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01-26-2012, 12:04 PM   #1
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Why is ISO Limited in Bulb Mode?

Does anyone know why ISO settings higher than 1600 can't be used when in Bulb mode? I sometimes take astrophotography pictures and higher ISO settings would be very useful to me as I must use Bulb mode for such photos.

01-26-2012, 12:09 PM   #2
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Might be related to the noise that arises as a result of the long shutter speed- but if you're shooting bulb, there should be enough light to properly expose the photo even at ISO 1600, right?

BTW, I was not aware of this restriction, as I've never pushed the ISO that high in bulb mode.

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01-26-2012, 12:12 PM   #3
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Adam,
Not necessarily enough light when shooting the night sky. Higher ISO, while adding noise, would reduce exposure time and star trailing if you have the camera on a standard tripod instead of a tracking mount. I shoot in RAW so can deal with noise via Noise Ninja.
01-26-2012, 12:15 PM   #4
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I imagine it's more of the heat issue - since heat production on the sensor isn't a linear relationship to the sensitivity.

01-26-2012, 12:48 PM   #5
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QuoteOriginally posted by atnbirdie Quote
Does anyone know why ISO settings higher than 1600 can't be used when in Bulb mode? I sometimes take astrophotography pictures and higher ISO settings would be very useful to me as I must use Bulb mode for such photos.
Update your firmware. That restriction was removed a couple of firmware revisions ago, if I recall correctly.
01-26-2012, 12:56 PM   #6
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I just got the camera this week and have installed the latest firmware. However, there is no mention of this issue in any of the firmware update notes on the Pentax site. I haven't had a chance to try setting ISO in B, but based my question on note in the K-5 manual that mentioned the ISO 1600 limitation in B mode.
01-26-2012, 01:03 PM   #7
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QuoteOriginally posted by atnbirdie Quote
However, there is no mention of this issue in any of the firmware update notes on the Pentax site.
As for version 1.02 they state "All ISO sensitivity setting can be used on Bulb mode."

see Latest K-5 Firmware Update : Software Downloads : PENTAX
01-26-2012, 01:18 PM   #8
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ISO settings beyond 1600 just cause the camera to push the RAW data.

Note that the sensor sensitivity is fixed and that an increase in sensitivity is emulated by increasing the gain applied to the sensel signals before analogue to digital conversion. This gain is maxed out at ISO 1600 and you can achieve the effect of higher ISO settings by pushing in post-production just as well.

That's why it made sense to limit bulb mode to ISO 1600 (for other modes higher ISO values are still useful to influence shutter speed calculations, etc.).

01-26-2012, 01:54 PM   #9
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Gimbal, Thanks for pointing out I need new glasses. LOL. Yes, I see now ISO is not limited. Thanks
01-26-2012, 03:58 PM   #10
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QuoteOriginally posted by atnbirdie Quote
Gimbal, Thanks for pointing out I need new glasses. LOL. Yes, I see now ISO is not limited. Thanks
In fact, if you set Expanded Sensitivity in the Custom Settings, you can have Bulb and ISO all the way up to 51,200 (!!!). Not that I am saying that this would make any sense..........
01-26-2012, 05:35 PM   #11
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QuoteOriginally posted by atnbirdie Quote
Yes, I see now ISO is not limited.
Did you see my earlier post?

Removing the ISO 1600 restriction does not change a thing for you, except that the image will automatically appear pushed in a RAW converter. In terms of image quality, you don't gain anything by using ISO settings greater than ISO 1600. You can just as well shorten the bulb exposure time yourself and push in post.
01-26-2012, 05:48 PM   #12
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QuoteOriginally posted by atnbirdie Quote
Does anyone know why ISO settings higher than 1600 can't be used when in Bulb mode?
Please read the answer provided by Class A.

I would even propose to shoot in ISO 80 (raw and underexposed of course) and push RAW from there. The reason is that a night shot always has blown highlights: the stars. And you have max. reserves in the highlights by staying at ISO 80. OTOH, the additional noise in the darks with a K-5 (after you pushed ISO in post processing) is minimal. And to further reduce noise, you can stack images. Which doesn't recover clipped highlights.
01-27-2012, 12:31 PM   #13
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Class A, Thanks for the explanation of the rationale for the original ISO limit. Will consider that as I shoot the stars.
01-27-2012, 02:03 PM   #14
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QuoteOriginally posted by Class A Quote
Did you see my earlier post?

Removing the ISO 1600 restriction does not change a thing for you, except that the image will automatically appear pushed in a RAW converter. In terms of image quality, you don't gain anything by using ISO settings greater than ISO 1600. You can just as well shorten the bulb exposure time yourself and push in post.
Actually you go backwards because above ISO1600 the camera applies noise reduction....
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