the Dawes Limit (MOA).

At less magnification the resolution is limited by the user's eye. Above this magnification the resolution is diffraction limited by the objective diameter to the Dawes Resolution Limit (MOA).

Match Magnification = 1MOA ÷ Dawes Resolution Limit (MOA)

NOTE: As long as resolution is limited by the users eye this is what the brain has been trained to deal with by long experience. Above match magnification the target image appears larger but not more detailed and not what the brain has learned to expect. To properly interpret and process this type of image the brain has to learn this new condition by practice.

Above match magnification is called "empty magnification" by astronomers. However, for us shooters, the larger target image is valuable to allow us to "aim small miss small".

 

[Turbulent Turtle]

@Fred Bohl I really like your explanation. I've known for a while that it takes some time to get used to a riflescope, especially at higher magnification. It's not difficult and does not require understanding or anything like that; it's just time for your brain to understand what it's seeing. So much of vision depends on the brain's interpretation of what's being fed to it.

Also, looking at your great table, I would point out that if you have a 56mm objective and you use a modifier disk or a reducer of some kind, you have to use the numbers that correspond to that size aperture. This explains why while gaining DOF with a smaller aperture, you also lose resolution.

 

[Fred Bohl]

As to the questions for what value should be used for the diffraction limit? The cited values include:

Raleigh's Resolution Limit (MOA) = 2.33/Objective Diameter (mm), approximate 1/4 wave error, Strehl Ratio = 0.82

Dawes's Resolution Limit (MOA) = 1.93/Objective Diameter (mm), approximate 1/5 wave error, Strehl Ratio = 0.88

Sparrow's Resolution Limit (MOA) = 1.17/Objective Diameter (mm), approximate 1/8 wave error, Strehl Ratio = 0.95

All three are based on observational experiments reflective of the time frame and quality of instruments used by the experimenters. I chose to use the Dawes value in my calculations as being the most representative of amateur level optics of today. Sparrow’s very optimistic value has only been verified by the very big and super quality major observatory all mirror telescopes using high end cameras or digital image sensors not human eyes. Note, excellent individual mirrors and lenses are being made at 1/10 wave error, Strehl Ratio = 0.97, but actual telescopes have more elements so that an overall 1/5 wave error is very good. For example, even the simplest fix power telescopic sight has 4 lenses so 0.97 x 0.97 x 0.97 x 0.97 = 0.88 overall. Also all mirror telescopes with direct film or digital image sensors have at least 2 mirrors so 0.97 x 0.97 = 0.94 overall.

 

[Fred Bohl]

Sorry, I probably should have included this in my post #109 above. For those that asked:

Strehl Ratio (From Edmund Optics)

The Strehl ratio of an optical system or component compares its real performance with an ideal version. The Strehl ratio of focusing optics, including spherical and aspheric lenses, is the ratio of maximum focal spot irradiance of the actual optic from a point source to the ideal maximum irradiance from a theoretical diffraction-limited optic (Figure 6). A Strehl ratio of 1 would indicate that an optic is perfect and aberration free. Common industry practice considers a lens “diffraction-limited” when the Strehl Ratio is greater than 0.8.


Figure 6: This lens has a Strehl Ratio of 0.826, which is considered diffraction limited because it is greater than 0.8

An optic’s Strehl ratio is approximately related to RMS transmitted wavefront error by Equation 8 where S is the optic’s Strehl ratio and σ is optic’s RMS wavefront error in waves.8 This approximation is valid when the wavefront error is <0.2 waves

 

[Fred Bohl]

Also, looking at your great table, I would point out that if you have a 56mm objective and you use a modifier disk or a reducer of some kind, you have to use the numbers that correspond to that size aperture. This explains why while gaining DOF with a smaller aperture, you also lose resolution.

