SMC Pentax-A Zoom 35-105mm Varifocal or Parafocal?

ddhytz · 04-23-2010, 06:25 PM

Originally posted by hangu

Those first two sentences made no sense at all, what exactly are you trying to say?

I'm going to have to disagree with you completely on every point. There is no difference in focusing systems between digital and SLR bodies, since the lens itself is doing all the focusing.
....

Try it on a film body with split image focusing screen, you will know what I was trying to say.

A digital SLR body has a focusing sensor underneath the mirror for auto focusing. That sensor does not exist on a manual focusing film body. A lens cannot do focusing by itself, only part of a focusing system.

Last edited by ddhytz; 04-23-2010 at 06:31 PM.

SOldBear · 04-23-2010, 06:30 PM

Originally posted by ddhytz

It is varifocal for sure. Especially when it is used on a manual focusing film body you need to zoom in first for focusing, then zoom out for composition.

If it is varifocal, you can't "zoom in first for focusing, then zoom out for composition."

Because you will lose focus when you zoom.

Originally posted by ddhytz

On digital bodies it seems you don't need to do so. I guess that is due to different focusing system, optical vs electonical.

Not sure what "need to do" here!

If the lens is varifocal, the need to re-focus after zooming still exists, whether by the operator or by the camera or by the lens.

ddhytz · 04-23-2010, 06:44 PM

Originally posted by SOldBear

If it is varifocal, you can't "zoom in first for focusing, then zoom out for composition."

Because you will lose focus when you zoom.

Not sure what "need to do" here!

If the lens is varifocal, the need to re-focus after zooming still exists, whether by the operator or by the camera or by the lens.

Regardless it is varifocal or not, this lens needs to zoom in for focus and zoom out for composition.

I have this lens on a MX body now, set to 35mm focal length, and try to focus infinity. When the lines in the split image screen aligned perfectly the distance shows 3m!

SOldBear · 04-23-2010, 06:58 PM

Originally posted by ddhytz

Regardless it is varifocal or not, this lens needs to zoom in for focus and zoom out for composition.

ddhytz,

I don't really like going back and forth, and I realize that we are straying off original topic now. But let me try to state what I know again, maybe we all can learn something together:

- A varifocal lens loses focus when changing focal length. For this lens, the user needs to zoom first to the desired focal length, then focus. Every time (s)he changes the focal length, (s)he needs to re-focus the lens.

- A parafocal lens does not lose focus when changing focal lengths. For this lens, it is recommended (not required) that the user zoom in to focus because focusing is easier (image in the viewfinder is larger), then zoom out to the desired focal length, taking care not to lose focus while zooming.

There is no lens that "needs to zoom in for focus and zoom out for composition."

Originally posted by ddhytz

I have this lens on a MX body now, set to 35mm focal length, and try to focus infinity. When the lines in the split image screen aligned perfectly the distance shows 3m!

Care to elaborate? I'm not sure this behavior has anything to do with "zoom then focus" or "focus then zoom."

Based on your description, I suspect there is something wrong with the lens or with the camera body.

ddhytz · 04-23-2010, 07:08 PM

Originally posted by SOldBear

ddhytz,

.... Care to elaborate? I'm not sure this behavior has anything to do with "zoom then focus" or "focus then zoom."

Based on your description, I suspect there is something wrong with the lens or with the camera body.

I am not sure whether there is anything wrong with my copy. I have tried on a Super A as well. All I know is I need to use 135mm end for focusing and zoom out to whatever focal length I want to use for composition, if I want to have a perfectly focused picture.

I am hoping someone else who has the same lens could confirm.

ddhytz · 04-23-2010, 07:10 PM

I am going out to take some test shots using K20D.

wlachan · 04-23-2010, 07:24 PM

It's parafocal I am certain. I used to have 3 of these lenses and 2 of them stayed in focus throughout the zoom range, one was not and quite bad. If yours varies slightly, it could be factory calibration. If it varies too much, the lens was unprofessionally serviced b4 since this is a complicated zoom to work with.

writeb · 04-23-2010, 10:48 PM

The copy I have is actually not that off at all. If I focus zoomed in and pan out, it remains clear, or at least pretty on as far as my tests. But if I focus zoomed out and zoom in on a subject, it seems a little soft and requires fine tuning. Heck, it could be my eyes not focusing properly, but It's pretty consistent.

What I would love to know if anybody knows for sure via the actual optical formula/specifications instead of anecdotally, which so far seems to lean towards varifocal. If I wrote Pentax, would they answer/know?

