how do you calculate short-distance focusing in foot/feet unit of measurement? (2 Viewers)

[iseegeorgesstar]

Hey,

I apologize if this is in the wrong place. But I've always wondered how do you convert the stated specs for close-up focusing of certain binoculars into feet? Sometimes, on certain product pages it says it such as "minimum/close focus at 6ft or 8ft" But other times it's in yards and that's when I get confused...

For example, on the Nikon M7 8x30 it says "144.4 m @ 1000 m" . How would you put in that in feet? Comes to focus in X-feet?

Thank you for the help.

 

[Ted Y.]

Multiply 144.4 by 3 for the number of ft; replace 1000m by 1000 yds.
433.3 ft @ 1000 yds
But this is FOV (linear) not the close-up focus.

The close-up focus is a linear measurements: 2m (or 2*3.28 ft)

 

[tenex]

Close focus will be stated simply in meters or feet, typically around 1.8-3 m or 6-10 ft these days. Convert to feet by multiplying by 3.3.

"144m @ 1000m" is field of view. For some reason we express that in ft @ yds, so multiply by 3 to get 432 ft @ 1000 yds. (You're not converting meters to feet, just adjusting for the difference between feet and yards.) FOV being dimensionless may best be expressed as an angle, in this case about 8.3° -- divide feet by 52.4.

By the way from another post of yours I just read, FOV is not related to objective size at all (it varies roughly inversely with magnification). You have some reading to do.


Edit: I was not pointlessly repeating Ted, he edited post #2 to add two more lines after I posted this.

 

[iseegeorgesstar]

Thank you very much for the replies.

I see now on the Nikon M7 product page it actually says the close focus: 6.6' / 2 m. I guess I just then have to convert if they only state the meters.

I didn't realize fov wasnt related to objective size. I always hear about 1.25" astronomy EPs have a max fov due to the barrel size so assumed it was the glass size.

Follow up question. So I understand you're getting 8.3degrees by dividing 432ft by 52.4

But where's the 52.4 coming from and what is that?

 

[Pinewood]

...

Follow up question. So I understand you're getting 8.3degrees by dividing 432ft by 52.4

But where's the 52.4 coming from and what is that?

Hello,

At 3000 ft [1000 yards] each degree of the field of view subtends 52.4 ft. I came up with a slight smaller figure, but close enough.

Stay safe,
Arthur

 

[Ted Y.]

Edit: I was not pointlessly repeating Ted, he edited post #2 to add two more lines after I posted this

I edit my post short before you published yours.

I think you explanation is clearer, therefore cannot be considered "pointlessly"

 

[wllmspd]

The max field of view from 1.25” astro eyepieces is due to when you want low powers you need a long focal length… which is sort of correlated to the required opening size in the bottom of the eyepiece…. (For wide apparent fields of view especially.
So if you want a very wide low power view you need the wider 2” eyepiece diameter. For telescopes with low focal ratios (focal length divided by aperture) and moderate to high powers this won’t apply and 1.25” eyepieces are just fine and often there won’t be a 2” option.

Peter

 

[Binastro]

Spotting scope prisms aren't wide enough for 2 inch or 3 inch width eyepiece barrels.
These larger eyepieces are for astro telescopes where prisms aren't generally used and experienced observers view inverted images, which are not degraded by prisms.

The field of view is determined by the eyepiece not the objective.

A radian is 57.3 (57.295) degrees.
A metre is bigger than a yard.
About 1609 metres in a mile or 1760 yards.
The actual standard is 2.54 cms is 1 inch.

So 57.3 x 1609 divided by 1760 gives about 52.4.

(52.391) (1609.344 metres in a statute mile).(57.2958 degrees in a radian).

So the difficulty lies in the metric system and Imperial system, plus the strange notion of fields in ft per 1000 yards.
A very mixed up calculation.

I much prefer fields in angular measure, i.e degrees or fractions of degrees.
Or with high magnification arc minutes.

Regards,
B.

 

[iseegeorgesstar]

Thank you for all the explanations!

 

[tenex]

The field of view is determined by the eyepiece not the objective.

Yes.

A circle of 1000yd=3000ft radius has a circumference of 18,850ft, divided by 360° = 52.36.

Of course real FOV comparisons only work well between instruments of the same magnification, because there's a general decline with increasing magnification, as one would intuitively expect from "zooming in" (although the apparent field may even increase, another subject...)

 

[iseegeorgesstar]

Since we're talking about fov. I hear people (myself included) sometimes describe the view in through a bins as being a bit constricting and you wouldn't initially think so because it has a nice fov. Is this the interplay of afov and fov?

 

[Binastro]

Yes, Tenex, I made a mistake in my calculation in post #8.

3,000 ft divided by 57.29578 is 52.3599 or 52.36ft.

360 degrees divided by 2 Pi is 57.29578 degrees or one radian.

Your calculation in post #10 is correct.

Regards,
B.

 

[Canip]

Since we're talking about fov. I hear people (myself included) sometimes describe the view in through a bins as being a bit constricting and you wouldn't initially think so because it has a nice fov. Is this the interplay of afov and fov?

Yes, sort of of.
An 8 degree FOV in an 8x binocular (which corresponds to about 64** degrees AFOV) is nice, in a 6x bino (where it‘s only about 48** degrees AFOV) it appears narrow

** very rough approximations

 

[iseegeorgesstar]

That makes sense. So to compare the afov across magnification when the fov is the same or close.

Is there some general afov number/threshold that people like on average? For example, 10x42 binoculars tend to be a very general purpose binoculars on average for most people because the view is stable enough. 32mm binoculars tend to be average a nice portable size because theyre small. People on average tend to be happy when afov is 60degrees or more because it doesnt feel constricting? (Just throwing out a number/example)

 

[Canip]

My opinion (very generalized): 60 degrees and more is nice in an 8x bino, 65 degrees and more is nice in a 10x.

 

[tenex]

60° seems to be some sort of industry standard for "wide-angle", though I'd prefer 65° myself, and Swaro NLs reach ~70° (as many older Porros also did, or more, though the outer field wasn't sharp). Most 6x/7x bins (and pocket models) can't manage much more than 50° without larger prisms. But fans of those even claim to like a smaller AFOV for eliminating distractions, whereas many others find larger AFOV more "immersive". I don't think that averages, it's just varying taste.