Uniform grey backgrounds

Uniform grey backgrounds

Post by Fred Athear » Sat, 01 Jun 2002 21:47:08



I have a lot of digital photos of art objects to process. They are all
taken under identical light conditions and camera settings and with
the identical neutral gray background.

But the color and brightness of the background in the shots changes a
lot depending upon depending the overall brightness and colors of the
object being photographed.

Nothing I  have figured out how to do with the camera --  a Nikon
CoolPix 950 -- seems to get around this effect.  It is like the camera
is just deciding to "improve" each shot by compensating the background
to balance brightness and color of the object.

For a while I used the method of making a reference point in the gray
background and calculating the average of the red, green, and blue and
then using curves to set each of the inputs to output the average.
But some of the backgrounds were still darker and some lighter.

Then I just started to do this:

1.  Put a number like 140 or 150 onto the clip board.
2. Hit control-M to open a curves adjustment.
3. With shift and control held down, click on a part of the image that
should be neutral gray.
4.  With control down, hit 1 and then double click in the output box
and hit v.
5. Do the same thing with 2 and 3 to set the other channels.

The result is a uniform neutral background that does not seem to
distort the colors of the object.

Two questions:

1.  Why should this be necessary?  Shouldn't there be a way to tell  a
fancy camera like the Nikon CoolPix 950 to shoot in a way that gray is
gray?  If so how?  I can't find a way.

2. Given the problem, and the need for uniform gray backgrounds in the
result, is the above the best way to go so far as keeping the colors
of the objects  undistorted?
Fred Athearn

 
 
 

Uniform grey backgrounds

Post by Bob Shomle » Sat, 01 Jun 2002 23:46:35


Quote:>Shouldn't there be a way to tell a fancy camera like the Nikon
>CoolPix 950 to shoot in a way that gray is gray?  If so how?

There might be two issues: gray as gray (no color cast) and to capture
the gray at constant density across many pictures (which interacts with
the density of the rest of each picture).  With the lighting constant
the gray could photograph the same if 1) white balance is not
recalculated for each shot, and 2) auto exposure does not changed
shutter speed, which could change density of gray in the captured image.

You might experiment with setting the camera's white balance to Preset
White (which uses a white reference), maybe that together with the Tone
Curve selection.  This might keep the background a neutral gray (vs
having a color cast).  Try shutter priority; see if a single setting
will adequately capture the tonal range of the different art works while
maintaining a constant density in your gray background.

Bob Shomler
www.shomler.com


> I have a lot of digital photos of art objects to process. They are all
> taken under identical light conditions and camera settings and with
> the identical neutral gray background.

> But the color and brightness of the background in the shots changes a
> lot depending upon depending the overall brightness and colors of the
> object being photographed.

> Nothing I  have figured out how to do with the camera --  a Nikon
> CoolPix 950 -- seems to get around this effect.  It is like the camera
> is just deciding to "improve" each shot by compensating the background
> to balance brightness and color of the object.

> For a while I used the method of making a reference point in the gray
> background and calculating the average of the red, green, and blue and
> then using curves to set each of the inputs to output the average.
> But some of the backgrounds were still darker and some lighter.

> Then I just started to do this:

> 1.  Put a number like 140 or 150 onto the clip board.
> 2. Hit control-M to open a curves adjustment.
> 3. With shift and control held down, click on a part of the image that
> should be neutral gray.
> 4.  With control down, hit 1 and then double click in the output box
> and hit v.
> 5. Do the same thing with 2 and 3 to set the other channels.

> The result is a uniform neutral background that does not seem to
> distort the colors of the object.

> Two questions:

> 1.  Why should this be necessary?  Shouldn't there be a way to tell  a
> fancy camera like the Nikon CoolPix 950 to shoot in a way that gray is
> gray?  If so how?  I can't find a way.

> 2. Given the problem, and the need for uniform gray backgrounds in the
> result, is the above the best way to go so far as keeping the colors
> of the objects  undistorted?
> Fred Athearn


 
 
 

Uniform grey backgrounds

Post by Fred Athear » Sun, 02 Jun 2002 07:58:23



>You might experiment with setting the camera's white balance to Preset
>White (which uses a white reference), maybe that together with the Tone
>Curve selection.  This might keep the background a neutral gray (vs
>having a color cast).  Try shutter priority; see if a single setting
>will adequately capture the tonal range of the different art works while
>maintaining a constant density in your gray background.

Thanks for the tips.  I had not understood exactly what the white
preset did, but clearly that is the way to turn off the automatic
color balance thing that is giving all those color casts to the gray
background.

As to using shutter priority to work around the automatic brightness
the problem there is that the CoolPix 950 does not have a fully manual
mode. If you set the shutter speed the aperture will adjust
automatically and if you set the aperture the shutter will adjust.  In
program mode they both adjust.

One way around that would be to set Metering option to "spot" mode.
Then before taking each shot I turn to the side to put the spot over
the gray background but with enough of the edge of the object showing
to set the auto focus and then turn back and frame the shot with the
shutter still 1/2 down to hold exposure and focus.

