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Testing prime focus digiscoping setups (3 Viewers)

New Stuff

Make and model of DSLR: Canon EOS 40d
Make and model of scope: Takahashi FSQ 106 w/ Extender Q
Type, make and model of camera adapter: AP PFCT w/ Canon EOS t ring
Length of spacer needed to reach focus: none
Focal length of complete setup in mm: 850mm f/8
Measured distance between target and telescope in meters: ~16m
Evaluated resolution number:
Speed setting of camera: .3 sec
ISO: 100
Original picture format (RAW, JPG) and resolution (mpx) RAW 12mpx
Date and time: 07/11/2008
Weather conditions (sun and wind): N/A
Other pertinent information: shot in basement- cool, slightly humid, like all basements.

i also shot the ISO target. post those up in a bit.

the smaller chip made a BIG difference! - i'm still a little short, however of 12 mp, and it shows.
 

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All of the tests reported in this thread have ignored a fundamental parameter necessary for any meaningful interpretation of test chart results. The easiest way to explain is to refer to the 'Koren 2003' target because this has been explained in detail on Norman Koren's site; before his development of Imatest he used the sine pattern chart together with ImageJ (which can be obtained free from NIH web site) to plot the frequency response of his sine pattern. He clearly states that the 250mm chart should form an image of 5mm on the sensor. This is achieved by photographing from a distance of 50 focal lengths.

A simple equation shows the relationships between the variables:
F = (S/W)*D
Where:
F= Focal length
S = Width of object on Sensor
W = Width of Object.
D = Distance to Object

If the value of D is less than 50 focal lengths then a correction factor has to applied to allow for the change in magnification.

It is this parameter of magnification (S/W) intimately linked with distance D that is not being considered.

I used the Koren sine chart for a number of years testing my 35mm lenses on an istD DSLR before moving on to Imatest. The sine test chart is a good approximation to the newer slant edge analysis of spatial frequncy response where MTF is defined as the normalized magnitude of the Fourier Transform of the imaging system’s point spread function. Alternatively, the MTF of the sine pattern describes the attenuation of sinusoidal waveforms as a function of spatial frequency.

I am prepared to analyse a few complete 25cm sine pattern images (as original jpegs)in Imatest, if anyone would care to send them to me. The software is now quite smart and all that is required is an image of a complete sine pattern chart at full original size no cropping.
 
Dave,

Thanks for your comment about our test protocol. Your offer to evaluate some tests with Imatest is very kind and I am sure some will take advantage of it.

However, I think that the sine wave pattern that needs to form an image of 5mm on the sensor is not what we are looking for. Our test target does not use the the Koren target and we are only interested in finding the smallest distance between 2 lines that we can resolve. We also want to test for chromatic aberration, a very important factor for digiscoping. IMHO, if our test is flawed, all photographs that are not taken at a distance of 50 wave lengths must also be flawed... Your comment is correct if one is interested in making a scientific test of a lens to test all of its caracteristics, which is not the case for most of us.

Yes, one can analyse a lens using a software that costs more than a good scope and obtain scientifically correct results but we are only interested in doing a simple comparison between different digiscoping setups, using simple tools and without having to be a guru in optics. So far, testing has been done in this forum by posting photographs of birds - large and small - close and far away - colorful and grey... It is simply impossible to do a meaningful comparison this way. There must be a better way, somewhere between the Imatests and the "Look, my bird is sharper than yours". This is exactly what we are trying to accomplish. I don't think we are there yet but I think it can be done.

Once we have a decent tool to compare different digiscoping setups, we will have accomplished a lot. It should bring significant improvements in digiscoping and more enjoyment of this fine hobby, which has so far been the outcast, in French "parent pauvre", of bird photography.

If we all use the exact same test, we will be able to meaningfully compare the results. Yes, the test won't be scientific, the results won't be expressed in optic terms. Yes also, one will be able to evaluate his digiscoping setup and compare it to others - and it will be acccessible to all without cost.
 
Hello Jules
Your test target is of course based on the ISO 12233 standard which requires that the active chart area (20cm 35.6cm) had to fill the full frame of the camera's ccd sensor using the framing arrows provided. The points I made still apply to your target. I have taken the liberty of analysing the 100% crops of 2 of the early targets just to illustrate the usefulness of Imatest. The resolution had to calculated in an unusual way because only cropped images were available.

You are obviously not aware of the earlier work I have produced on digiscoping resolution, no matter; I am intrigued by the high quality images from astro-scopes and am still willing to use Imatest of a full size image of your original ISO12233 copy which includes the 5 degree sloped squares which are there for the purpose of slant edge analysis. Thus you would obtain MTF values and CA giving you all the information you desire and some you are not aware of yet.
 

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Dave, some interesting read here,, not much on math these days and age but will re-read a couple times and try to figure out your data,,

Derry
 
Dave, some interesting read here,, not much on math these days and age but will re-read a couple times and try to figure out your data,,

Derry

Impressive test Dave, but IMO too complicated for our needs. Unfortunately, I don't think we need nor want that sophistication.

