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Diopters and depth of field

depth of field Diopter

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#1 christian mann

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Posted 26 November 2015 - 03:19 PM

Hi guys,

 

when shooting with diopters the other day I noticed that you can actually not focus objects in a distance, only close and middle range distances.

What is the proper mathematical/physical explanation for this?

I know that  diopters bring the close focus "closer" but why can't one focus throughout until infinity?

 

And is there a way to determine the "maximium focus" (opposite of close focus) ?

Let's say you're shooting with an ALEXA 16:9 2K with a 50mm f2.8 and a diopter 1.....apparently close focus will be 1m but can I predict the "maximal" focus?

Would be handy to know. And does it change according to lens, format, f stop?

 

If a diopter 1 brings the close focus to 1m - does that apply to any focal lenght? Hard to believe...

 

Thanx for your input :)

 

M


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#2 Stuart Brereton

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Posted 26 November 2015 - 03:44 PM

That's how diopters work. You gain extra inches of close focus, but lose infinity. You don't even need diopters for this. It's easy to construct a lens that will focus almost to the front element, but the trade off is that there will be no infinity focus. All lenses have to compromise between Close focus and maintaining Infinity


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#3 dan kessler

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Posted 26 November 2015 - 04:36 PM

If you are familiar with basic lens geometry, then you can understand what diopters do.

You know that parallel lights rays coming from an object at infinity pass through a lens and get focused to a point at the lens focal length on the other side. That's where the image is formed.   However, this arrangement works in reverse, too.

Think of a diopter as a lens working backwards.  Light rays from an object one focal length away from the diopter will pass through the diopter and emerge as parallel rays on the other side.  

When these parallel rays now enter the main photo lens, they get focused just as they would if viewing an object at infinity.   So, anything that is one focal length away from the diopter will be in sharp focus.  Closer or farther, out of focus.


Edited by dan kessler, 26 November 2015 - 04:36 PM.

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#4 Mark Dunn

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Posted 26 November 2015 - 04:50 PM

Hi guys,

 

when shooting with diopters the other day I noticed that you can actually not focus objects in a distance, only close and middle range distances.

What is the proper mathematical/physical explanation for this?

I know that  diopters bring the close focus "closer" but why can't one focus throughout until infinity?

 

And is there a way to determine the "maximium focus" (opposite of close focus) ?

Let's say you're shooting with an ALEXA 16:9 2K with a 50mm f2.8 and a diopter 1.....apparently close focus will be 1m but can I predict the "maximal" focus?

Would be handy to know. And does it change according to lens, format, f stop?

 

If a diopter 1 brings the close focus to 1m - does that apply to any focal lenght? Hard to believe...

 

Thanx for your input :)

 

M

You can think of the dioptre as compressing the focusing range so that 1m (for a no. 1) is the greatest distance for sharp focus, but it also shortens the minimum focusing distance quite a bit. The formula for that is

Xmin = X/(DX+1)

where X is the normal close focusing distance and D the dioptre value. So if your lens focuses down to 0.5m normally, it will focus down to 0.5/((1x0.5)+1) or 0.16m with a no. 1 dioptre.

It's the same deal for any lens, format or stop.


Edited by Mark Dunn, 26 November 2015 - 04:58 PM.

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#5 christian mann

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Posted 27 November 2015 - 03:53 AM

I see, I see.

So, does that mean I can work out the close focus with the formula Xmin = X/(DX+1) and when it comes to the greates disctance for sharp focus the follwoing applies:

 

diopter 0,5=2m

diopter 1=1m

diopter 2=0,5m

with any lens, format, stop?

If so, what is the formula for this?

 

Or how else would I determie the greates disctance for sharp focus with diopters ?

 

M


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#6 Mark Dunn

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Posted 27 November 2015 - 05:07 AM

There's no formula as such. As you state the distance in metres is 1/D where D is the dioptre number. 1m for a no.1, 0.5m for a no. 2 and so on.

The dioptre's number is the reciprocal of its focal length in metres.


Edited by Mark Dunn, 27 November 2015 - 05:08 AM.

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#7 davide sorasio

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Posted 28 November 2015 - 04:01 PM

sorry but I cannot make the formula work, maybe I'm wrong but after doing the math it would come to 0.5/ 1.5 that comes to 0.33, I do not see how you got to 0.16 meters at the end.


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#8 christian mann

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Posted 29 November 2015 - 07:01 AM

and on another note: why would I use a diopter instead of a macro lens ( which does wfocus to infinity) ?

if it's about a "closer" close focus a macro seems the way forward, not?

 

m


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#9 David Mullen ASC

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Posted 29 November 2015 - 11:27 AM

Macro lenses are more expensive and often only come in certain focal lengths; diopters are a quick and cheap way of getting the lens to focus closer.


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#10 Dom Jaeger

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Posted 29 November 2015 - 02:18 PM

sorry but I cannot make the formula work, maybe I'm wrong but after doing the math it would come to 0.5/ 1.5 that comes to 0.33, I do not see how you got to 0.16 meters at the end.

  

Yes, 0.33m is correct. Which shows how the entire lens focus range shrinks down to a range of only 17cm.

and on another note: why would I use a diopter instead of a macro lens ( which does wfocus to infinity) ?
if it's about a "closer" close focus a macro seems the way forward, not?
 
m


To expand on David's points:
Macros are usually better in terms of image quality (unless you use something like Master Diopters), but also more expensive and bulky and may not be part of a lens rental package for the duration of a shoot. By contrast a set of close focus diopters could be easily included in a rental package (or owned by a DP) for both planned and unexpected shots needing closer focus. They can be used singly or stacked, on different focal lengths or a zoom, to get the right magnification or to allow greater distance between lens and subject than a particular macro might.
With anamorphic lenses the minimum focus is not very close and even the rare macros don't get all that close, so diopters are the only real option for very close focus or high magnification shots in anamorphic.
Zooms also don't usually have terribly close minimum focus, and macro zooms don't really exist (in the sense of still having a zoom function at very close distances), whereas diopters allow zooms to focus closer and still act as zooms.
Macros are slower aperture lenses or have limited apertures at very close distances, whereas close focus diopters lose virtually no light and can be used on fast lenses.
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#11 christian mann

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Posted 01 December 2015 - 01:58 AM

ok, cool. thanx for the detailed explanation re. macro/diopter!

making sense :)

 

m


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