Kowa BD XD Prominar 8x32... for reading texts (1 Viewer)

I know I've chosen a weird title for the thread, but I'd like to explain something (good) I experienced with the Kowa BD XD Prominar 8x32... and at the same time gain some knowledge about the technical/optical reasons that may explain it.

I've done a completely and utterly unscientific comparison between binoculars of the 8x30/32 range; two porros and three roofs.
Kowa BD XD 8x32
Kowa YF 8x30
Nikon EII 8x30
Nikon M7 8x30
Meopta Meostar B1 8x32

I've checked for sharpness, brightness and "the usual suspects" when comparing binoculars in a completely and utterly unscientific way (I'll spare you the deails since this is not the subject of this thread).

So I was reading a word on a sign about 50 meters away (it was a brand, in big bold capital letters) when something surprised me. If I focused the text on the very center of the field of view and then moved the binoculars horizontally as to place the text on the very edge of the field of view (left side, in this case), the first two or three letters of the text became almost impossible to read, it was absolutely blurry. This happened with all the binoculars in the test... except when I grabbed the Kowa BD XD! I was shocked to find that binoculars more (or way more) expensive than the little BD XD (such as the Meostar, M7 or EII) performed so much worse in this particular test. So I've conducted another completely and utterly unscientific test.
This time I've placed a letter at about 7 meters (25 feet) and sat down with all the binoculars in the test to read the fine print. The letter contained extremely interesting information sent by my mobile phone provider. This was a more demanding test, and some binoculars displayed worse performance than others (the little YF was probably the one with the narrowest sweet spot). But again, I could see the same result among the "top contenders". When reading a sentence placed at the very edge of the field of view (placed on the very left, at the beginning of the sentence) the first or first two words where little more than a blurry spot of ink in all the binoculars except for the BD XD. So the Kowa have been crowned "best binoculars of the day for reading text" 8-P

I have limited knowledge about optics (to put it mildly). I do however have some experience using binoculars and over the years I've used and looked through a number of binoculars of different quality and price. As everyone else I guess, I more or less know the things I like and don't like in a pair of binoculars (and the things that annoy me). Personally, I've never been very concerned about edge performance, since I usually concentrate on the very center, so the view through (let's say) the EII seems to me really magnificent, while someone used to field flattener lenses might think otherwise.
My question (besides acknowledging this remarkable performance) is: How do you call this effect/feature/performance? Is this lack of distortion? Edge performance? (I very much doubt that the little BD XD 8x32 are record-breaking binoculars in any way, let alone this particular field). Any hints or comments are more than welcome.

It’s good to keep in mind that some binoculars have a much wider field of view than others. Although the image may appear to degrade at the edges, the edge of a Nikon EII 8x30 displays objects that can’t even be seen in the Kowa. (7.5 v 8.8 rfov, 131 v 154 meters)

Best,
Jerry

Jerry, you are most certainly right, haven't thought about it. It is true that some of the binoculars in this test do have particularly wide fields, the 8,8º of the EII are impressive, as are the 8,3º of the M7; and it is also true that the BD XD have the narrowest fov with 131 m/1000. However, given the price and "status" difference between e.g. the BD XD and the Meostar (131 vs 139 m/1000 m) I would have expected a lesser difference. Actually, what really surprised me is that the BD XD was the only one where you could read the first word of the sentence without any problem.

So, for a revealing “text” test, I’d mount some dense pages filled with characters (newspaper?). Then how much of a paragraph or page is readable - which binocular returns the most information. It’s possible that the instrument with the poorest edges is still returning more usable information because it’s useful field is wider.

In some ways your finding is surprising at the start but then makes perfect sense. Binocular manufacturers have to spend a lot of money and effort to perfect edge performance. Surely the easiest way to improve your edges is not showing them. Just mask the blurry part and call it a day. Meanwhile your competitors are trying to sell a more expensive product with ratty edges. Brilliant!

Best,
Jerry

Best,
Jerry

The Kowa BD XD 8x32 is a very good glass for the price in my opinion. I prefer it, compared to the Swarowski 8x30 CL, which is much more expensive.

The Kowa YF 8x30 is usable in the center but blurred to the edges.

The others I don't really know.

Hi Yarrellii,

The Nikon 8x30 E2 has a large amount of pincushion distortion at its edges which can be dialed into sharpness. Dialing it into focus will cause the center of the view to go out of focus on closer objects. This distortion was deliberately designed into the E2 by Nikon. The E2 doesn't have a field flattener designed into its eye pieces like the Nikon SE Porro prisms have.

Focusing the center of the view of the 8x30 E2 on a letter in the center of a sign 50 meters away will very likely result in the edges of the view being out of focus but you will be able to focus those edges into sharpness. If you do this the center of the view through the binocular will go out of focus.