On a slightly different note, where does one go to have old lenses recalibrated anyway? Would Pentax do this, or do specialty camera shops take shots at this?

ddhytz · 04-24-2010, 12:58 AM

Originally posted by writeb

The copy I have is actually not that off at all. If I focus zoomed in and pan out, it remains clear, or at least pretty on as far as my tests. But if I focus zoomed out and zoom in on a subject, it seems a little soft and requires fine tuning. Heck, it could be my eyes not focusing properly, but It's pretty consistent.
...

My copy does the same.
Is it possible that Pentax released two versions of this lens at some stage?

ddhytz · 04-24-2010, 01:00 AM

Some sample shots today.

Macro shots

ddhytz · 04-24-2010, 01:02 AM

One more

stillshunter · 04-24-2010, 04:00 AM

So what's the answer people? Is the A35-105/3.5 a par- or vari- focal? There seems to be differing opinions out there on the wider web...

From:

Quote:

My SMC Pentax-A 35-105mm f/3.5 (constant) manual focus lens is
varifocal. The focus varies significantly throughout the zoom range.
It doesn't matter too much because it's a one-touch lens, with a
combined zoom/focus ring, but the change in focus is fairly dramatic.

And:

Quote:

(Hmmm... I guess a couple
of users have had problems with the 35-105/3.5 staying parfocal when
zooming, but I had never noticed that problem with the samples that
I had used.)

To:

Quote:

Focus shift while zooming could be caused by faulty mechanisms. I used to have three A35-105/3.5 and only one of them shifted focus when zoomed. Given its poor phyical condition, I believed the lens was not properly calibrated when put together.

And:

Quote:

[EMAIL PROTECTED] writes:
I'm confused by my lack of memory. Is this lens varifocal ?
For one reason or another I keep thinking I DON'T have to refocus if I zoom
OUT from 105 to 35.

[EMAIL PROTECTED] writes:
David,
This lens is not varifocal.
You should be able to focus at 105 and zoom to 35,
all the while staying in focus...
Regards, Bob S.

[EMAIL PROTECTED] writes:
Hi Bob,
Both you and Alan Chan state the same thing. I'm going to run a quick test
with whatever film I have remaining in the LX right now (HP5) and see.
In the viewfinder it looks like it goes a bit out of focus as I zoom out to
35 (after focusing at 105). I'm not sure whether this is reflected in the
photos as the last time I was out using the Lens (with Jeff and Frank @ our
August TOPDML get together) the images were still in focus after the fact.
So.... I can live with the "it looks out of focus" in the viewfinder @ 35mm
as long as the images remain sharp. I'll perform the test with the lens
wide open at f3.5.
Cheers,
Dave

Bottom-line is...I AM CONFUSED!!! So does anyone have the light on this?....
....or maybe ddhytz is right and there are two versions...or we have some pretty confused folks out there people.

JeffJS · 04-24-2010, 09:45 AM

All I can tell you for certain is that mine stays in focus. I do have to be careful though because the zoom ring is actually a little on the small side an with the size of my fingers, I can hit the focus ring while turning the zoom. The quoted person who states it's a one touch lens either has lost any memory of actually using the lens, or has never held one.

stillshunter · 04-24-2010, 01:59 PM

Appears consensus is the A35-105/3.5 is parfocal. It seems as possible those who have claimed it a varifocal had a copy with a dodgy moving element group? It's a possibility, if there is truth to this statement from a previous photo.net user:

Quote:

Just make sure the focus does not shift when zoom from tele-to wide (very slightly is okay due to manufacturing tolerance). If it does, something is wrong (not uncommon with this lens).

BTW: I can find no reference to Pentax releasing this lens in two differing focal varieties...and it seems unlikely.

Another quote from a current photo.net user :

Quote:

If you're getting one of these lens [A35-105/3.5], treasure it.... the separate focus and zoom functions can be annoying to some but given that it will not lose its focus when zooming in and out is a definite plus.

Wonder if the OP finds their question answered?

hangu · 04-24-2010, 03:43 PM

Originally posted by ddhytz

Try it on a film body with split image focusing screen, you will know what I was trying to say.

A digital SLR body has a focusing sensor underneath the mirror for auto focusing. That sensor does not exist on a manual focusing film body. A lens cannot do focusing by itself, only part of a focusing system.

I still have no clue what you're trying to say as most of your statements contradict each other.

