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True Aperture? (1 Viewer)

G

giosblue

Well-known member
Quite a bits been said about the true aperture of spotting scopes in the past and recently. In particular the Pentax PF80ED.

I have both the Pentax PF80ED and the PF65ED. With the PF80 you can clearly see the exit pupil reduce in size as you close focus. I haven't measured it, but it's obvious. This doesn't happen on the PF65.

When you look down the objective lens and focus the scope, you can't see anything moving, however, I did the same thing with a Kowa TNS884 that I had for a week and you can clearly see the aperture reduce when looking down the objective lens into the scope. I didn't look at the exit pupil though, so I don't know if what I was seeing was having any effect on exit pupil size with the Kowa.
Could anyone with a Kowa 883/884 or any scope take a look at theirs and post their findings? Just curious.

With regards to the Pentax, as soon as you start to focus out and get to where you would be using a scope anyway, to me it makes no difference. And to be honest, I can't see the reduction in brightness anyway.

I had the Kowa 884 with the 25x60 zoom in direct comparison for week with the Pentax PF80 using fixed EPs and Baader zoom. You might think that the Kowa would blow the Pentax away and it would be no contest. Well it didn't, the PF80ED more than held it's own, and I decided to keep the Pentax and returned the Kowa. Yes the Kowa was better, but unless you had them side by side you wouldn't tell. And even then, for me it would hard to justify the price difference.

So I'm happy with my scope, despite all it's failings, size, weight, reduced aperture, it is an excellent scope, real value for money and highly recommended.

I would still like to know it the PF80 is the only scope that suffers from a reduction in aperture when you close focus though

Ron
 
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The Pentax is not the only scope that looses clear aperture at close focus. Every scope I've seen that uses a moving prism has the same problem, some worse than others. The Pentax PF65ED, Kowa 88/77 and most other alpha scopes use a moving focusing lens rather than a prism. I haven't noticed any aperture reduction in the scopes I've seen that use that method, but it could happen if the focusing lens were too small to clear the objective's light cone at the focusing lens' forward position. For scopes that use negative focusing lenses (most) that would result in reduced aperture at infinity focus. I have seen that happen in a few binoculars.

When you sight down the front of the Kowa the "aperture" you see is not the exit pupil. It's either the eyepiece fieldstop or some other internal stop. The change in its size you observed when rotating the focuser is not related to aperture. It results from the changing distance between the stop and the moving focusing lens.
 
Ron,
Confirming your findings, I have measured reduced aperture in my Pentax PF80a scope at close focus and also noted that this reduction does NOT occur with the Pentax PF65IIa scope I have. I agree with your opinion that this is not a big deal, as I don't view much with my bigger scope at 17 feet and don't notice the dimming when I do. I suspect this could be important for those who digiscope, however.

My small contribution here is that although your determination using exit pupil is correct, it is pretty hard to measure the tiny exit pupil accurately. The folks over in the Binocular Forum on the Cloudy Nights site have had long threads in the past regarding the "effective aperture" of binoculars and use an easier measurement of effective aperture they call the "flashlight test". Simply set up your scope on a tripod with the scope pointed towards a smooth wall, a foot or so away. Then shine a flashlight into the eyepiece when the scope is focused for infinity. Most scopes will give a full aperture diameter illuminated disk on the wall for this data point.

(For one that does not, see the 80th post in this thread: http://www.birdforum.net/showthread.php?t=316197&page=4
about the closeout Bresser 60 mm spotter available last November. It has an effective aperture of just 44 mm at infinity focus!)

I use an adjustable focus flashlight and set it for a parallel beam, but that doesn't seem to make much difference so long as the flashlight is kept a foot or so from the eyepiece. This measurement is easy to do with just a millimeter scale. Hold the flashlight in one hand and scale by the wall in the other. My PF80a is 80 mm at infinity and 61 mm at close focus of 17'
PF65IIa is 65 mm at both infinity AND at close focus of 14'
My Nikon ED50a shows 50 mm at infinity and 38 mm at its close focus of 10'.

