I've set up my Canon EOS 600D to save my photos as RAW+JPEG. What file is shown when I preview the images on the back screen of the camera?
You are almost certainly seeing a JPEG preview file. Even if you only save RAW files, the vast majority of cameras generate a preview or thumbnail JPEG and that is what you see on the LCD on the back of your camera.
RAW files contain monochromatic luminance values for each photosite. Since the sensor is masked with a pattern of filters that allow different colors of light to pass through adjacent pixel wells (usually Red, Green, and Blue), there is no color information until the RAW data is demosaiced so that an R, G, and B value can be interpolated for each pixel.
With most cameras if you only save to JPEG what you see on the rear LCD is also a smaller thumbnail preview of the full JPEG. Most cameras have sensors with much higher resolution than the LCD on the back of the camera.
It depends on the camera, but generally the JPEG is shown and the histograms correspond to the JPEG as well. RAW isn't actually an image file, it is sensor data. Without further image processing it can't be displayed as a particularly meaningful image since it would lack color information and would not be true black and white either due to color filters on the sensor.
As already answered, it depends of the camera (I know mine is using the jpg because if I shoot in raw only, the preview isn't available).
If you really want to know, you may test it easily:
- take two easy to tell apart picture (ie one of the sky and one of the ground)
- transfer the jpg files to your computer
- rename each of them with the name of the other one
- transfer the files back to your camera
- check the preview:
- if they still are correct, your camera preview is using the raw file.
- if they are switched (preview of the sky picture shows the ground and vice versa), then your camera preview uses the jpg file.
But as stated, both the raw and jpg file embed a lower resolution jpeg thumbnail, and I'm pretty sure that's what the camera is using for practical reason (it's faster to decode and display a small jpg than a big one or a raw file, and on your camera LCD, you couldn't probably see the difference)
Now, if you really really want to know about this, you could corrupt the jpg or raw file with an hexadecimal file editor. If you change some bytes in the middle of a jpg, it should look very corrupted. If you put it back on the camera, you will know:
- if the preview looks fine, then the camera is using the thumbnail
- if the preview is corrupted, then the camera is using the full image
I never tried that on a raw file, but I assume they are still compressed (with lossless compression) so this may work.
If you go the full way to answer your question, please post the result (we know how it is probably done, but it won't hurt to know for sure :o) )
RAW is a data format. It is not an image. Anything you see in the preview is an image. If you shoot JPEG, the choice is obvious. If you shoot RAW. what you see is the data transformed into an image by the camera processor and stored for convenience as an embedded JPEG in the file.
As for your question, you see, it does not matter. Whichever one is being viewed is exactly the same except that it may come from the separate JPEG or the embedded one. In any case, the camera shows you an image which looks identical. Nearly every camera also calculates the histogram from that and so you may see clipping that is not actually happening in the RAW file.