You are correct, for the March Scopes:
52mm Objective w/35mm Modifier Disk, DRL drops from 0.039 to 0.055 MOA
56mm Objective w/45mm Modifier Disk, DRL drops from 0.035 to 0.043 MOA

Also, using the Modifier Disk also reduces light through the scope by about 50%

 

[Turbulent Turtle]

Yes. The March MDs provide for one full stop difference, so 50% reduction is correct.

 

[Turbulent Turtle]

I'm bringing this thread back so we can discuss observations gleaned from the recent F-Class National Championships in Phoenix, AZ earlier this month. I have some videos to share as soon as I clean them up.

The videos were taken through my Kowa 883 using the Kowa adapters for my Nikon D7500 during the team matches.

In the meantime, you might want to go look at the March website for the latest in high magnification precision riflescope.

 

[6MMsteve]

I have the 40x60 ep zoom HM I really like it, clarity on another level

 

[Ned Ludd]

Denys, I'm curious about the numerous references I've seen in the last couple years to scopes with ED glass that "cut through" mirage better than scopes commonly found among F-Class shooters 10 years ago or so.

From my [limited] knowledge of optics, I would guess that the glass or coatings themselves have less to do with this effect than does some internal arrangement of lenses or perhaps the parallax assembly. I surmise this because the the lenses and coatings themselves would appear to be more about light transmission; i.e. clarity, color, brightness, etc. Mirage/shimmer is a physical object, and as such, it could be envisioned how "better glass" could actually increase the effect of mirage/shimmer as viewed through a scope. In other words, I could see how increasing resolution with better glass and/or lens coatings might conceivably make mirage worse.

On the other hand, it seems as though some of the newer high-end scopes not only have better glass/coatings, but the parallax adjustment is much tighter. In other words, the depth of field at the focal plane is much tighter. The difference between mirage/shimmer and solid objects is that it is translucent. So it seems as though larger depth of field would make mirage/shimmer seem greater because the user would be looking through a thicker slice. Is it possible that the beneficial effect of some of these new scopes with respect to mirage is really more of an effect of a narrower depth of field than of the lenses/coatings?

I ask this because I just recently purchased a new IOR-Valdada scope with Schott glass, which is very good glass, and compared it side-by-side to one of my NF 12-42x56 NXS scopes. The optics of the Valdada were excellent to be sure, but certainly not orders of magnitude better than the NF. The main difference between the two as I could see it was that the mirage/shimmer looked totally different between the two scopes. It was much stronger in the NF scope. Notably, the parallax turret settings on the NF scope are not what I would call "tight", meaning I can turn the knob a bit in either direction and still remain reasonably "in focus". The parallax settings on the Valdada are as tight as tight can be. It's either in focus, or completely out of focus. That suggests to me a tighter depth of field, which could account for the apparent decrease in mirage/shimmer as one would be looking through a lesser distance of it before it was lost to being out of the depth of field.

 

[Turbulent Turtle]

Hello Ned, always a pleasure talking with you.

I started this thread about 2.5 years ago, and making the totally unsupported by existing science at the time, that ED glass and more so with Super ED glass, I was observing that the IQ degradation due to the phenomenon we incorrectly call "mirage" was retarted or somewhat contained compared to regular (non ED or Super ED glass.

Early on in this longish thread I was challenged to get confirmation from Deon about this. In post 79, I announced, a year after the prior post, that Deon had indeed confirmed that Super ED was doing a better job with shimmer (mirage) than their ED-glassed scopes. They had no regular glass scopes with which to compare as all non-LPVO riflescopes they make all have ED or in the case of a few models, Super ED glass.

The shimmer protection (mirage retardation) has nothing to do with coatings; it's inherent in ED but mostly Super ED glass.

Higher quality glass does do a better job in mirage compared to lesser quality glass because the resolution of the glass is better and that seems to help retard the degradation a little bit. But it ain't ED and definitely no Super ED glass.

Here is a link to Glass VS ED VS Super ED comparison.