04-24-2010, 04:00 AM	#27
stillshunter Veteran Member Join Date: Nov 2009 Location: Southern Tablelands NSW Photos: Gallery \| Albums Posts: 406	So what's the answer people? Is the A35-105/3.5 a par- or vari- focal? There seems to be differing opinions out there on the wider web... From: Quote: My SMC Pentax-A 35-105mm f/3.5 (constant) manual focus lens is varifocal. The focus varies significantly throughout the zoom range. It doesn't matter too much because it's a one-touch lens, with a combined zoom/focus ring, but the change in focus is fairly dramatic. And: Quote: (Hmmm... I guess a couple of users have had problems with the 35-105/3.5 staying parfocal when zooming, but I had never noticed that problem with the samples that I had used.) To: Quote: Focus shift while zooming could be caused by faulty mechanisms. I used to have three A35-105/3.5 and only one of them shifted focus when zoomed. Given its poor phyical condition, I believed the lens was not properly calibrated when put together. And: Quote: [EMAIL PROTECTED] writes: I'm confused by my lack of memory. Is this lens varifocal ? For one reason or another I keep thinking I DON'T have to refocus if I zoom OUT from 105 to 35. [EMAIL PROTECTED] writes: David, This lens is not varifocal. You should be able to focus at 105 and zoom to 35, all the while staying in focus... Regards, Bob S. [EMAIL PROTECTED] writes: Hi Bob, Both you and Alan Chan state the same thing. I'm going to run a quick test with whatever film I have remaining in the LX right now (HP5) and see. In the viewfinder it looks like it goes a bit out of focus as I zoom out to 35 (after focusing at 105). I'm not sure whether this is reflected in the photos as the last time I was out using the Lens (with Jeff and Frank @ our August TOPDML get together) the images were still in focus after the fact. So.... I can live with the "it looks out of focus" in the viewfinder @ 35mm as long as the images remain sharp. I'll perform the test with the lens wide open at f3.5. Cheers, Dave Bottom-line is...I AM CONFUSED!!! So does anyone have the light on this?.... ....or maybe ddhytz is right and there are two versions...or we have some pretty confused folks out there people. Last edited by stillshunter; 04-24-2010 at 04:21 AM.

04-24-2010, 01:59 PM	#29
stillshunter Veteran Member Join Date: Nov 2009 Location: Southern Tablelands NSW Photos: Gallery \| Albums Posts: 406	Appears consensus is the A35-105/3.5 is parfocal. It seems as possible those who have claimed it a varifocal had a copy with a dodgy moving element group? It's a possibility, if there is truth to this statement from a previous photo.net user: Quote: Just make sure the focus does not shift when zoom from tele-to wide (very slightly is okay due to manufacturing tolerance). If it does, something is wrong (not uncommon with this lens). BTW: I can find no reference to Pentax releasing this lens in two differing focal varieties...and it seems unlikely. Another quote from a current photo.net user : Quote: If you're getting one of these lens [A35-105/3.5], treasure it.... the separate focus and zoom functions can be annoying to some but given that it will not lose its focus when zooming in and out is a definite plus. Wonder if the OP finds their question answered? Last edited by stillshunter; 04-24-2010 at 02:15 PM.

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04-23-2010, 06:25 PM	#16
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	Originally posted by hangu Those first two sentences made no sense at all, what exactly are you trying to say? I'm going to have to disagree with you completely on every point. There is no difference in focusing systems between digital and SLR bodies, since the lens itself is doing all the focusing. .... Try it on a film body with split image focusing screen, you will know what I was trying to say. A digital SLR body has a focusing sensor underneath the mirror for auto focusing. That sensor does not exist on a manual focusing film body. A lens cannot do focusing by itself, only part of a focusing system. Last edited by ddhytz; 04-23-2010 at 06:31 PM.

04-23-2010, 06:30 PM	#17
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	Originally posted by ddhytz It is varifocal for sure. Especially when it is used on a manual focusing film body you need to zoom in first for focusing, then zoom out for composition. If it is varifocal, you can't "zoom in first for focusing, then zoom out for composition." Because you will lose focus when you zoom. Originally posted by ddhytz On digital bodies it seems you don't need to do so. I guess that is due to different focusing system, optical vs electonical. Not sure what "need to do" here! If the lens is varifocal, the need to re-focus after zooming still exists, whether by the operator or by the camera or by the lens.

04-23-2010, 06:44 PM	#18
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	Originally posted by SOldBear If it is varifocal, you can't "zoom in first for focusing, then zoom out for composition." Because you will lose focus when you zoom. Not sure what "need to do" here! If the lens is varifocal, the need to re-focus after zooming still exists, whether by the operator or by the camera or by the lens. Regardless it is varifocal or not, this lens needs to zoom in for focus and zoom out for composition. I have this lens on a MX body now, set to 35mm focal length, and try to focus infinity. When the lines in the split image screen aligned perfectly the distance shows 3m!

