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DSLR questions?

 
 
PeteD
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      09-25-2006
Hi,

I recently bought my first DSLR (D80) and am getting to grips with
differences from film and my digital P&S.

1. I was reading about hot pixels and maybe I shouldn't have. Being an
engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
At 100% I could see a small + sign at one point and a couple of other
coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
photography will be at 1/60 sec and faster. I don't want to get carried
away looking for trouble!!

2. Also spotted two dust spots when doing this as I had a small
aperture. Further testing on sky shows the dust spots visible down to
around f14. I know this is par for the course but what do most people
do. Ignore it and post process or try and keep aperture wider?? I'm not
really up for having a go at sensor cleaning two weeks into ownership!!


This leads me to a comment I saw on Ken Rockwell's site.

3. Article title:
"How to Select the Sharpest Aperture Considering the Simultaneous
Effects of Depth-of-Field and Diffraction"

http://www.kenrockwell.com/tech/focus.htm

The following statement puzzles me?

"If you are a beginner or just shooting a 35mm or digital camera then
this article addresses issues which won't bother you at reasonable
apertures. Just use a tripod and choose the smallest aperture you have
if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
digital cameras. "

Why the aperture constraint?? Surely for good depth of field in a
landscape shot you would stop it down further.

Apologies if this all beneath you guys, I'm just trying to enjoy moving
to digital without getting stressed over a few issues.

Thanks
Pete

 
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frederick
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      09-26-2006
PeteD wrote:
> Hi,
>
> I recently bought my first DSLR (D80) and am getting to grips with
> differences from film and my digital P&S.
>
> 1. I was reading about hot pixels and maybe I shouldn't have. Being an
> engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
> At 100% I could see a small + sign at one point and a couple of other
> coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
> photography will be at 1/60 sec and faster. I don't want to get carried
> away looking for trouble!!
>

Normal - does the D80 have long exposure noise reduction (dark frame
subtraction)? If so, then use it for long exposures.
>
> 2. Also spotted two dust spots when doing this as I had a small
> aperture. Further testing on sky shows the dust spots visible down to
> around f14. I know this is par for the course but what do most people
> do. Ignore it and post process or try and keep aperture wider?? I'm not
> really up for having a go at sensor cleaning two weeks into ownership!!
>
>
> This leads me to a comment I saw on Ken Rockwell's site.
>
> 3. Article title:
> "How to Select the Sharpest Aperture Considering the Simultaneous
> Effects of Depth-of-Field and Diffraction"
>
> http://www.kenrockwell.com/tech/focus.htm
>
> The following statement puzzles me?
>
> "If you are a beginner or just shooting a 35mm or digital camera then
> this article addresses issues which won't bother you at reasonable
> apertures. Just use a tripod and choose the smallest aperture you have
> if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
> digital cameras. "
>
> Why the aperture constraint?? Surely for good depth of field in a
> landscape shot you would stop it down further.
>
> Apologies if this all beneath you guys, I'm just trying to enjoy moving
> to digital without getting stressed over a few issues.
>

Diffraction is explained well here:
http://www.cambridgeincolour.com/tut...otography.htm#
Ken may not have intended it, but diffraction effect isn't limited to
digital - perhaps he meant "digital slr with cropped sensor" not
"digital". Of course you can still use smaller apertures - but will
lose resolving power to diffraction. That might be justified depending
on what you are trying to achieve. For landscape where you probably
want maximum detail retained as well as depth of field, then you
probably want to avoid visible diffraction losses, and may benefit from
focusing manually and calculating parfocal distance. That is further
explained here:
http://www.cambridgeincolour.com/tut...calculator.htm
 
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PeteD
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Posts: n/a
 
      09-26-2006
Thanks for that,

The D80 has long shutter NR for long exposures from around 8secs... it
also has High ISO NR which is normally off for ISO's up to 800. I'll
have a play....I read something about the + meaning it's probably a bad
photosite or something.

Thanks for the pointers to the articles too.

