Author Topic: A850 Long Exposure Issue  (Read 3773 times)

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Offline lothian

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A850 Long Exposure Issue
« on: March 21, 2010, 10:19:19 AM »
Hi All

I've been out getting used to my new Sony kit - a big change from film but I'm enjoying it! 

I was really impressed with the camera's long exposure ability (on a tripod at low ISO) until I tried my bargain Minolta 28mm f2.8 in low light.  Attached is a small linked copy of the problem image, unfortunately the problem doesn't really show up at this resolution so I'll try to explain.

Basically, as the sky darkens around the bridge,  the graduation between tones isn't at all subtle,  like shaded rainbows or bands.  Examining the pixels in Photoshop at 100%, it looks rather blocky.

I can see a potential problem in the image, the bright floodlight on the left does seem to be causing some flare,  could this be the problem?  I've had good results from this lens in in other situations but this is the first time I've used it for an exposure over 1 second.  Also, I was using Cokin ND grad and I'm wondering if this may have contributed to the problem.
 
Exposure setting: 30 seconds at f11,  ISO 100, dynamic range, noise reduction, sharpening etc off.  I try not to push this lens and keep it at f8 - f11 to avoid diffraction problems.

Any advice would be much appreciated.

Steve
A850, 80-200mm/2.8 G, 28mm/2.8, 50mm/1.4, 24-70mm/2.8 ZA, Tamron 90mm/2.8, Metz 48 AF-1


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Offline REX (aka TG)

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Re: A850 Long Exposure Issue
« Reply #1 on: March 21, 2010, 10:50:26 AM »
Remove the filter and retest. If is still the same replace the lens and see.
My opinion it can not be from the camera more likely from the filter
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Offline Rob aka [minolta mad]

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Re: A850 Long Exposure Issue
« Reply #2 on: March 21, 2010, 12:36:56 PM »
Hi there,
Ive had the same problem at times, shooting with and without filters.
From what ive seen with my images, and it sounds very much like the scenario that you are having is that its posterisation.
Why this is happening i dont know, as i have never found out why.
I do know it happens if an image is processed heavily, but i have had it on images that have had minimal processing.


Rob

Offline lothian

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Re: A850 Long Exposure Issue
« Reply #3 on: March 21, 2010, 01:57:42 PM »
From what ive seen with my images, and it sounds very much like the scenario that you are having is that its posterisation.


Rob

Hi Rob

Thanks for the hint.  I googled posterisation and the symptoms seem to match.  In my case, banding seems to be the problem.  According to Wikipedia, this is caused by insufficient bits in an 8bit per channel image.  In an attempt to prove it, I used the sony raw converter to output to a 16 bit tiff,  I then converted this to 32 bits in Photoshop (it produced a monster file!).  I could see no banding in the 32 bit image.  Converting back to 8 bit,  I could see banding again.  I guess if I need to print an image with this problem, then I just increase the bits per channel and pass it to the lab.  For web display,  I'm not sure what's best. 

Thanks again for the replies 
Steve
A850, 80-200mm/2.8 G, 28mm/2.8, 50mm/1.4, 24-70mm/2.8 ZA, Tamron 90mm/2.8, Metz 48 AF-1


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Offline Stef.

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Re: A850 Long Exposure Issue
« Reply #4 on: March 21, 2010, 03:35:28 PM »
I am with Rob on this one. I had this happening and again have no clue why. I usually select parts of the sky and use a bit of gaussian blur on it.
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Offline rogprov

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Re: A850 Long Exposure Issue
« Reply #5 on: March 21, 2010, 04:48:10 PM »
There's a PS plugin to deal with this 8 bit problem ...

http://powerretouche.com/Histogram_Repair_plugin_tutorial.htm

...  but it will cost you £20 / $32  :(


I've just found this page on how to minimise/fix the problem  ...

http://www.ronbigelow.com/articles/posterization/posterization.htm

... lot of reading but may be helpful.
« Last Edit: March 21, 2010, 04:53:17 PM by rogprov »
Roger

Offline Rob aka [minolta mad]

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Re: A850 Long Exposure Issue
« Reply #6 on: March 21, 2010, 05:36:05 PM »
Informative tutorial, but one thing that i cant understand is that why i never had this on any of my A700 files, espescially as i do less PP now with the A900 than i did previously ???

Perhaps someone could explain ???


Rob

Offline lothian

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Re: A850 Long Exposure Issue
« Reply #7 on: March 21, 2010, 10:57:13 PM »
Yes, an interesting tutorial on posterisation, thanks for the links.  I tried adding a little noise and that improved the image.  I'll try the blur as Stef suggested next time.   

Just fiddling around with the raw converter, I checked the show out-of-gamut colours option and the highlight almost perfectly matched the contour of the banding.  It is certainly  true that the problem area lacked tonal range - just a very dark night sky with no clouds - so a further fix could be to wait for a better evening! 

Steve

A850, 80-200mm/2.8 G, 28mm/2.8, 50mm/1.4, 24-70mm/2.8 ZA, Tamron 90mm/2.8, Metz 48 AF-1


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Offline vaughaag

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Re: A850 Long Exposure Issue
« Reply #8 on: March 21, 2010, 11:58:46 PM »
I have been following this thread with great interest. One because I have had similar results from my 200 when shooting with longer exposures and two, because I have been and still am looking to upgrade to a FF body in the near future.

Can I just say that removing the posterisation from the equation, that is a stunning picture, lovely light and comp!

Rgds,
Dave
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Offline lothian

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Re: A850 Long Exposure Issue
« Reply #9 on: April 04, 2010, 10:23:55 PM »
Hi All and Happy Easter

First of all,  thanks for your input on this.

I figured out what was going on this evening during my photo shoot down at the beach.  The posterisation started appearing after an hour or so when the light really started to fade and my camera was flashing 30 seconds at me no matter what I did to the ND filters or aperture setting.  The problem was that the photographer did not read the manual for his new toy.  If I had studied the specs, I would have picked up on the fact the aperture priority mode is limited to 30 seconds max!  My old OM4Ti could go into several minutes but that's a different camera. 

When I got home I loaded up my flash card with the original problem raw, and guess what the exposure was?  Yes, 30 seconds.

Ok,  for serious long exposure work, I'll be using manual and bulb in future with my trusty Gossen meter.

So the diagnosis in my case is user error.

Kind regards

Steve
A850, 80-200mm/2.8 G, 28mm/2.8, 50mm/1.4, 24-70mm/2.8 ZA, Tamron 90mm/2.8, Metz 48 AF-1


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Offline Rob aka [minolta mad]

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Re: A850 Long Exposure Issue
« Reply #10 on: April 04, 2010, 10:52:21 PM »
Glad to hear that you have it sorted.
I have still been having some problems with posterisation, but reading the link in the article that rogprov addded, it looks like the curves adjustments that i normally use could be the main cause of the issue.
Will have to try it out and see.

Still find it strange that the A700 doesnt show any of this even with hard processing.


Rob