Inner Focus is a lie. About DA* 60-250 f/4 IF

**Lowell Goudge** · 05-29-2017, 04:36 AM

Originally posted by bscott

Is focus breathing a new issue with digital zooms or has this been a problem for a long time with our old film zooms?

Originally posted by clackers

Nothing to do with analogue vs digital, Bscott!

Originally posted by mikesbike

It goes way back. As clackers indicates. One example was the Pentax (by Tamron) FA 28-200mm, an AF lens for 35mm film bodies, which broke new ground in how close it could focus at longer focal lengths back around 1997. Unheard of closeness back then at 200mm. The only caveat being the FL shortening phenomenon now known as "focus breathing".

It actually has nothing at all to do with zooms. The first report I saw for the issue, although it was not called focus breathing way back in the mid 1970's was the Pentax K28/2 where they reported that the fixed rear element design led to the reduction in focal length as you focused closer.

I suspect you will also find that many new 1:1 macros have a similar issue.

As someone noted, the best way to deal with the issue, is to present curves of either magnification vs focus distance or focal length vs vocus distance, and put it out in the open. For this, you can fault not only manufacturers , but all the equipment reviewers who should on a lens review report this.

BrianR · 05-29-2017, 05:31 AM

Originally posted by Lowell Goudge

I suspect you will also find that many new 1:1 macros have a similar issue.

Yes, unfortunately I can't seem to find a 'live' version of this page anymore, but check out the diagrams of the nikon 85mm macro, which behaves like a 71.5mm lens on 71.5mm of extension at minimum focusing distance::

Focal Length and Magnification, by Pierre Toscani

From putting the DFA100mm on tubes, I've figured it to be about 75mm at minimum focusing distance. I keep meaning to attempt to measure it's focal length, but haven't gotten around to it.

Also of some interest to this thread is his page on telezooms. It's Nikon centric, but also discusses the tamron 70-200mm and has lots of pretty lens diagrams to look at:

Telephoto Zoom Lenses with Donders-type Afocal System, by P. Toscani

normhead · 05-29-2017, 05:53 AM

It occurs to me that the lenses that extend their lenses focusing also have more problem with purple fringing, especially with TCs. So you have to ask, is this a function of internal focus, or a bi-product of the attempt to reduce purple fringing? After all magnifying an image also magnifies any PF that may be present.

**Lowell Goudge** · 05-29-2017, 07:39 AM

Originally posted by normhead

It occurs to me that the lenses that extend their lenses focusing also have more problem with purple fringing, especially with TCs. So you have to ask, is this a function of internal focus, or a bi-product of the attempt to reduce purple fringing? After all magnifying an image also magnifies any PF that may be present.

i suspect that it is neither. see my comment above, what is now called "focus breathing" has been noted as far back as the mid 70"s. purple fringing is relatively new, being a digital issue,

in reality, it is much more related to lens design, lens cost, reduction in the mass of focusing elements, etc, and individual lens design criteria, although I am sure when designers look at the overall behavior, the level of aberrations from internal focusing to focusing by moving the entire group are considered.

normhead · 05-29-2017, 11:07 AM

I'm not sure you presented a different reality than I did. You just labelled the steps in the design process, used to reduce CA and purple fringing to make lenses that look good on the test charts.

**Lowell Goudge** · 05-29-2017, 11:17 AM

Originally posted by normhead

I'm not sure you presented a different reality than I did. You just labelled the steps in the design process, used to reduce CA and purple fringing to make lenses that look good on the test charts.

agreed.

i just think there is no one driving criteria. surely IF is very interesting for WR, since it removes relative changes in pressure due to changes in lens volume, etc...... as i said, it is a design decision driven by a ton of inputs.

normhead · 05-29-2017, 11:26 AM

Originally posted by Lowell Goudge

agreed.

i just think there is no one driving criteria. surely IF is very interesting for WR, since it removes relative changes in pressure due to changes in lens volume, etc...... as i said, it is a design decision driven by a ton of inputs.

Having taken an introductory course in lens design, I know just enough to know I know practically nothing about lens design, which when you think about it, s probably the best thing for a photographer to know. As photographer, you just have to be able to pick and choose which lens suits you best.

**Lowell Goudge** · 05-29-2017, 11:32 AM

Originally posted by normhead

Having taken an introductory course in lens design, I know just enough to know I know practically nothing about lens design, which when you think about it, s probably the best thing for a photographer to know.

im not so sure, if you understand a little about lens design, you might walk away with understanding at least why your lens that you just bought behaves as it does, otherwise, you end up like the OP raising this question

Quote:

As photographer, you just have to be able to pick and choose which lens suits you best.

again how do you pick one without knowing what it will do? i know, you are expected to have LBA and just buy it to try out

