The effect of camera type and focal length on digiscoping resolution

[dahyon]

Last summer, I spent a week's holiday in Wales stuck on a kneeling chair with a bad back. Each day at about 9am a buzzard would settle onto a pole which turned out to be 118 paces from my location. I could not get a high quality photo using my Z85 and cp4500 with electronic remote rig. Eventually, I borrowed my son's canon A95 and handheld it to the 20x-60x eyepiece. This photo was one of the results:
Buzzard on pole - Canon A95 (http://www.birdforum.net/pp_gallery/showphoto.php/photo/135476/sort/1/cat/500/page/1)

I was shocked at the difference in detail. Why was this? I had to find out. Initially, I used Noman koren's lens test chart then tried slant edge analysis, first using the free SFRwin application and then finally Imatestlight also by Norman Koren. I photographed slant edge targets at 10 and 30 metres using a cx990, cx4500 and a canon95. Each focal length of the A95 was tested and matched by the cx cameras. The Imatest output calculates MTF50% at lp/mm and cycles/pixel amongst others. I then used digiscoping calculations Excel spreadsheet by George Raiche, www.digibird.com (http://www.digibird.com/) to calculate the effective focal length @35mm. The results were collated and graphed which I have embedded in this post.

My conclusions
1 Maxview 40mm eyepiece loses 15% resolution and is unaffected by focal length
2 Zeiss 20-60x eyepiece loses about 24% resolution for each 10mm focal length
3 My cx 4500 is at best has only 72% the resolution of the A95
4 Effective focal length of cx4500 of ~3000@35mm obtained at the maximum focal length of 32mm has only half the resolution of that using 23.4mm focal length
The MTF50 is not an intuitive measurement not is the slant edge method.

[Harlequinduck]

Last summer, I spent a week's holiday in Wales stuck on a kneeling chair with a bad back. Each day at about 9am a buzzard would settle onto a pole which turned out to be 118 paces from my location. I could not get a high quality photo using my Z85 and cp4500 with electronic remote rig. Eventually, I borrowed my son's canon A95 and handheld it to the 20x-60x eyepiece. This photo was one of the results:
pp_gallery/showphoto.php/photo/135476

I was shocked at the difference in detail. Why was this? I had to find out. Initially, I used Noman koren's lens test chart then tried slant edge analysis, first using the free SFRwin application and then finally Imatestlight also by Norman Koren. I photographed slant edge targets at 10 and 30 metres using a cx990, cx4500 and a canon95. Each focal length of the A95 was tested and matched by the cx cameras. The Imatest output calculates MTF50% at lp/mm and cycles/pixel amongst others. I then used digiscoping calculations Excel spreadsheet by George Raiche, www.digibird.com to calculate the effective focal length @35mm. The results were collated and graphed which I have embedded in this post.

My conclusions
1 Maxview 40mm eyepiece loses 15% resolution and is unaffected by focal length
2 Zeiss 20-60x eyepiece loses about 24% resolution for each 10mm focal length
3 My cx 4500 is at best has only 72% the resolution of the A95
4 Effective focal length of cx4500 of ~3000@35mm obtained at the maximum focal length of 32mm has only half the resolution of that using 23.4mm focal length
The MTF50 is not an intuitive measurement not is the slant edge method.

Thank you for posting this information. Digibird was a site I had referenced priot to puchasing my present equipment. I'm presently utilizing a fixed eyepiece for my Pentax PF 80ED. In fabricating a collar type adapter I used the riied and true eyeball method. In further modification of adapters I will remember to refer to this reference.
Thanks.

[Cyclops]

Wow look at those lines fall as the FL goes up! And a mamximum on that first chart of 7000mm focal length in 35mm terms! That woukd be some camera lens magnifying 140X!

[dahyon]

Wow look at those lines fall as the FL goes up! And a mamximum on that first chart of 7000mm focal length in 35mm terms! That woukd be some camera lens magnifying 140X!

Hello
The highest mag I went to with this testing was Zeiss zoom 40X @ 32mm focal length of Cp4500 which equates to 6240~35mm when rounded is 125x. Of course the highest mag with the zoom eyepiece is 60x which equates to 9360mm~35mm rounded to 187x with 50mm(35mm) as abitrary x1. the attached chart shows the range that can be obtained from this setup. i have also attached a photo taken at 187x as it happens this too was of a buzzard, compare this with
http://www.birdforum.net/pp_gallery/showphoto.php/photo/135476

which was taken at 118 paces ~ 100 metres.

This Zeiss 85 with zoom eyepiece is awesome.
Cheers, Dave