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Resolution and the Irrelevance of MTF (1 Viewer)

Tringa45

Well-known member
Europe
Sometimes a naive question on birdforum can provoke an interesting discussion and I stumbled on a recent one on "sharpness" rather late as the OP is on my ignore list. There were nevertheless some misconceptions on that thread and it has since gone OT, so I thought I would start anew.

The potential resolution of an objective lens is defined by its diameter, as light gets diffracted at its edges.
William Dawes' criterion was 4,56/objective dia. in inches or 116/objective diameter in mm. Accordingly, an excellent 116 mm telescope would be able to resolve 1 arcsecond (1") and a 58 mm telescope would be able to resolve 2".

Dawes' observations were based on the ability to separate double stars with a minimum loss of brightness of 5% at their merger but today we can use a 1951 USAF chart with ever decreasing separation of line pairs (increasing frequency of line pairs/mm) to measure resolution. With increasing frequency the contrast diminishes until the lines merge into a grey blur. At higher spacings (lower frequency) the contrast will always be higher than at the resolution limit and it is nonsense to suggest that a lens with high resolution might show inferior contrast at higher spacings than another lens with lower resolution.

There is often a tendency to make comparisons between binocular or telescope lenses and camera objectives. The latter are focal devices and project an image on to a film or sensor. Binoculars are afocal devices and the image is focussed on the observer's retina. To achieve this the focal planes of the objective lens and eyepiece are brought to, or near to coincidence, so that the observed image for normal vision appears at infinity or, in the case of corrected vision, at the eye testing distance of 20 ft/6 m.

Binoculars seldom have a FoV exceeding 9° but even a full format 50 mm camera lens has a 46° FoV and the requirement is not only to provide acceptable centre sharpness but also acceptable edge sharpness. If this were achieved at f/5,6 then the aperture would be a mere 9 mm, offering a maximum potential resolution of 13", a miserable value for a binocular.
"Binastro" on this forum has often stated that adding a prism and eyepiece (lens2scope) to even the best telephoto camera lens makes a very poor scope.
I measured the resolution of my little 65 mm Swarovski scope at 1,78", which is diffraction limited. To overcome the limitations of my own eyes I had to boost the magnification to 130x using a 3,5 mm astronomical eyepiece and was able to resolve 5 line pairs/mm or a line width of 1/10th of a mm at a distance of 23 m! I doubt that there is a commercially available camera lens capable of that, maybe though on a spy satellite. :)

MTF measurements on camera lenses show loss of contrast from the field centre to the field edge of specific frequencies of sagittal and tangential line pairs (10, 20, 30 lp/mm) at the focal plane (sensor) and do so for various apertures. One could not conduct these measurements on a binocular or scope because they are afocal instruments and edge performance could not be boosted due to the limitations of the booster or interchangeable eyepiece, which would itself introduce field curvature or astigmatism. Edge performance of binoculars and scopes is a matter of personal preference and will have to remain subjective.

John

PS:- Low frequency contrast can, of course, be bosted by digital processing and can occasionally be seen in the birdforum gallery when it has been used to excess. It can also be achieved in analogue photography by using a diluted negative developer so that boundaries are starved of developer and appear sharper.
 
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I think that when the average human struggles to resolve 1’ (minute of arc) it is silly to talk about the diffraction limited resolution of the objective lens on an instrument with 8 or 10 times magnification. It’s fun, and educational, but of limited relevance. (just my opinion)

No offense to John, or anyone else here who knows much more about the subject than I ever will.
 
I think that when the average human struggles to resolve 1’ (minute of arc) it is silly to talk about the diffraction limited resolution of the objective lens on an instrument with 8 or 10 times magnification. It’s fun, and educational, but of limited relevance. (just my opinion)
No offense to John, or anyone else here who knows much more about the subject than I ever will.
No problem, Richard, but if that little diffraction limited scope of mine can theoretically resolve a 0,9" object the contrast at such high spatial frequencies is very low, so at 60x magnification I would be getting close to 1' and seeing its limitations. By all accounts, some of the lemons amongst the scopes can be detected within the magnification range of their zoom eyepieces.

