F-number & Depth of field (1 Viewer)

Hi all,

The term "depth of field" is commonly used in birding optics, but I have found it quite hard to understand what it actually means in binoculars (or scopes), which cannot be "stopped down". Why do some binoculars/scopes with similar power and objective diameter have more depth of field than others - or do they? (A reason other than "good binos have a good DOF"). Is it a thing that can be measured or calculated or is it just about the individual perception? Is it related to the objective focal length (determines f-number), which would maybe mean that "long and narrow" tubes (light path) would also have a longer DOF? Manufacturers usually don't tell the focal lengths of binoculars and some say that it is usually meaningless, but could this information be useful after all.

What factors affect DOF in binos?

Ilkka

Ask me one on sport!

James

iporali said:

Hi all,

The term "depth of field" is commonly used in birding optics, but I have found it quite hard to understand what it actually means in binoculars (or scopes), which cannot be "stopped down". Why do some binoculars/scopes with similar power and objective diameter have more depth of field than others - or do they? (A reason other than "good binos have a good DOF"). Is it a thing that can be measured or calculated or is it just about the individual perception? Is it related to the objective focal length (determines f-number), which would maybe mean that "long and narrow" tubes (light path) would also have a longer DOF? Manufacturers usually don't tell the focal lengths of binoculars and some say that it is usually meaningless, but could this information be useful after all.

What factors affect DOF in binos?

Ilkka

Click to expand...

This is a good question . It can be used to explain a number of odd things we see in bins. First of all’ different people actually don’t see the same amount of DOF in the same bin.Young people with good eyesight have greater focus accomodation than older people who need reading glasses. They will see greater DOF in any binocular (and less field curvature), so while measurements could be done ,and would be valid in comparing one bin to another they would not predict the actual DOF for an individual.

A second factor is magnification. Simply put, if every thing else is equal lower magnification bins have more DOF.

A third factor (and the most interesting one) is “effective” focal ratio which varies with different lighting conditions. The focal ratios of the objectives of most hand held binoculars fall between F:3.5 and F:4. For the moment just assume all binoculars are F:4 no matter what the objective size. An 8x25 has an F:4 objective with a focal length of 100mm, an 8x50 has an F:4 objective with a focal length of 200mm. Assume these two binoculars are being used in daylight conditions so that the pupil size of the eye is closed down to 3.125mm, exactly the same as the exit pupil of the 8x25. That means the entire 25mm of that bin’s objective is being used, but only the central 25mm of the 8x50’s objective.The “effective” focal ratio of the 8x25 in this situation remains F:4 since the entire objective diameter is being used, but the “effective” focal ratio of the 8x50 becomes F:8 (the 200mm focal length divided by the 25mm central part of the objective actually being utilized). Not only does this effectvely higher f-ratio in daylight increase DOF compared to the smaller bin, but it also reduces the visible effects of chromatic and spherical aberrations.

iporali said:

Hi all,

The term "depth of field" is commonly used in birding optics, but I have found it quite hard to understand what it actually means in binoculars (or scopes), which cannot be "stopped down". Why do some binoculars/scopes with similar power and objective diameter have more depth of field than others - or do they? (A reason other than "good binos have a good DOF"). Is it a thing that can be measured or calculated or is it just about the individual perception? Is it related to the objective focal length (determines f-number), which would maybe mean that "long and narrow" tubes (light path) would also have a longer DOF? Manufacturers usually don't tell the focal lengths of binoculars and some say that it is usually meaningless, but could this information be useful after all.

What factors affect DOF in binos?

Ilkka

Click to expand...

Ilkka: That confused me for a while. In photography the DOF of a lens is a function of the F number and the magnification. The larger the F number, the greater the DOF. The greater the magnification, the less the DOF.

However a binocular is a more complex system consisting of an objective that forms an image, and an eyepiece that magnifies the image. (In a sense the photographic lens is equivalent to the objective.) Note that the prisms can be ignored.

Consider two 8x30 binoculars:

Binocular 1: 120mm focal length objectives and 15mm eyepieces.
Binocular 2: 160mm focal length objectives and 20mm eyepieces.

Unless I am mistaken the second binoculars will have greater DOF than the first because they have longer focal length objectives i.e. smaller F number.

So yes binoculars do vary in their DOF and this is not hard to see when comparing competing products.

Hopefully someone will correct the above if I am mistaken.

Henry: Your third reason is new to me and very interesting. That might help explain why some people prefer 8x50 binoculars despite the fact that they will only be using part of the exit pupil in normal lighting. I have noticed that 8x30 roof prism binoculars tend to show more chromatic aberration and less DOF than their larger cousins but was not sure why. BTW where did you find that out? The DOF question was once asked on an astronomy forum and your answer was not given.

Leif, I think you typed too fast about the f-ratios. Im sure you meant to say that the higher the f-ratio the greater the DOF. Henry

henry link said:

Leif, I think you typed too fast about the f-ratios. Im sure you meant to say that the higher the f-ratio the greater the DOF. Henry

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I corrected it before your post appeared! Yes you are of course right.

Leif said:

Henry: Your third reason is new to me and very interesting. That might help explain why some people prefer 8x50 binoculars despite the fact that they will only be using part of the exit pupil in normal lighting. I have noticed that 8x30 roof prism binoculars tend to show more chromatic aberration and less DOF than their larger cousins but was not sure why. BTW where did you find that out? The DOF question was once asked on an astronomy forum and your answer was not given.

Click to expand...

Leif, It probably wasn't mentioned in an astronomy forum because it isn't much of a factor in astronomy since the eye's pupil is opened wide enough in the dark to make the effect negligible. Of course DOF isn't that much of an issue in astronomy anyway. Henry

Henry & Leif: You rule!

You really have more than answered my fuzzy question - thanks!

Consider two 8x30 binoculars:

Binocular 1: 120mm focal length objectives and 15mm eyepieces.
Binocular 2: 160mm focal length objectives and 20mm eyepieces.

Click to expand...

That was exactly my point about the focal lengths. The specific examples in my mind were Swaro 8x32EL, Nikon 8x32SE and 8x32HG/LX, of which "long-tube" Swaros and porro-Nikons seemed to have more DOF than roof-Nikons. I just couldn't explain why.

And that "effective focal ratio" - just brilliant! The eye actually does that stopping-down - and the big exit pupil lets the eye (binos) to move and still form an undisturbed image.

I feel so relieved...

Ilkka