[ronh]

I was sitting out on the back deck as twilight deepened yesterday, looking around with binoculars, just enjoying the end of the day.

The 12x50 wandered upon the lights at a distant intersection, but the focus was set very close. The out of focus blobs were not circular, but shaped like vertical cateyes. This was about when my pupils enlarge to 4mm, so the fit of exit and entrance apertures was very close. The vignetting resulted from the IPD being set too close, and I found a very narrow range where the blobs looked circular. I thought, this must be my true IPD, how unfortunate that I had it set too narrow, views will be better now. But this morning, with that adjustment, I experienced blackouts and excessive sideways color fringing. Henry and others have observed that vignetting, although it sounds like a bad thing, often cleans up optical problems related to fast systems like a binocular. But I do get sharper star images at night with a wider IPD adjustment. I suppose balancing the aberrations all around gives the most symmetric and pleasing star spots. I concluded that setting the IPD, or probably any other adjustment, based on measurements alone is a bad idea, considering the complexity of instruments and of vision. Eyes don't need a ruler to tell them what looks good.

As twilight got deeper, I noticed the bright red blooms of the "Scarlet Runner Bean", against its big green leaves. By day these show up like flames but as it got darker, the leaves remained well visible and green, while the blooms faded to black. I have read that this "illusion" results from the eye's sensitivity peak shifting towards blue in low light, leaving red behind. Our eyes have evolved this way to better prepare for the transmission profile of the anticipated Zeiss HT. Sorry 'bout that. Our eyes adjust for better low light vision, the scattered light from the twilight sky being blue. Most of us know this in our heads, but it's impressive to actually see it!

Just sit out, like the old folks used to do.
Ron

[steve@37n84w]

Like most (I suppose) I simply adjust the IPD until I attain a nice comfortable view with no blackouts, etc. The only thing I find interesting is that the actual IPD setting where I find the sweet spot can vary with different binoculars.

I've measured the distance between the center of the oculars on some of my binoculars out of curiosity and found it to be slightly different between various models. Seems like an IPD that hits your sweet spot on one model would be your ideal setting for all your binoculars but it doesn't work that way for me.

I do a lot of twilight glassing for deer out at my farm and know what you mean about the color shift, by the time I quit observing it's all just black and shades of gray. I also don't think the "twilight factor" of your binocular is the most important consideration for lowlight terrestrial use....aperture rules when it's really dark.

Steve

[newfie ghost]

I agree with the above posts IPD is very important, especially with certain models. It can make a good image turn into a great one. Sometimes the image is best when you have the perfect circle with both barrels. Other models I find it better if the image is a slight oval.

It is nice to hear personal stories and experiences.

BTW what 12x50 are you using? That is some big glass.

[mooreorless]

Newfie, I hope Ron doesn't mind me answering. He has a Leica Trinovid BN 12x50 and it his second one. Here is a link to his excellent thread on it.

http://www.birdforum.net/showthread.php?t=228112

Reading that thread makes me want one.

[newfie ghost]

Originally Posted by mooreorless

Newfie, I hope Ron doesn't mind me answering. He has a Leica Trinovid BN 12x50 and it his second one. Here is a link to his excellent thread on it.

http://www.birdforum.net/showthread.php?t=228112

Reading that thread makes me want one.

Ah, I remember that thread. Awesome and rare, not many folks have one of those.

Thanks mooreorless.