Optical quality: What makes a scope good or bad?
There has been several threads in this forum with the theme “What is the best spotting scope?” and there have been many well-meaning consumer – based recommendations as to what constitutes a good scope. I have been down this road as a shooter; I certainly understand that money is not infinite and that we all want the best for the least, but there reaches a point where you are probably better off waiting than investing in inferior equipment . Having spent several years in the commercial graphics/optics industry, and having logged a great deal of time behing medium and large format cameras, Leica M Series rangefinders, line cameras, enlargers, SLR's, and being a zone-system worshiper, I have acquired both a formal and informal education on what makes an optical tool good and bad. For me this has been a bit of a curse, because I can't always afford the tools I know to be the best. I know I am not alone, and to that end we cannot all affors to go out and buy the best Leica or Swaro spotters, but hopefuly, this will help "re-focus" your own efforts at judging what is truly good and bad in the spotting scope market.
Kodachrome does not lie. What you see is what you took, or more correctly, what you took through. "Live" optics, like scopes and binos are a bit deifferent in how we preceive them, but they are identical in terms of what comes out the other end.
To the lay-consumer, looking through a pair of $19.99 “as-seen-on-TV” binoculars, they have little experience against which they can judge quality, although with the price being only 1% of what a high quality pair of binoculars costs, one would think that the axiom, “you get what you pay for” would be a clue for most. Well, for most people, top-end optical quality is not necessary. For us precision shooters, it is a bit different. We want/need a tool that will do that, for which it was purchased.
When it comes to objectively evaluating scopes and binoculars etc., the human brain compensates for many flaws, and tends to use its own set of color filters. (A 40 watt incandescent bulb such as you would use in a table lamp burns with a color temperature of barely 3000 degrees kelvin… it’s light is very orange, yet our eyes make it white. Likewise, fluorescent lights cast green light, yet our brain likewise filters out that hue.) We may perceive a scope to be better than it really is, because we never evaluate a scope in its intended environment before we buy them. By that, I mean, few if any have the opportunity to take their scope to a 1000 yard range on a hot July day and see what results it renders. likewise, what does it do on a cool overcast day, where a scope should be able to see not only bullets holes, but the staples that hold the target.
All scopes have flaws. The extent of those flaws is based upon the quality of the scope in terms of the glass used, the numbers of elements, the coatings, and the design; there is a direct correlation between cost and quality. (do your homework however, because charging too much for a scope does not transform it into a good scope)
Light is electromagnetic energy. It is essentially a continuum of the same type of energy as a radio wave, except where radio wave lengths are measured in meters, reflected, visible light is measured in billionths of meters. (visible light is anywhere from 400 – 700 bilionths of a meter) The red end of the spectrum is lower energy, whereas the blue end is higher. IR scopes and cameras are simply “radio” receivers for the frequency of wave energy in the Infra-red spectrum.
Light signals are similar to radio signals in that we need a “receiver” to interpret them. We happen to have been born with two light “radios” in our skull, and just as there is a difference between a top-end Harmon-Kardon stereo receiver and a Canadian Tire crank-powered survival radio, there are differences in the quality of “Light receivers” we call spotting scopes.
Our eyes are made up of a single lens. Because each side of that lens is bent – more specifically because the back side of the lens is curved, if our retina was not also curved, our vision would resemble looking through a magnifying glass, where only the center is clear, and the edges are distorted and blurry. We have a curved plane that matches the contour of the lens, and as such the images our eyes “see” are for the most part free from distortion. Our brain is the ultimate “Adobe photoshop” and filters or corrects the rest. (The image projected onto the retina is actually inverted… anyone who has ever used a view camera is familiar with the concept).
Every device that magnifies an image does so at a price in terms of the result. The simplest image magnifier is a plain old magnifying glass. That single simple lens makes the middle image larger, but even a superficial examination of that image reveals many optical flaws in terms of distortion and sharpness. It is possible to significantly reduce the flaws that come with attempting to magnify an image, but this takes good glass, good coatings, and multiple precision ground glass elopements. When corners get cut in trying to eliminate these flaws, several types of short-comings appear. The following is a break-down of many of the more common flaws found in cheap spotting scopes.
Color distortion (Chromatic aberration)
When we manipulate light in order to magnify an image (as in the case of a spotting scope, or a rifle scope) we are re- pointing that light at an angle different to that in which it would normally fall. This means that each different wavelength of light will land in a different spot. While yellow light may be perfectly focused, the blue and red wavelengths are not. One falls into focus beyond the yellow and one falls in front.