04-23-2010, 06:58 PM	#19
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	Originally posted by ddhytz Regardless it is varifocal or not, this lens needs to zoom in for focus and zoom out for composition. ddhytz, I don't really like going back and forth, and I realize that we are straying off original topic now. But let me try to state what I know again, maybe we all can learn something together: - A varifocal lens loses focus when changing focal length. For this lens, the user needs to zoom first to the desired focal length, then focus. Every time (s)he changes the focal length, (s)he needs to re-focus the lens. - A parafocal lens does not lose focus when changing focal lengths. For this lens, it is recommended (not required) that the user zoom in to focus because focusing is easier (image in the viewfinder is larger), then zoom out to the desired focal length, taking care not to lose focus while zooming. There is no lens that "needs to zoom in for focus and zoom out for composition." Originally posted by ddhytz I have this lens on a MX body now, set to 35mm focal length, and try to focus infinity. When the lines in the split image screen aligned perfectly the distance shows 3m! Care to elaborate? I'm not sure this behavior has anything to do with "zoom then focus" or "focus then zoom." Based on your description, I suspect there is something wrong with the lens or with the camera body.

04-23-2010, 07:08 PM	#20
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	Originally posted by SOldBear ddhytz, .... Care to elaborate? I'm not sure this behavior has anything to do with "zoom then focus" or "focus then zoom." Based on your description, I suspect there is something wrong with the lens or with the camera body. I am not sure whether there is anything wrong with my copy. I have tried on a Super A as well. All I know is I need to use 135mm end for focusing and zoom out to whatever focal length I want to use for composition, if I want to have a perfectly focused picture. I am hoping someone else who has the same lens could confirm.

04-23-2010, 07:10 PM	#21
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	I am going out to take some test shots using K20D.

04-23-2010, 07:24 PM	#22
wlachan Veteran Member Join Date: Sep 2006 Location: Canada Photos: Albums Posts: 2,625	It's parafocal I am certain. I used to have 3 of these lenses and 2 of them stayed in focus throughout the zoom range, one was not and quite bad. If yours varies slightly, it could be factory calibration. If it varies too much, the lens was unprofessionally serviced b4 since this is a complicated zoom to work with.

04-23-2010, 10:48 PM	#23
writeb Senior Member Join Date: Nov 2009 Location: San Jose Photos: Gallery \| Albums Posts: 150 Original Poster	The copy I have is actually not that off at all. If I focus zoomed in and pan out, it remains clear, or at least pretty on as far as my tests. But if I focus zoomed out and zoom in on a subject, it seems a little soft and requires fine tuning. Heck, it could be my eyes not focusing properly, but It's pretty consistent. What I would love to know if anybody knows for sure via the actual optical formula/specifications instead of anecdotally, which so far seems to lean towards varifocal. If I wrote Pentax, would they answer/know? On a slightly different note, where does one go to have old lenses recalibrated anyway? Would Pentax do this, or do specialty camera shops take shots at this?

04-24-2010, 12:58 AM	#24
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	Originally posted by writeb The copy I have is actually not that off at all. If I focus zoomed in and pan out, it remains clear, or at least pretty on as far as my tests. But if I focus zoomed out and zoom in on a subject, it seems a little soft and requires fine tuning. Heck, it could be my eyes not focusing properly, but It's pretty consistent. ... My copy does the same. Is it possible that Pentax released two versions of this lens at some stage?

04-24-2010, 01:00 AM	#25
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	Some sample shots today. Macro shots

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04-24-2010, 01:02 AM	#26
ddhytz Veteran Member Join Date: Sep 2007 Location: Melbourne Posts: 602	One more

04-24-2010, 09:45 AM	#28
JeffJS Inactive Account Join Date: Aug 2008 Location: Michigan, USA Photos: Gallery \| Albums Posts: 7,484	All I can tell you for certain is that mine stays in focus. I do have to be careful though because the zoom ring is actually a little on the small side an with the size of my fingers, I can hit the focus ring while turning the zoom. The quoted person who states it's a one touch lens either has lost any memory of actually using the lens, or has never held one.

04-24-2010, 03:43 PM	#30
hangu Veteran Member Join Date: Feb 2010 Location: NYC Photos: Gallery Posts: 1,071	Originally posted by ddhytz Try it on a film body with split image focusing screen, you will know what I was trying to say. A digital SLR body has a focusing sensor underneath the mirror for auto focusing. That sensor does not exist on a manual focusing film body. A lens cannot do focusing by itself, only part of a focusing system. I still have no clue what you're trying to say as most of your statements contradict each other.