I have recently bought a used Canon A550 and tried using CHDK with it. I firstly wanted to use it as a time-lapse camera, so I installed the Ultra Intervalometer on the camera and enabled RAW file support.

All went well, however, the DNG files that come out of the camera have no preview. I can not use my Mac's Quicklook on the files to check the contents.
The files do have small thumbnails though.
I need to import the DNG files into Lightroom and update the DNG previews & metadata (this function is found in the Metadata menu when in Library mode) to get the previews right. For several hundred DNG files this process takes a while as Lightroom takes around 2-3 seconds to process each image. Is there a way to command CHDK to generate these previews when writing the DNG to the card?

Here's a screenshot of the thumbnail to show the difference with a preview. This screenshot is taken in the Ligthroom import window. As you can see it has a lower resolution than a regular embedded preview.

enter image description here

The reason I want this is because LRTimelapse needs these DNG previews to build a pre-rendering of a timelapse. If the embedded DNG preview is not present it shows this error:

enter image description here

What strengthens my belief in that this is not an OS side problem but a problem of CHDK/the camera is that when I validate a CHDK DNG (Library -> Validate DNG Files), before updating the preview, Lightroom says that the DNG is not valid. After updating the preview it's shows the same image as valid. So there must be something "wrong" with the DNG before I update the preview.

I have uploaded two DNG's straight from the camera. These show fine in LR, as LR builds its own previews, but they lack an embedded preview image.

  • \$\begingroup\$ I'm a bit confused as to what you mean when you use "previews" vs "thumbnails"? Besides, if you're importing into LR, does it really matter if there aren't embedded thumbnails in the DNGs, since LR won't use them anyway after it's finished importing them. I've used CHDK for timelapses too with DNG output with good results; I can't say I noticed whether they had decent embedded thumbnails or not, since I too was importing into LR. \$\endgroup\$
    – Conor Boyd
    Commented Jan 16, 2014 at 2:51
  • \$\begingroup\$ I think "small thumbnails" means the small image in the Finder window. And by "preview" he probably means the embedded thumbnail of the DNG file. But the issue is probably more Mac OS X related. Maybe the version of Mac OS you are using doesn't support the Canon A550 files. \$\endgroup\$
    – Heyfara
    Commented Jan 16, 2014 at 7:40
  • \$\begingroup\$ Could you post a couple of these DNGs? That would let people check on other OSes. \$\endgroup\$
    – Philip Kendall
    Commented Jan 16, 2014 at 9:29
  • \$\begingroup\$ This would seem a good question to ask on the CHDK forums, as it's very specific to that software, as you only want an in CHDK solution. \$\endgroup\$ Commented Jan 21, 2014 at 1:34

1 Answer 1


I've asked the question over on the CHDK forum and got the reply that it's technically possible to embed the preview image when the DNG is extracted by CHDK, but it would slow down the process a lot.

The advise was to use my computer to batch embed all the previews, just as I was doing already.


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