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60x Resolution -- What Newspaper Reading Distance? (1 Viewer)
- Thread starter galt_57
- Start date
henry link
Well-known member
Dave, It's going to depend on the aperture of the scope. In bright daylight the resolution of a 120mm scope at 60x matches eyesght resolution. Scopes smaller than 120mm will require a shorter distance. The smaller the aperture the shorter the distance. Larger scopes won't do any better than the 120mm at 60x because the extra detail will not be visible unless the magnification is increased. I wonder if trying to read text as opposed to just discerning lines or spaces between lines might introduce an unpredictable variable in different observers. Henry
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alan_rymer
Well-known member
Rather than newspaper text, why not a page of text from a WP package with specific text and font types and pitch?. We can then be sure we're reading from the same "Hymn Book"!.
henry link
Well-known member
Dave,galt_57 said:
I tried the newspaper reading test today with two different size Takahashi telescopes. I set up the newspaper at about 250ft, the longest straight line of site I could manage before bushes blocked the view. In an FS-78 (78mm aperture) the column type was easily readable at that distance. I had to move the newspaper to about 200ft to read the type using an FC-50 (50mm).
I think a smaller target would be better. Distances would be shorter and easier to measure. On the front of a US 1 dollar bill there is a small letter toward the upper left and the lower right. Beside those letters are some tiny numbers. Using those numbers would bring the distance down to 100ft or less, and they would be consistently the same size on every bill. I don't know how easy it is to find $1 bills outside the US, but I assume they are cheaper and more readily available than resolution charts.
Henry
henry link said:
Well, the difficulty is that I don't know how much of the blame lies with my own rather poor eyesight. At 250ft (80m) all I can make out are a few words like "The." Otherwise the text is too indistinct for me to read. The size of the text in the 60x scope seems to approximately correcpond to reading a newspaper directly at a distance of about six feet (2m) -- which I also cannot do.
henry link
Well-known member
galt_57 said:
Divide 250ft by 60 to get the eqivalent naked eye distance, about 4' 2". Try reading the newspaper with one eye at that distance under exactly the same light as in the telescope.
henry link
Well-known member
galt_57 said:
I'm hesitant to try to interpret the image that you posted. I'm a purely visual observer with no experience in digital imaging. That said; however, what I see on my computer monitor would correspond to poor visual performance for an 85mm telescope at 60X, if that is exactly what you see through the telescope. The image is very soft and low contrast.
After viewing your image I tried my Takahashi 78mm scope on a newspaper clipping at 316X, because I felt the apparent image size on the computer screen, even when I stood back from it, was larger than what I see at the eyepiece at 60X. But even at 316X the image quality at the eyepiece of the Tak is far superior to the digital image. It has much higher contrast between the black of the letters and the white of the paper with better defined edges.
Since I don't know what sorts of problems might be coming from the camera, etc. I'm unsure what to make of this image except to say that I think you should see a much better image in the eyepiece of an 85mm scope at 60X than what I see on my computer screen. Another defective scope, perhaps?
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Jay Turberville
Well-known member
galt_57 said:
That optical combination is limited due to difraction (Rayleigh limit) to about 97 lp/mm. The sensor of the camera can resolve about 115 lp/mm. So if the scope and camera lens were operating at the limit of diffraction, the image would still be soft since the camera can resolve more detail than the optics can theoretically resolve.
Of course, a digiscoping setup can't realistically be expected to deliver a diffraction limited image. Add to that the likelihood of atmoshperic interference and a very long shutter speed (1/7 sec) and we couldn't hope even on the best day to get a sharp image at that distance and magnification.
To add further to the problem, Rayleigh limit means that the line pairs would be barely distinguishable. They would be light gray and somewhat darker grey lines that would be kind of fuzzing together.
The image you posted reflects that kind of softness. But when I zoom in in Photoshop, I can find detail resolved that is 3-4 pixels in width. A sharp image on Bayer mask digicam will show detail that is between 1-2 pixels in width. So my finger in the wind guess is that you are problably resolving - barely - about 60 lp/mm since the image appears to have about half the detail that the sensor can resolve.
That is also in keeping with the notion that we expect to observe something less than the theoretically possible 97 lp/mm. It is also in keeping with a very large body of practical experience that tells us to expect soft results at scope magnifications above 30x and digiscoping equivalent focal lengths greater than 3000mm or so.
