PF ED 65 vs. 80 mm? (1 Viewer)

[FrankD]

I just recently had the opportunity to pick up a Pentax 80 mm ED A. I have owned the 65 ED A for about two years now. Initial comparisons (today only) have proved interesting. First impressions are that the 65 is actually a hair sharper and, believe it or not, seems somewhat brighter. I know this flies in the face of conventional wisdom, and physics, so I am looking for some possible answers. I have been tinkering with several eyepieces on both scopes.....

Pentax XW 20 and 10 mm
Knight Owl EWA 20, 15, and 9 mm
Meade 25 mm plossl

It is difficult to get both scope at the same magnification with similar quality eyepieces. The closest I can get is with either the XW 20 or the EWA 20 on the 65 mm and the Meade 25 on the 80 mm. Despite the difference in quality level and despite which combination I switch back and forth the 65 mm seems to provide that brighter and sharper image under everything but the dimmest light conditions.

A few thoughts come to mind:

1) I had read where the original 65 mm had some optical "improvements" over the original 80 mm model. Could this be influencing my impressions? It seems somewhat far fetched.

2) I have also seemed to notice that there appears to be somewhat more product variation in the 80 mm versus the original 65 mm. Is it possible I have an "average" 80 mm model and an above average 65 mm?

3) Could the difference in magnification for any given eyepiece (on both scopes) be affecting the perceived level of brightness? I expected to find the 80 mm as bright at 26x as the 65 mm is at 19.5x. Is this an unreasonable assumption?

Any thoughts/comments would be appreciated.

Thank you.

FWIW I have the Vixen lanthanum zoom coming in at the end of the week. I wonder how it will perform on each scope and if I will end up with similar impressions as those I already mentioned.

 

[Oregon Jim]

Frank, I have the PF-65a II, the PF-80 as well as the PF-80a. Recently I tested all three with a military resolution chart that my brother sent me. I have the Pentax XW20, XW14, XW10, Pentax SMC zoom and a 17mm Baader Hyperion. About all I was interested in was sharpness and resolving capabilities. In summary, the two PF80's were virtually identical to each other with all the eyepieces, while the PF-65 was slightly less sharp. I noted that I could resolve better with the PF-80's, even at lower magnification.

This next weekend I'll be birding with a couple of friends who each have a PF-65a II. If we get a chance, I'll see if we can compare all three scopes. Who knows... maybe my 65 is sub-par. Regardless, if I had to choose only one scope and one eyepiece, it would be the PF-65a II and the Pentax XW14. It's just so much more handy to use, and lugging a PF-80 around gets tiring. Anyway, I'll report back on the 3 PF-65a II scopes if possible.

 

[Sout Fork]

3) Could the difference in magnification for any given eyepiece (on both scopes) be affecting the perceived level of brightness? I expected to find the 80 mm as bright at 26x as the 65 mm is at 19.5x. Is this an unreasonable assumption?

Brightness is a function of the exit pupil of the optical system limited by the exit pupil of the human eye at any given time.

For daytime viewing the human eye has an exit pupil of about 3.5mm.

Thus assuming all the Pentax scopes are using the same 22mm FL ep with an exit pupil of 3.5mm, and thus matching your eyes daytime exit pupil, they should all be transmitting about the same amount of light at the following magnifications...

65mm=18x
80mm=23x
100mm=28x

From my own practical experience these numbers are pretty much true.

SF

 

[henry link]

Frank,

You can see a diagram of the optical design of the 65mm at the Japanese Pentax website here:

http://www.pentax.co.jp/english/products/sougan/scope/

Basically a triplet up front with a moving doublet focusing element (their description is 5 elements in 3 groups). The 80mm is not illustrated, but is quite different. There is an air spaced doublet up front and a stationary singlet placed about where the focusing lens is in the 65mm (3 elements in 3 groups). Focusing in the 80mm is accomplished by moving one part of the Porro cluster. The designs are different, but I wouldn't necessarily say the 65mm is "improved". I notice that the focal length of the 80mm angled version appears to be longer than the straight. There is no focal length spec given on the website for the 80mm's and the eyepiece chart is inconsistent, varying from 504mm to 540mm for the angled.

