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Pixels v Jpeg File Size v Print Size??

 
 
PeterH
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      01-18-2004
New to digital photography and need to get it in my mind how pixels, file
size and print size work.

I have a Canon 300D (6MP) and prefer to take all shots at maximum resolution
settings.

Example: I take a maximum resolution photo producing a 3MB jpeg file. Can I
get a 6"x4" print using the full resolution of the 3MB file or does a 6"x4"
only print at say 200K?

In other words, does a print of any physical size only have a certain number
of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
200KB file and the remaining 2.8MB is wasted or can the full 3MB be used to
print a better detailed 6x4?

regards

PeterH


 
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Trev
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      01-18-2004

"PeterH" <reply to http://www.velocityreviews.com/forums/(E-Mail Removed)> wrote in message
news:F0kOb.16611$(E-Mail Removed)...
> New to digital photography and need to get it in my mind how pixels,

file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum

resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file.

Can I
> get a 6"x4" print using the full resolution of the 3MB file or does a

6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain

number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say

a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be

used to
> print a better detailed 6x4?
>
> regards
>
> PeterH
>


Where to start.
Digital images are measured in Pixels, width "n" X height "n" will give
the total in your case in mega pixels.
To set a print size we pack those pixels together to a Sq inch to
produce the ppi and a dimension in inches
So 6 x 4 " at 300 ppi = 6 x 300=1800 pixels By 4 x 300 = 1200 pixels
If you had picked 250 ppi there would have been less pixels and 400 ppi
would have needed more pixels to produce the same size print.
as You will have much more pixels then that you would need a higher ppi
to get it to 6 x 4 inch. And as the aspect ratio is deferent you will
need to crop a bit off.

I have not mentioned file size as the makes no difference to printing.
Each Pixel as 8 bits of red and 8 bits green plus 8 bits blue adding up
to 24 bits per pixel multiply that by the amount of pixels width and
multiply by height and you get the file size.
Jpeg is a loosey file format the examines you image in blokes of 16 sq
pixels 4 across and 4 down. it looks for similarities in the pixel makes
a note of there position saves one as a reference and discards the rest.
depending on your settings just how close the similarity has to be to be
saved. This will make the file size smaller for storage or
transportation be cause the are less of those 24 bit pixels. When that
file is opened, those missing pixels a recreated based on the saved one
and notes as to the location of where similar ones should be placed.
Its good but not perfect.


 
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Tom Thackrey
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      01-18-2004

On 17-Jan-2004, "PeterH" <reply to (E-Mail Removed)> wrote:

> New to digital photography and need to get it in my mind how pixels, file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum
> resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file. Can
> I
> get a 6"x4" print using the full resolution of the 3MB file or does a
> 6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain
> number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be used
> to
> print a better detailed 6x4?


Jpeg files are compressed so their file size has almost no relationship to
anything except that if you know the uncompressed file size you can compare
it to the compressed file size and determine the compression ratio.

When people (who know what they're talking about) compare image quality in
terms of file size, they are referring to the uncompressed file size. In
general, the uncompressed file size will be the dimension in pixels times 3
because it takes 3 bytes to hold the color information for one pixel (in
8bit/color RGB). A 3000x2000 pixel image will be an 18 MB file
(3000x2000x3).

Pixel dimensions are the issue. Depending on the printer, somewhere between
about 180 and 350 pixels per inch (PPI) will produce the best print that
printer is capable of printing. So, if you are printing a 4x6 inch image on
a printer that does its best work at 300ppi, you will need at least a
1200x1600 pixel image (4*300 x 6*300). That image would also be about a 6MB
uncompressed file. Note that the DPI printed on the outside of the printer
box is meaningless. Printers use multiple dots to make a pixel, so 1440 DPI
has almost no relationship to the PPI needed for a good print on that
printer.

--
Tom Thackrey
www.creative-light.com
tom (at) creative (dash) light (dot) com
do NOT send email to (E-Mail Removed) (it's reserved for spammers)
 
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Jeffrey
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      01-18-2004

"PeterH" <reply to (E-Mail Removed)> wrote in message
news:F0kOb.16611$(E-Mail Removed)...
> New to digital photography and need to get it in my mind how pixels, file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum

resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file. Can

I
> get a 6"x4" print using the full resolution of the 3MB file or does a

6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain

number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be used

to
> print a better detailed 6x4?
>
> regards
>
> PeterH
>
>


It really depends upon the maxixum effective resolution your printer is
capable of producing (not to be confused about how many dots per inch). Old
style 3 or 4 colour inject printers, had to do a lot of dithering (ie mixing
a lot of different colour dots in patterns) to simulate different colours,
so you needed a lot of dots to make an effective pixel. These printers may
have only acheived say 72 effective dots/pixels per inch.

Newer printeres these days, have more colours (HP7960 has , have variable
size drops, and shoot more than colour on the same dot. These printers can
therefore have a much greater effective resoluton, probably greather than
300 but less han 600 effective dpi.

Therefore, if you intending to print your 6x4's on an older style printer,
you would only need

6in x 72 dpi *4in *72dpi * 3bytes (24bit colour) =

432 x 288 x 3 = 373248 bytes (say 200K JPEG with compression).

However, with a new printer (lets say 300 effective dpi) you will need

6in x 300 dpi *4in * 300 dpi * 3bytes (24bit colour) =
1800 x 1200 x 3 = 6,480,000 bytes.

