Depth of field of monoculars

The budget monocular thread made me look through my monoculars and I noticed that some have a very nice large DOF and some have a very shallow one.

I'm not sure, but there seems to be a correlation to the lens diameter and to magnification, but this could also be the quality of the lenses. Larger lens diameter usually gives better DOF and of course larger magnification gives worse DOF.

Am I correct here? Is this characteristic of monoculars? Do binoculars have the same phenomenon?

Depth of field can be correlated to magnification more than any other factor. The higher the magnification, the more shallow the depth of field. Try a high powered spotting scope some time and you will see how it works. Also that is relative to the person. Humans will see slightly out of focus as in focus, thereby depth of field is relevant to the person. In other words the same monocular used by two different people may result in two different opinions on the depth of field of that monocular. The optics also have an effect, but magnification seems to have the most prominent effect.

Binoculars also operate in a similar fashion, so yes they are affected too.

I'll add that I have several 10x monoculars with various lenses sizes, so the magnification shouldn't affect DOF - my Nikula 10x21 monocular has worse DOF than the no-name DXoffers 10x30 mono which in turn has worse DOF than my VisionKing 10-25x42 mono which has a worse DOF than my Bresser 10x50 binoculars. Also my no-name 15-55x21 mono has roughly the same DOF as my Nikula 10x21 mono. My Tmart Raily 6x30 mono and Mystery 8x42 mono have about the same DOF as my Bresser 10x50 binos, which are about the same DOF as my "Russian" 2.5x17.5 monocular. My BIC 3x28 binos have about the same DOF as the DX 10x30 mono. IIRC the Celestron 8x21 binos I had for a short time have worse DOF than the Bresser 10x50 binos.

So I'm confused.

Most of my monoculars and binoculars are of low quality, so I thought this might affect DOF but not sure.

I hope my newly ordered Bresser 8x21 binoculars will have better DOF than my x21 monos...

DOF is affected by magnification. It's a known fact. Try a search on depth of field in binoculars and do a little reading.

If you want to do some testing, print out this test pattern on 8x10 or larger paper and print out 5 of them or 7 of them. Put them on something stiff, so you can put them out at different distances. Set them out staggered, so you can see all of them at once and put them out at 10 yards from each other. Now set yourself up, so you are 50 yards away from the middle target. Focus the best you can on the middle one and then start looking at the ones before and behind. Don't do any more focusing, just see how the others look. Test all your optics at the same time, in the same day (sunny day if possible). Now set back up at 100 yards from the middle target and do it all again.

If some of your optics can show all the targets clearly, then go to 15 yards apart from each other.

Now you should be able to see some real differences, instead of percieved differences.

You are correct, most of your optics are of low quality and also they are inconsistent. Untill you have a chance to look through some zeiss ot swarowski, you probably are wasting time worrying about DOF. If you wish to find budget optics with better DOF, there's a saying. I think you have heard it before.

You might want to settle for what you got. I spent years looking for budget optics, only to realize that there just aren't any that are even remotely close to high priced ones. After looking through my wife's 30 year old Zeiss (30 year old technology and nothing "budget" could even come near it), I stopped looking and got rid of all my budget optics.

A larger objective, like 50mm compared to 21mm will usually give you the feeling of a larger DOF, because you have a larger FOV. It can be decieving sometimes.

low quality means less crisp images -- less of the light is going exactly where you want it to be.

That has the same visual effect as less depth of field; they image is not as good at best focus, and worse on either side of best focus, compared to a higher quality optic.

hazna, that's what I suspected, and the correlation between overall better optics and DOF seems higher than to lens diameter, although maybe the latter does have a factor in DOF? Only the "higher quality" Bresser 8x21 will tell...

And thanks Old-Lumens, that's a great test you suggested, but the DOF effect is visible even without a test pattern. For close range leaves 50cm apart need to refocus the monocular, and far objects 2-3km away are harder to focus on with the lesser quality monoculars.

Looks like I'll need to buy some quality optics (Vortex? :)) when I have the money...

I don't know about Vortex, I was not impressed with their binoculars. I had a pair and sent them back.

I really liked Zen and I own a Vanguard Venture Plus 8x42 roof prim binocular.

The Zen-Ray are awesome but I couldn't afford two different binoculars and had already bought the Vanguard.

I do not know about their monoculars. I only use Binoculars.

I do like roof prism binoculars, but porro prisms are supposed to have a larger DOF. The Vanduard have (I feel) a very nice DOF and FOV. I am a birder and they work well for that.

I only said Vortex because of the other thread... :)

The Vanguard and Zen-Ray are tempting as well.

My mom bought a Zeiss Victory FL 10*32 last year, and its pretty awesome. Never worried about DOF when using it, because I didnt have to. But its not exactly budget..^^

hehe 1500€ :p

You get what you pay for. :D

The Bresser 8x21 [roof] binoculars arrived yesterday afternoon, so I tested it at low light and also this morning with good light.

It is of course less bright than the Bresser 10x50 [porro] binoculars or the Mystery 8x42 [roof] monocular, also slightly lower resolution than both, but it has the DOF problem I mentioned before regarding other low objective diameter monoculars - its in-focus range is much lower. While with the larger objective similar magnification Bresser 10x50 and Mystery 8x42 I can see at least 1.5km to 2.5km using the same focus setting, with the Bresser 8x21 I can see only one distance in focus.

All in all I'm pleased with the Bresser 8x21, but it's nowhere near the quality of the Bresser 10x50 or the Mystery 8x42. Both the latter are Bak-4 glass, the 8x21 is BK-7, maybe this is has a bigger effect than I thought.

Maybe I'm just a large objective guy :)

Increasing the aperture reduces the DOF. Increasing the magnification reduces the DOF.