In a proper riflescope design, the manufacturer strives to control the dispersion of one or two specific wavelength(s). ED glass, and especially Super ED glass reduces the dispersion of ALL wavelengths (Super ED more so.) The best dispersion control is with pure crystal fluorite lenses, which are just not found in riflescopes for various reasons. (Cost, fragility, susceptibility to environmental conditions.)

But fluorite glass is available in some camera lenses and spotting scopes. Kowa Prominars (883/884, 99A/S) have that type of glass. I bought a Kowa 883 just for that reason. More on that in time.

Now that DEON had realized that I was correct in my long-term observations that Ed and especially Super ED did in fact retard IQ degradation due to shimmer/mirage, they also discovered a few more things that further enhance this quality in a riflescope and they produced the March-X 8-80X56 HM WA with these qualities in mind.

The result was in the announcement to which I linked a few days ago above.

At the recently concluded US F-Class Nationals LR in Phoenix, Tim Vaught shot the entire LR championship with that riflescope set at 75X-80X, and won. Tim is an awesome shooter and has won numerous events but read the article again to see what he said about his experience with the riflescope.

I shot in the same conditions with my March-X 10-60X56 HM set at 50X all week long. It performed exceedingly well. I also looked through the March-X 8-80X56 HM WA in the same conditions and I was amazed that I could get to 80X with an IQ as good or better than the one in my faithful 10-60X56HM set at 50X.

 

[paulT]

Enjoying reading this thread,
Are you guys who are shooting longrange F class with the March 60x high master scopes using the march Mod discs with All mirage conditions or just when the mirage conditions are very soupy/Heavy to help improve target clarity
Also have you found any difference with mirage/target reading with the MD Placed withinn the sunshade or scope lens, thanks.

 

[Turbulent Turtle]

Hello @paulT. I rarely use the modifier disk on my 10-60X56 HM. When I do, I place it at the end of the sunshade, which is always on my riflescope. On the other hand, I know several people who use it quite a bit and to good effect.

The modifier disk cuts down the incoming light by 50%, which represents on F-stop in photography parlance. This has 2 effects: it increases the depth of field (DOF), and it reduces the resolution of the image. The DOF increase is certainly something you can actually see. The decrease in resolution is imperceptible in my opinion, unless you are at very high magnification, in which case it may "soften" the image.

If I may, let me explain DOF a little bit. The depth of field of an image is a function of 4 things:
1- F-stop
2- Focal length
3- Focus distance (distance to object, such as the target)
4- CoC (Circle of Confusion)

Let's say the distance between you and the object is set, you cannot get closer or further away.

In a camera you have control over the F-stop, the focal length (if you are using a zoom lens), and the focus distance (you can focus at a different distance between you and the object.) You cannot change the CoC.

In a riflescope, you have control over the focus distance (side focus) and the CoC (zoom lever/ring.) You can change the F-stop with a modifier disk. You cannot change the focal length.

The rules are:
The greater the F-stop number (smaller aperture), the larger the DOF.
The greater the focal length, the smaller the DOF.
The greater the focus distance the larger the DOF.
The smaller the CoC (higher zoom) the smaller the DOF.

There are formulas and calculators to figure all these things for you and they work very well for camera lenses. They are more difficult to use with riflescopes because we do not know the focal length (of the objective lens group) nor the F-stop of the lens. If we knew the focal length, we would also know the F-stop number of the lens because we know the diameter of the objective lens.

Here is a calculator that I like to use. Figure a CoC of between .030mm and .010mm depending on the zoom value.

 

[paulT]

Hello @paulT. I rarely use the modifier disk on my 10-60X56 HM. When I do, I place it at the end of the sunshade, which is always on my riflescope. On the other hand, I know several people who use it quite a bit and to good effect.

The modifier disk cuts down the incoming light by 50%, which represents on F-stop in photography parlance. This has 2 effects: it increases the depth of field (DOF), and it reduces the resolution of the image. The DOF increase is certainly something you can actually see. The decrease in resolution is imperceptible in my opinion, unless you are at very high magnification, in which case it may "soften" the image.