Regards
Jules
 
Jules
I can't simplify that output because I do not have the original files.

To illustrate: I have taken the ISO12233 image from the DPReview test of the Canon450D and analysed it with Imatest and present a simplified summary output showing change of resolution across the frame the change in chromatic aberration across the frame.

I have used the slant edge analysis method on all my digiscoping rigs the summary results of some have been placed in this forum some considerable time ago.

You have embarked on a scientific enterprise where standardisation and reliability are mandatory requirements.
 

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You have embarked on a scientific enterprise where standardisation and reliability are mandatory requirements.

Sorry Dave but I don't agree with you. We ONLY want do see if a setup will be better than another one to take the picture of a bird at a distance. If one can resolve 20 on the target and the other one is limited to 15, then the first one will be doing a better job. That's all...

If people abide by the protocol, standardisation and reliability should be good enough for our purpose. There is nothing scientific about that. Our test should not be compared to Imatest or Koren in anyway. It will probably be an interesting leap forward for digiscoping compared to what has been done so far.

Of course there are people like you who will want to be more accurate and that are interested in scientific results. There are tools to do that, like you have demonstrated, but IMO they are unfortunately not for most of us.

Regards
Jules
 
Good morning Jules
You are of course entitled to your opinion. Do others agree?

For myself I am only trying to help clarify some misunderstandings on test chart measurement. For instance the ISO chart 12233 being used in your protocol has a maximum resolution of 2000 lw/ph but the Canon 450D as shown in my previous post can resolve upto 2500 lw/ph. This explains in part why some of our digscoping group are passing the maximum on your protocol. The updated ISO 12233 chart being used by DPReview now has a maximum of 4000 lw/ph to cater for the latest generation of DSLRs. This kind of detail cannot be reached with a home printer.

For those who have tested the Koren sine pattern and apparently exceeded the 200 lp/mm think about this: A Canon 450D is 2848 pixels high and is 14.8mm high, the maximum the sensor can resolve is therefore 96 lp/mm. The discrepancy is due to the magnification factor as mentioned in my earlier post.

It is not my intention to browbeat anyone; I really want to know how good these astro-scopes are. If one of the contributors such as Derry sends me an original uncropped jpeg of the ISO12233 I will run Imatest for that individual, the data can remain private if they so choose.
 
Good morning Jules
You are of course entitled to your opinion. Do others agree?

For myself I am only trying to help clarify some misunderstandings on test chart measurement. For instance the ISO chart 12233 being used in your protocol has a maximum resolution of 2000 lw/ph but the Canon 450D as shown in my previous post can resolve upto 2500 lw/ph. This explains in part why some of our digscoping group are passing the maximum on your protocol. The updated ISO 12233 chart being used by DPReview now has a maximum of 4000 lw/ph to cater for the latest generation of DSLRs. This kind of detail cannot be reached with a home printer.

For those who have tested the Koren sine pattern and apparently exceeded the 200 lp/mm think about this: A Canon 450D is 2848 pixels high and is 14.8mm high, the maximum the sensor can resolve is therefore 96 lp/mm. The discrepancy is due to the magnification factor as mentioned in my earlier post.

It is not my intention to browbeat anyone; I really want to know how good these astro-scopes are. If one of the contributors such as Derry sends me an original uncropped jpeg of the ISO12233 I will run Imatest for that individual, the data can remain private if they so choose.

Thanks for the valuable info Dave. We know that the last version of the target and protocol is not correct. If it can stop raining :C I will do more tests. I think that, by testing at a fixed distance, let's say 25 meters, we won't exceed the maximum resolution of the target.

Regards
Jules
 
did two test today,,

1. camera - scope,,

2. camera - Olympus TC 1.4 - scope,,

upper right corner crop is from the original raw file,,


Make and model of DSLR: Olympus E3
Make and model of scope: Televue TV85
Make and model of converter or barlow: Olympus TC1.4
Type, make and model of camera adapter: Olympus 4/3 to T mount
Length of spacer needed to reach focus: 50mm
Focal length of complete setup in mm: 600mm ((840 with TC1.4))
.....................................................Camera has 2X crop factor (1200mm) ((1680 with TC1.4))
Distance to target: 10m ((14m with TC 1.4 ))
Speed setting of camera: 1/400 ((with TC 1.4 1/160))
ISO: 100 on all photos
Original picture format (RAW 10.7MP) to Jpeg 5.5MP
Date and time: 14th july 16:09 / 16:15
Weather conditions (sun and wind):sunny with clouds blowing through,, wind in the 5 knot area


----- camera ------------ corner crop ------------ camera with 1.4 ------- corner crop
 

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Last edited:
I think that we will have to increase the distance in order to stop resolving at 20 max. Tomorrow, I will test 20-25-28-30m.
 
agree Jules,, if it is sunny here tomorrow I'll try the same distances with and without the 1.4 converter,,

Derry
 
Good morning all
I now understand that some of you are striving to develop a qualitative test chart for relative comparison of astro-scopes.
I know that there has been a lot of time invested in developing the ISO 12233 chart here in this group but this chart was designed for scientific measurement and I suspect designed to make it difficult to reproduce at home.