Pete

frederick wrote:
> PeteD wrote:
> > Hi,
> >
> > I recently bought my first DSLR (D80) and am getting to grips with
> > differences from film and my digital P&S.
> >
> > 1. I was reading about hot pixels and maybe I shouldn't have. Being an
> > engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
> > At 100% I could see a small + sign at one point and a couple of other
> > coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
> > photography will be at 1/60 sec and faster. I don't want to get carried
> > away looking for trouble!!
> >

> Normal - does the D80 have long exposure noise reduction (dark frame
> subtraction)? If so, then use it for long exposures.
> >
> > 2. Also spotted two dust spots when doing this as I had a small
> > aperture. Further testing on sky shows the dust spots visible down to
> > around f14. I know this is par for the course but what do most people
> > do. Ignore it and post process or try and keep aperture wider?? I'm not
> > really up for having a go at sensor cleaning two weeks into ownership!!
> >
> >
> > This leads me to a comment I saw on Ken Rockwell's site.
> >
> > 3. Article title:
> > "How to Select the Sharpest Aperture Considering the Simultaneous
> > Effects of Depth-of-Field and Diffraction"
> >
> > http://www.kenrockwell.com/tech/focus.htm
> >
> > The following statement puzzles me?
> >
> > "If you are a beginner or just shooting a 35mm or digital camera then
> > this article addresses issues which won't bother you at reasonable
> > apertures. Just use a tripod and choose the smallest aperture you have
> > if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
> > digital cameras. "
> >
> > Why the aperture constraint?? Surely for good depth of field in a
> > landscape shot you would stop it down further.
> >
> > Apologies if this all beneath you guys, I'm just trying to enjoy moving
> > to digital without getting stressed over a few issues.
> >

> Diffraction is explained well here:
> http://www.cambridgeincolour.com/tut...otography.htm#
> Ken may not have intended it, but diffraction effect isn't limited to
> digital - perhaps he meant "digital slr with cropped sensor" not
> "digital". Of course you can still use smaller apertures - but will
> lose resolving power to diffraction. That might be justified depending
> on what you are trying to achieve. For landscape where you probably
> want maximum detail retained as well as depth of field, then you
> probably want to avoid visible diffraction losses, and may benefit from
> focusing manually and calculating parfocal distance. That is further
> explained here:
> http://www.cambridgeincolour.com/tut...calculator.htm


 
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frederick
Guest
Posts: n/a
 
      09-26-2006
PeteD wrote:
> Thanks for that,
>
> The D80 has long shutter NR for long exposures from around 8secs... it
> also has High ISO NR which is normally off for ISO's up to 800. I'll
> have a play....I read something about the + meaning it's probably a bad
> photosite or something.
>
> Thanks for the pointers to the articles too.
>
> Pete
>

Are you sure it's 8 seconds?
It just strikes me that you would probably expect to see such noise at
exposures longer than 1 second or so.
If it is disabled when exposure is shorter, then that is good. The D70
didn't do that - so when long exposure NR was left on, users wondered
why the burst frame rate was well "below spec".
 
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acl
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      09-26-2006

frederick wrote:

> Are you sure it's 8 seconds?
> It just strikes me that you would probably expect to see such noise at
> exposures longer than 1 second or so.


On the D200 at least (and presumably also on the D80) I have found that
it makes no (positive) difference up to around ISO 640 for exposures up
to 30s. I have not tested longer exposures systematically. For ISO 800
and around 30s, it's slightly better to have dark frame subtraction on.
I have tested exposures of several minutes at various ISOs out of
boredom, and at eg ISO 400 and 6 min it's better to have it on (removes
purple blobs due to heat from electronics). It's just a matter of
testing your camera.

I may remember some of these numbers incorrectly; anyway, testing is
for free!

> If it is disabled when exposure is shorter, then that is good. The D70
> didn't do that - so when long exposure NR was left on, users wondered
> why the burst frame rate was well "below spec".


Well, if dark frame subtraction is on, the D200 shows a smaller number
of available exposures before the buffer fills, although DFS does not
occur unless the exposure is over 7s (or thereabouts). I have no idea
if it actually does stop taking pictures if the counter reaches zero
with DFS on, but I doubt it.

 
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frederick
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Posts: n/a
 
      09-26-2006
acl wrote:
> frederick wrote:
>
>> Are you sure it's 8 seconds?
>> It just strikes me that you would probably expect to see such noise at
>> exposures longer than 1 second or so.

>
> On the D200 at least (and presumably also on the D80) I have found that
> it makes no (positive) difference up to around ISO 640 for exposures up
> to 30s. I have not tested longer exposures systematically. For ISO 800
> and around 30s, it's slightly better to have dark frame subtraction on.
> I have tested exposures of several minutes at various ISOs out of
> boredom, and at eg ISO 400 and 6 min it's better to have it on (removes
> purple blobs due to heat from electronics). It's just a matter of
> testing your camera.
>
> I may remember some of these numbers incorrectly; anyway, testing is
> for free!
>

I have since checked specs - it appears to be automatically "off" at
exposures shorter than 1/2 second on the D80. As well as correcting for
the "purple blobs" from heat, it should correct for hot pixels. Perhaps
if you shoot at lowest ISO you will see an advantage with exposures
shorter than 30 seconds.
 