This creates a particular problem in the spotting scope used for shooting, because whenever there is an area of sharp contrast, such as the fringe between a white and black area on a target, (unless corrected) the result is a fringe of pink/purple blur. This aberration is less noticeable when looking at textured surfaces such as a stucco wall or a chimney, because there is less contrast. It may appear sharp, but that fringe will obscure sharpness in areas of high contrast (such as a bullet hole against a contrasting background. )
To correct this phenomenon, scope makers can use a number of different techniques. First of all, they may employ glass that is specifically manufactured to mitigate the degree of chromatic aberration. “ED” refers to “Extra Low Dispersion” glass or glass coatings, and is associated with scopes made by Nikon and Pentax. (Nikon actually manufactures its own glass). APO is a term generally associated with APOCHROMATIC correction of chromatic aberration, where a compound element (multiple lenses on close proximity) focuses each wavelength differently, or lenses incorporating different compounds (UD, SUD, CaF2, LD, SLD, ED etc.). Some use aspherical lenses, or lenses with a deliberate form of astigmatism that helps to focus light on one plane of focus.
All quality scope makers useone, or a a combination of these techniques. Low dispersion and achromatic glass, aspherical elements, multiple achromat elements, and proper lens coatings.
To PROPERLY judge the degree of chromatic aberration in a scope, focus that scope on a piece of white paper, on which there are multiple black lines, such as an eye chart. Pay close attention to the fringe between light and dark, or to the areas between black lines. A proper resolution chart used with a spotting scope reveals such defect quite dramatically
Sharpness/contrast
Most people simply look through a scope evaluate its quality or usefulness purely by judging how sharp the image is. Fair enough.
The tendency is to simply look through the center of the scope, and ignore the periphery. (much less look at the areas of contrast transition as mentioned previously) The target shooter needs an image that is sharp from edge to edge to help evaluate shooting conditions, including mirage, blades of grass, flags etc.
When scope makers economize, they do so in a number of areas…they use poor quality glass, or use only enough elements to render the middle portion of the image as sharp. The result is often a combination of things:
- The outer fringes may appear less sharp
- The outer edges distort the image (“pin-cushion” or barrel shaped curves to vertical and horizontal lines) also called spherical distortion.
- Poor image density, with the image appearing “grainy”
- The viewing area angle is narrow.
- The image is washed out or lacks contrast.
The typical “cheap” spotting scope has a narrow viewing angle, and in terms of its sharpness, do not judge it by the middle 25% of the image, pay attention to the outer edges. Aim that scope off center and try and focus your target in the outer 50% of the circle.
The shooter's spotting scope is a tool that is used for more than just looking at bullet holes in the middle of the target. It is used to watch waves of mirage, and to watch dust, cotton and other airborne debris, to evaluate wind conditions. A scope that does not focus properly in its periphery does not render mirage well at all.
Mirage, by its very nature, is distorted and diffracted light. Using a poor quality scope to evaluate a degraded quality of ambient light only makes that function more difficult.
The majority of serious shooters tend to use eyepieces of a fixed magnification. It renders a higher quality image, as it requires a less complicated optic in order to render the image. It also results in a wider field of view. The technology to produce a variable power eyepiece with minimal distortion and high resolution is extremely expensive. Most low-end scopes come with variable power eyepieces. This is merely degrading rather than augmenting any quality the scope may have had.
Viewing angle is likewise important. Shooters using a .308 at 1000 yards will be lobbing their bullets as much as 35-40 feet above the line of the bullseye. This is often above the area viewed by the spotting scope, and yet this is the area where we want to evaluate mirage. A wide, SHARP viewing angle is an essential tool for long distance precision shooting.
Scope Brightness.
If you are a bird watcher and your venue is the darkness of dawn or dusk, then a scope with maximum brightness is essential.
For the shooter, the need for a bright scope is slightly different. Trying to look through a dim scope on a bright sunny summer day is very difficult. Your pupils constrict in response to high ambient light, and their ability to transmit light is physiologically limited. Dim scopes are very difficult to use on those sorts of days. It is also the same reason why dim rifle scopes are difficult to use… it has nothing to do with using in low, light, it has everything to do with using them on bright days.
Good light transmission is really important. Variable scopes and compact spotting scopes generally suffer in this regard.
Ultimately, target shooters on a budget would be far further ahead to combine their spotting scope and rife scope budgets together and simply invest in a high-quality rifle scope.
I feel the most useful technique for evaluating scopes is to look through the several different kinds you'll see at a match. Everyone wants to boast of their toys, and you can even put them side-by-side. This was a truly interesting exercise for me, because I found the Pentax PF80ED out-performed or matched the best Swaro, Leica and Kowa scopes on the range. It made my own choice very clear.
