SR and extension tubes question

Michael Piziak · 07-22-2014, 09:52 AM

Is 150mm correct to set camera that has the Asahi Pentax 100mm f4 macro lens and the 3 Asahi Pentax extension tubes ?

The camera always asks me to set it for this manual set up. I just guess that it's about 150mm length. I know it asks this for SR.

OregonJim · 07-22-2014, 10:46 AM

No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance.

50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR.

bluestringer · 07-22-2014, 11:01 AM

That lens already does 1:2 alone, wouldn't 50mm tubes give it 1:1 at min. focus?

Na Horuk · 07-22-2014, 11:30 AM

The other thing you can do is disable SR, if you are taking photos with 2 sec timer and tripod (which you probably should be doing, if you can)

Michael Piziak · 07-22-2014, 01:36 PM

Originally posted by OregonJim

No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance.

50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR.

ok I'll set it at 100mm from now on
thanks

---------- Post added 07-22-14 at 01:37 PM ----------

Originally posted by bluestringer

That lens already does 1:2 alone, wouldn't 50mm tubes give it 1:1 at min. focus?

well, 1:1 is closer in than 1:2, thus the use of the extension tubes on occassion for smaller subjects

bluestringer · 07-22-2014, 06:33 PM

Originally posted by Na Horuk

The other thing you can do is disable SR, if you are taking photos with 2 sec timer and tripod (which you probably should be doing, if you can)

A tripod is just in the way and too slow to setup when shooting insects in the brush. Handheld and a flash is the way to go.

---------- Post added 07-22-14 at 09:34 PM ----------

Originally posted by Michael Piziak

Is 150mm correct to set camera that has the Asahi Pentax 100mm f4 macro lens and the 3 Asahi Pentax extension tubes ?

The camera always asks me to set it for this manual set up. I just guess that it's about 150mm length. I know it asks this for SR.

I've always added the tubes to the focal length. Works fine for me.

**Lowell Goudge** · 07-22-2014, 06:50 PM

Without knowing the exact shake reduction formula, it is all emulation, but, when you consider that really all focal length changes is magnification, adding extension tubes greatly increases magnification, so although many recommend setting for the native focal length, I would likely recommend trying a longer focal length to compensate for the magnification increase

**dms** · 07-22-2014, 08:57 PM

I also believe using extension tubes, which increases magnification, is equivalent to a longer focal length (FL). I have not done testing--in part because at significant magnification I use a tripod--so it is/was a mute question.

My inclination would be to say, for example, at m=1/2, the extension is 50 mm for a 100 mm lens, and I would use 150mm for the FL. The FL usually refers to the value when focused at infinity, but effectively you are increasing the FL--although many people dislike saying so--as it is not focused at infinity any more.

Some newer lenses actually reduce the FL as one focuses closer--but that is a different story--as it is not about adding extension.

Na Horuk · 07-23-2014, 03:33 AM

Do extension tubes directly increase magnification, or just minimum focus distance (which in turn increases magnification)?

reeftool · 07-23-2014, 08:24 AM

I have been adding the extension tube length and haven't noticed any issues. I have also just used the lens length and those were OK also.

zapp · 07-27-2014, 01:24 AM

Originally posted by OregonJim

No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance.

50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR.

At close range the image distance has significant effect on focal length 1/f = 1/i + 1/o. For large object distance (large o) a change in i is negligible, but at close range i may become even larger than o and thus quite important. Focal length typically decreases with increasing extension and the printed focal length is only true for infinite focus.

The simple formula answers all other questions as well.

07-22-2014, 09:52 AM	#1
Michael Piziak Closed Account Join Date: Dec 2013 Location: West Virginia Posts: 2,815	SR and extension tubes question Is 150mm correct to set camera that has the Asahi Pentax 100mm f4 macro lens and the 3 Asahi Pentax extension tubes ? The camera always asks me to set it for this manual set up. I just guess that it's about 150mm length. I know it asks this for SR.