My guess is that your rig is doing about as well as you could expect a rig to do. Not bad at all.
henry link
Well-known member
Jay Turberville said:
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henry link said:
It does seem that from what I've read digiscoping is always done at 20x-30x but as a beginner I'm wishing to try everything to establish the rules for myself. Perhaps there is an article that describes what happens to the image quality above 30x ?
Dave
Jay Turberville
Well-known member
henry link said:
That "almost but not quite" corresponds to the Rayleigh numbers. At around 6000mm equivalent, the scope can almost, but not quite provide as much detail as the camera CCD can resolve. But remember that Rayleigh is base on the ability to distinguish two closely spaced stars. These are very high contrast objects. Rayleigh is just above the level where the MTF is barely distinguishable. At that level of detail, the contrast should be suffering.
I did an informal experiment some time ago because someone had asked the question about why it was that the scope's visual image was "always" sharper than the digiscoped image.
The following is a slightly edited copy of my "report" as posted to the Digiscoping Birds Yahoo Group.
..............................................................
Roy Halpin raised this question a month or so ago. The question was,
"why is it that we look through a scope and see wonderful detail, but
when we digiscope the image we get a lower level of detail?" Most of
us seem to agree that this seems to be the case, but I was wondering
if it really is the case or if we just aren't being careful
observers. While (unsuccessfully) waiting for a Bald Eagle or two to
land in a tree that I'm pretty sure they like, I happened on my opportunity.
Across the river (on the Bald Eagle's side) there was a Forest Service
sign identifying that side as a "sensitive area" and forbidding entry
during the current season. This turned out to be a pretty good subject
for a test.
The sign was about 50 meters away. I looked at it with my Rubinar and
homebrew eyepiece (about 25x) and noticed that there was an emblem of
some kind in the lower right. I couldn't quite make out what it said
on the bottom side but it seemed like it was almost legible. I was
pretty sure it said "Department of Something-or-Other". I was
guessing it was "Department of the Interior", but I couldn't be sure.
It was a bit hard to read because the scope image was upside down and
backwards. And at 25x in broad daylight, the Rubinar's central
obstruction introduces some slight variations in the image that can be
a bit distracting. An eyecup on the eyepiece might have helped by
providing shade and perhaps allowing my eye's iris to dilate open
wider thus minimizing the distraction from the obstruction shadow.
A shorter focal length (higher magnification) eypiece probably would have helped also, but I didn't have one available.
I had the Swarovski ATS80HD with me as well, so I mounted it using the
20-60x eyepiece and took a look. I couldn't make out the bottom text
at 20X or 25X. But at 30X I could finally read it for sure. It said
"Department of Agriculture". It was more clearly legible at 40x and
60x, but it wasn't exactly crystal clear. I could see atmospheric
disturbances as I viewed the sign.
So now I had a situation where I had a pretty objective piece of
evidence about the level of detail I was able to see with my eye
looking through the scope. So now the question was whether that level
of detail would be visible when I digiscoped the sign.
This is a full frame resized image of the sign that gives an idea of
the setting.
http://www.jayandwanda.com/digiscope/digi_v_eye/6578_Rub25x_21.4_resize.jpg
The Swarovski and 20-60X scope clearly gave me the best visual image.
The image was stable (except for the atmospheric disturbances). And the image had very good contrast and was color neutral. The Rubinar
always has a yellow cast (which I correct for when digiscoping by setting the white balance preset) and as mentioned before, the central obstruction can create little distortions in the image at low to medium powers that you have to kind of look around. The Rubinar can also be a pain to focus precisely while the Swarovski was smooth and easy. I get excellent digiscoping results from the Rubinar, but for visual observation, the Swarovski is the slam-dunk winner.
And true to form, the Rubinar turned in the overall sharpest image of
the emblem at the lower powers. I can't say for sure if this was the
luck of the drawwith the atmospherice disturbances or not. But I did take many different shots using the same settings for both scopes. Most of
the shots looked similar. But the best of the Rubinar were the best of
all images taken.
Which gets me back to the main point. I was able to take shots
through the Rubinar where the text was clearly legible even though I
was unable (barely) to read the text when viewing through the Rubinar.
Hmmmm - digiscoping seems to be beating the eyeball? BTW, this "best"
shot was taken with the camera aperture stopped down to f7.6. This is
something I have been experimenting with and I believe it can give a
slight edge in sharpness at times. Its subtle, but it really seems to
be a factor at times and this gives further support.
