Nikon D70 long exposure - purple blobs?

Archived from groups: rec.photo.digital (More info?)

Hey guys! So I just recently got a D70 and took it to a summer camp for a
few weeks. Got some great photos with it (in my opinion anyway, I'm still
very much a rank amateur...)

My brother and I tried taking some star photos, such as this one:

http://www.cherith.ca/gallery/week1/aai

(ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
very much shrunken down from the original camera JPEG, but the problem is
still very visible)

We notice that on many of these long exposure shots, there are purple
blobs in the upper left corner. Seems like the longer the exposure, the more
pronounced the blobs are.

What is this? (Camera itself, sensor, lens?) Is this normal, or do I have a
bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
think this is normal for the D70, as we were hoping to experiment with more
shots like this in the future...

I also notice a strong purple cast in some of my photos, though I have not
experimented to see what circumstances they show up in.

Any ideas? Thanks for your time...

Archived from groups: rec.photo.digital (More info?)

Kevin wrote:

> Hey guys! So I just recently got a D70 and took it to a summer camp for a
> few weeks. Got some great photos with it (in my opinion anyway, I'm still
> very much a rank amateur...)
>
> My brother and I tried taking some star photos, such as this one:
>
> http://www.cherith.ca/gallery/week1/aai
>
> (ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
> very much shrunken down from the original camera JPEG, but the problem is
> still very visible)
>
> We notice that on many of these long exposure shots, there are purple
> blobs in the upper left corner. Seems like the longer the exposure, the more
> pronounced the blobs are.
>
> What is this? (Camera itself, sensor, lens?) Is this normal, or do I have a
> bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
> think this is normal for the D70, as we were hoping to experiment with more
> shots like this in the future...
>
> I also notice a strong purple cast in some of my photos, though I have not
> experimented to see what circumstances they show up in.
>
> Any ideas? Thanks for your time...

Don't know if the other pictures in the series you took were with your
D70, but I didn't notice any purple hue in them...at least not at that
resolution. Anyway, I took some night shots w/ my D70s not long ago
that turned out pretty noisy, and I figured out it was due to two
things: 1) I didn't have long exposure noise reduction (Menu->Camera
Icon->Long Exposure NR) on, and 2) I had VR on in my lens (24-120mm VR
AF-S Nikkor) w/ a tripod...a no-no. Next time out, no VR w/ long
exposure NR, and the shots were fine. Nothing as long as 2:00, but
fine, nonetheless.

Note: Once you are done shooting longer exposures, make sure and turn
the NR back off - it affects write time in normal exposures, IIRC.
Also, be patient while it writes to the card in NR mode, as well - it
takes a few seconds to do so, so don't worry about the longer wait to
review the shot(s). HTH...

Good luck,

Ben

Archived from groups: rec.photo.digital (More info?)

Sheldon <sheldon@xxxxxxxxsopris.net> wrote:
> It looks like some stray light or a reflection of somekind. Do you get the
> problem regardless of where you aim the camera? To see if it's the camera
> just repeat the exposure with the lens cap on. Keep us posted.

Hi Sheldon,

Thanks. I'll try the lenscap-on trick later to see what happens. Meantime
I've uploaded a couple of other photos to http://www.cherith.ca/jay, where
you can see the purple-blob problem is occurring regardless of camera
orientation.

I'll also try the NR feature and see what happens..

Archived from groups: rec.photo.digital (More info?)

"Kevin" <kevin@nospam.invalid> wrote in message
newscmqnc$q15$1@driftwood.ccs.carleton.ca...
> Ryan <quakeserver149@yahoo.com> wrote:
>> I forget the number, but I think around 40 seconds or so, is the point
>> at which star motion blur begins to show on a non moving exposure.
>> Perhaps someone can correct that if I am mistaken.
>
> Around that. I was mistaken; though we did take exposures as long as 2
> minutes (which definitely show signs of star motion), the picture I posted
> was probably only 30-45 seconds.
>
> Anyway, based on advice from Sheldon, I did some more tests, and it looks
> like it is not light pollution. I did 30-second and 1-minute exposures
> with
> the lens cap on, just now in my home, and the same purple blobs appear in
> the
> same positions. When I turn on noise reduction, the purple blobs are no
> longer present in the image. At least I have a workaround, but it would
> still be nice to know whether this is considered "normal" behaviour for a
> D70, or whether I should take it back...
>
> Thanks for your help and advice.

It is quite normal, many cameras do the same thing and the software in their
camera will fix things up, use a lower ISO and noise reduction for long
exposures, you don't need the higher ISO anyway, thats why you are using a
long exposure.

Archived from groups: rec.photo.digital (More info?)

