Stopping down a binocular (1 Viewer)

[james holdsworth]

I’ve got a pair of Zeiss conquest HD 15x56 that I like quite a bit but, in daytime viewing, I see too much CA. There’s been some discussion on here about using the objective covers to stop down binoculars, to reduce aberrations like CA. I have the objective covers for this binocular and I’d like to try this but I’d like to get the opinion from someone here than has done it first. What sort of diameter hole do I need to cut in the objective covers to potentially reduce CA but not affect overall viewing quality too much? Almost all of my viewing is daylight, mostly lake watching, so ultimate brightness isn’t necessarily a priority.

thanks in advance.

 

[Binastro]

Ideally an iris from a large lens or rather two are required.

I once suggested Vent Axia fan irises for really large scopes.

Some large aircraft lenses have irises bigger than 56mm.

I haven't looked on the web to see if irises are sold.

Much simpler to try black cardboard cut outs.

I suggest 48mm and 44mm, but choose what you think might work.

In bright sunshine the 15x56 is probably stopped to 38mm by ones eyes.

If CA is still visible then try 32mm or so.

Once a suitable size is found then cut the objective covers.

Telescopes use cutouts, even elliptical ones for certain observations.

Regards,
B.

 

[adhoc]

I have done this with two binoculars.

In the Alpen Teton 15x50 CA control is improved dramatically by such caps. See the first photo. In the new bino CA was so bad it was to me unusable, in good light at times I could not tell the purple flowers on a tree from its green leaves (literal truth), but now the image is "alpha" class!

The caps were machined locally, of nylon, and weigh 6 g each.

The great reduction in obj. area and exit pupil does bother me, but the striking increase in information useful for bird ID, i.e. spacial detail and coloration, over the best 10x instruments, handheld (my hands are on the steadier side), make it very viable.

In dimmer light I take these caps off. This works much of the time. Even in low light, though, certain conditions make the CA then reappear.

First I trialed models cobbled of black paper, and decided 30 mm was safe. In the well-made present items a bigger aperture may work, but I'm hesitant to try. Maybe we'll go up 2 mm at a time. But then when the CA reappears I'll need to order and pay again for two caps, with aperture stopped back down by that last 2 mm to optimum diameter! Maybe I should ask the machine place whether they can somehow do a trial pair at lower cost and also then indulge my requests for incremental widening.

Next has been the Zeiss Victory Pocket 8x25. In this bino I am bothered by "brightness washout" especially against a very bright background but to a lesser degree at all times. In the former condition glare and harsh contrast obliterate detail. Sometimes CA bothers. In other conditions, to me, vision feels stressed and also color rendition appears too "cold".

A couple weeks ago I had a pair of caps machined by the same place for this instrument at 20 mm aperture. See photos. There's a bit of a snag. As the first pic, cropped from an image from Scopeviews (thanks!), shows the outer surface of the barrel is molded a little farther out for a quarter of the circumference. The wrap side of the cap needs to be cut out for this. See the other two pics. The caps weigh 3 g each.

There's a striking improvement on both counts. I see distinctly more detail against a bright sky. CA then is slightly less. In other and less bright conditions, to me, vision is relaxed, colors are more natural, and the image seems even sharper. The slight narrowing of the very wide field of view is noticeable to me only when comparing it with caps on vs off. In dim light I take them off.

As above, maybe I'll try to open out the aperture to an optimum, some later time.

 

[Binastro]

For a 50mm doublet objective CA is said to be controlled at f/6, although Conrady's stricter opinions says f/10.

For a 30mm doublet objective f/3.6 with Conrady saying f/6.

A 25mm doublet binocular objective should be largely CA free.

But one's eyes might also perform better stopped down, so it is not only the binoculars to be considered.

The magnification is also a factor.

So, trial and error is a good approach to finding a suitable mask size.

There are variable aperture irises available on the internet, which might be suitable for many binoculars.
Two are needed.
They are available in different sizes.

