Why don't field flatteners guarantee edge to edge sharpness? (1 Viewer)

[agus_m]

I understand that a field flattener is a lens that goes right after the prisms that has the property to flatten the field, where the image forms. Ideally, if we focus the subject at the center and move it to the sides, it should stay focused (and sharp) without needing eye accommodation. I am not talking about distortion, or if a straight line stays straight through the binos, as this is a common misconception of the term.

This property is closely related to having a large sweet spot, which is usually a nº 1 priority in the optical design. So my first question is: How come not all binoculars have a FF? I have in seen several cutaway images that some eyepieces consist of up to 6-7 lenses. However, for many binos, none of these lenses is a FF (Nikon EII, Habicht, SLC, Zeiss porros, etc.). I don't understand what the goal would be in these designs if not to prioritize the biggest sweet spot possible. Nowadays, the NL pure, Swaro SV and SF still have 6-7 lens eyepieces, but 1 or 2 lenses are FF.

The second question is why these elements don't guarantee edge sharpness. The MHG has a FF, however the consensus here is that its edge sharpness is slightly inferior to the CHD, which doesn't have FF (Same with SFL vs SLC). What is the role of the FF in the former if it doesn't do a better job than a bino without it? An observation I had was that only alphas would sport a field 100% sharp. Although this seems to only apply to Swaro since even the Zeiss SF is not super flat. Now, a several times cheaper Sky Rover Banner Cloud has an even flatter field than the NL Pure according to some here. The Canon 10x42L is flatter than SF 10x42 too.

According to Henry Link, some binos with FF only correct for astigmatism up to the edge, not necessarily field curvature. In this way, the whole field would be usable if one has good accommodation, or if we refocus the image. It can also be that some binos artificially stop the field so that the edges are sharp, so a bino that's not sharp to the edge has a field stop too wide. Maybe this question is too technical, but that's why I post it here, I enjoy reading these things in the forum. Any ideas?

 

[NDhunter]

I am wondering if you have experience with any of these binoculars you mention, or are just a curious
sort that likes to pick and choose loose bits during your internet search for discussion ?
Many of the things you post are subjective and not really factual.
It does matter.
Jerry

 

[agus_m]

I am wondering if you have experience with any of these binoculars you mention, or are just a curious
sort that likes to pick and choose loose bits during your internet search for discussion ?
Many of the things you post are subjective and not really factual.
It does matter.
Jerry

I have a MHG 10x42 and I notice the field curvature. Some days my eyes have good accommodation and if I focus the image at 60-70% of the field center, the image basically looks sharp from center to edge. But in general, the curvature is noticeable. I have tried the NL Pure, EL SV, SF, etc at shops and I immediately noticed the sharpness, but I haven't owned one of those unfortunately to test them under different conditions.
I have the impression that many here don't have good eye accommodation, in this case field curvature is more evident, so I trust reviews here that find differences between alphas. There are many threads of comparisons between NL Pure, EL SV and SF, the consensus is that the EL wins (but has the Absam ring), followed by the NL Pure with a wider field, and then the Zeiss SF.

 

[Maljunulo]

Everything in optics involves compromises, and everything interacts.

Fixing one thing may be at the expense of another.

 

[Tringa45]

I am wondering if you have experience with any of these binoculars you mention, or are just a curious
sort that likes to pick and choose loose bits during your internet search for discussion ?
Many of the things you post are subjective and not really factual.
It does matter.
Jerry

Well, I thought it was an interesting question, so you have posted a superfluous and unpleasant personal criticism.

So here are my 2 cents open to correction as always:
Some of the demands on binocular eyepieces may include wide AFoV, sufficient eye relief for glasses wearers, lack of astigmatism and field curvature, all within the dimensional constraints of providing acceptable minimum IPD. The higher the demands, the higher the complexity.

All binocular objectives exhibit field curvature and the radius is dependent on the focal length of the objectives. I don't think astigmatism is a significant problem in objectives but is seen in most eyepieces. It can be detected by viewing a squared pattern at the field edge, when one can either focus the vertical or the horzontal lines, but not both simultaneously.
Astigmatism is a form of field curvature. It's just that the field curvature of tangential and sagittal rays differs.
If you see a loss of edge sharpness in an eyepiece where there has been an attempt to correct the field curvature of the objective, then the cause is most probably astigmatism and the designers have adopted a compromise.

John

 

[tenex]

It is a good question; the answer is just messy, how much one needs to know about optical systems to truly appreciate all the factors involved and the resulting difficulty of producing a wide, well-corrected view, in reasonably compact form, usable with eyeglasses, at an acceptable price. Consider Nikon WX, which in attempting to meet the other criteria fails on size and price for most people or purposes.

But there is something about this equation that nags at me too: why don't more binoculars try harder, and how much can it really cost? (Swaro NL is impressive but obviously overpriced.) I think it's an odd industry overall, very conservative or even lazy, that really doesn't expect most customers to notice or care. Even this forum is full of people who say they don't look at the field edge, or can't. So does it matter whether it's sharp? In any case, it's not a merely technical question.

 

[Binastro]

The reason why edge performance reduction is tolerated is, I think, because the eye has less resolution off axis.

There is also the blind spot in each eye, which is not noticed at all.

It is only when the so called sweet spot is very small that it bothers me.

I do like Nagler eyepieces in scopes as I can move my observing eye off axis to follow say Jupiter.

But with a binocular I just move the binocular.

The eye is very good at detecting side movement.

Regards,
B.

 

[Conndomat]

One should also mention the aspect that many people don't want field flattening because they find the view unnaturally flat, not all people want to use NL, EL, EDG...

Andreas