Is Depth of Field Affected By Objective Lens Diameter?

[PracticalTactical]

I have a question for some of you optics gurus out there...

March scopes and other companies offer these Mirage Disks that reduce the effective objective lens diameter to increase the depth of field to make mirage more easily readable.

So I'm wondering if this trend toward ever larger objective lenses is resulting in a reduction in the depth of the field ?

Would a scope of a given magnification with a 40 mm objective lens by some mathematic optical fact perhaps have a greater depth of field than a scope of the same magnification with a 56mm objective lens?

Or is depth of field affected by other factors in the way the lenses are organized within the scope.

I like to see the mirage to assess the conditions and a scope with a shallow depth of field does not appeal to me.

 

[_Windrider_]

If I’m not mistaken, length of the tube is one of the biggest deterring factors in depth of field.

 

[PracticalTactical]

That's a great point and thanks for bringing it up. I almost stated that in my original post.

Personally I find shorter scopes to have a tighter range of tolerance for eye position to get a good clear image as well.

I've been shooting a 5-25 ATACR and a 5-25 Kahles lately and find the longer Kahles to be easier to get behind that the ATACR.

For this reason, I am drawn to longer scopes.

 

[TheHorta]

Objective, magnification, aperture and distance to subject/target largely determine DoF.

Focusing on camera lenses, for a second, increased optical magnification affects DoF, but so does aperture. All things being equal, a 200mm/F2 will have similar DoF to a 400mm/F4 lens.

Also, distance to subject impacts DoF.

DoF at 10-feet, with certain lenses, may be only an inch or less. Whereas with the same lens at the same aperture, the farther away you get from the subject the closer you get to "hyperfocal" distance and your DoF increases linearly as you approach hyperfocal distance.

 

[PracticalTactical]

Agreed but that is not to say that depth of field is the same for all scopes at a given distance. I have this one particular short spotting scope with such a shallow depth of field that its hard to focus at any distance. I have others that are much more tolerant and it makes reading mirage easier, even viewing the target is easier.

But yes, depth of field with any particular scope will increase as the viewing distance is extended, but I'm looking for scope characteristics we can identify that result in a greater depth of field, largely so I can eliminate scopes from consideration that would have a shallow depth of field.

 

[_Windrider_]

Agreed but that is not to say that depth of field is the same for all scopes at a given distance. I have this one particular short spotting scope with such a shallow depth of field that its hard to focus at any distance. I have others that are much more tolerant and it makes reading mirage easier, even viewing the target is easier.

But yes, depth of field with any particular scope will increase as the viewing distance is extended, but I'm looking for scope characteristics we can identify that result in a greater depth of field, largely so I can eliminate scopes from consideration that would have a shallow depth of field.

In all honesty though, in today’s market of high end scopes, that kind of stuff is all relative. The differences are so minute it’s almost a waste of energy to sweat over it. I mean if I’m looking at a kahles, an ATACR and an S&B, it really comes down to price and which reticle I like better. I’m not concerned with depth of field.

 

[TheHorta]

Light transmission figures (aperture) will come into play in your consideration.

A PMII Ultra Bright 4-16x56 will have a large aperture (small F-stop number) which will decrease DoF at closer distances, but will let significantly more light in at overlapping magnifications with the same 56mm scope in SB's 5-45x High Power variant. The objective and tube may be identical in all other ways, but the optical formula will greatly affect defocus characteristics / DoF.

So, in the above example -- if I'm understanding scopes in the same way I do camera optics -- at 16x on both scopes, the DoF of the 4-16 should be noticeably more shallow.