Here is a full resolution crop of that shot showing the emblem. There
was no tweaking done to the shot.
http://www.jayandwanda.com/digiscope/digi_v_eye/6578_Rub25x_21.4_crop.jpg
But as a counterpoint, here is an otherwise identical shot except that the camera was set to f4.8.
http://www.jayandwanda.com/digiscope/digi_v_eye/6568_Rub25x_21.4_crop.jpg
You can see that the f/4.8 image is not as sharp.
Now here is a shot with the same scope but with the camera zoom at
14.9mm F 7.6 providing less overall magnification. With less
magnification, the text isn't really legible. Though you might guess
at what it says.
http://www.jayandwanda.com/digiscope/digi_v_eye/6574_Rub25x_14.9_f7.6crop.jpg
The same shot but at f3.8 is ever so slightly less sharp.
http://www.jayandwanda.com/digiscope/digi_v_eye/6572_Rub25x_14.9_f3.8crop.jpg
This is a shot with the Swarovski ATS80HD at 30x on the zoom and
21.4mm F7.6 on the camera. With the extra magnification, the text is
pretty legible.
http://www.jayandwanda.com/digiscope/digi_v_eye/6589_Swa_30xZ_21.4_f7.6.jpg
This shot is with the Swarovski ATS80HD at 20x on the zoom and 21.4mm
F7.6 on the camera. The text becomes barely legible at these
settings. Its in the area of guessing based on previous knowledge here.
http://www.jayandwanda.com/digiscope/digi_v_eye/6594_Swa_20xZ_21.4_f7.6.jpg
And finally, here is an image with the Swaro at 60x and 21.4mm F4.8 on
the camera. BTW, its worth noting that once you get past 30x on the
Swaro and a camera zoom of 21.4, the effective f-number for the
combination is 8 or more. At 60X, it is f16. So stopping the lens
down should have no effect.
http://www.jayandwanda.com/digiscope/digi_v_eye/6594_Swa_60xZ_21.4_f4.8.jpg
OK. So this tells us that a digiscoping combination can resolve about the
same level of detail that our eyes can through the scope. It is
necessary to have the right amount of total magnification via the
camera zoom though. This is what was needed in so that the Rubinar's
image could be legible through the camera but not visually. The
camera lens provided that extra bit of magnification needed to make
the detail resolvable. But why then do almost all of us think that
digiscoped images aren't as sharp? Well - probably because they aren't.
Perceived sharpness isn't only about resolution. It is also about
image contrast. What our digiscoped images lack is high contrast in
these areas of high detail and in areas of less detail. So our images
don't "pop" like the image we see in the scope. The whole process of
going through the camera lens and having the image recorded, interpretated and interpolated by the camera CCD and electronics hurts contrast. In
fact, no camera can capture the brightness range that our eyes can and no camera has a processor as complex as our brain helping to interpret the data.
But it seems that the digiscoping combination - while able to preserve
resolution to a large extent - suffers in the contrast of its images.
What we are seeing is that the resolution performance at
50%MTF isn't very good. This is why we sharpen images. Sharpening
won't put detail in the image that wasn't there. But it can improve
image contrast and the resolution point at which we get a 50% MTF.
(Go to www.normankoren.com if you wan't to learn more about MTF and
image sharpness). This is why "sharpened" images pop more even though
they don't resolve any more detail. Sharpenng compensates for a lack of
contrast between resolved details in the image.
This is the "best" image after some fairly heavy sharpening. Probably
more than what you'd want to do with a bird image for the web, but a
level that makes the text appear much more legible and probably about
right for printing. This image is also more like the the view through
the scope than the unsharpened images.
http://www.jayandwanda.com/digiscope/digi_v_eye/6578_Rub25x_21.4_crop_sharp.jpg
This excercise also demonstrates two other things. That stopping down
the camera lens at relatively low scope magnification may be worth
trying and that image sharpness while digiscoping is rather fragile. Very small variables can have significant impact.
...........................................................................
