Binocular depth of field ?


December 8, 2013, 08:15 PM
Looking to upgrade my compact binos because they have a very short depth of field. They are easy to quickly focus to a crisp edge, but still I am fed up with twirling the focus knob.

The problem is that when still hunting in forest, while scanning ahead between the trees, I have to frequently adjust focus as distance changes even by as little as 20 feet.

Can you guys suggest a 'compact' roof prism coated lens bino, in the 8x33 range, that gives excellent depth of field (and excellent eye relief) ?

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Andrew Leigh
December 9, 2013, 01:43 AM
Depth of field (DoF)is determined by the f stop. The f stop is the ratio between the primary lens and the focul length (indirectly the magnification) of the binocs.

So you would need to find the focul length of your binocs from the data sheet but as they are compacts lets assume they are 125mm. You can closely approximate the focul length by focussing the sun on a piece of paper, where the image becomes the smallest and clearest measure the length from the front of the objective lens to the piece of paper and you will have your focul length to within a couple of mm.

Then f=Objective diameter / focul length = 33/125 = f 0.264

So if you had a 50mm objective with the same focul length the f number would increase to f 0.4 or a significant improvement in DoF. Conversely if you reduced the magnification you would improve the DoF.

I think it would be fair to say that given lens design that ALL 8X32 will have about the same DoF so moving manufacturers for the same given spec will not help. You want either 42 or 50mm objectives or lower power and or both.

December 9, 2013, 04:52 AM
Hi twofifty,

maybe you want to take a closer look at one of these:


December 9, 2013, 08:46 AM
Andrew Leigh explained it well. Just to add. For best results the front objective should be 5X greater than the magnification. That means you should be looking at 6X30, or 8X40's etc. An 8X30 is certainly better than the 8X23's I see a lot of guys trying to use. An 8X30 works well enough in the middle of the day, but the smaller the front objective is relative to the scopes magnification the worse they will be in low light and the more critical eye position becomes.

High end, $1000+ optics with much better glass tend to do a little better than mid priced optics with the same size glass. But all things being equal the 5:1 ratio of lense size to magnification will always be better.

December 10, 2013, 01:04 AM
Thank you guys for the tech primer on f-stops and the 5xobj.dia. ratio

Those Steiners look like nice glass but perhaps a bit heavy to have around my neck.

December 10, 2013, 10:04 PM
Narrow depth of field is a "feature" of roof-prism binos. It just goes with the territory. That's why you never see a pair with individual focus ocular lenses. It's because you have to constantly fool with the focus.

December 11, 2013, 10:01 PM
Leupold's 6x30 Yosemite BX-1 has caught my attention.


It's a porro prism optical path (thx Kernel) so likely has decent depth of field.
Fully multi coated, nitrogen filled.
At 6x30 it will be no slouch picking up twilight (exit pupil 4.6mm).
6x is plenty of power for the woods & short distances I hunt.
FOV at 100yds - 42 feet (8 degrees).
Only weighs 17oz., yet armored body.
Length 4.6", but wider than a roof prism bino.
Generous eye relief - 18.5mm - great for eyeglasses.
No hassle service dept.

December 12, 2013, 11:52 PM
Voretex makes a very similar 6.5x30 bino that I think is even better then Leupolds and less expensive. I have two pairs, one for each son. They have a lifetime no-questions-asked replacement policy. IIRC they're about $100 and WAY better than some I've seen at twice the price.

Personally, I think 6x or 7x is ideal for most North American hunting. 8x at the most. In my binos I want: depth of field, field of view (wide angle), large exit pupil, and long eye relief. You lose all of that when you buy high magnification binos.

Plus, poro prism binos will have the same optical quality as roof prism designs that cost twice as much. So, if you want decent glass, and you're on a tight budget, poro prism is the smart way to go. For example, if you spent $500 on a nice poro prism bino it would have optical qualities like $1000 roof prism bino. The down side is size, poro's are bigger. But not always heavier. Roofs need more lenses inside which can make a smaller bino heavier.

December 13, 2013, 08:32 PM
Thank you Kernel for educating me on depth of field & the real world advantages of porro Vs roof prisms.

jmr40 and Andrew Leigh, thank you as well.

I've definitely narrowed it down to the 6.5x30 Vortex as well as Leupold's equivalent model in 6x30.

December 14, 2013, 09:00 AM
Carrying around your neck is another problem. Once you go with the shoulder straps you will not even know you your field glasses on.

I had forgotten my rather heavy leica trinovids after I sat in the truck before an evening hunt with my two clients. I got out of the truck, told them I had to go back to the lodge and get them. I went in my room, searched all around and was dumbfounded at where they could be. Standing hands on my hips looking at the top of the dresser I could see in the mirror I was wearing them.

It was an impressive moment to realize those big European glasses could be carried with no effort at all.

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