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acl
Guest
Posts: n/a
 
      09-26-2006
frederick wrote:
> I have since checked specs - it appears to be automatically "off" at
> exposures shorter than 1/2 second on the D80. As well as correcting for
> the "purple blobs" from heat, it should correct for hot pixels. Perhaps
> if you shoot at lowest ISO you will see an advantage with exposures
> shorter than 30 seconds.


No, the higher the ISO, the shorter the exposure that necessitates use
of DFS. Not surprisingly: the idea is that the DFS adds noise to the
image (the random part that is not common to the two frames). So it's
only useful if there are systematic noise sources (to wit, the purple
blob and hot pixels mentioned, or some kind of banding etc). These
appear at longer exposures and higher ISOs, for obvious reasons.
 
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frederick
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Posts: n/a
 
      09-26-2006
acl wrote:
> frederick wrote:
>> I have since checked specs - it appears to be automatically "off" at
>> exposures shorter than 1/2 second on the D80. As well as correcting
>> for the "purple blobs" from heat, it should correct for hot pixels.
>> Perhaps if you shoot at lowest ISO you will see an advantage with
>> exposures shorter than 30 seconds.

>
> No, the higher the ISO, the shorter the exposure that necessitates use
> of DFS. Not surprisingly: the idea is that the DFS adds noise to the
> image (the random part that is not common to the two frames). So it's
> only useful if there are systematic noise sources (to wit, the purple
> blob and hot pixels mentioned, or some kind of banding etc). These
> appear at longer exposures and higher ISOs, for obvious reasons.


I would have thought that as DFS removed noise that was repeatable
patterns from hot pixels etc, then it would be rendered more useless as
ISO increased random noise patterns which obscured those repeatable
patterns. Then again I got that idea from observing results.
 
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acl
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Posts: n/a
 
      09-27-2006
frederick wrote:
> acl wrote:
> > frederick wrote:
> >> I have since checked specs - it appears to be automatically "off" at
> >> exposures shorter than 1/2 second on the D80. As well as correcting
> >> for the "purple blobs" from heat, it should correct for hot pixels.
> >> Perhaps if you shoot at lowest ISO you will see an advantage with
> >> exposures shorter than 30 seconds.

> >
> > No, the higher the ISO, the shorter the exposure that necessitates use
> > of DFS. Not surprisingly: the idea is that the DFS adds noise to the
> > image (the random part that is not common to the two frames). So it's
> > only useful if there are systematic noise sources (to wit, the purple
> > blob and hot pixels mentioned, or some kind of banding etc). These
> > appear at longer exposures and higher ISOs, for obvious reasons.

>
> I would have thought that as DFS removed noise that was repeatable
> patterns from hot pixels etc, then it would be rendered more useless as
> ISO increased random noise patterns which obscured those repeatable
> patterns. Then again I got that idea from observing results.


It seems we're saying the same thing. I suppose my point was that these
repeatable patterns (hot pixels, mainly) are more likely to appear at a
given exposure time if the ISO is higher. Yes, random noise (not
removed by DFS, as you say) will increase, but hot pixels will also
increase. I have never seen hot pixels at up to 30s on ISO 100 and 200
on my camera, for example.

Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
and up to 30s, switching DFS on actually degrades the image on my
camera (it has more random noise; mind you, the difference is small).
So I thought a bit about it and realised that since both frames have
random noise, and you subtract them, the result will have even more
random noise (assuming it is zero-average, addition and subtraction are
the same, for our purposes: the noise is as likely to be up as down;
ignoring some details). Not really enough to worry about, but the extra
time is irritating.

 
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Paul Rubin
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Posts: n/a
 
      09-27-2006
"acl" <(E-Mail Removed)> writes:
> Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
> and up to 30s, switching DFS on actually degrades the image on my
> camera (it has more random noise; mind you, the difference is small).
> So I thought a bit about it and realised that since both frames have
> random noise, and you subtract them, the result will have even more
> random noise (assuming it is zero-average, ...


But in this case the camera could compensate by averaging together a
bunch of dark frames. The random components would cancel out and the
systematic components would stay.
 
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