-
The DPI Issue
I’ve seen questions about there here and on every other forum about editing (and web design for that matter).
The general advice is that DPI is irrelevant in any case except print (where it is essentially a metadata indicator setting physical print size).
However I also saw this thread where Walter is claiming that a simple change of DPI without resampling his image is having an effect on file size.
Now I know Walter knows his stuff, and I love Good Eats, so I want to believe this – but I just can’t.
I’ve scanned a photograph of mine at 2400dpi and cropped it slightly. Then I’ve changed the DPI in Photoshop, being sure to untick ‘Resample’ – here’s what I got:
2400dpi PSD
Pixels: 14100×9144
Print Size: 149.23×96.77mm
File Size: 372,347,051 Bytes (355MB roughly)300dpi PSD
Pixels: 14100×9144
Print Size: 1193.8×774.19mm
File Size: 372,347,051 Bytes (355MB roughly)72dpi PSD
Pixels: 14100×9144
Print Size: 4974.17×3225.8mm
File Size: 372,347,051 Bytes (355MB roughly)In short, the only thing that has changed in doing this has been the print size. This fits with everything I know (having been a print designer, web designer and editor).
Walter, you sure you’re not mistaking something here?
I can make a picture 720×576 in Photoshop and save it as 1 DPI, 72 DPI or 3000 DPI – and in Avid or Final Cut there is no difference at all. It’s just a plain old full-frame PAL image.
-
9 Replies
-
DPIs are relevant when scanning and when printing. In the video world is completely irrelevant. What matters in the number of pixels.
If you have to scan a very small picture to fill your screen you need scan with a high DPI. If you are scanning a big picture that will fill only a part of your screen you can scann it even with less than 72 DPIs.
rafaelPPC G5 2x2Gh 4GbRAM/BlackMagic SD/PMBP 17″Core2Duo 4GbRAM
JVC DTV-17″/FCS2/AE CS3/COMBUSTION/SHAKE -
[Sycophant] “Then I’ve changed the DPI in Photoshop, being sure to untick ‘Resample’ – here’s what I got:”
Go back and tick the Resample and then change the dpi. You’re not changing a darn thing when you untick it. No wonder you not seeing any changes.
David Roth Weiss
Director/Editor
David Weiss Productions, Inc.
Los AngelesPOST-PRODUCTION WITHOUT THE USUAL INSANITY ™
A forum host of Creative COW’s Business & Marketing, and Indie Film & Documentary forums.
-
That was my point… My understanding is that DPI will never have any impact on the size of the image as far as my editing software is concerned, nor will it affect filesize… However I’ve seen others (most recently and notably Walt Biscardi) claim that they have different results.
I’m just throwing it out – maybe I’m tossing my gauntlet at Walter’s feet…
-
I’ve said all I needed to say in the previous, and rather long, thread about this very issue. You can see my results posted in that thread.
Walter Biscardi, Jr.
Biscardi Creative Media
HD and SD Production for Broadcast and Independent Productions.STOP STARING AND START GRADING WITH APPLE COLOR
The new Color Training DVD now available from the Creative Cow! -
In video do not exist dots, do not exist inches.
Even when scanning a 35mm film we express the scanning precision in other way: 2K, 4k, 6K..In fact we only increase the DPIs of the scanner but we use other terms.
rafaelPPC G5 2x2Gh 4GbRAM/BlackMagic SD/PMBP 17″Core2Duo 4GbRAM
JVC DTV-17″/FCS2/AE CS3/COMBUSTION/SHAKE -
that said, it is amazing how many people struggle with images in something like Final Cut, slamming the processor by shifting around 300dpi photos when a simple re-size to the video pixel dimensions, either making it near to 1920 pixels wide (In an HD example) or dropping the dpi to something like 72dpi, (only because it makes the filesize smaller with resampling) I think 72 is PC screen res and 96 is mac screen res, can make moving still images in FC so much faster…
-
FCP!! You want to see some of the ‘slamming’ that goes on by naive users in PowerPoint on their laptops!
I’ve seen ‘megapixel camera’ direct inserts, eight up on a single PPT frame. Users say ‘Hmmm it did seem a bit sluggish!’
Alan
-
Depending on your requirements, it’s usually best to resize images to the appropriate dimensions (1920×1080, 1280×720, etc…) but the DPI of that image will make no difference at all.
-
Indeed, I did see your comments in that thread, but I cannot reproduce them, and nothing I understand about the mechanics of digital imaging supports what you say you observe.
The file size of an image is directly proportional to the number of pixels and the number of bits per pixel. No other factor is going to affect that.
So an RGB image of 1920×1080 at 8 bits per pixel will be approximately 6,220,800 Bytes (plus overhead for metadata and file headers). Obviously if the image has compression applied (either as a lossy format like JPEG, or with a lossless compression like Zip in a Tif file).
Reply to this Discussion! Login or Sign Up