Can we 'twist' a digital picture to be square with the camera?

What 'trick' am I missing to take a photo of my watercolor paintings?
Can we 'twist' the digital photo to be square with the camera?

I must be doing something wrong. When I edit my digital photographs of my
watercolor paintings, I need to crop the photo but I can NEVER get the crop
edges to be exactly square with the digital camera photograph of the
painting.

Therefore I always find myself losing a few millimeters of painting so that
the photograph is square. What am I doing wrong?

Here is how I take the picture.
a) I put my unmatted 30 inch by 22.5 inch watercolor on a white towel lying
on the ground
b) I stand over the painting, sometimes on a step stool, and position the
Canon PowerShot A95 5MP camera over the center of the painting
c) I snap the shot using flash & macro focus making sure I am as square as
possible and taking an extra inch or so of border all around to facilitate
cropping

Back inside the office, each and every time, when I crop the 2592 by 1944
pixel digital results using Irvanview 3.98 on Windows XP, I end up with
edges that are not square at first. I have to lose some of my precious
painting just to get square edges when I crop.

What am I doing wrong?
What's the trick to get a photo that is head-on with the painting?
Is there a way to twist the painting with software so I don't have to crop
edges and lose painting data?

 

It seems you have a viewfinder that is not accurate - shows more coverage
than is there, and/or distortion is having its way. You should not need
macro-mode for such pictures, either.

Hang the painting on a wall. Shoot with a tripod. Leave the camera on the
tripod and check the photos. Adjust until it is correct and leave it there
for the pictures.

 

Well you can rotate if its not SQ or maybe you have a ratio of width to
height that's the problem As I cant see your image I dont know What size are
you cropping the 2592*1944 image to.

PSP has straightening and parallax correction tools. Does your picture
have more white border at one side then the others.

 

What am I doing wrong?

Your description suggests you're seeing barrel distortion, which is an
effect of the lens. Have a look at this page from DPReview's review of
your camera:

http://www.dpreview.com/reviews/canona95/page5.asp

Scroll down to the Macro Focus and Barrel and Pincushion Distortion
sections - is that what you're seeing?

To minimise the effect, start by switching macro mode off. I don't know
why you're using that. Second, if possible get further away from your
painting and zoom in on it a bit. This will be easier if you hang it on
a wall. Your camera shows less distortion when it's not at its widest
zoom setting. Failing that, you'll need to use an image processing tool
to correct the distortion.

 

Either that, or the subject and sensor planes are not parallel.

That is easy to get quite accurate by replacing the paintings by, or
put in front of the painting, a mirror (tile),absolutely parallel to
the painting's surface. If the camera lens is imaged dead center in
the viewfinder (and assuming parallax shift is taken into account),
every thing shouls square out except for lens distortions.

Bart

Bart

 

You need to use tools to make sure that the picture and the camera are
absolutely square, and that the camera is centered on the image. Handy tools
are an easel for the painting, a tripod for the camera, a good level, a
measuring tape, and a carpenter's square.
If you get something like Photoshop or PhotoImpact OTOH, you can correct the
misalignment in software.
Cheers;
CB

 

In newer editions of photoshop and others you can "twist" the image but
there will be some loss of image quality (possibly extremely minor but
it will be there). If this is something you do a lot you might want to
consider a copy stand although a copy stand large enough to accomodate
your canvas and give you enough height to use a lens that will not
cause distortion (I would aim for using a 40 - 60mm after taking crop
factor into account) will not be cheap. A tripod and a wall will
probably be better than the sytem you are using but you will still have
geometry problems and repeatability will be difficult. That is where
copy stands come in. Copy stands will also have facilities for various
lighting set-ups.

 

There are two problems you are running into:
1) Alignment of the painting to the sensor plane
2) Distortion from the lens
What to do?
There are several ways to align a painting/sensor. The mirror method
has been posted, I find it inaccurate (does work for some). One thing
needed is a tripod so you can fix the camera in one place, shooting by
hand means every picture will have a different alignment. I place the
painting on an easel, make sure the center and edges are parallel to
the camera lens, use an angle finder (available for $6-$10 at a
hardware store) Make sure the lens and painting are at the
corresponding angles. This is more involved than what can be covered
here and is described in a Kodak book on photographic reproduction.
The lens problem comes from using a zoom lens, most zoom lenses have
distortion throughout their range, most common are barrel and pincusion
distortions. Depending on the quality of the lens these distortions are
better controlled with the more expensive lenes are almost distortion
free. The other distortion in copying flat objects is field curvature,
this is maybe desirable in general picture taking, but annoying in
reproduction work. Both of these problems are cured by using a true
macro lens. this unfortunately is not an option with your A95.
Best thing for you to do is get a decent tripod and work on getting the
painting and sensor aligned. Do not use software to correct the
alignment as that enters other distortions into the image.