One problem with the 80mm is that the moving prism of the focusing system increasingly stops down the objective at distances closer than infinity. I tested one of these a few years ago and found that the clear aperture at closest focus was reduced to about 68mm. That specimen was not very good optically with considerable pinching and astigmatism. I star tested another one later that was much better.

If you really want to know what's going on you're going to have to star test these scopes. If you don't know how to do it now is the time to learn. There is really no substitute if you want to know how good a telescope really is. Comparing them to each other will not reveal the absolute quality of either. Measuring resolution and comparing that to diffraction limited resolution for their respective apertures would also be informative. At least star testing is free and you can buy a cheap USAF 1951 resolution plastic card from Edmund Optics.

Henry

 

[npos]

[QUOTE=
2) I have also seemed to notice that there appears to be somewhat more product variation in the 80 mm versus the original 65 mm. Is it possible I have an "average" 80 mm model and an above average 65 mm?
**********************************

I "auditioned" a Pentax 80mm at a tent optics fair a few years ago and its optical performance was abysmal. The enthusiastic and friendly manufacturer's rep seemed totally oblivious of his inferior sample on display and I did not have the boldness to betray my opinions. This spotter, though, has a loyal following so there must be good quality specimens in the field.

Norm P
Massachusetts, USA

 

[FrankD]

Jim,

Regardless, if I had to choose only one scope and one eyepiece, it would be the PF-65a II and the Pentax XW14. It's just so much more handy to use, and lugging a PF-80 around gets tiring.

Even after having only owned the 80 mm for a day I think I would have to agree. I keep both of the scopes downstairs in the den. I had to lug both of them upstairs today and the difference was quite noticeable, both in terms of weight and overall size. I thank you for sharing your comparison experiences. I would be eager to hear more comments once you try your friends' scopes.

SF,

Your calculations seem to match my initial expectations which leaves me a little bit confused as to why I was noticing so much of a difference. Something that I know was discussed in the binocular forum in the past was the effect that apparent field of view and the subsequent size of the object within that field of view has on perception. The object I had been comparing the scopes on was a freestanding, domed birdfeeder at about 20 yards. While attempting to compare both scopes at the same magnification I was forced to use an eyepiece with a narrower apparent field of view on the 80 mm. In which case the birdfeeder took up much more of the field of view. I have a feeling that if I would have picked an object at a much further distance then my experiences may have been different.

Henry,

As always your insight is appreciated. I will pick up one of the USAF charts and attempt to test the resolution of both scopes. As you mentioned I am sure that would speak volumes to their individual performance and should point out any optical deficiences.

Measuring resolution and comparing that to diffraction limited resolution for their respective apertures would also be informative.

You lost me a bit here with this statement. Can you explain what the diffraction limited resolution is to a layman?

Norm,

When posting this thread I had not given thought to the fact that it might appear I was dissatisfied with the 80 mm. If I had just purchased the 80 mm first I think I would have been extremely impressed with it optically. The 65 mm at lower powers spoiled me a bit in terms of perceived brightness, field of view, etc... (see some of my above comments on possible misconceptions based on the apparent field of view). I only had a brief time to again tinker with the 80 mm again today. Tomorrow isn't going to provide any opportunity. This weekend should give me plenty of time to get a better feel for it.

Thank you all for your thoughts.

 

[Sout Fork]

Frank,
Just a little anecdote:

My wife was asked to do a three day dawn to dusk survey of an area where it was suspected that a rare bird was nesting.

We set up both the 80mm and the 100mm side by side and just sat and observed. We were both using eps that gave us a wide field aprox 30x view in our respective scopes (a 17mm ep in the 80 and a 20mm in the 100).

The long and the short of it was that the 100mm was only "brighter" at the extreme margins of the day - the first hour before sunrise and the last hour after sunset. Any other time of day and there was no practical difference in brightness.

However after the 80mm became useless at dusk the 100mm was still able to give us stunningly detailed views for another 30-45 mins later into the evening. This extra evening time was very important to us because it is known that this particular birds most active feeding time is very late in the day and this might be the only time we would be able to get a positive identification. As it turned out we were never able to make a positive ID.

