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Old Dec 9, 2013, 1:20 PM   #1
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Hi,
Does anyone know of a 'file size tutorial' that will explain the basics, wrt to images? I have never really understood the meaning of the various terms used, so much so that I can't even tell you what I don't understand about it.
Thanks,
...... john

Last edited by Shinnen; Dec 9, 2013 at 1:21 PM. Reason: clarification
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Old Dec 9, 2013, 1:31 PM   #2
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Larger file sizes mean greater detail in the images.




That's about it, really.
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Old Dec 10, 2013, 1:59 PM   #3
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G'day John

The [computed] file size for a photo will vary depending upon several factors

Early cameras of say, 3mpx created JPG file sizes of <1Mb
As the mpx size increased to 6mpx, the JPG file size was around 2Mb

When the mpx got to 12mpx, the JPG file size moves to about 4Mb

If the camera can save to RAW format [ie- with no compression used], then a file size for 12mpx goes from about 4Mb to about 12Mb

Then we come to software pp after editing
During the SAVE process, you can choose any of 'Lo-Med-Hi' quality options. Each of these operates in such a way that some compression occurs, and the more the compression, the more fine detail is sacrificed

so as TCav states ... "Larger file sizes mean greater detail in the images."

Hope this helps a bit
Phil
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Old Dec 10, 2013, 5:54 PM   #4
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'File size" refers to the number of bytes, or the space it will take on your computer's hard drive.
"Image size" refers to the number of pixels.
Images are formatted in various ways, some of which are compressed formats, which allow smaller file sizes, (.jpg is the most common) and can use more or less compression, depending on the needed image quality. Better quality corresponds to less compression and larger file sizes.
When you open an image to view on your PC, the file is decompressed (if it's a compressed image), and converted to a bit map, which is the viewable format. You could also save the image as a bit map, which would have no compression.
A picture from my 12MP camera, full size, is 4000 x 3000 pixels. If I save it as a .jpg, with best quality from my camera, the file size is around 4.7 MB. (more or less, depending on how much detail is in the photo). When I open it to view on my computer, it is converted to a bitmap which is about 36 MB. (4000 x 3000 x 3colors x 8bits per color) If I edit the image and save it as a bit map, it remains 36MB, regardless of how much or little detail it contains.
If I resize it and save it as a .jpg file, the file will be correspondingly smaller.

(hopethis wasn't too basic for your)

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Old Dec 11, 2013, 6:28 PM   #5
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Hi Brian,
So .... are you saying that a jpg is just a compressed Bitmap (which has lost some of it's information due to compression ..... the amount lost being contingent on how much the bitmap was compressed); and that when the jpg is decompressed back to a bitmap (for viewing), it does not regain the lost information caused by the original compression?
..... john
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Old Dec 11, 2013, 6:53 PM   #6
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Yes, the compression used in JPEG is not a lossless compression, like ZIP, for instance. Once a JPEG file is compressed, it loses some of the detail, and it will never be recovered. And while you can't ever NOT compress a JPEG file, you can limit it to a very slight compression.

But that makes larger file sizes.
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Last edited by TCav; Dec 12, 2013 at 5:12 AM.
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Old Dec 11, 2013, 9:48 PM   #7
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There are lossless compression schemes. In most cases I am aware of, though, the compression ratio is not very good, and the decompression takes considerably longer. Jpeg provides very rapid opening of images, and the losses at the lowest compression are very slight. It also provides for comprehensive metadata. As with most things in life, there is always a tradeoff.

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Old Dec 12, 2013, 12:26 PM   #8
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Alright then. So..... I should set my editor's compression to 100%, the best quality for storing, assuming I have the disk space?
I just read up on the various image file types on Wiki. There's a lot of them. But it seems like my options, camera wise, are either raw or jpeg?
..... john
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Old Dec 12, 2013, 12:53 PM   #9
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yes - you always want to save with the best jpeg quality. The key is not to keep re-saving it. You want to get your workflow to a point where you're really only saving the image 1 or 2 times after you load it to your PC. And yes, there are a lot of different file extensions out there. Usually a camera will have JPEG and whatever that brand calls raw (for my Canon DSLRs the raw files are .CR2 - other brands use different extensions. Then there are some other extensions - Adobe has some lossless file extensions it can use.

I've personally never had a use for all the proprietary stuff except when I am doing some graphical stuff - posters, etc where I build with layers in photoshop. Then I will save in their lossless file type PSD - to keep the layers. Unfortunately you can't display those image types online and you really don't print from them. So everything gets written as a jpeg before being sent to print for me.
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Old Dec 12, 2013, 1:31 PM   #10
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Something you may want to consider: When doing a lot of post-processing, don't keep saving your work in JPEG format, as that will slowly but surely wash away all the detail. Instead, use a file format that doesn't use compression, or that does, but uses a lossless form of compression. That will allow you to preserve as much of the detail as possible. There are two common file types that will satisfy that requirement:
  • Tagged Image File Format (TIFF or TIF) - TIFF has been widely used for years to store bitmap (or raster) graphic images (as opposed to vector graphic images, like those from AutoCAD, for instance), and can use lossless compression.
  • Photoshop Document (PSD) - PSD is widely used, not just by Photoshop, or even just Adobe applications, but by many photo editing applications to store images.
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