Canon EOS XTI compression?

I'm shooting highest quality setting "L" with the Canon Digital Rebel XTI
using a Sandisk Ultra II card. When editing the dpi is always 72 dpi. When
I use my Nikon D50, the dpi is 300 dpi. I usually edit my photos to 800x?
and 72 dpi. But.....the question is: is Canon doing a compression to
72dpi???

 

Hi!
Did you notice the size of the photos?
Yes you see them at 72 pixels per inch, but the size is much greater than
what one thinks he might need.
To illustrate my point, my Canon EOS D30 shoots photos at 14,6 x 9,733
inches @ 240 PPI.
This translates into 6 x 4 inches @ 584 pixels per inch.
Yours must show something like this: 48,6618 inches x 32,440 inches @ 72
PPI.
This translates into 6 x 4 inches @ 584 pixels per inch. Mind you, my Canon
is only 8MP. Since yours is 10 MP, it might even show larger initial
numbers, but the math is still the same.
Take care,
Marcel.

 

As long as the photo remains in electronic form the dpi value is
meaningless. It only becomes relevant when actually printing a photo on some
media and it is set to the appropriate value at that time.

For technical reasons cameras have to put something in that field and
traditionally that is a value of 72. Apparently Nikon decided on 300, but
they could just as well have picked 42 or any other number.

jue

 

The size is 3888x2592

 

So that's the size then. DPI is meaningless while the file is still
digital.

Greg

 

On what devise? A black&white newspaper print? An old dot-matrix printer?
72DPI is about the resolution of those.

A sensor 14.6x9.7 inches large? How do they fit such a beast into a handhold
DSLR?
A 30D actually has a sensor size of 22.5 x 15.0mm with 3504 x 2336 pixels.
That translates to a resolution of about 155 pixel/mm or roughly 3900
samples per inch.

However even that is irrelevant in the context of this discussion because
PPI and DPI are two totally different things.

Whatever you are trying to explain here, it has no relevance for the DPI
used when such a photo is printed.
Please see http://en.wikipedia.org/wiki/Dots_per_inch for a reasonably good
explanation of what DPI is and how it is different from PPI (which usually
can be significantly lower than DPI).

And neither has any relevance for taking a photo. As long a the photo is not
displayed in some way, either on a specific monitor or printed on a specific
printer, the terms PPI and DPI are meaningless, because both relate to the
resolution of a specific physical devise.

The only relevant number when taking a photo is the total size in pixels,
i.e. 3504 x 2336 for the 30D. Anything else depends onthe output/display
device.

jue

 

This size Image will yield a print size of:

12.96 x 8.64" at 300dpi and
54 x 36" at 72 dpi whether the photo is taken with a Canon, Nikon,
Pentax or any other camera.

The dpi reported by the software or exif data is meaningless as
applied to the electronic original. This reporting is based on the
anticipated resolution of the viewing device. 72 dpi is/was the
approximate resolution of a computer monitor (roughly a 15" monitor at
1024 x 768 resolution - my 19" monitor at 1600 x 1200 resolution
yields about 114 dpi). 300 dpi is considered to be the minimum
printed resolution for photo quality prints (and that is debatable).

This dpi reporting is a holdover from the early days that should be
eliminated from all software. The ONLY thing it does is confuse new
users.

My $.02 worth
Bill

 

Actually it will print with 300 resp. 72 _pixel_ per inch. Neither inkjet
nor laser printers can change the lightness (= intensity) or the hue of the
color pigments, therefore they msut use several dots to print a single
pixel.

There is one application where DPI is meaningful: when scanning a document
you want to now the resolution of the scan, such that you can reproduce the
document at a given magnification, e.g. 1:1. However even in that case it
should better be called samples per inch.

Again, that is pixels per inch.
DPI is really a property of the printer and modern printers have a much
higher dots per inch count.

I whole-heartily agree with that.

jue

 

Inkjet will resolve 1 dot of a colour at 300 dpi. The "several" dots at
300dpi per dot is "many" dots (perhaps hundreds?). They have variable
dot size (typically same nozzle, different signal amplitude / wavelength
with Epson Piezo, same nozzle with several different size heating
elements with Canon thermal, different sized individual nozzles with HP
thermal). Ink coverage density will affect lightness - pigments are
not opaque.

 

Following that idea, cameras could label the dpi measured at the sensor:
more like 4,000dpi DSLRs & 8,000dpi compact cameras. Than we could tell
if it's a *real* 1:1 macro