Sheldon wrote:

> "Kevin" <kevin@nospam.invalid> wrote in message
> newscmilb$h24$1@driftwood.ccs.carleton.ca...
>
>>Hey guys! So I just recently got a D70 and took it to a summer camp for a
>>few weeks. Got some great photos with it (in my opinion anyway, I'm still
>>very much a rank amateur...)
>>
>>My brother and I tried taking some star photos, such as this one:
>>
>>http://www.cherith.ca/gallery/week1/aai
>>
>>(ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
>>very much shrunken down from the original camera JPEG, but the problem is
>>still very visible)
>>
>>We notice that on many of these long exposure shots, there are purple
>>blobs in the upper left corner. Seems like the longer the exposure, the
>>more
>>pronounced the blobs are.
>>
>>What is this? (Camera itself, sensor, lens?) Is this normal, or do I have
>>a
>>bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
>>think this is normal for the D70, as we were hoping to experiment with
>>more
>>shots like this in the future...
>>
>>I also notice a strong purple cast in some of my photos, though I have not
>>experimented to see what circumstances they show up in.
>>
>>Any ideas? Thanks for your time...
>
>
> It looks like some stray light or a reflection of somekind. Do you get the
> problem regardless of where you aim the camera? To see if it's the camera
> just repeat the exposure with the lens cap on. Keep us posted.

It is and it is entirely normal behaviour in an uncooled CCD camera on
long exposures. IR photons and thermal effects from the on chip read out
control electronics affect one corner of the display. Modern chips are
better than they used to be. Take a dark frame shot for the same time
exposure of the inside of your lens cap and subtract it to get something
closer to the true image. NB ideally taken at the same time with the
sensor at roughly the same temperature.

The Kodak DC-120 was one of the first to allow long exposures and
suffered from considerably more warm corner effects on 16s exposures.

http://www.nezumi.demon.co.uk/astro/dc120/dc120.htm

To get away from the problem you need to actively cool the chip -
impractical on consumer cameras but routine in scientific work.

Regards,
Martin Brown

Archived from groups: rec.photo.digital (More info?)

"Owamanga" <owamanga-not-this-bit@hotmail.com> a écrit dans le message de
news: d9que1lfl3k9ve4jvgld1onijg9euk4sfc@4ax.com...
> On Tue, 02 Aug 2005 05:38:09 -0400, Ed Ruf <egruf_usenet@cox.net>
> wrote:

>
> I was under the impression that the viewfinder cover's purpose is to
> allow for accurate metering without viewfinder light effecting the
> outcome. Once the shutter is tripped the mirror blocks the light from
> the viewfinder doesn't it?

It should, but how to be absolutely sure? Some mirrors are smaller than they
used to be... (more recessed in more compact models?)
According to Canon (EOS D. Rebel for example) the use of the viewfinder
cover should be seriously considered.

Anyway, as said here after, if the flares show up in every night long
exposure regardless of the orientation of close sources of artificial light,
the viewfinder cover should not be the solution.


>
> Anyway, as others have suggested, this is caused by on-camera chip's
> thermal radiation affecting the sensor. Dark frame subtraction can
> help, but it, added to the hot-pixel filter stuff the D70 does
> automatically doesn't lend itself to scientific-quality astronomy
> images.
>
> I remember reading somewhere that there is a trick of turning the
> camera off during it's RAW write that will prevent it from running the
> hot pixel filter (which kills most of the fainter stars). I don't
> recall the details though.
>
> --
> Owamanga!
> http://www.pbase.com/owamanga

Archived from groups: rec.photo.digital (More info?)

Okay, this thread was driving me nuts, so I took a few 2 minute exposures
with my D70 and the lens cover on. Completely black image from side to side
and corner to corner. Some were taken with the viewfinder convered, some
not.

Sheldon

Archived from groups: rec.photo.digital (More info?)

Kevin wrote:
> Sheldon <sheldon@xxxxxxxxsopris.net> wrote:
>
>>Okay, this thread was driving me nuts, so I took a few 2 minute exposures
>>with my D70 and the lens cover on. Completely black image from side to side
>>and corner to corner. Some were taken with the viewfinder convered, some
>>not.
>
>
> Sheldon,
>
> Glad to hear that.
>
> If there are any other D70 users out there who would be willing, I would
> appreciate additional data points. Please let me know if you have any
> noise problems with long exposures.
>
> Thanks guys, you've been most helpful.
>
>
Ok, I had a bash as well. At 2 minutes with NR Off & lens cap on I got a
purple flare at the top left corner while viewing it on the camera's LCD.

With NR On there was no purple flare.

BUT .... looking at both pictures ( RAW ) in Nikon Capture there was no
purple flare in either of the shots ... now I'm lost ??
Bernard

Archived from groups: rec.photo.digital (More info?)

Kevin <kevin@nospam.invalid> wrote:
> We notice that on many of these long exposure shots, there are purple
> blobs in the upper left corner. Seems like the longer the exposure, the more
> pronounced the blobs are.

As others have pointed out, the simplest solution is to turn
on noise reduction.
See also
http://www.ooblick.com/text/noise-reduction/
for some pictures of this phenomenon.

--
Andrew Arensburger, Systems guy University of Maryland
arensb.no-bloody-spam@umd.edu Office of Information Technology
A problem can be found for almost every solution.