Swaro ATX/STX 65mm vs 85mm Objective Lens

[jmb1269]

As much as I know *facts* are not high in people's list these times, I will provide a few. Before, terminology: cherry == an optically perfect scope; lemon == a scope that should have not passed quality control.

1) With the exception of one model [1], all scopes use a fixed objective lens and a zoom eyepiece (or a fixed eyepiece in some cases). The objective lens (the big lens at the front) determines how much light the scope can get, the zoom determines magnification, field of view and exit pupil

2) exit pupil determines in its entirety how much light the image we see has. End of. This is not an opinion, it is physics. Exit pupil is calculated as objective_lens_diameter/magnification. This means that at the same magnification a scope with a larger objective lens will have a more luminous image. At the same time, if we use two different scopes at their largest exit pupils, the images are different in FOV and magnification, but the image coming from the larger exit pupil is brighter.

3) everything being the same (the Swaro ATX is perfect for this statement, since the only variable is actually the objective lens), a larger objective lens gives greater detail and resolution. Once more, this is physics, not an opinion. If your see more detail out of a 65mm scope than a 95mm scope, you are comparing a moderns HD glass cherry with an old, not HD glass lemon.

4) our eyes gather images, our brains see. Our eyes gather images that are upside down, have a hole in them, and have no perspective correction. Our brains put together two partially overlapping images, give us 3D vision, put the image right side up, hide the blind spots and correct for perspective. Again, a fact.

5) image perception is determined by how luminous, detailed and contrasty an image is, plus colour perception.

The upshot is that at 25x the 85mm gives a larger exit pupil than the 95mm at 30x (if you do not believe me just check the specks on Swaro' site). So, at the maximum possible exit pupil the 85mm gives a brighter image in low light, fact. For my use, the loss of a 5x mag (going from 25x to 30x) is less important than losing ~15% of the exit pupil surface.

I have no doubts that, comparing two good samples, the 95mm will give a more detailed and contrasty image. I have no doubts that comparing scopes at the same mag will advantage a larger objective lens in all metrics. Yet, in low light one goes to the lowest zoom point to gather as much light as possible -- in low light I care about exit pupil size above all other parameters. Hence, the 25x mag of the 85mm gives it an advantage in low light compared to the 30x of the 95mm, accepting lower mag and overall less resolution and contrast (at all mags).

Having said that, I have no doubt that detail and contrast can make up for loss of light for some people in some circumstances. If someone is colourblind the loss of brightness might be less of a problems compared to the loss of detail (as one example). All eyes are different, that's why our subjective perception through a scope is ours only, yet it matters above the specs on paper. Once one has gathered the information about the specs of different scopes the rational course of action is to go to the store, to see how our eyes and brains interact with the scope, and what actually matters most. Ideally spend hours in the store looking through the scope, to let your eyes adjust to it (eyes adjust to scopes, and change the performance over a few hours).

If you find the image coming from the 95mm better at low light, that is what you see. But that is your own experience, not a physical fact. At the same time your own personal experience of the image is really the only thing that matters

[1] the Zeiss Harpia has a zoom lens and a fixed eyepiece. The advantage is a very large FOV, the disadvantage is a small exit pupil size.

Great info! Thanks for sharing.

 

[fatrascal]

Some really great information in this post. When you decide to compare the 85 vs 95 you may as well compare the 65 at the same time. If you are a backpack hunter then I highly suggest the 65. I went through this same ordeal about 15 years ago and started with the swaro 80mm which was what was available at the time. After several years I switched to the 65 and have seen many backpack hunters do the same thing over the years. Backpack space and weight matters. And you really don't lose much at all between the 65 to 85 and even 95. If you truley need the extra light and resolution in the morning and evening then yes, go ahead and compare all 3 sizes. The great thing about the atx system is that you can get the 65 for backpacking and the 95 for glassing from the truck. At that point you don't need anything in the middle which makes the 85 obsolete. Just my opinion. Fatrascal.

 

[Elkangle]

I've spent a decent time with all 3...the 85 is my choice for all around hunting

I really enjoy the 25 power vs the 30 power..also enjoy the size of the 85 for packing multiple different ways on my pack

The 65 just doesn't get used anymore...seems like all the animals I want to look at don't come out until the last few seconds of light...if your sheep hunting sure but if your looking into the deep dark timber then the extra light is much appreciated

I don't care about the size or weight...if it works il use it...Il be adding the 115 to the mix and hopefully its has the same light the 85 does on 25x but at 30x


These are with the 85

 

[SecretSpot]

As much as I know *facts* are not high in people's list these times, I will provide a few. Before, terminology: cherry == an optically perfect scope; lemon == a scope that should have not passed quality control.

1) With the exception of one model [1], all scopes use a fixed objective lens and a zoom eyepiece (or a fixed eyepiece in some cases). The objective lens (the big lens at the front) determines how much light the scope can get, the zoom determines magnification, field of view and exit pupil

2) exit pupil determines in its entirety how much light the image we see has. End of. This is not an opinion, it is physics. Exit pupil is calculated as objective_lens_diameter/magnification. This means that at the same magnification a scope with a larger objective lens will have a more luminous image. At the same time, if we use two different scopes at their largest exit pupils, the images are different in FOV and magnification, but the image coming from the larger exit pupil is brighter.

3) everything being the same (the Swaro ATX is perfect for this statement, since the only variable is actually the objective lens), a larger objective lens gives greater detail and resolution. Once more, this is physics, not an opinion. If your see more detail out of a 65mm scope than a 95mm scope, you are comparing a moderns HD glass cherry with an old, not HD glass lemon.

4) our eyes gather images, our brains see. Our eyes gather images that are upside down, have a hole in them, and have no perspective correction. Our brains put together two partially overlapping images, give us 3D vision, put the image right side up, hide the blind spots and correct for perspective. Again, a fact.

5) image perception is determined by how luminous, detailed and contrasty an image is, plus colour perception.

The upshot is that at 25x the 85mm gives a larger exit pupil than the 95mm at 30x (if you do not believe me just check the specks on Swaro' site). So, at the maximum possible exit pupil the 85mm gives a brighter image in low light, fact. For my use, the loss of a 5x mag (going from 25x to 30x) is less important than losing ~15% of the exit pupil surface.

I have no doubts that, comparing two good samples, the 95mm will give a more detailed and contrasty image. I have no doubts that comparing scopes at the same mag will advantage a larger objective lens in all metrics. Yet, in low light one goes to the lowest zoom point to gather as much light as possible -- in low light I care about exit pupil size above all other parameters. Hence, the 25x mag of the 85mm gives it an advantage in low light compared to the 30x of the 95mm, accepting lower mag and overall less resolution and contrast (at all mags).

Having said that, I have no doubt that detail and contrast can make up for loss of light for some people in some circumstances. If someone is colourblind the loss of brightness might be less of a problems compared to the loss of detail (as one example). All eyes are different, that's why our subjective perception through a scope is ours only, yet it matters above the specs on paper. Once one has gathered the information about the specs of different scopes the rational course of action is to go to the store, to see how our eyes and brains interact with the scope, and what actually matters most. Ideally spend hours in the store looking through the scope, to let your eyes adjust to it (eyes adjust to scopes, and change the performance over a few hours).

If you find the image coming from the 95mm better at low light, that is what you see. But that is your own experience, not a physical fact. At the same time your own personal experience of the image is really the only thing that matters

[1] the Zeiss Harpia has a zoom lens and a fixed eyepiece. The advantage is a very large FOV, the disadvantage is a small exit pupil size.

Thank you!