Now if you want to develop a chart for qualitative comparison which can printed on a home printer then I suggest you have a look at this page:

http://www.clarkvision.com/imagedetail/relative-lens-sharpness

I have compared this method with Imatest and it works rather well for resolution. For relative comparisons you could just report the the value 8,6,4,3,2,1.

I have also been pondering about a qualitative test for chromatic aberration. One only has to photograph a black square on a white background to detect CA if present. With so many lens reviewers using Imatest there is a range of published images showing varying levels on chromatic aberration so it wouldn't take much effort to collate these to create a kind of strip chart showing the spectrum of CA. Thus it would be a simple matter of checking your own CA result with the pre-calibrated strip chart. Though I suspect the astro-scopes will have very little CA. In the mean time I will collate my own series of CA results an construct what will probably be the worst case scenario CA chart.
 
Dave, appreciate your continued input to this thread. The only problem I see with a chart like the one on the ClarkVision website in your last post is that the finest detail lines are right at the edge of the page. We are more interested in resolution at the centre of the lens so that chart would need to be chopped about a bit to be useful to us.

Although the current test isn't scientific and it wasn't meant to be, it does allow you form a simple visual interpretation such as sharpness, edge sharpness and CA. If there's something out there to satisfy a quick visual interpretation and anyone that wants to follow a more scientific approach then I'm sure we would all be happy to give it a try.

Paul.
 
Good morning all
I now understand that some of you are striving to develop a qualitative test chart for relative comparison of astro-scopes.
I know that there has been a lot of time invested in developing the ISO 12233 chart here in this group but this chart was designed for scientific measurement and I suspect designed to make it difficult to reproduce at home.

Now if you want to develop a chart for qualitative comparison which can printed on a home printer then I suggest you have a look at this page:

http://www.clarkvision.com/imagedetail/relative-lens-sharpness

I have compared this method with Imatest and it works rather well for resolution. For relative comparisons you could just report the the value 8,6,4,3,2,1.

I have also been pondering about a qualitative test for chromatic aberration. One only has to photograph a black square on a white background to detect CA if present. With so many lens reviewers using Imatest there is a range of published images showing varying levels on chromatic aberration so it wouldn't take much effort to collate these to create a kind of strip chart showing the spectrum of CA. Thus it would be a simple matter of checking your own CA result with the pre-calibrated strip chart. Though I suspect the astro-scopes will have very little CA. In the mean time I will collate my own series of CA results an construct what will probably be the worst case scenario CA chart.

Dave,

Thanks for the input and the link for this test.

I share Paul's opinion on this test and I don't think it would be more useful than ours once we find the sweet spot for the distance.

Regarding printing, I don't think it is a problem. With a simple Canon ink jet printer, I can easily print the Koren chart and resolve perfectly the maximum resolution of 200 lp/mm.

Regards
Jules
 
Hello Paul
I use the ClarkVision chart in tandem with slant edge analysis, mainly because I find it more intuitive to visualise than abstract shapes. What I had in mind for you was this kind of chart (see attached - reduced for web) which tests the whole image frame both for resolution and for CA.
What do you think?
 

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Hello Paul
I use the ClarkVision chart in tandem with slant edge analysis, mainly because I find it more intuitive to visualise than abstract shapes. What I had in mind for you was this kind of chart (see attached - reduced for web) which tests the whole image frame both for resolution and for CA.
What do you think?

Dave,

I see 2 difficulties with this testt - it is still scientific and requires math calculation behond the reach of many. Reading the Clarkvision site, it looks somewhat similar to the Imatest but less sophisticated and it doesn't require software to analyse the chart.

Like you said it so well, we are trying to achieve a QUALITATIVE test. Even if I can understand those calculations if I make the effort, I don't really have an interest in optics nor the in depth knowledge to understand the full meaning of the results. All I want is a simple number that will reasonably describe the resolution of a given digiscoping setup and that will make comparison possible.

Second, focusing on it may be quite difficult at 50 ft. Unfortunately, we don't have access to a full resolution file to print and test it.

I have tested our target derived from the ISO one yesterday at 20-25-30 meters and found it difficult to focus accurately. Results below - the mm value includes a 1.6 sensor crop factor.

2 findings from these tests:

1- The more magnification, the easier it is to focus.
2- The maximum resolution value increases with magnification, which does not make sense for our tests :C

IMO, this clearly demonstrates that our initial parameter of using a distance that will fill the width of the sensor completely with the target is the way to go. This is also a parameter of the Clark test.

In order to achieve what we are trying to do, we will need a target that is not as easy to resolve as our version of the ISO chart. The maximum resolution number should be impossible to reach. Unfortunately, it may be difficult to print.

Opinions please !
 

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As it's feather detail we are trying to resolve why not have a black box of a set dimension on the chart and stick a feather to it, not to exceed the edges of the box. The detail on a feather is much finer than we can print. To make the chart easier to focus on we could have some bigger, bolder black areas that are easier to see in the viewfinder. Just some random thoughts.

Paul.
 

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