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10-26-2008, 12:03 AM   #1
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Shake reduction on ext tubes ?

Hi guys,
I was wondering if my super tak 50mm F1.4 on extension tubes should still have 50 entered for shake reduction ?
Seems to be ok ...
Pete

K100D iso400 1/90 f5.6 12 mm tube


10-26-2008, 03:53 AM   #2
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The formula

QuoteOriginally posted by Transit Quote
I was wondering if my super tak 50mm F1.4 on extension tubes should still have 50 entered for shake reduction ?
The answer is no.

You have to use the thin lens formula (cf. Lens (optics) - Wikipedia, the free encyclopedia):
1/f = 1/S1 + 1/S2
where S1 is the distance to subject (measured from about the point where the aperture blades are), f your lens focal distance (50mm), and S2 is the value you have to enter for shake reduction, i.e.:
S2 = 1 / ( 1/f - 1/S1 )
Only for infinite S1 will this equal f.

If S1 approaches f, you are better off using another formula based on magnification.

UPDATE:
As pointed out by Dave (newarts) in this post: https://www.pentaxforums.com/forums/449848-post8.html, this formula becomes more practical to use when using the magnification:

The magnification m is the ratio of distances of lens to subject and focal plane, respectively:
m = S2 / S1
Express S1 in terms of m and you get:
S2 = f (1 + m)

Last edited by falconeye; 01-16-2009 at 04:12 AM. Reason: update with magnification added
10-27-2008, 04:07 AM   #3
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Thanks falconeye...
so the the shake reduction input will change with distance to subject ?
heck !
I read the link thanks
Maybe I will just turn off SR and use a tripod

Pete






QuoteOriginally posted by falconeye Quote
The answer is no.

You have to use the thin lens formula (cf. Lens (optics) - Wikipedia, the free encyclopedia):
1/f = 1/S1 + 1/S2
where S1 is the distance to subject (measured from about the point where the aperture blades are), f your lens focal distance (50mm), and S2 is the value you have to enter for shake reduction, i.e.:
S2 = 1 / ( 1/f - 1/S1 )
Only for infinite S1 will this equal f.

If S1 approaches f, you are better off using another formula based on magnification.
10-27-2008, 04:49 AM   #4
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QuoteOriginally posted by Class A Quote
Even if the distance to subject is infinity, doesn't cause the extension tube a different field of view and shouldn't he use the focal length that corresponds to the effective field of view?
If the distance to subject is infinity (1/S1=0), then he won't be able to focus at all using extension tubes

QuoteOriginally posted by Transit Quote
so the the shake reduction input will change with distance to subject ?
heck !
Yes, because the Field of View does. But except for very high magnifications exceeding 1:1 will the error be small enough so leaving SR on would still be better than turning it off. And for magnifications exceeding 1:1 you should reverse mount your lens anyway...

Using my formula, if S1=S2=2f, you get a 1:1 magnification (S2=S2) and then have to use 100mm (2f) rather than 50mm. So, in practice, it will be anything between 50mm and 100mm.

10-27-2008, 04:53 AM   #5
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QuoteOriginally posted by Class A Quote
Even if the distance to subject is infinity, doesn't cause the extension tube a different field of view and shouldn't he use the focal length that corresponds to the effective field of view?
If the distance to subject is infinity (1/S1=0), then he won't be able to focus at all using extension tubes

QuoteOriginally posted by Transit Quote
so the the shake reduction input will change with distance to subject ?
heck !
Yes, because the Field of View does. But except for very high magnifications exceeding 1:1 will the error be small enough so leaving SR on would still be better than turning it off. And for magnifications exceeding 1:1 you should reverse mount your lens anyway...

Using my formula, if S1=S2=2f, you get a 1:1 magnification (S1=S2) and then have to use 100mm (2f) rather than 50mm. So, in practice, it will be anything between 50mm and 100mm.

Another point is this: SR only compensates rotational shake, not translational shake. If you are very close to the subject, translational shake may at least be as important. So, the effectiveness of SR is degraded anyway...


