When producing HDR (High Dynamic Range) renderings you rarely need to save above 16-bit. This is applicable whether you are using TGA, PNG, TIF(F) or EXR. It is a common misconception that the file 'must' be 32-bit. With a 16-bit file you will still have more than enough of a colour range to manipulate in Post. (PhotoShop/Composite/After Effects etc)
In one of my previous posts discussing OpenEXR, you should have noticed that is the default setting within 3ds Max.
16-bit is often referred to as 'half-float.' This float/half float term refers to the fact that the fraction part or decimal point can "float" anywhere within the number. This produces a broad range of decimal point numbers, resulting in a greater range of numbers being possible for colour manipulation. Because of this a 32-bit file often contains too much data, and in turn means the file is too large to work with efficiently, and takes up more storage space. A 16-bit, half float, file has the benefits of more detail being held in the file (ie highlights and dark shadow detail) but without the excessive file sizes. Normally when images are created they use whole numbers. For example, a JPG is an integer file format with a much smaller range of colour information, resulting in a much poorer representation of the CG information you've created in your file. (0-255 range). I, personally, recommend you always use a format that supports half-float information, to ensure your work is accurately represented, ie NOT .jpg
In case you werent aware the EXR (OpenEXR) file format was developed by ILM (Industrial Light Magic) around 2000. ILM is part of Lucasfilm Ltd. This is now their standard file format, which they utilise in all their motion pictures.
One of the strongest benefits of float/half-float data is that it allows you to change the F-stop in post. Below are three links from ILM that clearly demonstrate, this advantage.
(This can be achieved with the "Exposure" tool in Photoshop, or the "Photolab" tool in 3ds Max Composite)
I strongly recommend having a look at ILM's website - by clicking here. to see an example of the great movies they have created.