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Calculating Diopter Strength


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#1 Albion Hockney

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Posted 25 October 2016 - 09:12 PM

I have a shoot coming up and we are shooting on anamorphic (Todd AO). We are mostly in medium/medium wide shots but we do have a few close ups and I'm trying to sort out what diopters we need. 

 

Im waiting back on close focus for our set of lenses but it seems most Todd AO sets people are saying about 3ft-6ft. What Diopter strengths do people typically carry?

 

and further more how do you calculate close focus of a lens with a given strength of diopter. 


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#2 David Mullen ASC

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Posted 25 October 2016 - 10:04 PM

https://en.wikipedia...Close-up_filter

When you add a close-up lens to a camera which is focusing at the shortest distance at which the objective lens can focus, and you don't change the focus adjustment, the focus will move to a distance which is given by following formula:  X = X / (DX + 1)

X being the shortest distance at which the objective lens can focus, in m, and D being the Diopter value of the close up filter. This is the minimal working distance at which you will be able to take a picture with the close-up lens.

For example, a lens that can focus at 1.5 m combined with a +3 diopter close up lens will give a closest working distance of 1.5/(3*1.5+1)=0.273 m.

 

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This is what I found online -- I haven't checked it against what might be in the ASC Manual yet, but if I understand this formula correctly, then if your lens focuses to 4' minimum, then a +1/2 Diopter should change the minimal focus to 1' 4, for example.  But I'm not sure I'm doing the math correctly because it seems you could never focus the lens to 2' because every focus distance I toss into the formula with a +1/2 diopter keeps getting me a distance that never reaches 2', which can't be correct.

 


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#3 David Mullen ASC

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Posted 25 October 2016 - 10:51 PM

I must be misunderstanding the formula because the ASC Manual has a chart and it says that with a +1/2 Diopter, the focus at 4' becomes approx. 3' 7"...


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#4 Dom Jaeger

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Posted 26 October 2016 - 04:29 AM

Because the diopter value is a metric term you have to use metres in the formula.

So 4' is approx 1.2m.

1.2/(0.5x1.2 + 1) = 0.75m or approx 2' 6"

Which is not what that ASC chart says either, but it is what the formula gives.
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#5 Dom Jaeger

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Posted 26 October 2016 - 06:54 AM

Another way of working out the minimum focus distance of a lens with a diopter is to convert the lens minimum focus to a diopter value, and add the close-up diopter value.

So using a 4' or 1.2m minimum again, the diopter equivalent of 1.2m is 1/1.2 or 0.833. If we add a +1/2 diopter, the diopter value becomes 1.333 which equates to 1/1.333 or 0.75m.
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#6 Marcel Zyskind

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Posted 26 October 2016 - 01:56 PM

I usually carry +1, +2 & +3 diopters when shooting anamorphic (also with spherical), occasionally also +.5. Don't forget you can also stack diopters. But then you're into macro world.
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#7 Miguel Angel

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Posted 26 October 2016 - 04:49 PM

I would say that you don't really need to worry about what diopter you need in most cases if you ask for +1/2, +1, +2 and +3.

Then, when shooting the camera tests you just place them in front of the lens with a subject to the distance you think you are going to have him / her on the day and see the effect.

On set, in my experience it is usually faster to ask for the diopters when they are needed and place them in front of the camera till you achieve the right distance than calculating the formula.

Although, when you keep using them you kind of know the diopter you are going to need anyways.

I usually ask for +1/2, +1, +2 and +3 when shooting anamorphic and spherical and if I really need something higher than that, it would be for an effect which won't be possible with a macro lens like placing the +4 and moving it with the +3 moving in front of the diopter.

Have a good day
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#8 Gregg MacPherson

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Posted 26 October 2016 - 05:07 PM

Miguel's approach gets my vote. Have a few on hand and choose them on the fly.
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#9 Albion Hockney

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Posted 31 October 2016 - 03:02 AM

Thanks for the replies 


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