Tom

 

The guy who shoots my slides first puts up a black drape on the wall larger
than the work, then mounts the work in the center of the drape. The photos
have a black border, look great, and there is no cropping involved.

In my experience (before I gave up and had a professional do it) you can
*never* get it so square that you won't lose a little of the work.

 

For one thing, the fact that you're hand-holding the camera (especially
while standing on a step stool) and trying to shoot straight down must make
it extremely difficult to get the painting square with the camera. Isn't
there some way you can put the watercolor against a vertical surface so you
can use the camera on a tripod? This would make it far easier to get
everything square.

With that size subject I don't know why you're using the macro setting,
though that probably isn't related to your problem. Also unrelated to that,
but isn't flash causing you some problems as well?

There is, though there may be a slight loss of image quality (it may not be
noticeable, so might not matter to you at all -- especially with a
watercolor which is usually not a highly detailed subject anyway). Paint
Shop Pro, for example, has a Straighten tool which does exactly what you
want, very easily. No doubt there are other kinds of software that will do
this too.

 

As others have mentioned, it's probably barrel distortion that you
are seeing. It is probably most apparent at the wide-angle setting of
your camera.

So I would suggest:

1. Turn off your flash.

2. Turn off macro mode.

3. Hang the painting on a wall, preferably on a north wall outdoors so
that you're shooting it in shade.

4. Zoom the lens out to a medium position, probably the equivalent of
70 or 80 mm in a 35mm camera. Put the camera on a tripod and line it up
with the center of the watercolor.

5. Bracket your exposures if you know how to do that. (You may have to
switch the camera to P mode rather than Auto to use exposure
compensation). Check the histogram which is available on your camera to
make sure you are getting a full range of exposure response.

You're fortunate in that your 30 x 22.5 watercolor is an exact match
for the 4:3 sensor dimensions in your camera, so not much cropping
should be required. I presume you are using the LCD display to frame
your picture. The optical viewfinders in Canon Powershot models show
only about 85 per cent of what you are actually shooting.

My wife shot a bunch of pictures of her favorite Impressionist
paintings in the d'Orsay museum in Paris, using a Canon A40. All of
them show barrel distortion, since she was using wide-angle mode. She
doesn't really care; she's just glad to have them in the album


-- Ron

 

I would:

Hang the painting, if I didn't have a copy stand. Use a tripod for the
camera. Align both painting and the camera's back as close as
practical.

Use a medium zoom and mid-aperature settings, not macro.

I'd experiment with lighting and white balance settings. It seems to
me flash might wash out the colors in a watercolor and maybe cause some
reflection.

Use the timer to trip the shutter.
-----

- gpsman

 

.. . .

Definitely, though that doesn't seem to be what her complaint is. If it is
barrel distortion that's the problem, that can usually be corrected in
software.

 

Correcting barrel or pincushion distortion in software is the last resort of
the scoundrel. I don't know why folks put up with such a waste of time. Get
good lenses and quit spending your life sitting on your butt at a computer
'fixing' things that can be done right the first time.

 

Oh please. Now tell us exactly what P&S with some optical zoom is free
from such distortion. Or even which higher end DLSR lens zooms are
completely free of it. Not everyone is using perfect primes.

________________________________________________________
Ed Ruf Lifetime AMA# 344007 ()
http://EdwardGRuf.com

 

You pay one way or the other. Make your choice. It's still just plain nutz
to be spending hours fixing a picture that could have been made once,
properly, in a few minutes.

 

PSP makes corners that are mathematically exact right angles. If your
subject doesn't crop to right angles, then the problem is not with
PSP.

1. Lining up your camera and painting may need to be more precise.
It's not easy to get those suckers exactly parallel. 2. Or (perish the
thought!), your painting may be a lttle off-square. 3. Or, maybe your
less-than-$5000 camera lens is producing a small distortion.

In any case, you might not have to reshoot your subject. PSP9 (which
is the version I'm familiar with) has a Perspective Correction tool
that could be enough to satisfy your needs.

 

What a nonsense.

With the PTLens filter in Photoshop fixing barrel or pincushion
distortion takes a only few seconds. You can even set up an action to
batch correct hundreds of photos.

The only thing you pay is a slight loss in resolution due to the
recalculation of the pixels, but in the era of megapixel cameras with
tons of resolution this is a minor issue.

 

Croping to right angles is a trivial operation.
I think you're trying to say something else. Perhaps a
trapezoid->square transform?

 

Yes. There are two kinds of distortion to be corrected. PSP does
both.

The first is pincushion or barrel distortion caused by the lens.
Before any cropping or other distortion adjustments, correct it
(from the Adjust > Photo Fix menu) to make the edges of the painting
straight.

The second is pure geometry, a perspective shift caused by the
camera's view point being just a little off center from the
painting. Fetch the Perspective Correction tool. Move the corners of
its rectangle to the four corners of the painting (or any other
places that you know should outline a rectangle). Double click and
magic happens.