Digital file sizes

I apologise if this is an elementary question but new to digital I am

trying to get some points of comparison with film. If I scan one of

my B&W 35mm negatives at 3200 dpi I get a file size of 11.5mb. What

would the file size be of a RAW file from say a Leica Digilux 2 5mp

camera ?

RAW= little processing, possibly some slight reversable compression, but a RAW file from a 5 MP camera should be pretty darn close to 5MP.

 

Scanning film negatives at high PPI creates much bigger files than a RAW file from a 5 MP camera, which isn't necessarily better.

Raw files typically contain 1 byte per pixel, sometimes as much as 1.5. Count on 5MB to 7.5MB.

 

I wouldn't think about comparing "quality" in terms of file sizes. In my experience (others may disagree) for the same file size a picture from a digital camera contains more usable information that a film scan.

The real qualitative difference is that the digital file from the camera is the original while

the scan of the negative is a second generation - -and how good that second generation

version is dependent on the mechanical quality of the scanner and the skill of the scanner

user.

 

the size of the A "raw" file from a camera isn't what you want to look at anyway -- it is the

size of the processed image when opened as a 16 bit per channel (AKA 48 bit) TIFF or PSD

file. if you want to compare sizes at this point , make sure you are also looking at a 16 bit

per channel scan as well.

 

My experience is that conservatively a scanned image needs to be about three times the

size of an image made with a digital camera to make a more direct comparison in image

quality at a given print size-- which leads back to how well the scan was made in the first

place

The raw file has 8 or 12 bits per pixel. The Bayer sensor records intensity for each pixel and color for each group of 4 pixels. The conversion process from raw to grey scale tiff generates a file of the same size. The Digilux 2 has a sensor of 2560 x 1920 which would yeild a tiff file of 7.4 MB uncompressed at 12 bits/greyscale and 4.9 MB uncompressed at 8 bits/greyscale. In saving a tiff file, most software has the option to compress. I think your files are compressed or the scan is from the center of the frame. At 3200 dpi a 36 mm x 24 mm grey scale scan would produce 4535 x 3023 pixel image which would be 13.7 MB at 8 bits or 20.5 MB at 12 bits.

 

The quality of the image has more to do with the sensor diminentions and technology than the pixel count. The 2/3" 8.8 mm x 6.6 that is 18 months old in the Digilux 2 is an issue. If you want a rangefinder camera, the Epson R-D1 with it's 23.7 mm x 15.6 mm sensor might be a better choice.

It's not elementary, but it's not a complete mystery either.

 

3200 dpi stands for dots per inch. You can muliply the length by width to determine the total number of dots your scan will end up with. Additionally, each dot (pixel) has a red, green, and blue component. If the image is scanned at 8 bits per channel, each pixel represents 24 bits or 8 bytes. A 16 bit scan then is 48 bits or 16 bytes per pixel. From that, you can calculate the exact file size. (8 bits = 1 byte, 1024 bytes = 1 KB, 1024 KB = 1 MB)

 

Where this gets tricky is with current digital cameras. Current sensors can record luminosity only and can't distinguish color. The solution to this is to put tiny red, green, and blue filters over each sensor in an alternating mosaic pattern. Thus, in a camera's raw file, there is 8 or 16 bits of data for each pixel (instead of 24 or 48); software on the computer assembles these and compares neighbor pixels to guess at each pixels missing intensities in red, green, and blue. It's not perfect, but it works surprisingly well.

 

Back to the original question, a 5 mp camera does indeed have 5 million pixels, but each encompases only one channel. Also, the raw file may be compressed to save space, but this have absolutely no effect on the image quality (called lossless compression). When you open the image in photoshop, it's interpolated into a 24 or 48 bit file depending on the camera and settings.

 

The larger question of which has better quality is much more complicated, and the debate continues.

Joe - I recall sseeing somewhere that the Epson RD-1 doesn't focus exactly with Leica lenses which markedly affects the image quality. I can't remember where I saw this but there was a follow up from someone who said it explaned a problem he was having.