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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...

"Kevin" <> wrote in message
news:dcmilb$h24$...
> 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.

Sheldon

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

> 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.


It looks like light pollution to me. With a long exposure, the
slightest light source makes a big difference, at least in my
experience.

I shoot lightning a bit and a street light down the block gives me loads
of trouble.


Did you use a lens hood? If not, do so. If there are light sources
within direct ray view, try to find a shadow and put the front of your
lens in that shadow. This can be a tree, a utility pole, an upright on
a porch, a mailbox, whatever you can find. If that is stray light
from a nearby city, then... I don't know, hide behind a wall perhaps?

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.

Sheldon <> 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..

Ryan <> 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.

"Kevin" <> wrote in message
news:dcmqnc$q15$...
> Ryan <> 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.

On 2 Aug 2005 01:35:39 GMT, in rec.photo.digital 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...

Try the les cap photo as Sheldon suggested. Also, don't forget to use the
viewfinder cover as any stray light captured by this will be recorded as
well. The purple cast you mention is in normal exposure shots as well?

Other things to possibly try, upgrade to latest firmware, use the factory
reset switch on the bottom.
----------
Ed Ruf Lifetime AMA# 344007 ()
See images taken with my CP-990/5700 & D70 at
http://edwardgruf.com/Digital_Photog...ral/index.html

Sheldon wrote:

> "Kevin" <> wrote in message
> news:dcmilb$h24$...
>
>>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

On Tue, 02 Aug 2005 05:38:09 -0400, Ed Ruf <>
wrote:

>Try the les cap photo as Sheldon suggested. Also, don't forget to use the
>viewfinder cover as any stray light captured by this will be recorded as
>well. The purple cast you mention is in normal exposure shots as well?

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?

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