Most binocuculars with their fast focal ratios fail to get close to the diffraction limit but at 8x or 10x it is admittedly unlikely that a loss of resolution could be detected.

Regards,
John
 
Most binocuculars with their fast focal ratios fail to get close to the diffraction limit but at 8x or 10x it is admittedly unlikely that a loss of resolution could be detected.
This has more or less been my own opinion also. But kabsetz's comments here (link) were quite thought-provoking. I would really like to compare an average example of say a Zeiss Conquest 10x42 HD to a "cherry" (blind testing, of course) to see if I could notice any differences in perceiving detail at distance under sub-optimal viewing conditions.
 
I would really like to compare an average example of say a Zeiss Conquest 10x42 HD to a "cherry" (blind testing, of course) to see if I could notice any differences in perceiving detail at distance under sub-optimal viewing conditions.
This is an interesting thread, John. And post, Patudo.

I compared my 32mm CHD against several other binoculars, including 40-42mm, over several hours (different lighting conditions) and was surprised how well I was able to resolve makeshift targets at ~80 yards with the 32mm compared to the bigger objectives. It's made me wonder if my 32mm sample is typical or exceptional as it also has really noticable focus snap compared to all other binocs that I have used. But maybe it's the faster focus mechanism.

I did not use a common resolution chart, as I suspect that our brains can fill in missing information on a known target. I used a target unknown to me prior, that could barely be resolved with my vision.

Anyway, I ordered a booster for the next round! And maybe it'll be done blind, but I had so many samples on hand that I lost track of make and model and had to stop and look at what I just viewed through.

I did spend several months comparing an 82mm scope to like/same 82mm. The reference is a known good sample. I would call it a cherry based on numerous star tests at various distances, but would not claim to be an expert.

Anyway, the 82mm test sample was pretty good, subjectively from observation in the field. I compared under different conditions and distances over the months and it would sometimes come close to the reference scope but never fully match or exceed it. I did swap EP around but unfortunately only had one high power zoom so that was a limitation. Still, I had other fixed power EP too.

I finally had enough and did a star test using the EP off the reference scope and immediately saw issues with the diffraction rings. No surprise, but I really wanted to see what my subjective opinion would be from months of painful comparison before performing the star test.

I was really hoping that the test sample would fully match or exceed the reference too.
 
I compared under different conditions and distances over the months and it would sometimes come close to the reference scope but never fully match or exceed it.
I'd be really interested to know how you went about these field tests - what were you looking for, over what distances and conditions (and magnifications) and also in what ways did you feel the reference scope was better?
 
Anyway, I ordered a booster for the next round! And maybe it'll be done blind, but I had so many samples on hand that I lost track of make and model and had to stop and look at what I just viewed through.
I hope you got something suitable. Sometimes a tripod mounted second binocular behind the binocular to be tested will suffice, but this might result in intolerably small exit pupils.
Henry has often stated that the quality of the booster is not really important, so I think something like a 4x12 Vixen monocular or Opticron Gallery Scope would be ideal. You would have to adapt it to the eyecups (masking tape?) and ensure that the monocular objective is not too close to the eye lens to avoid vignetting.
Here USAF-Auflösungstestbild Taschenformat | Edmund Optics is the test card everyone should have. I oversaw it and bought the expensive glass slide. :(
The diffraction limit for your 32 mm Conquest is 3,6" but I think you could be more than happy if it managed 5"
I got a lot of help from "typo" on this forum when I started doing resolution measurements. Unfortunately he hasn't been seen for some time, but if you need any help, you could PM me.

John
 
I'd be really interested to know how you went about these field tests - what were you looking for, over what distances and conditions (and magnifications) and also in what ways did you feel the reference scope was better?

I could consistently see more detail with the reference scope. And achieve very good focus more easily. I had to fiddle with the test sample and it never seemed quite right.

