Re: DPI
- From: Nospam <nospam@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 1 May 2007 20:39:35 +0100
In message <I6GdnWlniIkP1avbnZ2dnUVZ_sSmnZ2d@xxxxxxxxxxxx>, DBLEXPOSURE
<celstuff@xxxxxxxxxxx> writes
Patrick Ziegler ImageQuest Photography
"Nospam" <nospam@xxxxxxxxxxxxxxxxxx> wrote in message
news:w4tsYrCY3hNGFw+C@xxxxxxxxxxxxxxxxxxxxx
Could some one tell me in simple language or point me to a web site to
answer the following question.
I have read that cameras work on 72 dpi
so how do u print to 300 dpi?
Not sure about the technical side yet.
Maybe I'm mixing up the wrong things!
thanks
--
Nospam
OMG not another DPI thread...
Just kidding, here is the answer I posted in a thread above.
It is really simple so all you folks who are scratching your head, pay
attention.
The pixel originates in your camera. If you have an 8MP camera then guess
what? There are roughly eight million Pixels on the sensor in your camera.
Those pixels are laid out in a matrix, so many wide by so many tall.
I'll use the ever-popular EOS 400D as an example. It sensor is 3888 x 2592
pixels.
Guess what? 3888 x 2592 = 10,077,696 or 10.1 million.
SO now we have this image we have captured with our 10MP camera and it is
3888 x 2593. It's a big file.
A typical 20inch monitor might have 1600 x 1200pixels, (Native resolution).
Now wait, how am I going to fit 3888 pixels across a screen that is only
1600 pixels wide?
When display that large file on that relatively small screen, whatever
software you are using to display the image will re-sample it and squish it
down to a manageable size to display.
In Photoshop and other programs, this can lead to an image that has a bit of
aliasing and other artifacts do to an algorithm that is designed to be fast.
But no worries, the algorithms has not changed the file, it has only
manipulated the file existing in RAM.
Okay, so we still have our large image and we want to keep that image if we
intend on printing it. Why? You ask.
Because the more pixels we have the larger we can print and retain all the
quality and detail that was captured by the camera.
300PPI, sometimes written 300DPI, Yes, people do use the wrong terminology
from time to time but lets face it, a pixel is a dot, is the highest
resolution discernable by the human eye. 300 is used as a reference point
when we talk about making prints of our images.
If we print our 3888 pixel wide image at 300ppi it will be nearly 13inches
wide. 3888pixels/300pixels per inch = 12.96
But wait, I need it to be 15inches wide. No problem, it will then print at
259ppi
3888 / 15 = 259
3rd grade math.
How low can we go? It is up to you and the size of your printer. If you
have a photo of a black square on a white wall, you can print it at 80ppi
and it will still have sharp edges and nice corners.
Now, if you shoot a wedding and print the brides portrait at 80ppi your
probably going to end up in court. Detail and the viewer's eye for quality
determine the threshold. But remember, nobody can see better than 300ppi.
Okay, but I am not going to print this photo I only want to use it on my Web
site. Re-size it in Photoshop. Image>image size>resample your screen
resolution and the size you want it to appear on that screen will determine
what pixel dimensions you choose. I typically make mine either 800wide or
600tall depending on if they are horizontal or vertical compositions.
But wait, my new printer says in has a Max Resolution of 4800 X 2400DPI you
said nobody can see better than 300, what is up with that?
Totally different spec, somewhat related but different.
Long story short, This is what I tell people so I don't have to get that
deer in the headlights look, Printer resolution is how many dots of ink the
printer uses to your image's pixels. So, if we print at 300ppi on this new
printer at 4800dpi we will get 16 dots of ink in ever pixel that was
captured by the camera. This means better tonal quality not necessarily
sharper image.
4800 / 300 = 16 more 3rd grade math.
That's it, long winded but not overly complicated.
PZ
www.imagequest.ifp3.com
Thanks for that info, easy to follow, explained well - if only I could
get the process of photosynthesis over to my year 9 student, very
helpful info.
Question
So if my printer is printing 16 dots of ink per pixel.
Well this seams a lot of dots of ink to a pixel, the dots of ink must be
very small.
What number of dots per inch can the eye resolve? I'm not sure if this
is actually a sensible question as I think it is linked to the original
resolution.
I have to admit to being a digital freak when it computers, DVD etc.,
love the stuff.
But when it comes to photography I am still trying to hang onto my
'analogue' equipment, I love my T90 and my darkroom with all its
chemicals etc..
But...
while on holiday I did, test the digital waters so to speak, and bought
an Olympus mju 7600.
Just a small compact happy snappy unit, able to take with me anywhere I
go. Produces some nice pics but I have a lot to learn about it yet, only
had it 3 weeks.
I know my T90 and A1 inside out.
So a lot to learn about the 7600 and digital photography yet.
But I can see lots of advantages with this new fangle technology.
I may move to a DSLR later. But for now still lots to get my head
around.
Now just for fun,, and to clarify my understanding a bit...
The spec of my camera says it has a ccd sensor size of
1/2.33" CCD (primary colour filter), 7,380,000 (gross)
I assume that means the diagonal is 1/2.33" = 0.429"
so this gives an aspect ratio of 3072 × 2304 (1.3333 : 1)
so I think this must mean that the CCD sensor is
about 8.72 mm X 6.54 mm approx. (25.4 mm to the inch)
the camera spec say;
SHQ 3072 × 2304 Low compression
HQ 3072 × 2304 Standard compression
(and some other I have not bothered with why have
7MP and not use them xD cards are cheap now.)
I have been using the SHQ mode only.
so that mean I have 3072 pixels over 8.72 mm so that means
I have a CCD sensor resolution of
3072/8.72 * 25.4 = 8948 ppi
if I'm right, but it does sound a bit high!
I realise this is sensor resolution and not image or printing
resolution.
BTW I have had a look at your web site, nice photos, the quality is
nearly as good as 'real' photography ;-) I love my T90, all that money
now obsolete, boo hoo.
--
Nospam
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