Scope Question

T

Top

Guest
I have a Kowa 663 Prominar Spotting Scope with a 20 x 60 Eye Piece and a Leupold 45 x 45 Competition Scope. At 100 yards the target is clearer, more defined with the Kowa when set at 45x as opposed to the Leupold Rifle Scope. Is this a good indication of the glass quality or lack of with the Leupold? Or are we talking apples and oranges here?

Thanks,

Tom Kennedy
Dublin, New Hampshire
 
Top said:
I have a Kowa 663 Prominar Spotting Scope with a 20 x 60 Eye Piece and a Leupold 45 x 45 Competition Scope. At 100 yards the target is clearer, more defined with the Kowa when set at 45x as opposed to the Leupold Rifle Scope. Is this a good indication of the glass quality or lack of with the Leupold? Or are we talking apples and oranges here?

Thanks,

Tom Kennedy
Dublin, New Hampshire
Click to expand...

Besides being one of the clearest spotting scopes available due to its excellent glass, the Kowa's much greater light gathering ability due to the larger circumference of the objective lens gives it a clear advantage over a rifle scope.
 
An increase in contrast

virg said:
Besides being one of the clearest spotting scopes available due to its excellent glass, the Kowa's much greater light gathering ability due to the larger circumference of the objective lens gives it a clear advantage over a rifle scope.
Click to expand...

The Kowa optics will let in more light than the scope. Increased light transmission will result in a larger degree of contrast particularly when viewing something like a target which is largely black ink on white paper anyway. The human eye is easily tricked. A higher contrast image is usually interpreted as being sharper than a lower contrast image even if the number of line pairs per mm both optical systems can resolve is the same.
Andy.
 
better resolution

Scope resolution is limited by and is proportional to objective diameter. Assuming quality to be very similar, the 60 mm spotter will provide 33% better resolution than the 45 mm scope (60/45 = 1.33).
 
90

It's actually proportional to the diameter squared, so it's 3600(60 squared) divided by 2025 (45 squared) or about 1.78. It's the same in astronomy - if you double the diameter of the scope objective, you get 4 times the area (and light gathering ability) and proportionally more resolution if the optics are any good at all!

Dennis
 
Sounds like a flow problem to me, a plumbing issue. Bigger will gener'ly win....'Course I'm in construction :)

al
 
DeltaBravo said:
It's actually proportional to the diameter squared, so it's 3600(60 squared) divided by 2025 (45 squared) or about 1.78. It's the same in astronomy - if you double the diameter of the scope objective, you get 4 times the area (and light gathering ability) and proportionally more resolution if the optics are any good at all!

Dennis
Click to expand...

I think you are confusing light gathering and resolving.

Light gathering and resolution (Britannica.com)

Light-gathering power: This capacity is strictly a function of the diameter of the clear objective—that is, the aperture—of the telescope. Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared; for example, a 25-cm (10-inch) objective will collect four times the light of a 12.5-cm (5-inch) objective ([25 × 25] ÷ [12.5 × 12.5] = 4).

Resolving power: This is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve. The resolving power of a telescope can be calculated by the following formula: resolving power = 11.25 seconds of arc/d, where d is the diameter of the objective expressed in centimeters. Thus, a 25-cm-diameter objective has a theoretical resolution of 0.45 second of arc and a 250-cm (100-inch) telescope has one of 0.045 second of arc.


Please also see:

http://www.rocketmime.com/astronomy/Telescope/ResolvingPower.html
 
Fred,

You're right about resolving power, but I wasn't addressing that side of it, altho it is as or more important. I was merely pointing out, as you did in your "light gathering" explanation that the relationship is based on the square of the diameter, not just the diameter. My understanding is that resolving power, outside the theoretical, is based on the quality of the glass in the optics. Better glass, better/more resolution. This holds true for astronomy, photography, bird watching, etc. My 80mm Celestron spotting scope is not nearly as clear/resolvable (is that a word??) as my Pentax 80mm spotting scope - it cost about 1/6th as much, so that's understandable!! My Nikon SLR lenses give me a much better picture than my point-n-shoots, megapixels being the same. You get what you pay for (usually) with optics; $$=clarity/resolution IMHO.

Dennis
 
$$$ versus Quality

Dennis,

As long as we've highjacked this thread...

The diffraction limit of resolution by objective diameter is only usefull as a comparison for as I stated "very similar" quality scopes. The other limitation on resolution is how well the scope design (glass quality, coating selection, lens grinding precision and lens positioning/alignment) corrects for the various aberations (spherical, color, internal reflections, etc.) that plague the design of optical systems.

The cost of design labor, assembly labor and materials is clearly more for the best quality designs than for the merely adequate (blister pack on a rack post). The same can be said for the cost of mechanical design and reliability to be expected.

Fred
 
I may be wrong, but IIRC, I read something a few years back that said the human eye cannot gather anymore light than a 44mm lens can let through. Now resolution is probably a completely different matter.
 
All true

I think you are confusing light gathering and resolving.

Light gathering and resolution (Britannica.com)

Light-gathering power: This capacity is strictly a function of the diameter of the clear objective—that is, the aperture—of the telescope. Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared; for example, a 25-cm (10-inch) objective will collect four times the light of a 12.5-cm (5-inch) objective ([25 × 25] ÷ [12.5 × 12.5] = 4).

