Dpi questions

[sally_05]

Hello,

Just wondering,

If I shoot with a basic DSLR (so the images are shot at 72dpi), when I edit these images, how do I save them at the highest quality resolution?

At the moment, I simply save as a JPEG at highest quality setting (12).

I heard that changing the dpi up to 300dpi increases quality, but surely if it's only shot at 72dpi you can't possibly increase the pixels available? Wouldn't this create a false 300dpi image?

Thanks to anyone who can help!

 

[table1349]

The difference between PPI and DPI

 

[Ysarex]

Hello,

Just wondering,

If I shoot with a basic DSLR (so the images are shot at 72dpi), when I edit these images, how do I save them at the highest quality resolution?

At the moment, I simply save as a JPEG at highest quality setting (12).

I heard that changing the dpi up to 300dpi increases quality, but surely if it's only shot at 72dpi you can't possibly increase the pixels available? Wouldn't this create a false 300dpi image?

Thanks to anyone who can help!

You mean PPI or pixels per inch. DPI refers to printer dots not pixels. The key to clearing up your confusion is in that inch factor. I'm sitting working on a photo right now in Photoshop and it's 300 ppi. At 300 ppi it will make a print 10.12 X 13.08 inches. If I change it to 72 ppi it stays the same image and it's total pixel count remains unchanged but it will then make a print 42.16 X 54.5 inches. Both prints would be made from my 3036 x 3924 pixel photo.

Joe

 

[Light Guru]

Hello,

Just wondering,

If I shoot with a basic DSLR (so the images are shot at 72dpi), when I edit these images, how do I save them at the highest quality resolution?

At the moment, I simply save as a JPEG at highest quality setting (12).

I heard that changing the dpi up to 300dpi increases quality, but surely if it's only shot at 72dpi you can't possibly increase the pixels available? Wouldn't this create a false 300dpi image?

Thanks to anyone who can help!

In addition to the things said about the difference between dpi and ppi you seem to be lacking a understanding of other aspects that effect "quality"

First jpeg is a highly compressed format that is really only good for final delivery not foe preserving quality.

Second simply changing the dpi of an image does not magically increase quality.

 

[KmH]

Second simply changing the dpi of an image does not magically increase quality.

A digital image has no dpi that can be changed.
Dpi and ppi are not interchangeable terms.

PPI only has meaning for physical prints and does not effect the electronic display of a digital image - as long as the digital image is not re-sampled using one of the several re-sampling interpolation algorithms that are available.
Changing the ppi value doesn't change the digital image quality, or the digital image size (the pixel dimensions) - until a physical print is made.

 

[apaflo]

Hello,

Just wondering,

If I shoot with a basic DSLR (so the images are shot at 72dpi), when I edit these images, how do I save them at the highest quality resolution?

At the moment, I simply save as a JPEG at highest quality setting (12).

I heard that changing the dpi up to 300dpi increases quality, but surely if it's only shot at 72dpi you can't possibly increase the pixels available? Wouldn't this create a false 300dpi image?

Thanks to anyone who can help!

The "DPI" listed in the Exif data and put there by your camera or as listed in any image editor is exactly the same as PPI.

The only time the term DPI is used as other than a direct replacement for PPI is in describing inkjet printers where it refers not to an entire pixel but to the ink dots that make up the pixel. Rest assured that any reference to DPI in the 50 to 300 range is exactly as if they were saying PPI, and equally that any reference to goes above about 800 has to be ink nozzle squirts and not pixels.

DPI for a monitor actually refers to the "Dot Clock" and it is 1 for 1 with the pixel rate. (It is the "Dot Clock" for a printer too, but it isnt' a 1:1 relationship.)

The DPI associated with an image is an Exif tag. It has nothing to do with the quality of the image, and changing it does not affect the image data in any way.

The only image data that affects print size are the pixel dimensions.

And if you have an image with a specific pixel dimension it can only generate a single sized print on a given printer without being resampled.* If the printer mechanically moves the print head at a 300 PPI rate, a 3000x2400 image will print out at 10" by 8". The way print size is changed is not by changing the PPI of the printer, which is fixed, but by resampling the image to a different pixel dimension.

Printers do not make two different sized prints directly, without resampling, from the same image data. Any time you make a print you either resample the image to exactly match the printer's PPI rate, or the print driver will resample it for you. (For large differences in size it does make a difference in image quality for sharpening; to get it perfect it should be manually done rather than let the print driver to it.)


