Working with large files...

[Ron Wilson]

Ok I am new at this but I learn quick. My Rebel XT has a learning curve without a doubt but I also realized my PC and PC skills may need to be upgraded as well. I don't want to get into PC specifics but I would like to know what you people have found as a useful routine in going from a large file (large for me is between 3-5MB) to something smaller and more manageable. Most of my shots are going to be seen on the PC, uploaded to a forum, printed at home in 8x10 format and lastly inserted into a Video CD.
When I uploaded my first 10 shots last night with my new Rebel XT, I was aghast by the size. Even when imported into PS 5.0 it was larger than I was used to working with, image size was 42" by 33" or thereabouts. I decreased the image size right away down to 8.5 x 11. Was this stupid? Am I confusing file size with image size? Help. Thx in advance.:oops:

[etaf]

yes you are confusing here
The resolution is all about pixels and how many you have
The screen will display them at around 72pixels per inch PPI

I have been used to using Photoshop and edit files of arond 25MB {.tiff} as these I want to print at A4 size and had been scanned in from a Nikon 35mm scanner.
They printed out great at A4 size 300PPI

so you need a few files to do what you want to do
printing 8" x 10" @ the normal recommend 300PPI {this varies and loads of advice here}

then 300 Pixels per inch = 300 x 8 = 2400 300 x 10 = 3000
so you need an image 2400 x 3000 pixels

to upload to a forum then you need a lot less pixels
I carried out a survey on another forum to see what screen size most people have {some years ago it was something like 45% @ 800 x 600 and 45% @ 1024x768} the majority was 1024 in that survey
so to display on a screen change the pixels to about 900 if landscape or 700 if portrait.

much smaller file size

hope that makes sense

resolution
http://www.ltlimagery.com/resolution.html
http://www.blaha.net/Main%20Picture%20Resolution.htm
http://www.scantips.com/basics03.html


for PC's
Then thats a different story on memory, swap file size etc etc
but to help there it would be useful to know current PC specs - software etc

[Poco]

Just, whatever you do, don't overwrite the original files just because they are too big. Always keep the originals. Disk space is cheap (compared to your camera), but you can never retrieve the data that is lost when you convert or resize your photos just to make them smaller. If you want to make changes then save them as a new file.

When it comes to size, the simplest thing (this is the system I use) is to, as suggested above, use the pixels when dealing with screen output (online) and keep everything as large as possible when dealing with prints.

[Ron Wilson]

I understand about different sized pixels for different purposes. What does this do to my file size? Does it make it smaller, ie. I have less pixels now. How is image size, on my PC effected? Do I lose resolution? Is this the same as detail? I have seen amazing pics on websites that are no greater than 100K.

[Ron Wilson]

TY Poco. I just followed your link to a shot of a little Goslings on the beach in North Vancouver. It is a very pretty shot. The properties are 33k file size 408x275 pixels. Would the shot "look better" at 70K? or 600x 450 pixels? Which factor determines the answer? Are the two factors mathematically linked?

[etaf]

yes less pixels = small file size

do you have photoshop at all

File size is related to the number of pixels and the bit depth

lets assume 8 bit per channel (RGB = 24bit)
http://www.luminous-landscape.com/tutorials/bit-depth.shtml

Tiff is a lossless format - so every pixel is saved to the file.

Now the file size will be how many RGB pixels you can save

4050 x 2650 pixels is a 10.7 megabyte resolution
and will create a file size of about 31.7MB
and @ 300 DPI / PPI will produce a print 8.8 x 13.5 inches

http://www.luminous-landscape.com/tutorials/understanding-series/und_resolution.shtml

This is a good site with tables - showing filesize and resolution
http://www.ltlimagery.com/resolution.html

If you have photoshop - you can play with image size and see the filesize.

D1H has a max res of 2012 x 1324 file size 7.6Mb
DH2 I think is that the max resolution is 2484 x 1636 - if this is correct than the filesize would be around 11.5Mb

approx
Take the horizontal pixels and multiply by vertical pixels and multiply by 3
total pixels x 3 = filesize for a 8 bit depth per channel (24bit image)
http://www.phototechmag.com/previou...-mj/sharma.html
RGB only has 3 channels
so 35MB divided by 3 approx 12mB per channel
or CYMK diveided by 4 approx 9MB per channel
Hope this helps

then theres compression and JPG files using a compression technique to reduce the filesize
__________________

[Poco]

TY Poco. I just followed your link to a shot of a little Goslings on the beach in North Vancouver. It is a very pretty shot. The properties are 33k file size 408x275 pixels. Would the shot "look better" at 70K? or 600x 450 pixels? Which factor determines the answer? Are the two factors mathematically linked?

If you click on that link you will be taken to a higher res (and larger file).

The math of how big an uncompressed file is given already so I won't repeat that. When it comes to JPG files it is also about the compression. JPG uses a lossy compression algorithm. They work by essentially removing information that is harder to see. The more you compress the file the smaller it gets but the more noticeable the difference.

So the size is affected by the pixel dimensions (408x275) and the amount of compression (usually seen as a quality slider when you save a JPG in PS). If you reduce the dimensions you will get a smaller file because of the math discussed in the previous post (widthxheightxbpp). If you reduce the quality of the file with the same dimensions then the file size will also reduce.

In general (not always) the larger the file the higher the quality. It is either taller, wider, or less compressed. Either way, there is likely more information in the file.

There is also such a thing as lossless compression (Which I believe that the Canon RAW uses) to keep the file size smaller while not losing any quality.

I'm sure there is a good example to use from the physical world, but I can't think of one right now.

Kaz