Perhaps this simplified method will encourage more folks to measure the effective aperture of their optics. - Bill
 
Well! very illuminating, there's more to these scopes than meets the eye. lol

Sorry, couldn't resist.

My findings with the light test are exactly the same as yours Bill. 80mm and 61mm. At my "normal" viewing distance, about 150yds or so it's about 76mm. My scopes a straight, don't know if that makes any difference though. To be honest I'm a bit disappointed, but it seems it's not only the PF80ed that suffers from this.

With a 12mm EP, which gives 42x on the PF80, this reduces the exit pupil from 1.9mm to 1.4mm.

It would be nice if more folks did this test and posted their results so we could get a better idea what's out there and what we are actually buying. Ron
 
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henry link said:
The Pentax is not the only scope that looses clear aperture at close focus. Every scope I've seen that uses a moving prism has the same problem, some worse than others. The Pentax PF65ED, Kowa 88/77 and most other alpha scopes use a moving focusing lens rather than a prism. I haven't noticed any aperture reduction in the scopes I've seen that use that method, but it could happen if the focusing lens were too small to clear the objective's light cone at the focusing lens' forward position. For scopes that use negative focusing lenses (most) that would result in reduced aperture at infinity focus. I have seen that happen in a few binoculars.

When you sight down the front of the Kowa the "aperture" you see is not the exit pupil. It's either the eyepiece fieldstop or some other internal stop. The change in its size you observed when rotating the focuser is not related to aperture. It results from the changing distance between the stop and the moving focusing lens.
Click to expand...

Thanks Henry, I don't know what happens with the Kowa, but when turn the focus wheel you can see what looks like a diaphragm closing in. Just like in a camera lens when stop it down. If you say it doesn't effect the aperture that's good enough for me. Wouldn't mind trying the light test though.
 
billb9430 said:
Ron,
Confirming your findings, I have measured reduced aperture in my Pentax PF80a scope at close focus and also noted that this reduction does NOT occur with the Pentax PF65IIa scope I have. I agree with your opinion that this is not a big deal, as I don't view much with my bigger scope at 17 feet and don't notice the dimming when I do. I suspect this could be important for those who digiscope, however.

My small contribution here is that although your determination using exit pupil is correct, it is pretty hard to measure the tiny exit pupil accurately. The folks over in the Binocular Forum on the Cloudy Nights site have had long threads in the past regarding the "effective aperture" of binoculars and use an easier measurement of effective aperture they call the "flashlight test". Simply set up your scope on a tripod with the scope pointed towards a smooth wall, a foot or so away. Then shine a flashlight into the eyepiece when the scope is focused for infinity. Most scopes will give a full aperture diameter illuminated disk on the wall for this data point.

(For one that does not, see the 80th post in this thread: http://www.birdforum.net/showthread.php?t=316197&page=4
about the closeout Bresser 60 mm spotter available last November. It has an effective aperture of just 44 mm at infinity focus!)

I use an adjustable focus flashlight and set it for a parallel beam, but that doesn't seem to make much difference so long as the flashlight is kept a foot or so from the eyepiece. This measurement is easy to do with just a millimeter scale. Hold the flashlight in one hand and scale by the wall in the other. My PF80a is 80 mm at infinity and 61 mm at close focus of 17'
PF65IIa is 65 mm at both infinity AND at close focus of 14'
My Nikon ED50a shows 50 mm at infinity and 38 mm at its close focus of 10'.

Perhaps this simplified method will encourage more folks to measure the effective aperture of their optics. - Bill
Click to expand...

Hi Bill,
I got the same measurement as you with my straight version Nikon 50ED. 50mm at infinity and 38mm at close focus. I never noticed a dimming view at close focus, although I don't normally use spotters for close up viewing.
 
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