Regards,
B.

 

[jafritten]

For a 50mm doublet objective CA is said to be controlled at f/6, although Conrady's stricter opinions says f/10.

For a 30mm doublet objective f/3.6 with Conrady saying f/6.

A 25mm doublet binocular objective should be largely CA free.

But one's eyes might also perform better stopped down, so it is not only the binoculars to be considered.

The magnification is also a factor.

So, trial and error is a good approach to finding a suitable mask size.

There are variable aperture irises available on the internet, which might be suitable for many binoculars.
Two are needed.
They are available in different sizes.

Regards,
B.

Binastro,
I read your contributions with great interest. What you say about controlling CA by stopping down the objectives is very interesting.
I love my Ultravids very much. The only drawback I can see is CA (and price ...). I have often wondered why CA is virtually absent in the 8x20 configuration but only so acceptable in the 10x42s. Having read your post, I see why.

Which opening diameter would you suggest for stopping down the 10x42?

Is there a reason why Leicas are less good at controlling CA than others? Has it to do with their compactness and faster f/stops?

Cheers

 

[adhoc]

Thank you Binastro, that's good news about the variable aperture devices. You did mention these in your previous post but I thought you were doubtful that they are available now so I did not look for them on the internet. On reading your last I have done so, and there's at least one manufr. with a suitable device. Out of their range of items, I will get two of the item that I see spans the aperture range relevant to me. Then we need to design and make two copies of a device which enables that to be fitted to binocular barrels of varying diameter. Regards, Adhoc.

 

[tenex]

Is there a reason why Leicas are less good at controlling CA than others? Has it to do with their compactness and faster f/stops?

Hmm, one can also ask why Leicas can't quite match the central resolution of other alpha bins. Is it a compromise favoring compactness or some other optical quality, or just a low R&D budget?

 

[Binastro]

Post #5.

The easy way is to make black card masks with different apertures and see which is best.

The objective may be a triplet.
The eyepieces and our eyes also have CA.

CA may vary with distance.

In bright light the pupils of our eyes stop down the binocular, so a 10x42 may become a 10x25.

Post #7.

My 8x32 Leica BA has very fine central resolution.
I doubt that it is much inferior to any binocular.

A shorter binocular is likely to have more CA than a longer one.

The Foton 5x25 has 30mm objectives, possibly from 8x30 Porros.
These have a longer focal length than standard 25mm binocular objectives.
The aperture seems to be stopped down to 25mm internally.
My example has very high resolution.

Much of the resolution capabilities of a binocular depends on the observer's eyesight, assuming the optics are made to high standards.
Some of the lower priced binoculars have very poor, maybe molded, optics.

The resolution of binoculars seems to vary from one example to the next.
Lemons to cherries to everything in between.

What I don't understand is why people who have cherry optics sell them to buy something newer, which will probably be of poorer optical quality.

Regards,
B.

 

[Troubador]

Hmm, one can also ask why Leicas can't quite match the central resolution of other alpha bins. Is it a compromise favoring compactness or some other optical quality, or just a low R&D budget?

I have always suspected that Leica prioritised compactness.

Lee

 

[Binastro]

Stereo cameras have a linkage that stops down both lenses by the same amount with one control.

With most binoculars this would be difficult as the objective spacing varies.

There are a few binoculars, such as some Canon IS that have fixed separation objectives where a link would be easier.

B.

 

[tenex]

My 8x32 Leica BA has very fine central resolution.
I doubt that it is much inferior to any binocular.

This cannot be said of my 10x32 BN, well though it served me. Is the 8x generally better, or was mine simply not a cherry?

 

[dorubird]

I have always suspected that Leica prioritised compactness.

Lee

Yes, and also design

 

[Bino Steve]

If you can design it either digitally or drawing it, a cap with precise holes in it is perfect for 3D printing. There are many places online that with take your design and print you one-off samples pretty cheap. Try out different diameters that way.