SF

 

[henry link]

The object I had been comparing the scopes on was a freestanding, domed birdfeeder at about 20 yards. While attempting to compare both scopes at the same magnification I was forced to use an eyepiece with a narrower apparent field of view on the 80 mm.

Frank,

At 20 yards the 80mm Pentax is severely stopped down by the focusing prism. The angled version might be restricted to 65mm or less.

FWIW I find star testing more useful than resolution testing. Resolution testing can tell you there is a problem, but star testing can tell you why there is a problem.

If you decide to buy the USAF 1951 resolution chart there is a formula for converting line pairs per mm to arc seconds of resolution. Once you've done that you can determine how close a scope is to "perfect" resolving power for its aperture. You multiply the resolution in arc seconds by the aperture in millimeters and the resulting number indicates how far from perfect the scope is. Ideally the number you want to see is around 115-120. For example the resolution of a diffraction limited 80mm scope using the USAF chart in daylight would be around 1.5 arc seconds (1.5 x 80 = 120). I've never tested a birding scope that good. The best cherry samples of birding scopes would mostly fall between 130-135. More typical scopes between 145-155. Anything over 160-165 I would consider to be a true lemon. Obviously none of us want a lemon, but I don't want a mediocre scope either. What I'm really after is a cherry and those are hard to find.

Henry

 

[FrankD]

The long and the short of it was that the 100mm was only "brighter" at the extreme margins of the day - the first hour before sunrise and the last hour after sunset. Any other time of day and there was no practical difference in brightness.

SF,

Your findings mirror my own initial findings with the 80 mm and the 65 mm. Only at the very extreme end of the day did the 80 mm seem to overtake the 65 mm for brightness. I have been tinkering with both scopes and an inexpensive Celestron I picked up yesterday. My initial findings still appear to hold true but I am betting alot of it is the result of the brighter/sharper image that a lower power setting exhibits.

Henry,

Thank you very much for that explanation. A friend is being kind enough to send me a USAF resolution chart. I will follow your instructions and see if I can come up with some numbers before the week is out. Star testing, and just testing at further distances, will also follow.

 

[FrankD]

FWIW I star tested the scope tonight and it seems fine. The out of focus image on both sides of perfect focus seemed as one would expect. I lack the proper terminology to describe it but there wasn't any unusual shape to the image. I am waiting on the resolution chart and will post more as I have time to tinker with it.

I did have more time to experiment with the scope and I think I have found some simpler answers to my apparent concern. For starters it seems that eye placement is more critical on the 80 mm with any given eyepiece. If I happen to be looking through the outside "edge" of the exit pupil then the image seems less sharp and almost seems to swim in and out of focus. With perfect eye placement the image is razor sharp and entirely comparable to the 65 mm...and brighter to boot.

Second, and this is my fault for not considering it or mentioning it earlier, I was comparing all three scopes while looking out the back window off of my deck. For whatever reason the image in the 80 mm is drastically sharper while not looking through any intermediate glass. I do not have an explanation as to why but I am sure some of you could suggest some.

Three, focusing seems much pickier on my 80 mm. The focusing knob is faster and therefore it is a little bit more difficult for me to not roll right over "perfect focus". With a little tinkering and patience I was able to get a nice, sharp image.

Lastly, I think I am going to need to invest in a heavier/sturdier tripod for it. I took the 65 mm off of my 3130/3001 combo and it seems somewhat adequate for the 80 mm but with the center column fully extended (I stand 6'4" tall) there is just too much vibration while trying to focus the scope. I have to focus and then wait for the image to settle down before trying to determine if the image is in perfect focus. Suggestions in this regard would be appreciated.

Thank you again folks.

 

[Tero]

I have had it with tripods too. One leg keeps falling off my Manfrotto. Usually before I have it set up, so no disaster. The plate that attaches to the bottom of the scope also came apart once.

The tripods are steady enough with no wind and no touching, but...

I do not want an even more expensive tripod with the same problem. It does not take a rocket scientist to design one.

