Diopters

Michael Garcia · October 16, 2008

Say I am using a #1 Diopter, my maximum focus distance is 1 Meter, so when focused at infinity I will only have focus up to 1 meter away, no further.

That being said does this actually shift my focus closer?

For example if I am shooting on a 35mm lens with a minimum focus distance of 20" and I place a #1 diopter in my infinity focus moves to 1 meter, but does my minimum focus move closer as well? And if so is there a mathematical relationship there? So that if I use a #2 Diopter, moving infinity to 1/2 meter, does it move my minimum focus forward proportionately?

I hope this makes sense. Because I was under the impression that my minimum focus Would change, however in some recent tests that really didn't seem to be the case. there is image magnification that seems to be proportional between the diopters but the minimum focus seemed to stay the same. Thanks.

Chris Keth · October 16, 2008

As far as I know, the near focus distance gets nearer but to be honest I've never tested it and very rarely use them. I'll be curious to see if someone here knows the full answer to your question.

Mike Lary · October 16, 2008

I used diopters on a K3 stock Meteor lens. The minimum focus distance on the lens was around six feet if I remember correctly. When I placed a diopter on the minimum focus distance got closer. When I used all three diopters (1, 2, and 3) I was shooting macro from about six to eight inches away. DOF was almost non-existent. I don't know the relationship, but I know I used a formula from the ASC Manual to determine DOF. It might address the min focus distance as well, but I don't have the book handy to check.

Leo Anthony Vale · October 16, 2008

For example if I am shooting on a 35mm lens with a minimum focus distance of 20" and I place a #1 diopter in my infinity focus moves to 1 meter, but does my minimum focus move closer as well? And if so is there a mathematical relationship there? So that if I use a #2 Diopter, moving infinity to 1/2 meter, does it move my minimum focus forward proportionately?

Yes. A dioptre number is the recoprical of its focal length in meters. thus a #1 is 1/1meter,

a #2 is 1/2meters.

The focus of the lens can also be expressed in dioptres. So if the lens is focused at 1 meter, it will be focused at 1 dioptre, thus putting a #1 dioptre on that lens will change the focus to 1 + 1 dioptre, which equals 2 dioptres. The new focus is 1/2 meter.

putting a #2 on the lens focused at 1 meter will give you a new focus distance of 2 + 1 dioptre, 3 dioptres which is a third of a meter.

Michael Garcia · October 17, 2008

So I looked into the ASC manual and I found some conversion tables that showed the relative focus marks on any given lens and how they would change with different diopter strengths but I still haven't been able to determine if the closest focus mark on any given lens is theoretically moved closer with the use of a diopter or if it is just the illusion of closer focus through magnification. So even though I think we are getting closer to the answer, and I really appreciate all of the great input given, it has already helped to broaden my knowledge of diopters but I still haven't quite arrived at what I am looking for. So if anyone has any other ideas please let me know!

Michael Lindsay · November 7, 2008

I use diopters a fair bit.. (mainly the excellent Zeiss master diopters)

The rule that you mention (+1 brings infinity to 1m, +2 brings infinity to .5m etc) is what I use. There can be no rule for min focus as each lens is different.

The min focus is definatly changed. This change is obviously not as dramatically as the jump from infinity for max focus (the slight magnification achieved with diopters is not the way they achieve CF)

Max focus rules are important with tracking shots (or a moving subject)

hope that helps

Michael

Mike Lary · November 8, 2008

So I looked into the ASC manual and I found some conversion tables that showed the relative focus marks on any given lens and how they would change with different diopter strengths but I still haven't been able to determine if the closest focus mark on any given lens is theoretically moved closer with the use of a diopter or if it is just the illusion of closer focus through magnification.

I'm a little confused by your wording. The focus marks are still accurate in regards to the physical distance to the subject. If your new minimum focus with the diopter is six inches, then the six inch mark on the lens will define your minimum focus. There's no illusion of closer focus. Am I misunderstanding your question?

Chris Keth · November 8, 2008

I'm a little confused by your wording. The focus marks are still accurate in regards to the physical distance to the subject. If your new minimum focus with the diopter is six inches, then the six inch mark on the lens will define your minimum focus. There's no illusion of closer focus. Am I misunderstanding your question?

I think you must be misunderstanding the question. It reads as if you're saying that with a + diopter on the lens, that you can still use the focus markings on the lens as normal.

As an example of how far off the marks can be thrown, consider that with a +3 diopter on the lens and the lens focus set to the 4 foot mark, the system will actually be focussed at 10-3/8 inches.