BTW.
Many people forget about this. A DA 35mm Ltd. Macro, when used at 1:1, actually has the Field of View of a 100mm tele lens (compared to 35mm film, 35*2*1.53).
10-27-2008, 02:59 PM   #6
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Class A, I think it's all clear. So my response is meant for you and I'll try to be brief now.
QuoteOriginally posted by Class A Quote
I know. It was meant to be a hypothetical assumption. I should have written "were infinity".
You cannot make this assumption, not even hypothetically. As soon as the length of extension tubes is larger than zero the subject distance must become finite. Therefore, you miss to understand that the formula already takes focussing into account.
QuoteQuote:
Your equation is useful to illustrate the change in focal length but doesn't take focusing into account.
My equation describes the effect of focusing. You must miss something.
QuoteQuote:
Why is your original answer "no"?
It is no to the original question "if [...] 50mm [...] should still have 50 entered for shake reduction?"
QuoteQuote:
The combined system lens+tube just acts as if it were a lens with a different focal length. No?
No. The extension tube does nothing. There is no glass in it! It only allows to focus closer to the subject. Maybe, you better replace the extension tube by a bellow in your imagination.
QuoteQuote:
primarily it is designed for compensating translational shake. Can you please point us to your source that says otherwise?
As soon as you have an in-depth understanding of how the SR system works you'll see for yourself. The Pentax can only measure angular velocities around two axes and the sensor can be translated in two directions and rotated around one axis. However, because the camera cannot measure translational shake, the sensor translations are computed to compensate the rotational shake only (which the focal length is required for). Translational shake can be ignored for large enough subject distances. You find all the nasty details somewhere else, including some posts of myself.
QuoteQuote:
Perhaps you are referring to the fact that the sensor is rotated around the X- and the Y- axes to compensate shake?
The sensor isn't rotated.

I don't want to sound arrogant, but I will stop discussing this now. I wanted to give the OP some help about how to achieve less shake when using extension tubes.

Last edited by falconeye; 10-27-2008 at 03:10 PM.
10-27-2008, 04:28 PM   #7
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QuoteOriginally posted by falconeye Quote
The Pentax can only measure angular velocities around two axes and the sensor can be translated in two directions and rotated around one axis.
That's right. I got that one backwards. The sensor never tilts and due to the nature of the shake detection only rotational camera movements are detected. Thanks for clarifying.
10-27-2008, 08:13 PM   #8
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QuoteOriginally posted by falconeye Quote
I wanted to give the OP some help about how to achieve less shake when using extension tubes.
Thanks !
Apart from a tripod, I try to hold camera relaxed and gentle squeeze shutter release, brace against things etc ... I might change the focus function back to the shutter so I can catch focus. Any other things I could do would be greatly appreciated
As I get older the shaking seems to increase but the SR seems to cope wonderfully.
cheers
Pete

10-27-2008, 08:14 PM   #9
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QuoteOriginally posted by falconeye Quote
Class A, I think it's all clear. So my response is meant for you and I'll try to be brief now.
I posted to this thread since I misread the original question (as in "do I need to put in a different focal length?") and therefore thought the answer "no" was incorrect. I now (better late than never ) discovered that falconeye provided the answer that I thought was correct. Sorry for the confusion!
10-28-2008, 04:35 AM   #10
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QuoteOriginally posted by Transit Quote
Any other things I could do would be greatly appreciated
I don't know either. Focussing and keeping stuff in focus seems to be most difficult. Your focus trap idea might help.

If you have a K20D, you may find it helpful to use magnified LiveView to focus when working on a tripod. Unfortunately, LiveView uses the current aperture, so one first has to set it fully open for LiveView focussing and close down after LiveView to take the photo...

QuoteOriginally posted by Class A Quote
Sorry for the confusion
You are welcome.
10-28-2008, 05:05 AM   #11
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With plants there is always something in focus
so I'm not too worried .
Have a K100D and happily a seagull finder is in the post ...
Will be interesting to see if the 2.5x mag helps focus check
cheers
Pete
10-28-2008, 01:05 PM   #12
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QuoteOriginally posted by Transit Quote
Have a K100D and happily a seagull finder is in the post ...
You may also try a focusing screen with a split prism, although this more appropriate for shooting fast lenses wide open, when everything seems to be in focus, which is definitely not the case with macro work.

If you shoot static objects, I believe a focusing rail would be best to nail down the focus. Some bellows have one included.
10-28-2008, 02:23 PM   #13
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I tend to like the clear view rather than ground prismatic bits
rely on the camera focus indicater a bit too...
even started using that horrific peep noise to save looking for the indicator light

you are no doubt correct about rails,
I was eyeing a set up the other day...
must...stop...buying...gear
03-09-2009, 08:21 AM   #14
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As the Mars Polar Lander team would no doubt ask - in what units?
03-12-2009, 01:12 PM   #15
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Thanks! This has been very helpful.

I now understand why my painstaking comparisons between SR and non-SR showed pretty much no difference...
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