* Note that some printers can print in two modes, one at 300 and the other at 600 PPI or similarly different values. The point is that PPI rate for the printer is not flexible.

 

[apaflo]

Second simply changing the dpi of an image does not magically increase quality.

A digital image has no dpi that can be changed.
Dpi and ppi are not interchangeable terms.

A digital image has an Exif tag for DPI; which can be changed to anything... it does not change image quality.

DPI and PPI are almost always directly interchangeable terms.

PPI only has meaning for physical prints and does not effect the electronic display of a digital image - as long as the digital image is not re-sampled using one of the several re-sampling interpolation algorithms that are available.
Changing the ppi value doesn't change the digital image quality, or the digital image size (the pixel dimensions) - until a physical print is made.

PPI and DPI are the same thing, it is the pixel rate of a display device. That is true of an electronic display as well as with a printer. Changing a monitor's DPI rate will change the size of images displayed on the monitor, and changing the PPI rate for a printer will have exactly the same effect. Just note that to change DPI/PPI it is the physical display device that is changed, not the image data.

But in practice you can't easily change the device, and instead the image is resampled to an appropriate pixel dimension to get a different display size (on the monitor or on a paper print), and the DPI/PPI remains fixed.

 

[AceCo55]

First jpeg is a highly compressed format that is really only good for final delivery not foe preserving quality.

Interesting - I think there may need to be some qualifications to this statement.
I looked at quite a few websites concerning this and none of them described "compression levels 10-12" as highly compressed. In fact virtually all of them indicated that there would be little discernible difference (especially level 12). Is there a difference? Yes - especially in areas of colour tonal graduations ... but you would have to look for it.
Many of the sites reiterated do all your processing in a loseless format. Only use jpeg to save an image (if more processing is needed, don't do it again and again from a jpeg file).

 

[apaflo]

First jpeg is a highly compressed format that is really only good for final delivery not foe preserving quality.

Interesting - I think there may need to be some qualifications to this statement.
I looked at quite a few websites concerning this and none of them described "compression levels 10-12" as highly compressed. In fact virtually all of them indicated that there would be little discernible difference (especially level 12). Is there a difference? Yes - especially in areas of colour tonal graduations ... but you would have to look for it.
Many of the sites reiterated do all your processing in a loseless format. Only use jpeg to save an image (if more processing is needed, don't do it again and again from a jpeg file).

You have expressly confirmed what Light Guru said!

Consider an exampe where I just converted a D800 RAW file to a 16 bit TIFF formatted RGB image. The TIFF file is 174,900,750 bytes in size. So then I converted that to a JPEG formatted RGB image (at 100 percent quality, which is what you are calling compression level 12), and the file size is 32,746,243 bytes. The JPEG file, even at maximum quality, is just 18.7% the size of the TIFF file. Even if the 16 bit TIFF is first converted to an 8 bit TIFF before going to a JPEG, the JPEG is 68% the size of the 8 bit TIFF.

That is "highly compressed" any way we choose to look at it.

The sites you reviewed apparently added that the compression is lossy, and the original data cannot be recovered. That is exactly the reason Light Guru so very correctly suggested that JPEG should only be used for a final product, the "display copy" of an image, and never for an intermediate stage where editing will continue.

 

[AceCo55]

In practical terms when to look at that TIFF image and the 100% quality jpeg you will not see a significant different unless you go pixel peeping.
Sure the TIFF has more information and if you were going to do more processing then it would be preferable.
Also the TIFF has a larger file size that the equivalent .psd file but both of these are lossless file formats.

There are LOTS (if not most) of photos in sports magazines that not only were shot in jpeg but also processed and saved as jpegs.
The point I am trying to get across is that jpeg's may well be more than satisfactory for your purposes and saving to a jpeg doesn't necessarily mean that somehow you have a "terrible" photo because it is jpeg. The compression at the lowest level (highest quality) will still give you a high quality image and shouldn't be dogmatically discarded as a file format.
Not everyone wants to take RAW images, process as TIFFS and only use jpeg for posting on the web. They can still have beautiful images if they understand the limitations of jpeg. There is not one way - which is what Light Guru was suggesting. I was merely pointing out that many are completely comfortable with the image quality of high quality jpegs.
Now you can argue that RAW and TIFF are better/more latitude/more info retained etc and I wouldn't argue at all with that. However that doesn't mean everyone has to go down that path in order to get terrific images.