Size of .hdr file

[nwinspeare]

Hello, I am new to this forum so I will present myself briefly. I've been an amateur photographer for 10 years, I love landscapes and portraits mostly. I live in France and I own a beautiful d800.
I have been in hdr for a while but I just noticed something strange... If I make 5 raw files into tif format, I have 5 files of 220Mb each. (!!) Into photomatix they go and I save a .hdr file. This file is only 110Mb. Where did all the information go ???
Please help me to understand,
Nicolas

PS : Trying a demo of SNS-HDR at the moment, back with conclusions later.

 

[Tiberius47]

When you make an HDR picture from several different exposures, not all of the information is used from each exposure. The program only takes the bits that are properly exposed to make the finished picture from.

 

[Helen B]

I presume that the D800 TIFFs are stored as 16 bits (symbol b) per channel per pixel. That is two bytes (symbol B) per channel per pixel, or six bytes per pixel, hence the 220 MB files.

.hdr files have four bytes per pixel, because they store the numerical values in a different way - rather like scientific notation. They store one byte per channel, plus one byte for an exponent ("power"). Therefore the .hdr file can contain more information (in terms of a wider range of values - more of a difference between the highest and lowest values) albeit at a slightly lower precision (fewer individual values). This means that an .hdr file can be smaller than a 16-bit TIFF file of the same pixel dimensions even though it represents a wider range of brightness values.

I don't know how much you know about maths, so I don't know how far to go into the explanation. Apologies if I have dumbed it down too much. Ask if you want more.

 

[nwinspeare]

Thanks for the answer Helen B, actually I'm a maths teacher, so any technical explanation should be ok. I just hope that in the hdr file, I don't lose any information about detail and sharpness.

 

[markj]

Your up to bat, Helen! BTW, Isn't math singular not maths.

 

[Helen B]

I'm not sure what else there is to add - the .hdr file uses a significand (mantissa) that can hold an integer value between 0 and 255 for each of the red, green and blue channels, and a shared exponent (RGBE). Thus it can hold in four bytes what would take 24 bytes if exponent notation wasn't used, at the expense of precision.

Maths is short for mathematics.