07-22-2014, 10:46 AM	#2
OregonJim Veteran Member Join Date: Mar 2009 Location: Willamette Valley, Oregon Posts: 1,327	No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance. 50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR. Last edited by OregonJim; 07-22-2014 at 10:52 AM.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
manual lens question and extension tubes	crystax	Troubleshooting and Beginner Help	8	12-11-2010 04:22 PM
Using SR with TCs and/or extension tubes	Spazmatazz	Pentax DSLR Discussion	3	03-19-2010 07:11 AM
Question on extension tubes	ismaelg	Pentax SLR Lens Discussion	3	11-25-2009 10:09 PM
Extension tubes question	yeatzee	Pentax SLR Lens Discussion	22	05-09-2009 07:41 AM
Question about extension tubes	switters	Pentax DSLR Discussion	7	12-06-2007 01:21 PM

07-22-2014, 11:01 AM	#3
bluestringer Veteran Member Join Date: Nov 2011 Location: Cotton fields of South Georgia Photos: Gallery Posts: 2,748	That lens already does 1:2 alone, wouldn't 50mm tubes give it 1:1 at min. focus?

07-22-2014, 11:30 AM	#4
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	The other thing you can do is disable SR, if you are taking photos with 2 sec timer and tripod (which you probably should be doing, if you can)

07-22-2014, 01:36 PM	#5
Michael Piziak Closed Account Join Date: Dec 2013 Location: West Virginia Posts: 2,815 Original Poster	Originally posted by OregonJim No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance. 50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR. ok I'll set it at 100mm from now on thanks ---------- Post added 07-22-14 at 01:37 PM ---------- Originally posted by bluestringer That lens already does 1:2 alone, wouldn't 50mm tubes give it 1:1 at min. focus? well, 1:1 is closer in than 1:2, thus the use of the extension tubes on occassion for smaller subjects

07-22-2014, 06:33 PM	#6
bluestringer Veteran Member Join Date: Nov 2011 Location: Cotton fields of South Georgia Photos: Gallery Posts: 2,748	Originally posted by Na Horuk The other thing you can do is disable SR, if you are taking photos with 2 sec timer and tripod (which you probably should be doing, if you can) A tripod is just in the way and too slow to setup when shooting insects in the brush. Handheld and a flash is the way to go. ---------- Post added 07-22-14 at 09:34 PM ---------- Originally posted by Michael Piziak Is 150mm correct to set camera that has the Asahi Pentax 100mm f4 macro lens and the 3 Asahi Pentax extension tubes ? The camera always asks me to set it for this manual set up. I just guess that it's about 150mm length. I know it asks this for SR. I've always added the tubes to the focal length. Works fine for me.

07-22-2014, 06:50 PM	#7
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	Without knowing the exact shake reduction formula, it is all emulation, but, when you consider that really all focal length changes is magnification, adding extension tubes greatly increases magnification, so although many recommend setting for the native focal length, I would likely recommend trying a longer focal length to compensate for the magnification increase

07-22-2014, 08:57 PM	#8
dms Site Supporter Join Date: Aug 2011 Location: New York, NY Photos: Gallery Posts: 2,192	I also believe using extension tubes, which increases magnification, is equivalent to a longer focal length (FL). I have not done testing--in part because at significant magnification I use a tripod--so it is/was a mute question. My inclination would be to say, for example, at m=1/2, the extension is 50 mm for a 100 mm lens, and I would use 150mm for the FL. The FL usually refers to the value when focused at infinity, but effectively you are increasing the FL--although many people dislike saying so--as it is not focused at infinity any more. Some newer lenses actually reduce the FL as one focuses closer--but that is a different story--as it is not about adding extension.

07-23-2014, 03:33 AM	#9
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	Do extension tubes directly increase magnification, or just minimum focus distance (which in turn increases magnification)?

07-23-2014, 08:24 AM	#10
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,553	I have been adding the extension tube length and haven't noticed any issues. I have also just used the lens length and those were OK also.

07-27-2014, 01:24 AM	#11
zapp Pentaxian Join Date: Jul 2011 Posts: 2,385	Originally posted by OregonJim No. It should be set to 100mm. The extension tubes don't change the focal length, they just decrease the minimum focus distance. 50mm of extension on a 100mm lens gives you 0.5X magnification, but the field of view is still that of a 100mm lens, which is what matters for SR. At close range the image distance has significant effect on focal length 1/f = 1/i + 1/o. For large object distance (large o) a change in i is negligible, but at close range i may become even larger than o and thus quite important. Focal length typically decreases with increasing extension and the printed focal length is only true for infinite focus. The simple formula answers all other questions as well.