Comparisons were done with scopes side by side, mostly at 75x, but I had to move the EP between scopes, which was a limitation especially with varying conditions. Distances from across a room to several miles, plus the nighttime sky. Clean air, dirty air, low light, bright light, you name it. It was done over several months!
 
I hope you got something suitable. Sometimes a tripod mounted second binocular behind the binocular to be tested will suffice, but this might result in intolerably small exit pupils.
Henry has often stated that the quality of the booster is not really important, so I think something like a 4x12 Vixen monocular or Opticron Gallery Scope would be ideal. You would have to adapt it to the eyecups (masking tape?) and ensure that the monocular objective is not too close to the eye lens to avoid vignetting.
Here USAF-Auflösungstestbild Taschenformat | Edmund Optics is the test card everyone should have. I oversaw it and bought the expensive glass slide. :(
The diffraction limit for your 32 mm Conquest is 3,6" but I think you could be more than happy if it managed 5"
I got a lot of help from "typo" on this forum when I started doing resolution measurements. Unfortunately he hasn't been seen for some time, but if you need any help, you could PM me.

John

Thanks for the link and offer, John. I have a 3x and 4x booster now, but haven't done much with them except use them independently. They are handy to have in a shirt pocket!
 
I hope you got something suitable. Sometimes a tripod mounted second binocular behind the binocular to be tested will suffice, but this might result in intolerably small exit pupils.
Henry has often stated that the quality of the booster is not really important, so I think something like a 4x12 Vixen monocular or Opticron Gallery Scope would be ideal. You would have to adapt it to the eyecups (masking tape?) and ensure that the monocular objective is not too close to the eye lens to avoid vignetting.
Here USAF-Auflösungstestbild Taschenformat | Edmund Optics is the test card everyone should have. I oversaw it and bought the expensive glass slide. :(
The diffraction limit for your 32 mm Conquest is 3,6" but I think you could be more than happy if it managed 5"
I got a lot of help from "typo" on this forum when I started doing resolution measurements. Unfortunately he hasn't been seen for some time, but if you need any help, you could PM me.

John
Never one to hide my own obtuseness, I have a question: what is the need for a booster or second bino? Especially in view of what you said in your OP: "One could not conduct these measurements on a binocular or scope because they are afocal instruments and edge performance could not be boosted due to the limitations of the booster or interchangeable eyepiece, which would itself introduce field curvature or astigmatism."
I am about to order that mini test card in the hope it comes with instructions.
 
Never one to hide my own obtuseness, I have a question: what is the need for a booster or second bino? Especially in view of what you said in your OP: "One could not conduct these measurements on a binocular or scope because they are afocal instruments and edge performance could not be boosted due to the limitations of the booster or interchangeable eyepiece, which would itself introduce field curvature or astigmatism."
I am about to order that mini test card in the hope it comes with instructions.

The reason for a booster is that the centre resolution of most binoculars and good scopes would exceed the resolution capabilities of their users' eyes at standard magnifications.
Here is a thread that might help Do higher scope magnifications make any sense? although I started off with a false assumption. However, Henry soon set me straight. :)
MTF measurements are measurements of contrast for camera lenses at specific line frequencies across the field at the image plane (film or sensor) but could have no relevance to an afocal instrument like a binocular or scope.
If you get the Edmund 1951 chart this https://www.edmundoptics.com/knowledge-center/tech-tools/1951-usaf-resolution/ would show the line pair frequency, which would of course have to be set in relation to the viewing distance.
It should be possible to conduct binocular resolution measurements indoors with a well-illuminated chart but a good scope would easily resolve the 14,3 lp/mm at domestic distances and require an outdoor location.