There have been many good articles posted on the net regarding spotting scopes. I cannot over-state the benefits of becoming self-educated.
http://www.birds.cornell.edu/AllAboutBirds/gear/scopes/compare
http://www.spottingscopereview.net/target-spotting-scope
Regards,
Ian
There has been several threads in this forum with the theme “What is the best spotting scope?” and there have been many well-meaning consumer – based recommendations as to what constitutes a good scope. I have been down this road as a shooter; I certainly understand that money is not infinite and that we all want the best for the least, but there reaches a point where you are probably better off waiting than investing in inferior equipment . Having spent several years in the commercial graphics/optics industry, and having logged a great deal of time behing medium and large format cameras, Leica M Series rangefinders, line cameras, enlargers, SLR's, and being a zone-system worshiper, I have acquired both a formal and informal education on what makes an optical tool good and bad. For me this has been a bit of a curse, because I can't always afford the tools I know to be the best. I know I am not alone, and to that end we cannot all affors to go out and buy the best Leica or Swaro spotters, but hopefuly, this will help "re-focus" your own efforts at judging what is truly good and bad in the spotting scope market.
Kodachrome does not lie. What you see is what you took, or more correctly, what you took through. "Live" optics, like scopes and binos are a bit deifferent in how we preceive them, but they are identical in terms of what comes out the other end.
To the lay-consumer, looking through a pair of $19.99 “as-seen-on-TV” binoculars, they have little experience against which they can judge quality, although with the price being only 1% of what a high quality pair of binoculars costs, one would think that the axiom, “you get what you pay for” would be a clue for most. Well, for most people, top-end optical quality is not necessary. For us precision shooters, it is a bit different. We want/need a tool that will do that, for which it was purchased.
When it comes to objectively evaluating scopes and binoculars etc., the human brain compensates for many flaws, and tends to use its own set of color filters. (A 40 watt incandescent bulb such as you would use in a table lamp burns with a color temperature of barely 3000 degrees kelvin… it’s light is very orange, yet our eyes make it white. Likewise, fluorescent lights cast green light, yet our brain likewise filters out that hue.) We may perceive a scope to be better than it really is, because we never evaluate a scope in its intended environment before we buy them. By that, I mean, few if any have the opportunity to take their scope to a 1000 yard range on a hot July day and see what results it renders. likewise, what does it do on a cool overcast day, where a scope should be able to see not only bullets holes, but the staples that hold the target.
All scopes have flaws. The extent of those flaws is based upon the quality of the scope in terms of the glass used, the numbers of elements, the coatings, and the design; there is a direct correlation between cost and quality. (do your homework however, because charging too much for a scope does not transform it into a good scope)
Light is electromagnetic energy. It is essentially a continuum of the same type of energy as a radio wave, except where radio wave lengths are measured in meters, reflected, visible light is measured in billionths of meters. (visible light is anywhere from 400 – 700 bilionths of a meter) The red end of the spectrum is lower energy, whereas the blue end is higher. IR scopes and cameras are simply “radio” receivers for the frequency of wave energy in the Infra-red spectrum.
Light signals are similar to radio signals in that we need a “receiver” to interpret them. We happen to have been born with two light “radios” in our skull, and just as there is a difference between a top-end Harmon-Kardon stereo receiver and a Canadian Tire crank-powered survival radio, there are differences in the quality of “Light receivers” we call spotting scopes.
Our eyes are made up of a single lens. Because each side of that lens is bent – more specifically because the back side of the lens is curved, if our retina was not also curved, our vision would resemble looking through a magnifying glass, where only the center is clear, and the edges are distorted and blurry. We have a curved plane that matches the contour of the lens, and as such the images our eyes “see” are for the most part free from distortion. Our brain is the ultimate “Adobe photoshop” and filters or corrects the rest. (The image projected onto the retina is actually inverted… anyone who has ever used a view camera is familiar with the concept).
Every device that magnifies an image does so at a price in terms of the result. The simplest image magnifier is a plain old magnifying glass. That single simple lens makes the middle image larger, but even a superficial examination of that image reveals many optical flaws in terms of distortion and sharpness. It is possible to significantly reduce the flaws that come with attempting to magnify an image, but this takes good glass, good coatings, and multiple precision ground glass elopements. When corners get cut in trying to eliminate these flaws, several types of short-comings appear. The following is a break-down of many of the more common flaws found in cheap spotting scopes.
Color distortion (Chromatic aberration)
When we manipulate light in order to magnify an image (as in the case of a spotting scope, or a rifle scope) we are re- pointing that light at an angle different to that in which it would normally fall. This means that each different wavelength of light will land in a different spot. While yellow light may be perfectly focused, the blue and red wavelengths are not. One falls into focus beyond the yellow and one falls in front.
