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04-09-2017, 02:44 AM   #1
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Manual Focal Length Settings using Extension rings ???

Hi Pentaxians.

I have a K-3 using SMC Pentax Asahi -M 1:2 / 50 mm Lens
Extension Kit Panagor 12 mm 20 mm 36 mm

So if I just put the lens on its own I guess I have to set the Focal length to 75 mm
so when using an extension tube do i also need to combine the tube size mm + Lens 50 mm x 1.5.crop. Is my understanding of this correct ?

Also when Using a Manual zoom Lens do I need to change the focal length every time I zoom in tighter ? or do I just put in the Maximum focal Length x 1.5 Crop.

and Finally can extension tubes be used combined with a macro zoom lens in Macro Mode ?

Thanks In Advance

04-09-2017, 03:01 AM   #2
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Extension tubes don't change the focal length. So leave the 50 mm unchanged. You don't need to consider a crop factor, just use the focal length of the lens.
When using an old Zoom lens I usually select a focal length I use most frequently with that lens (or, sometimes, just leave the SR off).
04-09-2017, 03:40 AM   #3
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QuoteOriginally posted by Lee Malta Quote
So if I just put the lens on its own I guess I have to set the Focal length to 75 mm
No, just set the focal length to 50mm.
The crop factor is only important for comparison of field of view on different sensor sizes but must not be used for the shake reduction system (Otherwise you would overcompensate and might get more blur than without SR)

As for the extension tubes I usually type in the focal length of the used lens for the correct exif information and deactivate SR.
Yes, the higher your magnification gets (or the closer you get) the more influence has camera movement on the sharpness of the image and you could compensate for that by increasing the focal length for SR but as I said before when you set the wrong focal length you might end up with even more blur. So I either use a tripod for macro work (SR off anyway) or when photographing insects I use a higher shutter speed to decrease the effect of camera movement (this also helps freezing insect movement)

Last edited by othar; 04-09-2017 at 03:58 AM.
04-09-2017, 07:43 AM   #4
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I have experimented with this, I can say that using the increase in magnification as a rough multiplier to focal length seemed to help. So a 50mm with a 25mm extension tube used at 75mm SR worked better than using it at 50mm.

As other said with any lens without tubes use actual focal length, the camera already understands the crop factors and assuming that this is relevant it already takes that into account.

04-09-2017, 07:56 AM   #5
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QuoteOriginally posted by volley Quote
Extension tubes don't change the focal length. So leave the 50 mm unchanged.
That is not technically true. Extension, by definition, increases the effective focal length.* That being said, I am not sure what to say regarding the OP's question. The first answer that comes to my mind is that shooting hand held with all but the shortest extension tubes on a 50mm lens will very likely outstrip the capabilities of the SR system and is not a good idea.


Steve

* This results in a corresponding reduction in relative aperture. In the days before TTL metering it was necessary to manually calculate additional exposure to account for extension with bellows or tubes.
04-09-2017, 08:14 AM   #6
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QuoteOriginally posted by stevebrot Quote
That is not technically true. Extension, by definition, increases the effective focal length.* That being said, I am not sure what to say regarding the OP's question. The first answer that comes to my mind is that shooting hand held with all but the shortest extension tubes on a 50mm lens will very likely outstrip the capabilities of the SR system and is not a good idea.


Steve

* This results in a corresponding reduction in relative aperture. In the days before TTL metering it was necessary to manually calculate additional exposure to account for extension with bellows or tubes.
Thanks, today is a good day, I've learned something!
04-09-2017, 08:48 AM - 1 Like   #7
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QuoteOriginally posted by Lee Malta Quote
So if I just put the lens on its own I guess I have to set the Focal length to 75 mm
As others have already covered 50mm = 50mm, there is no need to allow for the crop factor. So you set the camera to 50mm when it asks. I have no input on what to use when using extension tubes with shake reduction. When using tubes I am always on a tripod so SR if off anyway.
I wanted to address why you need to set the focal length which you may or may not know. There are two reasons, the Pentax reason is to adjust the shake reduction mechanism to the lens being used. And the second is so that the proper focal length shows up in the image EXIF. So depending on how dependent you are on shake reduction this might be important or completely unimportant. If you are not using shake reduction you could use any focal length you want when the camera asks, using different numbers could then later be used to determine which extension tube length was being used. Just a thought and of course if you are using SR then likely not a good idea. Though it might be interesting to see if adding length to the focal length when using tubes improves the result.
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