I would add to that summary that part of the perceived sharpness versus digiscoped sharpness may have something to do with the overall magnification and the manner in which we view it. Once we take our digital image, we tend to zoom in on the details on the computer screen. My compter screen never looks as sharp as reality. It doesn't have the dynamic range. The particular detail we are examining may also subtend a much larger angle of our view than when we look through the scope. In other words, we are magnifying the image even larger than when looking through the scope. If we print the image, we definitely reduce the dynamic range as compared to what our eye can see. That is why artful manipulation of contrast and brightness is necessary in order to make truly fine print images. So the recorded image, by its very nature, can't really compete with the live view. The extra softness that high powered digiscoping adds just makes the problem even worse. If we want "sharp" images, we need to digiscope so that we have something close to 50%MTF response at the sensor's limit of resolution. That equates to using about half of the scope's visually usable magnification.
Furthermore, the eye sees differently than a camera. It doesn't take snapshots. The brain receives a stream of imagery and integrates it rapidly. It is a very active system. I wonder if in some ways it is like the astronomer's practice of stacking images. Random errors such as from atmospheric disturbances can, to some extent be averaged out by our brains.
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Thanks for the data dump Jay. This thread is actually related to another thread where Henry was discussing star testing results, which several Zeiss scopes had failed. I can't seem to perform the star test on my new Zeiss either. Since star testing is done at full magnification I thought a simpler target might also tell me something. I do not seem to be able to read a newspaper at 250 feet (80m) -- nor get a clear cp990 photo. Henry reported that he could see sharply at that distance. My poor eyesight and technique may yet be a factor.
Dave
Dave
henry link
Well-known member
Yes, thanks Jay, although that's a bit more than a non digiscoper like me can take in.
How about a test with fewer variables. Use a resolution chart combined with high magnification (could require 80-100X) to determine the visual limit of resolution with your ATS-80HD. Image the same chart at the same distance through the scope using whatever technique is required to produce the highest resolution digital image and compare the results.
Henry
How about a test with fewer variables. Use a resolution chart combined with high magnification (could require 80-100X) to determine the visual limit of resolution with your ATS-80HD. Image the same chart at the same distance through the scope using whatever technique is required to produce the highest resolution digital image and compare the results.
Henry
Jay Turberville
Well-known member
henry link said:
Hey - when the eagles don't show up, you can either nod off or find something useful to do.
I can only go to 60x on the ATS-80HD without buying an adapter and an additional high quality, high magnification eyepiece. High quality eyepieces matter a LOT when digiscoping. I just reminded myself of this today as I stupidly used a 25mm generic plossl on a distant shot. The price of the extra gear is not worth it to me.
Another problem is that is I need a better resolution chart. The chart I have won't hold up to high magnifications at around 50 feet and I have found that going further than 50 feet or so is hit or miss with getting good still air that won't munge up the results.
And since my main goal is digiscoping, I'm less concerned about the limits of resolution than I am about what provides good sharp digiscoped images and whether the optics are performing about as well as could be expected. Good sharp images definitely happen by operating well withing the limits of resolution. Recording detail at 10% MTF or so results in poor photographic quality.
Other than curiosity, the only reason I even pursued the question of "eye vs. camera" as far as I did was to verify that there wasn't some huge unexplained optical loss happening. It was a nice verification of theory excercise. I don't think there is any significant unexplained loss.
I think figuring out how to do star tests is probably the best way to verify the scope's optical performance. My problem with that is either tracking real stars at night or placement of a daytime star during the day. Maybe I'll see if I can place one of my ball bearings on our neighbors fence across the golf course and up the hill. I'll probably get arrested for stalking though.
But considering that my catadiotric tests showed pretty similar performance beetween the three scopes that I have, I strongly suspect that they are all operating at similar levels and that this is near what is commonly accepted as diffraction limited.
Sorry for the previous very long post. But I just wasn't up for a rewrite/reanalysis of that test.
The summary is that digiscoping does record very close to the same level of detail that we see visually. Sometimes perhaps even a bit more. But the contrast (MTF) appears to be less and that may be real, psycho-optical or perhaps a combination of both. So we shouldn't be surprised if the view through the scope looks better than the images from the camera.
rogerscoth
Well-known member
Jay,Jay Turberville said:Hey - when the eagles don't show up, you can either nod off or find something useful to do.
Sorry for the previous very long post. But I just wasn't up for a rewrite/reanalysis of that test.
There is no need to apologise for the long post - detail is surely required when only a full and concise explanation will answer the question.
Incidentally, may I ask what camera did you use for the test shots?
Roger
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