Dioptre/Magnification eh?

chuffalump

Member
I was looking for a low cost scope and found a couple in the ?25 range. Thing is, they were classified by magnification rather than dioptre.

So I looked it up. Ignoring the magnification vs lens power fudge. A lens dioptre is it's magnification x 4. I've seen people recommend dioptres from 0.5 to 0.75. These are tiny magnifications. How can cheaper scopes be classed as four or eight X magnification (16 to 32 dioptre)?

A .75 D lens focuses light at 1.3m, .5 D is 2m so it makes sense that I've seen recommendations of smaller dioptres for longer DL.

Hmmph. It occurs to me that the 4X or 8X could refer to the apparent magnification at a standardised DL rather than the actual optical physics definition.
 


Rik

Supporter
Supporter
Hear that sound? That's a can of worms being opened...
First, not an expert (don't even shoot compound much).
But my understanding is that it's not so simple. The actual magnification you get is dependent on the scope/eye distance as well as the dioptre. Simple formulae only work when you're using a simple lens like a magnifying glass - close up.
What you're doing with a compound sight is more like making a kind of shoddy telescope using the lens of your eye and the sight lens. "shoddy" because it will be more or less blurry, depending on your eye and the lens set up (hence the use of "clarifier" peeps to turn the thing into a proper simple telescope with a rear lens as well).
As a rule of thumb: bringing the scope lens towards you increases magnification, moving it away decreases. So declarations of "magnification" on advertising are more or less bunk. Based on assumptions about the setup. Dioptre should be an actual measurement of the lens, so should be more portable, but doesn't tell you immediately what the magnification will be for you.

I expect someone will step in to tell me where I've got it wrong... :)
 


bimble

Well-known member
Supporter
Fonz Awardee
Ironman
if I recall correctly x4 is a 0.5 diopter, x6 is a 0.75 and x8 is a 1.0 diopter... as you know the actual magnification you see it reliant on more factors than just the diopter. There is a handy chart on the interwebs that compares diopter/distance/magnification

 


KidCurry

Well-known member
The relationship of the eye to the scope lens and peep sight is not straight forward. If you are interested here is a very short research paper that explains the relationship of the eye to the scope lens and the peep (with or without clarifier lens) and why it is different for different archers. It explains quite nicely the role of the peep with or without a clarifier lens and the issue of magnification and dioptre. It is also correctly referenced which makes a nice change.

https://avehjournal.org/index.php/aveh/article/viewFile/122/91

Strydom B, & Farrerira J, (2010) Optical lenses and Magnification in Archery
 


chuffalump

Member
Thanks for that. I prefer to have a little understanding of these things, at least enough to know the limitations of the advice I receive or give.

After I posted this I decided I was probably correct in assuming it was magnification based on eye distance from the lens. Meaning 4X or 6X is actually going to vary slightly depending on DL but dioptres are forever. Hurr hurr. Sorry.

Bimble's chart shows the optic definition of magnification. So 0.5D is 2X at 1m viewing. 4X doesn't get a look in unless you have the arms of an orangutan. Next is to have a look at KidCurry's research paper.

Strap those worms back in and whisper "There's no place like home, there's no place like home".
 


jerryRTD

Active member
When I decided to use a scope I got the housing, tried various dioptres by holding them at arms length. I bought a .5 and a .75, tried both and settled on the .5. Nothing beats 'suck it and see'. worms back in can, can kicked down the road.

Now all we have to do is sneak off before some one mentions depth of field.
 


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chuffalump

Member
I ordered the equivalent of the 0.5 in 'cheap'. Mainly because theory says lower D for longer DL. Also should be a more gentle transition from a recurve sight.
 


OpaNL

Member
Added a pdf with the chart to the download section under manuals: (This file is in the moderation queue).
Don't know how long it will take though and if it's accepted.

Peter
 


Del the Cat

Well-known member
Just to add to the confusion.
What does X2 mean?
If you view a rule with one eye, whilst the other has the X2 lens in front. Does a 1mm division on the rule viewed through the lens appear the same length as 2mm viewed with the eye?
I suspect some manufacturers go by the apparent magnification of AREA... so if they would claim that as X4 (e.g a 1mm sided square is now a 2mm sided square which has an area of 4 sq mm).
Very much a case of caveat emptor.

I once drew up a label x2 (e.g with linear dimensions double size) I sent it off to the printer to be printed at half size.... it came back half the area!!!
They said it's their industry way of doing things... I was not amused.
Del
 


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