As the distance of the object you are focused on approaches infinity this distortion will disappear. A distant mountain ridge viewed at infinity will be sharply in focus whether looking at it in the center of the glass or at the edge of the glass.

Bob

I have tried similar but less controlled versions of this test with the binoculars I have available to me. I love my Genesis 8x33, but they are pretty bad at edge sharpness compared to the others. Noticeable blur starts almost immediately from center. Surprisingly, the best edge sharpness of my short list is Leupold Yosemite 8x30 though I suspect the exit pupils might be truncated a bit on those.

@OhWeh The Kowa BD XD is a little gem. It doesn't "shine", it's not particularly flashy... but it's really easy to use, gives a great quality feeling and the views, although not top-of-the-range, are always satisfaying (BTW, I've read you many times in the Jülich-Bonn forum, I didn't know you were here as well!).

@ceasar Really interesting remarks. Do you know the exact term/effect that would apply/explain the fact that the very edge of the fov in the BD XD is readable? Thanks again

@18000bph hmmm, puzzling to read this, since the Kowa YF 8x30 is supposed to be a sibling of the Yosemite and did not provide (in my unit) a readable text on the edge when I carried on the test. Said this, I think the YF is probably my favourite binocular if we only count value/money, and is the one I always recommend to anyone asking me for a pair of binoculars. They are amazing value for a mere 100 €/$, so light, nimble and fun to use. I wish Kowa created a YF+/YF-ED/YF II porro with improved optics for 250-300 €, those could be THE binoculars (for me at least).

Yarrelli,

I have tried all those binoculars at various times and your results do not surprise me in the least. As I recall, all present some degree of field curvature. It simply means the focus distance at the edge of the view is closer than in the centre. You should be able to get the text at the edges sharper by refocussing. For those with the same radius of curvature some of those with widest fields of view will need greater focal adjustment. If I remember rightly the EII and YF have more pronounced curvature than some of the others, yet will deliver a sharp image near the edge when refocussed.

For another subset the centre sweetspot may be larger, but the image at the edge will remain blurred even when refocussed. If you were to draw a cross on your target and attempt to bring it into focus at the edge of the view, you will find that you can get with the radial or tangential arm sharp but not both. This is called astigmatism. I seem to remember the Meostar was one of these.

The Nikon M7 has generally a flatter field for it's FoV than the others but has a more complex distortion pattern across the view and cannt be focussed to real sharpness at the edges.

Just to reemphasise that the FoV is an important point here. I recall comparing the Meostar and older Vortex Viper ED 8x32s. As best I could judge the optical designs were identical, with the same profile of field curvature and astigmatism. The Vortex simply had a smaller field stop reducing the field of view. They had simply traded field of view for a sharper edge.

With due respect to Bob, pincushion distortion is something else again. To keep it simple, it makes straight lines look like they are curved away from the centre at the edge of the view. It is not the same as field curvature.

I don't know of any binocular that is truly sharp edge to edge when viewing flat objects, though the Swarovski EL FP/SV is amongst the closest. Personally I've never used binoculars to examine an edge to edge two dimensional object, so I've never considered it an important factor.

All the designers juggle, FoV, ER, field curvature, resolution, CA and a collection of other distortion to deliver the best the best set of compromises they can offer at a price. Many deliberately choose to emphasise one feature over another to entice one kind of customer or another. The BD XD wouldn't be my favourite on that list, but if there is one thing I've learned from the forum, it would be that each of us has different priorities.

David

Wow, that gave me something to mull over during the coming days (I think I'll have to read it a few more times, though). Thanks, David! It is always a pleasure to read from all the knowledgeable folks that gather here at BF. I think I can see where the effect is coming.
My favourite from that bunch has to be the EII, probably followed by the M7. However, this particular performance of the BD XD was a stunning surprise, hence the question.

18000bph said:

I have tried similar but less controlled versions of this test with the binoculars I have available to me. I love my Genesis 8x33, but they are pretty bad at edge sharpness compared to the others. Noticeable blur starts almost immediately from center. Surprisingly, the best edge sharpness of my short list is Leupold Yosemite 8x30 though I suspect the exit pupils might be truncated a bit on those.

Click to expand...

I am just re-reading now and I realize I didn't word it properly.

The Genesis 8x33 has the smallest sweet spot of what's available to me. The magnitude of softness or blur really doesn't get any worse as you inspect from just off-center out to the edge. So when you check just the sharpness of the edges, they are still comparable to the others. Actually, the outer 10 or 20% is probably a little sharper than it is closer to center.

Where the little Leupold Yosemite 8x excels is with the size of the sweet spot, not necessarily edge sharpness.