05-29-2017, 05:31 AM	#32
BrianR Veteran Member Join Date: Dec 2010 Location: Ontario Photos: Gallery Posts: 3,332	Originally posted by Lowell Goudge I suspect you will also find that many new 1:1 macros have a similar issue. Yes, unfortunately I can't seem to find a 'live' version of this page anymore, but check out the diagrams of the nikon 85mm macro, which behaves like a 71.5mm lens on 71.5mm of extension at minimum focusing distance:: Focal Length and Magnification, by Pierre Toscani From putting the DFA100mm on tubes, I've figured it to be about 75mm at minimum focusing distance. I keep meaning to attempt to measure it's focal length, but haven't gotten around to it. Also of some interest to this thread is his page on telezooms. It's Nikon centric, but also discusses the tamron 70-200mm and has lots of pretty lens diagrams to look at: Telephoto Zoom Lenses with Donders-type Afocal System, by P. Toscani Last edited by BrianR; 05-29-2017 at 06:27 AM.

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05-29-2017, 04:36 AM	#31
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,891	Originally posted by bscott Is focus breathing a new issue with digital zooms or has this been a problem for a long time with our old film zooms? Originally posted by clackers Nothing to do with analogue vs digital, Bscott! Originally posted by mikesbike It goes way back. As clackers indicates. One example was the Pentax (by Tamron) FA 28-200mm, an AF lens for 35mm film bodies, which broke new ground in how close it could focus at longer focal lengths back around 1997. Unheard of closeness back then at 200mm. The only caveat being the FL shortening phenomenon now known as "focus breathing". It actually has nothing at all to do with zooms. The first report I saw for the issue, although it was not called focus breathing way back in the mid 1970's was the Pentax K28/2 where they reported that the fixed rear element design led to the reduction in focal length as you focused closer. I suspect you will also find that many new 1:1 macros have a similar issue. As someone noted, the best way to deal with the issue, is to present curves of either magnification vs focus distance or focal length vs vocus distance, and put it out in the open. For this, you can fault not only manufacturers , but all the equipment reviewers who should on a lens review report this.

05-29-2017, 05:53 AM	#33
normhead Pentaxian Join Date: Jun 2007 Location: Near Algonquin Park Photos: Gallery \| Albums Posts: 40,451	It occurs to me that the lenses that extend their lenses focusing also have more problem with purple fringing, especially with TCs. So you have to ask, is this a function of internal focus, or a bi-product of the attempt to reduce purple fringing? After all magnifying an image also magnifies any PF that may be present.

05-29-2017, 07:39 AM	#34
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,891	Originally posted by normhead It occurs to me that the lenses that extend their lenses focusing also have more problem with purple fringing, especially with TCs. So you have to ask, is this a function of internal focus, or a bi-product of the attempt to reduce purple fringing? After all magnifying an image also magnifies any PF that may be present. i suspect that it is neither. see my comment above, what is now called "focus breathing" has been noted as far back as the mid 70"s. purple fringing is relatively new, being a digital issue, in reality, it is much more related to lens design, lens cost, reduction in the mass of focusing elements, etc, and individual lens design criteria, although I am sure when designers look at the overall behavior, the level of aberrations from internal focusing to focusing by moving the entire group are considered.

05-29-2017, 11:07 AM	#35
normhead Pentaxian Join Date: Jun 2007 Location: Near Algonquin Park Photos: Gallery \| Albums Posts: 40,451	I'm not sure you presented a different reality than I did. You just labelled the steps in the design process, used to reduce CA and purple fringing to make lenses that look good on the test charts.

05-29-2017, 11:17 AM	#36
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,891	Originally posted by normhead I'm not sure you presented a different reality than I did. You just labelled the steps in the design process, used to reduce CA and purple fringing to make lenses that look good on the test charts. agreed. i just think there is no one driving criteria. surely IF is very interesting for WR, since it removes relative changes in pressure due to changes in lens volume, etc...... as i said, it is a design decision driven by a ton of inputs.

05-29-2017, 11:26 AM	#37
normhead Pentaxian Join Date: Jun 2007 Location: Near Algonquin Park Photos: Gallery \| Albums Posts: 40,451	Originally posted by Lowell Goudge agreed. i just think there is no one driving criteria. surely IF is very interesting for WR, since it removes relative changes in pressure due to changes in lens volume, etc...... as i said, it is a design decision driven by a ton of inputs. Having taken an introductory course in lens design, I know just enough to know I know practically nothing about lens design, which when you think about it, s probably the best thing for a photographer to know. As photographer, you just have to be able to pick and choose which lens suits you best.

05-29-2017, 11:32 AM	#38
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,891	Originally posted by normhead Having taken an introductory course in lens design, I know just enough to know I know practically nothing about lens design, which when you think about it, s probably the best thing for a photographer to know. im not so sure, if you understand a little about lens design, you might walk away with understanding at least why your lens that you just bought behaves as it does, otherwise, you end up like the OP raising this question Quote: As photographer, you just have to be able to pick and choose which lens suits you best. again how do you pick one without knowing what it will do? i know, you are expected to have LBA and just buy it to try out