If I may, let me explain DOF a little bit. The depth of field of an image is a function of 4 things:
1- F-stop
2- Focal length
3- Focus distance (distance to object, such as the target)
4- CoC (Circle of Confusion)

Let's say the distance between you and the object is set, you cannot get closer or further away.

In a camera you have control over the F-stop, the focal length (if you are using a zoom lens), and the focus distance (you can focus at a different distance between you and the object.) You cannot change the CoC.

In a riflescope, you have control over the focus distance (side focus) and the CoC (zoom lever/ring.) You can change the F-stop with a modifier disk. You cannot change the focal length.

The rules are:
The greater the F-stop number (smaller aperture), the larger the DOF.
The greater the focal length, the smaller the DOF.
The greater the focus distance the larger the DOF.
The smaller the CoC (higher zoom) the smaller the DOF.

There are formulas and calculators to figure all these things for you and they work very well for camera lenses. They are more difficult to use with riflescopes because we do not know the focal length (of the objective lens group) nor the F-stop of the lens. If we knew the focal length, we would also know the F-stop number of the lens because we know the diameter of the objective lens.

Here is a calculator that I like to use. Figure a CoC of between .030mm and .010mm depending on the zoom value.

Thank you for your very informative reply
Turbulent turtle
Great info, With regard to focusing when watching Mirage say at 1000y Range
Do you focus at different Ranges During shooting string e.g spotting scope set halfway to the Target and with rifle scope set to watch both the mirage and Target for score, Thanks.

 

[Turbulent Turtle]

A great question. My thinking on this is evolved over time. Before I go any further, let me state that I am not a great shooter with a string of medals, awards and so on. I've been doing this a long time but lately, I've encountered challenges with my marksmanship. Age. So please keep that in mind as you read on.

As I've been discussing in this thread, for several years now, I have been of the opinion that ED glass and its betters (for the lack of a better word) allow the user to deal with mirage differently than before. In my mind, the order of glass with increasing resistance to IQ degradation due to mirage is as follows:

Regular optical glass.​

Premium optical glass.​

ED glass.​

Super ED glass​

Pure fluorite crystal glass.​

I used to think that the spotting scope should not have ED or better glass in order to get the full effect on the mirage in image. Since we keep them down to 20-25X and they are not used to aim but to monitor the conditions, good non ED glass was the way to go.

After spending some time with my March-X 10-60X56 HM, (Super ED glass), I found myself using that one to monitor the conditions much more than my Kowa 82-SV. I have a large wheel on the side focus knob that allows for much greater, granular control of the focus and I could snap back and forth to check mirage at different distances and then get back on target to take the shot.

In the 10-60X56 HM, I could make out the faintest mirage and not have the IQ degraded anywhere near what happens with non-ED glass. I found I was not using the 82SV anywhere near as much as I used to because of that. So I recently bought a Kowa 883 Prominar with pure crystal fluorite objective lenses. Reading the mirage with that puppy is amazing. I have just started deploying the 883 in position and in fact, the Nationals is almost the first time when I did that. Now I have to retrain myself to use the spotting scope more when in position.

The Kowa design has a dual focus control, which I like but I am still learning it. Currently at the 1000-yard line, I use the 25X LER eyepiece and I set up the 883 right alongside the scoped rifle. I can look through the 883 and get behind the riflescope very quickly and I'm still developing the setup. I use the fine focus to go back and forth between 500 & 1000 yards. I'm trying to find a set distance at which I would focus and leave it, but that seems to be different every time.

The fluorite lenses allow me to discern the faintest mirage, without messing up the IQ. It's actually a joy to look through. I shot several videos through the 883 using their digiscoping adapter and my Nikon D7500. After I clean up the videos, I will post a link here. The maximum magnification of the wide angle zoom eyepice that I use for digiscoping, is 60X, which is equivalent to my March-X 10-60X56 HM but shy of the March-X 8-80X56 HM WA. I video and different mags to provide a good feel for the mirage.