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Exit pupil vignetting in BK-7 prisms binoculars (1 Viewer)

L

Leonardo

Member
Hi, guys,

I'm testing some entry level binoculars and I need to clarify some ideas about exit pupil vingnetting of BK-7 prisms binoculars.

This vignneting only affects the edge of the exit pupil, right?

Another question, should I see any shadowing at the edge of the field of view when I bring the binoculars close to my eyes?

By the way, I can clearly see the diamond shape shadow in the exit pupil of BK-7 Porro prisms binoculars, but I haven't seen this vignetting in roof prism binoculars. Any comment on this matter?

Thank you!

Regards,

Leo
 
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Leo,

Yes, to your first question. It only affects the edges.

Sometimes, to answer your second question. I own many Bk-7 binoculars right now. With some of them I don't notice any "shadowing" at the edge of the field. Others I do but they are all Bk-7.

As for your last question, are you saying that you have Bk-7 porros and BK-7 roofs? Just off the top of my head I don't remember any Bk-7 roofs in recent history. Is there a particular model you are referring to?
 
Yes, the vignetting only affects the area of the exit pupil that looks dimmer. This form of vignetting dims the image equally across the field, but only if your pupils are large enough to accept the shadowed area of the exit pupil. It has no effect at all if your pupils are small enough to fit within the fully illuminated area inside the diamond shape.

Any prism, roof or Porro, is affected. The determining factor is the focal ratio of the objective lens. If a light cone is below about f/5 one side of it will strike the prism reflecting surfaces at an angle that is below the critical angle for total internal reflection in BK7 glass. The diamond shape is formed by four such shadows, one at each reflecting surface. I believe BAK4 can accept a light cone down to around f/3.5 and still achieve TIR.
 
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FrankD said:
As for your last question, are you saying that you have Bk-7 porros and BK-7 roofs? Just off the top of my head I don't remember any Bk-7 roofs in recent history. Is there a particular model you are referring to?
Click to expand...

Well, I refer to basic roof prisms binoculars, like Tasco Essentials 8x42 (ES842) and Sierra 10x25 (TS1025D), but I'm not sure about the type of glass has been used.
 
henry link said:
Yes, the vignetting only affects the area of the exit pupil that looks dimmer. This form of vignetting dims the image equally across the field, but only if your pupils are large enough to accept the shadowed area of the exit pupil. It has no effect at all if your pupils are small enough to fit within the fully illuminated area inside the diamond shape.
Click to expand...

Ok, thank you! So, let's say we have a 10x50 (exit pupil: 5 mm) BK-7 prisms binoculars, then it will virtually only dim image slightly at night.
 
Yes, inside the diamond shape there is a clear exit pupil of probably around 3.5 mm. As long as your eye is open to no more than 3.5 mm there is no dimming of the image at all.
 
Leonardo,

I did a little digging. It appears there were at least a couple of versions of the Tasco Essentials 8x42. The one with the fancy bubble/ribbing on the outside was Bk-7. The other version which has more contemporary armoring appears to be Bak-4 at least from what I gathered from a few websites.

Truth be told I have never owned a roof prism binocular, that I can remember, which originally retailed for under $50 US. The least expensive 8x42 I can think of was the Zhumell Short Barrel at approximately $80 US. It has Bak-4 prisms. I gave it to my dad for him to use in place of an old pair of Tasco 7x35s that he lost. He loves their improved eye relief since he does wear glasses regularly.

Wish I could help more.
 
henry link said:
Any prism, roof or Porro, is affected. The determining factor is the focal ratio of the objective lens. If a light cone is below about f/5 one side of it will strike the prism reflecting surfaces at an angle that is below the critical angle for total internal reflection in BK7 glass. The diamond shape is formed by four such shadows, one at each reflecting surface.
Click to expand...

I can clearly imagine the situation in a double Porro prism system, but it's not that easy for me in a roof prism system like the Schmidt-Pechan.
 
Maybe it's easier if you just ignore the two roof reflections which are hard to visualize. The remaining four reflections in a Schmidt-Pechan are all flat surfaces just like a Porro. Three of the four are set at exactly the same 45º angle to the optical axis as a Porro, but one reflection is below the critical angle and has to be mirror coated. That reflection doesn't depend on TIR, so it doesn't really need BAK4 glass, but the other three and the roof surfaces do, so both prisms of the cluster need to be BAK4.

BTW, on another thread it was pointed out to me that the single highly obtuse angle of the first prism in an Abbe-Konig roof prism cluster probably doesn't need BAK4 for TIR.
 
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Hi!

I'm testing two 10x25 cheap binoculars from Tasco these days, a roof prisms one (168RB) and a Porro prisms one (ES1025). I can see the exit pupil vignetting in the Porro prisms model, but there is no vignetting in the roof prisms model. Therefore, I must say I haven't seen any exit pupil vignneting in roof prisms binoculars so far, regardless of the glass has been used for making their prisms.
 
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