About dimension and resolution.......

Franco68879 · May 16, 2002

I have bought a Canon G2 and I'm really satisfied with it,but I have a doubt I can't resolve.It' s about dimension of the photo and the resolution.

I know my camera can use differents of them,but I can't find an explication of which are the differences between the diferent formats.The use I do with the camera is store the photos on the computer and make video-cd that I watch on Tv.Can someone help me to understan whic is the right difinition and size is best for this?
Thanks a lot and excuse me for the mistakes,I write from Italy

Andrew Harvey · May 16, 2002

Franco,

You should always be taking your photos in the best resolution/lowest compression ratio that the camera supports. You may only be viewing them on a TV at the moment but in the future you may be using something better and would regret having taken your shots at a lower resolution. Photos (JPEG) are compressed when they are saved to memory and the greater the compression the lower the quality - another reason to use the best resolution.

Now for the technical bits (this is just theoretical): Televisions in the UK (and Italy?) have 625 horizontal scan lines on the screen so, in theory, your pictures do not need to have more than 625 pixels vertically, anything greater won't be visible so, full-screen, 800x600 pixels would probably be OK. Computer monitors have a resolution as set on your video card, 1024x768 is typical. Again, in theory, your picture does not need to be any bigger than your monitor resolution.

If the reason for your question is that you want to fit the maximum number of pictures on your CD then I would do the following:- Take the picture in the highest resolution, save these and make copies. Use an image editing programme to reduce the size (dimensions) of the copies and increase the compression ratio. Write these copies to the CD. because of the poor quality of TV pictures you will probably be able to compress the pictures quite a lot before you notice a difference in quality. Try experimenting to see what suits you best.

Ciao

Andy.

Franco said:
I have bought a Canon G2 and I'm really satisfied with it,but I
have a doubt I can't resolve.It' s about dimension of the photo and
the resolution.
I know my camera can use differents of them,but I can't find an
explication of which are the differences between the diferent
formats.The use I do with the camera is store the photos on the
computer and make video-cd that I watch on Tv.Can someone help me
to understan whic is the right difinition and size is best for this?
Thanks a lot and excuse me for the mistakes,I write from Italy

Franco68879 · May 17, 2002

Andrew Harvey said:
Franco,

You should always be taking your photos in the best
resolution/lowest compression ratio that the camera supports. You
may only be viewing them on a TV at the moment but in the future
you may be using something better and would regret having taken
your shots at a lower resolution. Photos (JPEG) are compressed when
they are saved to memory and the greater the compression the lower
the quality - another reason to use the best resolution.
Now for the technical bits (this is just theoretical): Televisions
in the UK (and Italy?) have 625 horizontal scan lines on the screen
so, in theory, your pictures do not need to have more than 625
pixels vertically, anything greater won't be visible so,
full-screen, 800x600 pixels would probably be OK. Computer monitors
have a resolution as set on your video card, 1024x768 is typical.
Again, in theory, your picture does not need to be any bigger than
your monitor resolution.
If the reason for your question is that you want to fit the maximum
number of pictures on your CD then I would do the following:- Take
the picture in the highest resolution, save these and make copies.
Use an image editing programme to reduce the size (dimensions) of
the copies and increase the compression ratio. Write these copies
to the CD. because of the poor quality of TV pictures you will
probably be able to compress the pictures quite a lot before you
notice a difference in quality. Try experimenting to see what suits
you best.

Ciao

Andy.
Dear Andy,

thanks a lot for your answer,it made me understand things that I didn't know.I have one more question for you:I have made several photos of the same subject with the different possibilities my camera can use,but when I control the properties on the pc's video the only difference I see is how big is the file.I mean the number of pixels por inch,doesn't change,and I was sure that more definition is more pixels......................I don't understand.Can you help me? Thanks a lot.
Ciao
Franco

Andrew Harvey · May 17, 2002

Sure I can help:-

More definition does not mean more pixels - more resolution means more pixels.

When you change the quality settings in the camera all you are doing is altering the JPEG compression ratio when that image is saved. This does not alter either the image dimensions or the PPI. The PPI is totally meaningless anyway until you actually print the image.

Let me be more specific; At G2 max resolution your images have the size 2272x1704 (3.8 megapixels), this is the same whether you use the Super Fine, Fine or Normal quality setting. Once the picture is taken it gets written to the media card, this is where the quality setting comes in - the lower the setting, the higher the compression and therefore the smaller the file size. The JPEG algorithm actually throws away some of the pixels to achieve this compression and the quality setting determines how many are thrown away. When you open each file in Photoshop it gets expanded to its original size and to do this the computer has to add back the pixels that were thrown away - the computer has to make an educated guess as to what colour those pixels were but there is enough information in the file to make a good guess. All you will see on screen if you look at the pictures closely side-by-side is a difference in image quality where the computer hasn't quite guessed correctly, the actual image size and PPI will be the same for all.