But perhaps if I set the Metering to "matrix" mode and include enough
of the gray background in each shot the exposure will remain constant
except for a few very light or very dark objects which it would be
easy to lighten or darken just using the lightness/contrast photoshop
adjustment.

Fred Athearn

 
 
 

Uniform grey backgrounds

Post by John Houghto » Sun, 02 Jun 2002 19:09:28




> But perhaps if I set the Metering to "matrix" mode and include enough
> of the gray background in each shot the exposure will remain constant
> except for a few very light or very dark objects which it would be
> easy to lighten or darken just using the lightness/contrast photoshop
> adjustment.

You coould use the auto exposure lock feature.  After this mode has been
set, the first shot you take determines the exposure and white balance
settings for all subsequent images (until you reset or disable the lock).
So you can take your first exposure using the background and then focus and
frame as you like for the following shots.

John

 
 
 

Uniform grey backgrounds

Post by Bob Shomle » Sun, 02 Jun 2002 23:26:06


Quote:>You coould use the auto exposure lock feature.  After this mode has
>been set, the first shot you take determines the exposure and white
>balance settings for all subsequent images (until you reset or
>disable the lock).
>So you can take your first exposure using the background and then
>focus and frame as you like for the following shots.

There is a risk in holding exposure constant, depending on how much the
art works displayed on the gray background will very in reflected
brightness.  One of the characteristics of digital cameras that I've
observed is that it's very easy to overexpose the highlights a high
contrast scene.  Once clipped, that highlight detail is gone forever.

Taking this back to Photoshop...  What I recall was desired was to hold
the gray background surrounding the art works constant, both neutral
gray (no color cast) and in density.  First question was if the "fancy
camera" (Nikon 950) could be made to do this.  Answer appears to be
probably yes for color cast - white balance, but no for
exposure/density.  Perhaps that could be accomplished in Photoshop by
the following.

Open image from camera in Photoshop and duplicate it on a new layer.
Under this duplicate image layer create another new layer and fill it
with the desired neutral gray (150-150-150?).  Now operate on the top
layer (image copy).  Perform whatever tonal and color adjustments are
desired, then select the photographed gray surrounding background in
that layer and erase it, which will let the gray layer underneath show
through as the background.  That method, repeated for each photo, will
produce the same 150-150-150 gray background in each.  

Bob Shomler
www.shomler.com

 
 
 

Uniform grey backgrounds

Post by Fred Athear » Mon, 03 Jun 2002 14:36:45


On Sat, 1 Jun 2002 11:09:28 +0100, "John Houghton"


>You coould use the auto exposure lock feature.  After this mode has been
>set, the first shot you take determines the exposure and white balance
>settings for all subsequent images (until you reset or disable the lock).

The only exposure lock feature I know about on the CoolPix 950 is the
one set by pressing down the shutter 1/2 way.

The 990 does seem to have that feature though see:
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/990/EXPOSURE/EV-AE-lo...

It looks like an excuse to upgrade but I think that if I can get rid
of the automatic color cast "feature" using the white preset I can
live with and/or easily adjust the overall exposure in photoshop.

Probably just a quick hit with Hue/Saturation/Lightness to bring the
background close to the photoshop background an then a boost to
saturation and a few hits with unsharp.
Fred Athearn

 
 
 

Uniform grey backgrounds

Post by John Houghto » Mon, 03 Jun 2002 15:26:37



Quote:> The only exposure lock feature I know about on the CoolPix 950 is the
> one set by pressing down the shutter 1/2 way.

> The 990 does seem to have that feature though see:

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/990/EXPOSURE/EV-AE-lo...

The 950 has just the same feature and is activated in a similar manner.  I
don't have my 950 any more so I cannot give you precise instructions, but
check the user documentation for auto exposure lock, or find the exposure
options in the menus while in M mode.

John

 
 
 

Uniform grey backgrounds

Post by Fred Athear » Mon, 03 Jun 2002 22:21:12


On Sun, 2 Jun 2002 07:26:37 +0100, "John Houghton"


>The 950 has just the same feature and is activated in a similar manner.  I
>don't have my 950 any more so I cannot give you precise instructions, but
>check the user documentation for auto exposure lock, or find the exposure
>options in the menus while in M mode.

You are right.  It is called AE-LOCK on the second page of the M-rec
menu.  That ought to make it a lot easer to do this.  Any adjustments
that are necessary should be the same on all the shots so I can use an
action and a few saved layers.

Fred Athearn

 
 
 

1. contouring of random/non-uniform grids to uniform grids

I've got this done using NCAR's bivar routine.  Just a kludge, but
it works...  It works with the X-windows program 'contour'.  

Let me know if there is any interest in posting, or I'll e-mail individual
requests.  It's just a little front end program.

-A.

--
-----------------------------------------------------------------------------
 Andrew Anselmo / Department of Mechanical Engineering / SUNY Stony Brook  

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