Most HDR Software can accept JPEG, TIFF or RAW files as input for the merge process, and HDR Express and HDR Expose are no exceptions. If you only have JPEG or TIFF legacy images that you want to process you don’t have an option, but going forward you will get better results if you shoot and process RAW image files for HDR.JPEG and TIFF files are great final output file types, but they have been processed causing clipped exposure ranges, artifacts due to applied contrast curves and in the case of JPEG files, artifacts due to compression.
When we merge individual images into 32-bit HDR images, we need to undo most of those changes to create the linear data to process the areas of exposure overlap. In many cases this works well, but in some cases such as extreme lighting conditions working with JPEG or TIFF source files can create areas of posterization where especially highlight data can be clipped in processed files. JPEG source images have the additional disadvantage of compression artifacts that will change the between the different exposures and create alignment issues.
Closeup of merged RAW image
Closeup of merged jpeg image with artifacts circled
You want to isolate the differences between the bracketed images to only exposure times. If each image has different JPEG compression artifacts in different areas of the image these changes will be exaggerated when merged.
RAW Files provide us much better source data to work with. The data is linear and has not been process or had tone curves applied. Also, RAW files typically have 12 or 14-bits of data with extended highlight information. This allows us to create 32-bit HDR images with much smoother transitions and less potential for posterization with no compression artifacts.
So if you have a choice going forward when shooting for HDR set the camera to RAW and do the HDR merge and tone mapping operations first on 32-bit HDR data. Then, save the image as a TIFF file. If you still have some local corrections or retouching to do, work on the output TIFF file then.
The fundamental workflow rule of digital photography processing is to start with the big items first and work your way down to the smaller details. In HDR, processing the merge and the tone mapping are the first order of business.
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