 

[stereotruckdriver]

FrankD, I'am having the same problem, same tripod-head as you have. I have problems using a 10mm or 7mm on my PF 65A. My 80mm is a straight scope and I'am 6'2" so I feel you're pain! I to would be interested in finding a different combo?

 

[FrankD]

I read some of the other suggestions on the threads in this forum as well as in the tripod forum. I guess I am going to have to read some more. I have one or two places locally I can check a few out. Maybe I will see if I can take the scope with me and try it out on a few combinations.

Thanks for sharing the similar experiences guys. Maybe collectively we can come up with something.

 

[mooreorless]

Frank, I have been thinking on this tripod thing. Some people have taken a piece of wood long enough to mount on the tripod like you would the scope and then mount the scope under this piece of wood so that the center of weight is closer to the pivot point instead of all the weight on top.This would lower it though. I will try to find picture of this.This probaly works better for astro scopes.
Regards,Steve

 

[jmepler]

FrankD and stereotruckdriver, you should check out the Manfrotto 055 series, they used to be the 3021 I believe.

I have a PF-80ED, straight model. I am only 5'9" but I need to only raise the center column an inch or two, of its 16" of travel, for comfortable viewing.

My tripod is a Manfrotto 055XWNB. The NB designation is for the retractable spike feet which will add another 2" of height which along with the 128RC (3130) head gives me a height range of 5'0" to 6'4" to the top of the quick release plate.

 

[Oregon Jim]

Until last spring, my only scope was a Pentax PF-80 straight body. My Bogen 3001 was totally inadequate, so I bought a 3021, but still struggled with stability. Once I extended the center column to match my eye level (I'm 5'11"), all bets were off. Anyway, knowing that I needed to find a way to make the center column more rigid, I came up with a cheap and simple addition made out of some 2" PCV pipe and two 1" reducers glued to either end. I have a small lathe that I used to turn the ends down slightly, but a file would do the trick too. Some black spray paint finished it up. Use considerable force to hold the column down while tightening the thumbscrew. Honestly, it made for a much more stable arrangement, and until I was able to move to angled bodied Pentax's, it served me quite well.

 

[young ian]

Good-day to everybody.
One quick/ cheap solution to
stop wobble/vibration. Fix a ' bungee 'rope from the
end of the scope to a convenient place on the tripod
leg, then tension as required.

Sadly, they talk of the strength / lightness of any
tripod under discussion but the never talk about the
'whip' dimension. Which in my opinion is the most
important part of the equation!!

Kindest regards,
Young Ian.

 

[Crake Dude]

Good zoom for 65 ED

I don't own a Pentax 65 ED but I had the chance to try one in an optics store recently. I use mainly zoom eyepieces, and the 'normal' zoom on this body was pretty rubbish. It was an astro store so they also had the very expensive, and expansive Pentax zoom. Not as great as I was expecting, but the Baader Hyperion 8-24mm zoom transformed the scope altogether...absolutely superb. Still edge CA visible wth the Baader(which may have been from the scope body?), and this eyepiece is half the price of the Pentax.

I used to have a Carl Zeiss 85 Diascope, and the Baader zoom was brill on that also, and it was more wide-angled through the powers, bitingly crisp, and showed less perimeter-edge distortion on the lowest power, but I already had the CZ zoom(vario-ocular, really)...unfortunately.

Steve

 

[FrankD]

Guys,

Thank you for all of the suggestions. I am going to give some of the less expensive options a try before going with a heavier duty tripod. However, it might eventually come to that. I will let you know what I end up with.

And, FWIW, the Vixen Lanthanum zoom seems to work entirely to my level of satisfaction with both the 65 and 80 mm models. I followed the advice on here and am glad I did. This zoom coupled with the XW 20 seems like an excellent combination to cover all bases.

 

[stereotruckdriver]

FrankD, how do you like the XW20? More importantly on the PF65? That is the one EP I don't have. I would be very interested to here you're thought's or anyone-else that has experience with that EP. The reason being I love the view at 20x on the XF ZOOM, just doesn't work for digiscoping! Thank's Stereo...