How to calculate non infinity tables for diopters/close up filters?

Helge · Oct 2, 2021

I have a stack of non marked diopter lenses for medium format folders that I’d like to use.
The focal length is pretty easy to make a good guess at with the usual infinity focus projection trick.
But it would be nice to get even closer to the subjects by focusing the lens to other distances.
Is there any formula for calculating the correct distance?

BrianShaw · Oct 2, 2021

I’ve done similar with Kodak Retina cameras. There are tables in the Retina reference materials that aid.

Maybe you will be as fortunate? Which folder are you using?

can you affix a ground glass and figure it out via experimentation? I think that’s where I would start.

Helge · Oct 2, 2021

BrianShaw said:
I’ve done similar with Kodak Retina cameras. There are tables in the Retina reference materials that aid.

Maybe you will be as fortunate? Which folder are you using?

can you affix a ground glass and figure it out via experimentation? I think that’s where I would start.

Just Zeiss Ikon folders with the standard press-on holders.
These are truly unbranded lenses. There is no chance of finding tables.
There has to some way of calculating tables?

BrianShaw · Oct 2, 2021

How about this: affix a ground glass (or something that reasonably can serve as a GG) to the film plane. Affix a diopter. Focus at a specific distance. Note what the "equivalent focus distance" is on the focus scale. Repeat at a closer distance. Repeat several times and graph the results.

Nicholas Lindan · Oct 2, 2021

Once you have figured out the diopter of the lens then any close-up lens table should work.

If you want to calculate your own try https://en.wikipedia.org/wiki/Close-up_lens

Helge · Oct 2, 2021

BrianShaw said:
How about this: affix a ground glass (or something that reasonably can serve as a GG) to the film plane. Affix a diopter. Focus at a specific distance. Note what the "equivalent focus distance" is on the focus scale. Repeat at a closer distance. Repeat several times and graph the results.

Plenty of ways wing it with empirical fudges. Put it in front of a TLR viewing lens etc. All of them imprecise and slow.

Nicholas Lindan said:
Once you have figured out the diopter of the lens then any close-up lens table should work.

If you want to calculate your own try https://en.wikipedia.org/wiki/Close-up_lens

Thanks. Do you have a set of tables?

AgX · Oct 2, 2021

-) The maximum image quality most lenses have at infinity or near that. Setting the primary lens in a close-up-lens combo shorter than infinity thus will reduce image quality. Better to instead choose a different close-up lens

-) To calculate the subject distance in case the primary lens is set shorter nonetheless, turn the distance you set into a diopter value, add this value to the diopter value of the close-up lens. Turn that result again into meters and you got the distance the subject to place at for this combo and focus setting.

wiltw · Oct 2, 2021

The diopter strength sets the subject distance...with the lens focused at Infinty. Closer than that focus distance can be obtained by using standard lens helicoid/bellows mechanism.

FL = 1m / Diopter

FL of closeup lens vs. Diopter Power = Subject Distance

2m vs. +0.5 diopter = 79" (2m)
1m vs. +1 diopter = 39.5" (1m)
0.5m vs +2 diopter = 19.75" (0.5m)
0.33m vs. +3 diopter = 13" (0.33m)

Unfortunately I don't know how to compute subject distance vs. lens focus distance (without diopter). With 1m FL (+1 dopter) at Infinity the subject distance is 1m (39.37") but with lens set to 3' the subject distance drops to 46cm (18")..

BrianShaw · Oct 2, 2021

Helge said:
Plenty of ways wing it with empirical fudges. Put it in front of a TLR viewing lens etc. All of them imprecise and slow.

LOL... I don't quite know how to interpret this: dismissive or ignorant.

Good luck for a speedier and more precise solution.

Helge · Oct 2, 2021

BrianShaw said:
LOL... I don't quite know how to interpret this: dismissive or ignorant.

Good luck for a speedier and more precise solution.

Sorry, didn't mean to sound dismissive. Ground glass in the film gate is just IMHE very imprecise. Especially at such close ranges.
Running numbers through a formula will just be faster, more easily transferable and better hit a "medium". Looking at ground glass with a loupe is hard work.
My fallback will be using a 75mm TLR viewing lens as reference.

