Sweet spot peculiarities; I can´t get the center as sharp as the periphery (1 Viewer)

In the Nikon 8x30 Eii?
I suspect 'equal-ripple' tradeoffs far more now.
I'm not sure how to alter that other than changing the
field lens spacing inside. That's inner surgery, spacing shims.

Most probably don't have the acuity to notice, but
certain visual patterns are affected beyond your acuity..

Plain Kelners and Plossls (in x-wides) can't do that....the focus just fades.

. Using averted vision, if one is experienced, it is possible to see that different areas of the field are brighter than others. In other words, it is possible to see fainter stars and fainter deep sky objects at certain positions in the field. I used to use this to maximise what I could see. However, I would not have thought that birdwatchers would experience this, although I suppose they might.

Initially, I was wondering if small cataracts could be part of the problem, but only a careful eye examination would confirm whether this is the case or not.

Even the most experienced observer, without the necessary testing equipment, will not be able to test their own eyesight as well as they could with the help of an experienced ophthalmologist.

My eyesight tests take about 2 1/2 hours in total. This is necessary, at least for me, because I'm older and want to get the best performance from my eyes. With the correct prescription eyeglasses my resolution is about 130% better than the so-called 20/20 or 6/6 perfect eyesight. In fact, many people do much better than this, with some achieving 250% better. On the other hand, many people do not achieve 20/20 vision, but they still do very well. There is a vast range of eyesight abilities and one has to make the best of what one has.

In my case, my optician for some reason enjoys testing my eyesight, which is why he is prepared to spend 2 1/2 hours. He says that I am the only person in his 30 years of testing, with whom he uses his one eighth dioptre lens. In other words I can focus to plus or minus one 16th of dioptre when I'm rested. And also fix the small amount of astigmatism to plus or minus 1.25°. Without glasses my resolution is quite low, so I need to use glasses generally, although even now if I am not doing anything special I don't wear glasses.

It is no surprise to me that somebody in their 40s should choose to have a thorough eye examination. I started using glasses in my late 40s. At this time I found that my driving was affected by my eyesight in subtle ways, which had not occurred when I was younger.
Eyesight examinations are important not just for ones eyes, but also for one's general health.

6130m,

I hesitate to enter these waters again as there are a few too many red herrings swimming around , but while I can't seem to find any of my old posts on the subject of a perception of oval shaped sweet spots it has come up here several times.

I have a pretty good idea of why you and your friend see this in various binoculars, even without eyesight astigmatism. It's not really an instrument characteristic (that's why you found that it doesn't rotate with the instrument). It results from the off-axis vignetting in every binocular combining with unconsciously asymmetrical eye movements. You can teach yourself to be aware of those movements and choose to induce or eliminate an asymmetrical sweet spot in virtually any binocular by consciously changing the position of your pupils in just the right way as you move your gaze toward the edge of the field. In my experience the darkening of blacks and apparent increase in sharpness along the horizontal axis happens because the eyeballs naturally rotate horizontally in a way that tends to produce more off-axis vignetting than vertical eyeball movements do.

There is a way to directly observe the effect of eyeball rotation and pupil position on vignetting by using a defocused artificial star. Set up an artificial star at 3-4 m from the binocular (glitter point in sunlight or pinhole in aluminum foil stretched over a flashlight beam). Set the binocular focus on infinity and observe the defocused disk of the star point. What you see is a focused image of the binocular objective lens or your own eye's pupil depending on which is smaller. Move the disk to the edge of the field and notice how it's size and shape change. What you see near the field edge is the vignetted pupil (eye or exit) that you actually view through when you look at the periphery.

I'll leave it to you to play around with the effects of pupil movement in this test, but I think you will quickly notice that lateral horizontal movements of your pupil have a large effect on the amount of vignetting along the horizontal axis and I believe you will notice that natural eyeball rotation tends to create more horizontal vignetting than vertical vignetting. Increased vignetting (of the exit pupil or the eye pupil) has the benefit of decreasing aberrations like field curvature and astigmatism in the binoculars as well as decreasing axial aberrations of the eye, so the periphery of the field in the direction with the most vignetting looks the sharpest and highest contrast.

Henry

Hi 6130rm,

I had noticed a similar situation with my 8x42 Zeiss Victory HT's. Looking at high contrast lettering on sign posts etc showed slightly more 'clear/sharper' images at about 25%+ away from centre. Increasing the IPD by approximately 1/2+ mm from where I had originally set it up, 'increased clarity returned' to the centre and remained so towards the edges of the field of view. Viewing more natural objects also benefitted from this wider IPD and so I left the IPD at this slightly wider position and everything is great. Make sure that it is not so wide that it begins to show the shadows at the edges. I believe that some binoculars may be quite sensitive to the the IDP position.

Doug..........

Very interesting!
Now I have something to play with during the weekend!

And a short binocular review at the end: I am very happy with the combination of 9x33 Granite and 8x30 Nikon Eii, they complement each other. The Eii more neutral in the colors (similar, but in my opinion, a little more color depth and contrast than my Conq HDs), the Granites a bit warmer, but still true to reality. The Conq HDs appear brightest, but after sunset it seems what really counts is the pupil size, not the (in my opinion kind of "artificial") brightness due to what I assume to be higher blue transmission in the HDs. After sunset I notice very little difference between the Eii and the HDs. The Granites are a tiny bit darker, but compensate this with the higher magnification. After sunset my 7x42 Dialyt rules despite warmer than the HDs during the day. All are about the same in "sharpness" and sweet spot, with maybe some advantage to the Eii due to color depth and contrast. The Dialyt feels a little behind in the group, except for their low light capacity, and some hard-to-grasp very attractive feeling of "optical instrument" (Reminds me of my experiences with levelling instruments). I would never separate from my Dialyt even with its faults. Maybe I will be able to share this love with the Eii. Both are old fashion, but with charm.

doug el10x32 said:

Hi 6130rm,

I had noticed a similar situation with my 8x42 Zeiss Victory HT's. Looking at high contrast lettering on sign posts etc showed slightly more 'clear/sharper' images at about 25%+ away from centre. Increasing the IPD by approximately 1/2+ mm from where I had originally set it up, 'increased clarity returned' to the centre and remained so towards the edges of the field of view. Viewing more natural objects also benefitted from this wider IPD and so I left the IPD at this slightly wider position and everything is great. Make sure that it is not so wide that it begins to show the shadows at the edges. I believe that some binoculars may be quite sensitive to the the IDP position.

Doug..........

Click to expand...

It seems IPD related. I did this test with the Eii, and it helped, the difference between center and peripheral sharpness disappeared. On the Granite it somewhat decreased the diameter of the zone of less sharpness/contrast in the center, but the negative effects of too much increase in IPD took overhand before I could enjoy any complete disappearance of the sharpness difference.
Nevertheless, my granite is pretty sharp in the center, and I´ll get used to move the object off center if I need it even sharper.