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Math for converting MP to MB ?


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is there a simple arithmetic formula for doing a quick calculation of

a cameras MP to the MB file size when opened in Photoshop ?

 

If you're shooting maximum resolution RAW files with a 11 MP 1DS, or

17 MP with a 1DS Mk II, how do you figure the size when the file will

be opened in PS?

 

Or for that matter, if you're shooting finest quality Jpeg images ?

 

thanks

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regarding RAW...on my 20D, which is 8MP, I can get anywhere from 6 to almost 12 MB files. As the above commenter indicated it depends on the amount of information that is in the scene. Now if you're talking about the conversion of the original RAW file, whatever size it happens to be, to TIFF..........I'm gonna guess that any RAW editting you do would change that also, plus the conversion to RGB, so I don't think there is a canned formula for that either, but I might be wrong on that account. I just know that I haven't seen a definitive correlation between RAW file size and TIFF file size, but haven't really worried about it either. I'm used to 50MG TIFF files from scanning film, the final TIFF of a RAW (20D) digital isn't past that, so It's a no nevermind for me.
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If you are talking about the uncompressed size (and it is uncompressed when working in Photoshop, whether it started as a jpeg or a tiff or whatever) without masks, extra layers etc then how about the approximate size in MB being:

 

number of MP x 3 (for the three colour channels) x 1 or 2 (1 for 8 b per pixel, 2 for anything between 10 b and 16 b per pixel because PS only works in 8 or 16 bits per pixel, 8 b = 1 B)

 

Best, Helen

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So....Helen and Mag's formulae are almost the same then - x2.87 for Mag and x3 for Helen (and double that again for 16 bit). Makes sense...everyone agree ?

 

I expect that as Thomas points out, the aount of detail/data info in the image will also affect the file size.

 

Also, the conversion from RAW software (Adobe, Phase I, etc) will probably have an effect on file size too I expect - correct ?

 

When you convert from a RAW file do you always have the option of saving an 8 or 16MB Tiff (or PSD) file ?

 

And when you convert from Jpeg is it always 8 bit?

 

Using the abovementioned math,would the following be acceptable "rules of thumb":

5MP camera = 15MB (8bit),

6MP = 18MB (8bit),

8MP =24MB (8bit),

11MP =33MB (8bit),

16MP = 48MB (8bit),

 

Tons of questions here I know - grappling to get a handle on it.

 

thanks for your patience

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In images; a pixel can be many different things ; ie 1 bit (black or white) ; 4 bits; 1 nibble; ancient graey scale; 8 bits greyscale ; 8 bits RGB in color; 8 bits CMYK ie color; 16 bits RGB ; 16 bits CMYK; etc etc. Unless you know what your pixel is; there is no conversion. A typical image might be 8 bit RGB; each pixel is thus 6 nibbles; ie 3 bytes. Here a 1 MP image is 3 megabytes.
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Frank,

 

The difference between Mag's number and mine is fairly simple: I'm converting MP to MB and Mag is converting pixels to MB. 'Mega' is not 1 000 000, it is 1 048 576. A 1000 x 1000 image in PS results in a 2.87 MB file, a 1024 x 1024 image results in a 3 MB file (if RGB, 8 bits per pixel per channel).

 

In Photoshop it doesn't actually matter how much image detail there is. There is a certain number of pixels and a certain number of bytes (three or six in RGB 8 bit and 16 bit respectively) for each pixel, and that is it, apart from any embedded information or extra layers etc.

 

Best, Helen

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3 bytes per pixel in RGB with 8 bits per component, 6 bytes per pixel in RGB with 16 bits per component. Use your local definition for MB (typically 1048576 for RAM, 1000000 for hard drives, and I've seen 1024000 and 1073742) and MP (typically between about 980000 and 1010000).

 

So, as you can see, it's not a precise science (between us computer guys counting in binary and marketing guys inflating figures).

 

Count 3MB per MP in 8-bit (JPEG), 6MB per MP in 16-bit (raw), and you'll be within a few percent.

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People can give you rules of thumb based on experience, but ultimately there is no precise answer regarding how big a RAW or PSD file from a certain camera will be on disk, because RAW and PSD files use (lossless) compression to reduce file size. Different files respond differently to compression, so shots of different scenes end up being different sizes.

 

For JPEG, the compression is lossy. This lets the file size be even smaller, but image data is actually discarded in the compression process. The "fineness" setting controls how aggressive the compression is. The smaller you want the file to be, the more fine detail suffers.

 

Helen quite rightly points out that the size of the uncompressed image data is a simple matter of multiplication. It's important to realize that Helen is not telling you how to calculate the file size on disk, since, again, it has compression applied.

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One last bit of pedantic-ness: the confusion around what "mega" (and "kilo") means stems from the fact that unfortunately the prefix can be used in two different ways.

 

In computer science, powers of two are useful, and "kilo" usually means 2^10 = 1,024, and "mega" usually means 2^20 = 1,048,576. However, this conflicts with the metric / SI meaning of these prefixes, which are based on powers of 10 (kilo = 10^3 = 1,000 and mega = 10^6 = 1,000,000).

 

Almost everything to do with computers uses the powers-of-two defition. Almost everything else uses the usual SI definitions based on powers of 10. The exception is storage, because storage manufacturers can claim larger capacity figures by choosing to use the SI definition.

 

To try to fix the confusion, there is an attempt to officially change the binary prefixes to "kibi", "mebi", "gibi", "tebi", etc, with the "b" indicating binary. It has not been very successful, but you can learn more by Googling "kibibyte".

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In December 1998 the International Electrotechnical Commission (IEC) approved as an IEC International Standard names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission, so that instead of kilo-, mega- and giga- bytes, we would have kibi-, mebi- and gibi- bytes for use in describing file and image sizes.

<br><br>

But since this initiative is way too late to be of practical use, just remember that within the world of computers, "kilo" means "1024" and not "1000" - etc. If you really care -

<a href="http://physics.nist.gov/cuu/Units/binary.html">check this NIST page</a><br><br>

 

 

.

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