John
 
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The reason for a booster is that the centre resolution of most binoculars and good scopes would exceed the resolution capabilities of their users' eyes at standard magnifications.
Here is a thread that might help Do higher scope magnifications make any sense? although I started off with a false assumption. However, Henry soon set me straight. :)
MTF measurements are measurements of contrast for camera lenses at specific line frequencies across the field at the image plane (film or sensor) but could have no relevance to an afocal instrument like a binocular or scope.
If you get the Edmund 1951 chart this https://www.edmundoptics.com/knowledge-center/tech-tools/1951-usaf-resolution/ would show the line pair frequency, which would of course have to be set in relation to the viewing distance.
It should be possible to conduct binocular resolution measurements indoors with a well-illuminated chart but a good scope would easily resolve the 14,3 lp/mm at domestic distances and require an outdoor location.

John
If the centre resolution exceeds the resolution of the eye at standard mag, ie. 30x of my BTX 85, why then does the higher resolution of 50x of my BTX 85 + ME 1.7x TC not exceed the resolution of theye even more. By my calculation the std. resolution of the BTX should be 40.9" and the resolution with the TC should be 68.2". The resolution of my o.8/20-25 vision should be around 48" in photopic conditions.
 
There's monochrome resolution, such as with the USAF charts, etc.; and then there is contrast and MTF.

Contrast and MTF in any objective will become important if you try to resolve minute details of low contrast, such as with astronomical observations of Jupiter (which is 47" across at best). That is, this matters when you are reaching towards the resolution limit and spatial frequencies above it. I realize this is seldom the case with binoculars, unless in areas where they are strongly aberrated (edge softness, edge CA, etc.)
 
20-25 vision US is 0.8 in the EU or any other country using a metric scale for measuring eye resolution (20-20 being 1.0); 48" is 48 arcseconds (the accepted standard resolution in daylight conditions for normal 20-20 vision being 1' (one arcminute) equal to 60" (sixty arcseconds)).
 
20-25 vision US is 0.8 in the EU or any other country using a metric scale for measuring eye resolution (20-20 being 1.0); 48" is 48 arcseconds (the accepted standard resolution in daylight conditions for normal 20-20 vision being 1' (one arcminute) equal to 60" (sixty arcseconds)).
Thank you, I wasn’t familiar with the EU format, ° ‘ “ I am familiar with.
 
20/25 visual acuity is less good than 20/20.

In the metric scale 20/20 is 6/6.

If one uses 0.8 as a measure then that is 20/16 or 6/4.8

The so called 60 arcsecond measure of resolution is a very loose term.

Resolution of what?
Double stars?
Unequal double stars.
Black spots on a white background.
White spots on a black background.
Etc. etc.

One also has to have optimum bright lighting conditions.

As to low contrast planetary features Jupiter is not really a difficult test.
Jupiter's moons are a difficult test.
I have not seen detail with a 12.5 inch scope, but some observers with good planetary eyes for faint detail do see detail on the moons.
Saturn is more of a test, as is cloud detail on Venus, best seen in twilight and especially in different colour filters.

Detail on Neptune and Uranus is difficult.

Regards,
B.
 
OT, but I disagree. Jupiter has wealth of features, barges, variously sized spots, details in belts in slightly differing hues, etc. You will always see something, even in a 50mm scope, but then you can always go deeper, seeing permitting.

I have seen details on Venus in a blue filter, no problem, similarly as various shades of cream and olive on Saturn, even the hexagon, orientation of the planet permitting. Uranus is barely 4" across and a bit uninteresting :)
 
If the centre resolution exceeds the resolution of the eye at standard mag, ie. 30x of my BTX 85, why then does the higher resolution of 50x of my BTX 85 + ME 1.7x TC not exceed the resolution of theye even more. By my calculation the std. resolution of the BTX should be 40.9" and the resolution with the TC should be 68.2". The resolution of my o.8/20-25 vision should be around 48" in photopic conditions.
The Dawes' limit is 116/D where D is the aperture of the scope in millimetres.
If the 85 mm objective module of your BTX were diffraction limited it should be able to resolve 1,36"
With a visus of 0,8 you should be able to resolve about 1,25' or 75". The scope resolution however is baseed on line pairs, so you would need a magnification of (2x75)/1.36 or about 110x to see the limitations of your scope.

John
 

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