If you change the camera resolution to a lower setting, say, 1600x1200 (1.9 megapixels) or 1024x768 (0.7 megapixels) then you will have less pixels and you will see a difference in image size on screen but the PPI value will remain the same. See some of my previous posts for an explanation of PPI.

Andy

Franco said:
Franco68879 said:

Andy.
Dear Andy,

Click to expand...

thanks a lot for your answer,it made me understand things that I
didn't know.I have one more question for you:I have made several
photos of the same subject with the different possibilities my
camera can use,but when I control the properties on the pc's video
the only difference I see is how big is the file.I mean the number
of pixels por inch,doesn't change,and I was sure that more
definition is more pixels......................I don't
understand.Can you help me? Thanks a lot.
Ciao
Franco

PatrickS · May 18, 2002

I have follow up question ... Looking at the galleries for the Canon D60, there are two specific images there:

(1) http://img.dpreview.com/gallery/engine.asp?photo=1&gallery=canoneosd60_samples1

(2) http://img.dpreview.com/gallery/engine.asp?photo=3&gallery=canoneosd60_samples1

I downloaded the "Original Large File" for each and loaded into photoshop. I interpret the info from photoshop as saying photo (1) has dimensions 42"x28" at 72 pixels/inch. (2) has dimensions of 17x11 and has 180 pixels/inch.

This does not seem to align with the explanation below. Am I missing something? If not, why would these originals be different?

Thanks,

Patrick

Andrew Harvey said:
More definition does not mean more pixels - more resolution
means more pixels.

When you change the quality settings in the camera all you are
doing is altering the JPEG compression ratio when that image is
saved. This does not alter either the image dimensions or the PPI.
The PPI is totally meaningless anyway until you actually print the
image.
Let me be more specific; At G2 max resolution your images have the
size 2272x1704 (3.8 megapixels), this is the same whether you use
the Super Fine, Fine or Normal quality setting. Once the picture is
taken it gets written to the media card, this is where the quality
setting comes in - the lower the setting, the higher the
compression and therefore the smaller the file size. The JPEG
algorithm actually throws away some of the pixels to achieve this
compression and the quality setting determines how many are thrown
away. When you open each file in Photoshop it gets expanded to its
original size and to do this the computer has to add back the
pixels that were thrown away - the computer has to make an educated
guess as to what colour those pixels were but there is enough
information in the file to make a good guess. All you will see on
screen if you look at the pictures closely side-by-side is a
difference in image quality where the computer hasn't quite guessed
correctly, the actual image size and PPI will be the same for all.
If you change the camera resolution to a lower setting, say,
1600x1200 (1.9 megapixels) or 1024x768 (0.7 megapixels) then you
will have less pixels and you will see a difference in image size
on screen but the PPI value will remain the same. See some of my
previous posts for an explanation of PPI.

Andy

Franco said:

Andrew Harvey said:

Andy.
Dear Andy,

Click to expand...

thanks a lot for your answer,it made me understand things that I
didn't know.I have one more question for you:I have made several
photos of the same subject with the different possibilities my
camera can use,but when I control the properties on the pc's video
the only difference I see is how big is the file.I mean the number
of pixels por inch,doesn't change,and I was sure that more
definition is more pixels......................I don't
understand.Can you help me? Thanks a lot.
Ciao
Franco

Click to expand...

Andrew Harvey · May 18, 2002

Hi Patrick,

Yes you are missing something... quite simple mathematics. If you take an image at 72 ppi and increase the ppi value to 180 but keep the image pixel dimensions the same you will decrease the PRINTED image size. Remember that the PPI value is only important to the printed image size. Since this value is irrelevant until printing most digital cameras set the ppi to a nominal 72 or sometimes 144. Whoever edited image (2) must have changed the ppi themselves.

Andy.

PatrickS said:
I have follow up question ...

I interpret the info from photoshop as saying photo (1)

PatrickS said:
has dimensions 42"x28" at 72 pixels/inch. (2) has dimensions of
17x11 and has 180 pixels/inch.

This does not seem to align with the explanation below. Am I
missing something? If not, why would these originals be different?

Thanks,

Patrick

PatrickS · May 19, 2002

Thanks for clarifying.
P.

Andrew Harvey said:
Yes you are missing something... quite simple mathematics. If you
take an image at 72 ppi and increase the ppi value to 180 but keep
the image pixel dimensions the same you will decrease the PRINTED
image size. Remember that the PPI value is only important to the
printed image size. Since this value is irrelevant until printing
most digital cameras set the ppi to a nominal 72 or sometimes 144.
Whoever edited image (2) must have changed the ppi themselves.

Andy.

PatrickS said:

I have follow up question ...

Click to expand...

I interpret the info from photoshop as saying photo (1)

Andrew Harvey said:

has dimensions 42"x28" at 72 pixels/inch. (2) has dimensions of
17x11 and has 180 pixels/inch.

This does not seem to align with the explanation below. Am I
missing something? If not, why would these originals be different?

Thanks,

Patrick

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