Size of File When Opened and Saved

[paul_brenner1]

Please forgive a perhaps dumb question:

 

I've noticed that if I have downloaded say a 5 megapixel image from my camera,

open it up, do some tweaking, and save it (at minimum compression/max

quality), it is a much larger file size (up to two times larger). I realize

that tweaking adds some size, so I tried an experiment:

 

I opened a file, saved it with NO changes at minimum compression, and it was

still significantly larger.

 

I had assumed that the file size from the camera was the maximum size,

and "minimum compression" would merely (with no other changes) maintain the

size. But apparently, the file is being uncompressed when opened, and when it

is saved with minimum compression, it is retaining this much larger file size.

 

Am I understanding and have I stated this correctly, or is something else

going on?

 

Thanks,

 

Paul

[bryantan]

The JPEG outputted from most cameras is compressed equivalent to a 10 setting in photoshop. That's why when you resave at 12, it's a larger size. If you sharpen (particularly a portion of smooth sky), the file size will increase as well.

[bobmichaels]

Paul: Don't worry about file size as it is essentially a non-significant number. It is only distantly related to the quality of image data stored there. Storage is cheap enough so you don't have to worry about file size.

 

I suggest you deal with more significant numbers, like how many pixels wide is your image data array and if it is rendered in 8 bit or 16 bit files.

[eyepasha]

Paul,

 

I am not sure if it is clear from the above that the major difference in file size is coming from compressed vs. uncompressed files and that there are various ways to compress that are trade offs between file size and image quality.

[Ed_Ingold]

When you open a JPEG file in Photoshop (or whatever), it is decompressed into what amounts to PSD format. The size you see in the status panel reflects that decompression. If you save the file in PSD or TIFF format, that is about the size of the resulting file. If you save in JPEG format, it will be recompressed but the size may vary from the original. The variation is due to the "quality" level you choose, and that the JPEG compression algorithm is proably different than the original. Each algorithm has it's quirks.

[robert_fisher4]

Bob, that's more than a little misleading. With JPEGs, file size is directly related to quality. A lower file size means a higher compression rate which means lower quality. And if you're working with JPEGs, 16 bit is irrelevant since JPEG is only an 8 bit capable format.

 

If you're working with RAW files, then the RAW file in camera is compressed but it's lossless compression. When you open it up and save it as a TIFF file (or JPEG), the file is uncompressed and you get the full file size.

[paul_brenner1]

All,

 

Thanks! A lot of good info. My original question was pretty basic; I just didn't realize that the file from the camera COULD be uncompressed. I thought it was the maximum size going forward, and could only be compressed (not expanded) from there.

 

Paul