Helge · Oct 2, 2021

AgX said:
-) The maximum image quality most lenses have at infinity or near that. Setting the primary lens in a close-up-lens combo shorter than infinity thus will reduce image quality. Better to instead choose a different close-up lens

-) To calculate the subject distance in case the primary lens is set shorter nonetheless, turn the distance you set into a diopter value, add this value to the diopter value of the close-up lens. Turn that result again into meters and you got the distance the subject to place at for this combo and focus setting.

Front cell focusers are mostly set for optimal optical performance at 40x the focal distance.
How do you turn a distance into a diopter value?

Nicholas Lindan · Oct 2, 2021

Wasn't there something about leading a horse to water...

From the above wikipedia article:

Close distance = (lens distance setting) / ((lens distance setting * close-up diopter) + 1)

AgX · Oct 2, 2021

Helge said:
How do you turn a distance into a diopter value?

The most basic thing in understandung reading glasses and close-up lens designations.
(Though some of the latter may have proprietary designations instead.)

1 / distancemeter = diopter

Helge · Oct 3, 2021

Nicholas Lindan said:
Wasn't there something about leading a horse to water...

From the above wikipedia article:

Close distance = (lens distance setting) / ((lens distance setting * close-up diopter) + 1)

Close distance in this case is measured from the front of the close-up lens. A spreadsheet program will make all the tables your heart desires.

Yeah I saw that thank you. Guess it will have to do. A general formulae seems not to be very well know or is too complex to be used by laymen.
Max and min setting will probably give me 90% of what I really need anyway.

BrianShaw · Oct 3, 2021

Min and max setting probably won’t give great results. At macro focus with diopter lens (or otherwise) the DOF is sliver thin. Unless your needs are quite superficial.

The formulas can indeed be daunting. That’s why I recommended empirical approach. Yes, it takes a bit of time and peeping into a small GG is a pain, but it actually is quite accurate.

Nicholas Lindan · Oct 3, 2021

The formula will give you distances for all the lens setting focus distances, not just max and min.

Example:

Lens 'focused' to 1.5 meters; +2 diopter lens. Subject-lens distance = 1.5 / ((1.5 * 2) + 1) = 1.5 / 4 = .375 meters
Lens 'focused' to 0.5 meters, +2 diopter lens. Subject-lens distance = 0.5 / ((0.5 * 2) + 1) = 0.5 / 2 = .250 meters

The formula will work in feet if you convert diopters to feet - a +1 1/m diopter lens = +1/3.3 1/ft diopter lens

Expect some error in the above - you do need to confirm the distances for your particular lens.

In my youth taking macro shots with an Agfa Sillette and close-up lenses I had a knotted string tied to a bolt in the tripod socket that gave me the distances for several focusing distances that were set on the lens. I used the frosted glass of a microscope slide at the film plane to find the knot positions - it isn't hard, there is very little depth of field with the lens wide open.

Be sure to close down to a small aperture when taking pictures.

Helge · Oct 3, 2021

Nicholas Lindan said:
The formula will give you distances for all the lens setting focus distances, not just max and min.

Example:

Lens 'focused' to 1.5 meters; +2 diopter lens. Subject-lens distance = 1.5 / ((1.5 * 2) + 1) = 1.5 / 4 = .375 meters
Lens 'focused' to 0.5 meters, +2 diopter lens. Subject-lens distance = 0.5 / ((0.5 * 2) + 1) = 0.5 / 2 = .250 meters

The formula will work in feet if you convert diopters to feet - a +1 1/m diopter lens = +1/3.3 1/ft diopter lens

Expect some error in the above - you do need to confirm the distances for your particular lens.

In my youth taking macro shots with an Agfa Sillette and close-up lenses I had a knotted string tied to a bolt in the tripod socket that gave me the distances for several focusing distances that were set on the lens. I used the frosted glass of a microscope slide at the film plane to find the knot positions - it isn't hard, there is very little depth of field with the lens wide open.

Be sure to close down to a small aperture when taking pictures.