Compared to the max resultion of your camera

3072 x 2048 x 3 = 18,874,368 bytes.

I also have a 300D, and after obtaining that camera, I bought the HP7260 for
my photoprinting, because my old Epson 760 was no longer good enough (but I
still use it for text printing).

When I am print 6x4 prints, I take advantage of the extra resolution, to
crop the photo, so that I only print out that part of the photo that I want
to print. I still give up a fair bit of the resolution available to me, but
I still produces a great result.

Jeffrey

PS. I have unfortunately not been able to use the cropping feature in
Photoshop Elements Print Preview, to do the cropping.



 
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Jim Townsend
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Posts: n/a
 
      01-18-2004
PeterH wrote:

> New to digital photography and need to get it in my mind how pixels, file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file. Can I
> get a 6"x4" print using the full resolution of the 3MB file or does a 6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be used to
> print a better detailed 6x4?


First:

Pixels and bytes are two different things. Pixels are picture elements
that were captured by your camera. As a mater of fact that's where the
term 'pixel' came from.. it's short for "picture element"

Bytes are what computers use to represent data. The only thing they
have in common is they use the same prefix, 'mega'. That designates
a million. A million volts is a megavolt.. But there are no
megapixels in a megavolt.

Music files are made of bytes, video files are made of bytes. Program
files are made of bytes. If you look on your hard drive, you'll see
*all* your files are made of bytes. Almost all these files have
megabytes but they have NO pixels.

The number of bytes required to represent a pixel varies depending on
the complexity of the pixel and whether it's color or not. For 8 bit
color files, you need three bytes to represent the data in one pixel.
So.. A 6MP file WILL equal 18 Megabytes.

But your JPEG images are only 3 megabytes !! Yes.. JPEG is handy
because it compresses the image data. This saves lots of space
on your compact flash card and on your hard drive.

Note that a 3 Megabyte JPEG still has 18 Megabytes worth of data..
It's only compressed when you save it to disk.. When you open a
JPEG file, it is uncompressed and exists as an 18MB file in your
computer's memory... If you save your 3 Meg JPEG files as
uncompressed TIFF files, you'll see they will be a full
18 Megabytes..

For printing.. I think this is the most misunderstood aspect of
digital photography there is..

Consider you have an image. It has pixels in two dimensions..
That's width and height. Megapixels are derived by multiplying
these two numbers together..

In the case of your camera, the image is 3072 wide by 2048 high.
If you multiply, 3072 x 2048 = 6.29 Million. Or 6.29 megapixels.

If you print your image 14 inches wide, then you're spreading
3072 pixels across 14 inches of paper. In doing this, you wind
up with 219 pixels in every inch of the image.. Or.. 219 Pixels
per inch. PPI.. Sometimes called DPI.. (Dots per inch).

If you print the 3072 pixel wide image 6 inches wide.. Then you
have 6 / 3072 = 512 Pixels spread out over each inch, or 512
pixels per inch.

That's *all* pixels per inch are.. The physical dimensions of
the image printed on paper, divided by the pixels in your image.
Pretty simple, but again.. It baffles many.

The more pixels per inch you have, the better your printed image
looks.. Makes sense.. There's more data.

But you say your camera or software says the image is 180 pixels
per inch (or some other value) !!!

Yes.. You can tag whatever PPI or DPI value you want on an image
file. This value has NO effect on the pixels or how they are
arranged or the quality of the image.

When you see a value of 'xxx' pixels per inch, you're looking at
the *proposed* size the image will be *when* it is printed on PAPER..

So if you see your editor showing your 3072 pixel wide image with a
value of 72 pixels per inch, then that means if you don't change
anything, it will print at 3072/72= 42 inches wide.. But again, if
you don't want it that big, just change the pixels per inch. If
you change it to 512 pixels per inch, then the image will print at
6 inches wide..






 
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PeterH
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Posts: n/a
 
      01-18-2004
Thanks to all who replied - all excellent answers.

I will read your posts a couple of times to be sure I have it right, but
it's making a lot more sense now.

Regards

PeterH


"PeterH" <reply to (E-Mail Removed)> wrote in message
news:F0kOb.16611$(E-Mail Removed)...
> New to digital photography and need to get it in my mind how pixels, file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum

resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file. Can

I
> get a 6"x4" print using the full resolution of the 3MB file or does a

6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain

number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be used

to
> print a better detailed 6x4?
>
> regards
>
> PeterH
>
>



 
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Don Stauffer
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Posts: n/a
 
      01-18-2004
The image inherently has a given number of pixels, regardless of whether
you print it or not. Once you decide to print, however, the pixels per
inch will be inverse to the size you print it. If you print it 4 x 5,
you will have more ppi than if you print it 8 x 10 (twice as many, in
fact).

PeterH wrote:
>
> New to digital photography and need to get it in my mind how pixels, file
> size and print size work.
>
> I have a Canon 300D (6MP) and prefer to take all shots at maximum resolution
> settings.
>
> Example: I take a maximum resolution photo producing a 3MB jpeg file. Can I
> get a 6"x4" print using the full resolution of the 3MB file or does a 6"x4"
> only print at say 200K?
>
> In other words, does a print of any physical size only have a certain number
> of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
> 200KB file and the remaining 2.8MB is wasted or can the full 3MB be used to
> print a better detailed 6x4?
>
> regards
>
> PeterH


--
Don Stauffer in Minnesota
(E-Mail Removed)
webpage- http://www.usfamily.net/web/stauffer
 
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