This creates a particular problem in the spotting scope used for shooting, because whenever there is an area of sharp contrast, such as the fringe between a white and black area on a target, (unless corrected) the result is a fringe of pink/purple blur. This aberration is less noticeable when looking at textured surfaces such as a stucco wall or a chimney, because there is less contrast. It may appear sharp, but that fringe will obscure sharpness in areas of high contrast (such as a bullet hole against a contrasting background. )
To correct this phenomenon, scope makers can use a number of different techniques. First of all, they may employ glass that is specifically manufactured to mitigate the degree of chromatic aberration. “ED” refers to “Extra Low Dispersion” glass or glass coatings, and is associated with scopes made by Nikon and Pentax. (Nikon actually manufactures its own glass). APO is a term generally associated with APOCHROMATIC correction of chromatic aberration, where a compound element (multiple lenses on close proximity) focuses each wavelength differently, or lenses incorporating different compounds (UD, SUD, CaF2, LD, SLD, ED etc.). Some use aspherical lenses, or lenses with a deliberate form of astigmatism that helps to focus light on one plane of focus.
All quality scope makers useone, or a a combination of these techniques. Low dispersion and achromatic glass, aspherical elements, multiple achromat elements, and proper lens coatings.
To PROPERLY judge the degree of chromatic aberration in a scope, focus that scope on a piece of white paper, on which there are multiple black lines, such as an eye chart. Pay close attention to the fringe between light and dark, or to the areas between black lines. A proper resolution chart used with a spotting scope reveals such defect quite dramatically
Sharpness/contrast
Most people simply look through a scope evaluate its quality or usefulness purely by judging how sharp the image is. Fair enough.
The tendency is to simply look through the center of the scope, and ignore the periphery. (much less look at the areas of contrast transition as mentioned previously) The target shooter needs an image that is sharp from edge to edge to help evaluate shooting conditions, including mirage, blades of grass, flags etc.
When scope makers economize, they do so in a number of areas…they use poor quality glass, or use only enough elements to render the middle portion of the image as sharp. The result is often a combination of things:
- The outer fringes may appear less sharp
- The outer edges distort the image (“pin-cushion” or barrel shaped curves to vertical and horizontal lines) also called spherical distortion.
- Poor image density, with the image appearing “grainy”
- The viewing area angle is narrow.
- The image is washed out or lacks contrast.
The typical “cheap” spotting scope has a narrow viewing angle, and in terms of its sharpness, do not judge it by the middle 25% of the image, pay attention to the outer edges. Aim that scope off center and try and focus your target in the outer 50% of the circle.
The shooter's spotting scope is a tool that is used for more than just looking at bullet holes in the middle of the target. It is used to watch waves of mirage, and to watch dust, cotton and other airborne debris, to evaluate wind conditions. A scope that does not focus properly in its periphery does not render mirage well at all.
Mirage, by its very nature, is distorted and diffracted light. Using a poor quality scope to evaluate a degraded quality of ambient light only makes that function more difficult.
The majority of serious shooters tend to use eyepieces of a fixed magnification. It renders a higher quality image, as it requires a less complicated optic in order to render the image. It also results in a wider field of view. The technology to produce a variable power eyepiece with minimal distortion and high resolution is extremely expensive. Most low-end scopes come with variable power eyepieces. This is merely degrading rather than augmenting any quality the scope may have had.
Viewing angle is likewise important. Shooters using a .308 at 1000 yards will be lobbing their bullets as much as 35-40 feet above the line of the bullseye. This is often above the area viewed by the spotting scope, and yet this is the area where we want to evaluate mirage. A wide, SHARP viewing angle is an essential tool for long distance precision shooting.
Scope Brightness.
If you are a bird watcher and your venue is the darkness of dawn or dusk, then a scope with maximum brightness is essential.
For the shooter, the need for a bright scope is slightly different. Trying to look through a dim scope on a bright sunny summer day is very difficult. Your pupils constrict in response to high ambient light, and their ability to transmit light is physiologically limited. Dim scopes are very difficult to use on those sorts of days. It is also the same reason why dim rifle scopes are difficult to use… it has nothing to do with using in low, light, it has everything to do with using them on bright days.
Good light transmission is really important. Variable scopes and compact spotting scopes generally suffer in this regard.
Ultimately, target shooters on a budget would be far further ahead to combine their spotting scope and rife scope budgets together and simply invest in a high-quality rifle scope.
I feel the most useful technique for evaluating scopes is to look through the several different kinds you'll see at a match. Everyone wants to boast of their toys, and you can even put them side-by-side. This was a truly interesting exercise for me, because I found the Pentax PF80ED out-performed or matched the best Swaro, Leica and Kowa scopes on the range. It made my own choice very clear.
There have been many good articles posted on the net regarding spotting scopes. I cannot over-state the benefits of becoming self-educated.
http://www.birds.cornell.edu/AllAboutBirds/gear/scopes/compare
http://www.spottingscopereview.net/target-spotting-scope
Regards,
Ian
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