Ok, that makes sense, now that I wrap my head around it. Thanks a lot man!
Always stopping way down. Although diopters work slightly better on medium format at similar apertures.

AgX · Oct 3, 2021

In case some other readers got confused by the terminology:

-) some Americans call an eyepiece a diopter

-) the alternative way to describe a focal length or focused distance is the unit Diopter (based on meters)

Petrochemist · Oct 22, 2021

AgX said:
In case some other readers got confused by the terminology:

-) some Americans call an eyepiece a diopter

-) the alternative way to describe a focal length or focused distance is the unit Diopter (based on meters)

The eyepiece is only called a diopter because it's strength is quoted in dioters.
Diopters are a convenient measure as they can be added.

If adding a diopter to a lens the resulting combinations focal length can be calculated by converting both to Diopters, adding them together the converting back to focal length. (as per Wiltw's post bellow)

wiltw said:
The diopter strength sets the subject distance...with the lens focused at Infinty. Closer than that focus distance can be obtained by using standard lens helicoid/bellows mechanism.

FL = 1m / Diopter
.

AgX · Oct 22, 2021

Petrochemist said:
The eyepiece is only called a diopter because it's strength is quoted in dioters.
Diopters are a convenient measure as they can be added

You likely think of those who use "diopter" for the accessory correcting-lens.
But I came across also "diopter" being used for the built-in rear part of the viewer, what I call "eyepiece" And at most eypieces we use, their diopter is not given, but must be established.

I rather think the origin for calling an eyepiece a diopter is that diopter next to that measuring unit also, or rather originally, meant an aiming tool, back from times when there were not even optics yet. And in part of the (English speaking) world thit term has been taken over for optical sights, thus eyepieces.

I came across this term in this meaning only here at Apug, and even was puzzled at first. Which of course must not mean that on some fields over here that meaning does not exists too, it might just slipped my attention.

wiltw · Oct 22, 2021

AgX said:
You likely think of those who use "diopter" for the accessory correcting-lens.
But I came across also "diopter" being used for the built-in rear part of the viewer, what I call "eyepiece" And at most eypieces we use, their diopter is not given, but must be established.

I rather think the origin for calling an eyepiece a diopter is that diopter next to that measuring unit also, or rather originally, meant an aiming tool, back from times when there were not even optics yet. And in part of the (English speaking) world thit term has been taken over for optical sights, thus eyepieces.

I came across this term in this meaning only here at Apug, and even was puzzled at first. Which of course must not mean that on some fields over here that meaning does not exists too, it might just slipped my attention.

The confusion is further compounded by the fact that camera manufacturers are not uniform in specifying the Diopter strength in the viewfinder

Some set the standard eyepiece of a certain distance (like 30" or 1m) and that eyepiece is called '0' when its actualy strength might be -0.5 Diopter; then accessory lenses that are put in place are called -0.5 (as a delta or change value) when the actual strength is actually -1.0 Diopter
Others set the standard eyepiece, and that eyepiece is the actual Diopter value, like -0.5 and accessory senses that are put into place which are half Diopter stronger are called '-1.0' actual value rather than a delta (change) Diopter strength

...and figuring out which of the two conventions applies for you camera is a puzzle which is best resolved if you can look at the manufacturer's Price List showing accessory eyepieces (vs. 'standard' eyepiece) to determine which applies. For example I have a Bronica Price List showing the standard '0' Diopter eyepiece is actually -1.5 Diopter.

AgX · Oct 22, 2021

Yes, when one reads at SLR manuals the respective paragraphs one is more confused than before. Best to forget about that and test oneself.
Take several of those cheap generic eyeglasses (in steps of 0.5 diopters) and check critically for charpest image of groundglass details, with the camera-lens best taken of. The diopter of those eyeglasses is the diopter of the correcting lens for the eyepiece.

Trouble only now starts if one cannot or does not want not to install a generic correction lens, but one from the range of the camera manufacurer. Then one has to calculate ones finding into the designation of the proprietary correction lens...

How to calculate non infinity tables for diopters/close up filters?

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How to calculate non infinity tables for diopters/close up filters?

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