Resolving power: This is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve. The resolving power of a telescope can be calculated by the following formula: resolving power = 11.25 seconds of arc/d, where d is the diameter of the objective expressed in centimeters. Thus, a 25-cm-diameter objective has a theoretical resolution of 0.45 second of arc and a 250-cm (100-inch) telescope has one of 0.045 second of arc.
___________________________________________________________________
The diffraction limit of resolution by objective diameter is only usefull as a comparison for as I stated "very similar" quality scopes. The other limitation on resolution is how well the scope design (glass quality, coating selection, lens grinding precision and lens positioning/alignment) corrects for the various aberations (spherical, color, internal reflections, etc.) that plague the design of optical systems.

The cost of design labor, assembly labor and materials is clearly more for the best quality designs than for the merely adequate (blister pack on a rack post). The same can be said for the cost of mechanical design and reliability to be expected.
____________________________________________________________________________________________

Yep all of the above true. As far as the eye is concerned it is a pretty lousy optical instrument. If your camera was fitted with a lens system with the same specs as your eye you would return it to the store complaining that the camera was faulty because of the picture quality it produced. The eyes saving grace is that its signals are processed and corrected by a computer so powerful it is aware of its own existence. Our brain.

However creating a larger diameter lens to transmit more light creates a lot more problems when keeping the aberrations to a minimum. It's quite easy to achieve a large aperature system that has lousy resolution capabilities.
Andy.
 
I did a very rough experiment some time back.

In the old days of film photography, when we read lens tests, it was pretty common to find that the maximum image quality did not coincide with the maximum aperture. It was usually a couple of stops down (half the diameter of the maximum aperture)

Along those lines, I cut a rough hole in a piece of tablet backing and went down the firing line one day (not at a benchrest match) asking if I could do a little experiment that involved target image sharpness. Of course in every case the image got a lot darker when I put my crude "stop" in front of the objective, but differences in image sharpness could still be seen. I tried it on rifle and spotting scopes. The whole process was brief and crude. A lot of the scopes' images were sharpened when stopped down, but not all. I tested my own 36X B&L last, and although the image was a lot darker, it was no sharper. I think that this illustrates that for rifle scopes, on a bright day, that sharpness may not be limited by objective diameter as much as quality of lenses, design, and quality of constructon. Of course not all of the scopes had adjustable objectives, so some of what we saw may have been the result of increases in depth of field when stopped down, compensating for fixed objectives not being focused for the target distance, but this would not apply in all cases, or to the spotting scopes.
 
Dusty Stevens said:
Scopes cannot gather light only transmit it.
Click to expand...

I disagree....

"to gather" is to collect, to pile up or bring together as in berries or cattle or crowds of people.

Or light.

When I useta' burn woodticks with the magnifying glass it weren't just "transmitting" the light, but gathering bunches of it into a small area.....

al
 
Spherical abbo v Diffraction

Boyd Allen said:
I did a very rough experiment some time back.

In the old days of film photography, when we read lens tests, it was pretty common to find that the maximum image quality did not coincide with the maximum aperture. It was usually a couple of stops down (half the diameter of the maximum aperture)

Along those lines, I cut a rough hole in a piece of tablet backing and went down the firing line one day (not at a benchrest match) asking if I could do a little experiment that involved target image sharpness. Of course in every case the image got a lot darker when I put my crude "stop" in front of the objective, but differences in image sharpness could still be seen. I tried it on rifle and spotting scopes. The whole process was brief and crude. A lot of the scopes' images were sharpened when stopped down, but not all. I tested my own 36X B&L last, and although the image was a lot darker, it was no sharper. I think that this illustrates that for rifle scopes, on a bright day, that sharpness may not be limited by objective diameter as much as quality of lenses, design, and quality of constructon. Of course not all of the scopes had adjustable objectives, so some of what we saw may have been the result of increases in depth of field when stopped down, compensating for fixed objectives not being focused for the target distance, but this would not apply in all cases, or to the spotting scopes.
Click to expand...

Yep when you make a larger diameter lens as free from aberrations as possible then only use the light rays produced by the center portion of the lens the light transmission drops but the image gains sharpness due to removal of the paraxial rays which come to a different focal point. Depth of field also increases as does depth of focus but the former only makes the image look sharper if the target is not positioned at infinity. For most scopes a target at 100 yards would not be. As you stop down further the image will eventually suffer from the effects of diffraction which makes the image look fuzzy again. Thats why real high end optics that require large diameter collectors will often make use of aspheric curves parabolics ( not just reserved for mirrors ) and the like to make better corrections.
Andy.
 
alinwa said:
I disagree....

"to gather" is to collect, to pile up or bring together as in berries or cattle or crowds of people.

Or light.

When I useta' burn woodticks with the magnifying glass it weren't just "transmitting" the light, but gathering bunches of it into a small area.....

al
Click to expand...

Thats not gathering thats concentrating. Gathering light up and sending it thru lenses is not possible unless my friends at CERN havent told me yet. Now a scope concentrating light into a smaller exit pupil is a whole different story but gathering up light is not physically possible. Schmidt and bender used to talk about this and ramble on and on about it
 
You must log in or register to reply here.
Back
Top