Spherical aberration why? (2 Viewers)

[Paultricounty]

Was using a three year old Swarovski 8x30 Habicht. The owner has taken very good care of these based on its appearance. They seem to be in good collimation and snap into focus well. The issue today was that the fall off was much more prevalent on the bottom of the field. I checked both barrels with each eye to rule me out as the possible problem. I have a pair of these Habicht’s but they are in another home in Florida, so no access for a side by side.

Looking at some objects the fall off or soft focus on the top and sides of the field starts about 80-85% from the center (center is extremely sharp). But the fall off on the bottom of the field falls off or blurs at about 70% of the field all the wa6 to the stop. This blur cannot be focused, it’s edge degradation. On a stop sign at about hundred yards the white letters stay sharp and in focus until about 10+ % near top and sides, but fall out of focus at about 30% give or take on the bottom of field.

What’s going on here?
Why so much more fall off on the bottom of the field of view?

Thank you
Paul

 

[henry link]

This has come up before. When the eyeball rotates toward 6:00 it tends to be less affected by vignetting from the prisms than when it rotates toward 9:00, 12:00 and 3:00. The vignetting is actually beneficial in suppressing off-axis aberrations by stopping down the exit pupils near the field edge, so the 6:00 position tends to look worse in most binoculars just because the exit pupils there remain more open to the eye. The aberrations are actually equal in each direction, provided there is equal vignetting in each direction. Try rotating the whole binocular 90º and then turn it upside down. At each position examine each telescope separately. I'll be very surprised if you don't find that the 6:00 position remains the "bad" one even after rotating. At least, that's what I just saw through my 8x30 Habicht.

For testing off-axis aberrations I always look down for consistent results that are not sweetened by vignetting.

BTW, that's not spherical aberration you're seeing. SA affects the entire field, including the center.

 

[Paultricounty]

This has come up before. When the eyeball rotates toward 6:00 it tends to be less affected by vignetting from the prisms than when it rotates toward 9:00, 12:00 and 3:00. The vignetting is actually beneficial in correcting off-axis aberrations, so the 6:00 position tends to look worse in most binoculars, even though the aberrations are actually equal in each direction, provided there is equal vignetting in each direction. Try rotating the whole binocular 90º and then examine each telescope separately. I'll be very surprised if you don't find that the 6:00 position is still the "bad" one even after rotating. At least, that's what I just saw through my 8x30 Habicht.

For testing off-axis aberrations I always look down for consistent results that are not sweetened by vignetting.

Thx Henry, I’ll check this out tomorrow. This just never popped out to me much in any binoculars. Today was kind of different , I was comparing the Habicht’s to an EDG 8x42 in different lighting conditions throughout the day and after a while I couldn’t get a sharp image on the bottom of the Habicht’s. And yes I know the EDG has field flattner and seems sharp all around.

Paul

 

[lilcrazy2]

This has come up before. When the eyeball rotates toward 6:00 it tends to be less affected by vignetting from the prisms than when it rotates toward 9:00, 12:00 and 3:00. The vignetting is actually beneficial in suppressing off-axis aberrations by stopping down the exit pupils near the field edge, so the 6:00 position tends to look worse in most binoculars just because the exit pupils there remain more open to the eye. The aberrations are actually equal in each direction, provided there is equal vignetting in each direction. Try rotating the whole binocular 90º and then turn it upside down. At each position examine each telescope separately. I'll be very surprised if you don't find that the 6:00 position remains the "bad" one even after rotating. At least, that's what I just saw through my 8x30 Habicht.

For testing off-axis aberrations I always look down for consistent results that are not sweetened by vignetting.

BTW, that's not spherical aberration you're seeing. SA affects the entire field, including the center.

Henry
I have noticed on more than a few roof binos, that the sweetspot seems oval instead of round. When looking toward the 6 or 12 position the bluriness starts much nearer the center than when looking at the 3 or 9 position. Is this what you were referring to?
Tom

 

[henry link]

Yes, but the effect is not related to prism type, only to vignetting.

I see it the same way you do, as worse off axis performance toward the 12:00 and 6:00 directions, but others only report it toward the 6:00 direction.

You can eliminate the effect by intentionally shifting your eye socket toward the 12/6:00 positions when you look in those directions.

 

[lilcrazy2]

Yes, but the effect is not related to prism type, only to vignetting.

I see it the same way you do, as worse off axis performance toward the 12:00 and 6:00 directions, but others only report it toward the 6:00 direction.

You can eliminate the effect by intentionally shifting your eye socket toward the 12/6:00 positions when you look in those directions.

Yes, but usually by tilting the binos for me.