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Image manipulation help please....

Jules

Formerly known as "nachoman"
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4,628
I know there are a lot of photographer types round here- can I ask for a bit of clarification on one aspect of digital work? My wife takes a lot of photos for print and sends them via email to the mags, etc. A high proportion of the time they are sent back as too low DPI (not helped by the fact we’ve never ever been able to work out how to check default DPI on a Mac). These images come up on my PC as huge size but 100 DPI (the overall pixel sizes are vast).
All I do it move the images over to my PC- stick them in photoshop- reduce the size of the image to 2/3 and- low and behold- their DPI has now cranked itself up to 300 and print shops are quite happy. Surely DPI is only a function of image size rather than an absolute so ‘not being the right DPI’ is irrelevant? Am I missing something here?
 

Pete Thomas

Chief of Stuff
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Could it be there is a default in your Photoshop that converts the DPI to 300 when you resize.

in this case what I do is use Photoshop resize, but don't put anything in the size parameters, change the dpi to 300 and untick resample.

What this does is just change the dpi, the image naturally changes dimesions but the quality stays exactly the same.

Think about it, say the image is 100dpi and is 300 pixels wide, it si therfore also 3 inches inches wide.

If you change the dpi to 300 (and no resampling) it is still 300 pixels wide, but they now are 1 inch (ie 300 dots per inch)

Print places seem to always insist you do this, although it's obviously very easy for them, but I think they do this because if anything is wrong with the end print, they can always say they didn't do anything to it, just printed how you supplied it. Which is fair enough I suppose.
 

Jules

Formerly known as "nachoman"
Messages
4,628
Thanks for the response. I'm resampling in photoshop so the DPI goes up by the amount the size goes down. My point was that a superbig, low DPI image when reduced to- say, squeeze onto a post card size- should be exactly the same as a signifantly smaller starting image which is at a higher DPI .... Neither are the same size as the post card they're ending up on and are being signifcantly reduced, so it shouldn't make any odds (but obviosuly does.... :shocked: ). By the way- any idea how to find ther default DPI on a Mac Image?
 

kevgermany

ex Landrover Nut
Subscriber
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21,947
There's a dpi flag in the image. It's intended to tell the program how to display/print the image.So progs that read it determine dimensions from pixel size and dpi setting.

Generally progs in Windows ignore it and work on actual pixel size. Don't know about Mac. Pete's method should reset the dpi flag without resampling.

My guess is that your problem is centred around the print shop guys not having sufficient control over their software - or not knowing enough to look beyind the flag setting.
 

Pete Thomas

Chief of Stuff
Commercial Supporter
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. :shocked: ). By the way- any idea how to find ther default DPI on a Mac Image?
Not sure what you mean by a Mac Image.

Thinking about it my previous answer was probably bonkers, because there is no default dpi in Photoshop resize window, it always shows the dpi if the image you open. ie you can't open the image size window unless there is an image to open it for, the menu is greyed out if you have no images opened.
 

milandro

Well-Known Member
Messages
2,483
I am not sure what you mean by default DPI on a Mac.

The way the sizing of a jpeg comes out from the camera and onto your computer via Photoshop depends on the device with which you photograph and not from the program or the platform that you use.

I’ve just done it to make sure , I opened 4 different pictures taken with phones and cameras they all came out differently depending on the device used. This means that you will have to resize them accordingly if you want to print them and resize both the printing size and the size of the file which will depend from it.

Naturally if you want to print it bigger than the files allows you you will be enlarging the pixels with consequent loss of quality because you enlarge the pixel.

Of course also the dimensions ratio of the pictures varies according to the devices and if you have a give size you might need to crop the picture

I have an easy trick, for lazy people (you just have to remember not to exceed the possibility of the file!). You can bypass any complex resizing by learning how to use the crop tool. Put the resolution to the size you need (300 dpi) then you put in the height and width the printing size, a cropping rectangle (or square) will appear maintaining the proportions that you have set by giving it the size, once you are satisfied on how big it is you can move it around within the pictures. Crop, save a copy of this image with a name different form the one it had out of the camera, then close the original picture WITHOUT saving the changes!!!! (so you keep the original unaltered file)
 

Jules

Formerly known as "nachoman"
Messages
4,628
Cheers guys. “I am not sure what you mean by default DPI on a Mac.”- Apologies, not explained well. On my PC I can right click an image, go to properties and it will display the size, pixels and DPI of the picture. On a Mac- properties gives dimensions in pixels but no sign of default DPI. Hence the only way to ensure the image is printer friendly is to transfer them all to my PC to check the properties (which can’t be the easiest way to go)
 

Pete Thomas

Chief of Stuff
Commercial Supporter
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Appears you are right. I always rely on opening in Photoshop to see the dpi.

There are two things to bear in mind also

Are you sending to somebody who wants the exact dimensions, or do they want the highest resolution in order for them to then reduce if necessary.

e.g. If I send images to a magazine for publication in an article, only the features editor can decide on the final dimensions when they format the article. In this case I do as stated above, convert to 300 with no resamplin, so the dimensions change with the dpi change but quality is absolutely the same.

If, as you say, it's going to end up smaller, then the printer is going to resample it and you really shouldn't be doing any resampling as each time it's resampled (in theory) it could be losing some quality. Cropping, as milandro says, is good as it doesn't lose quality and is worth doing if you think the final image may end up very small.

(However if it's going to be printed at a dimension you want (e.g. on a mug opr t shirt) then some printers may want you supply at the exact dimensions. If you aren't paying for a proof, some printers want to cover themselves legally if the final thing is wrong, they can always argue they just printed what you gave them.
 

kevgermany

ex Landrover Nut
Subscriber
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21,947
Guys, don't confuse the issue. The dpi that the printers are complaining about is a tag inside the image. It has NOTHING to do with actual image size/pixel count. It's just an instruction/suggestion to software to print/display at that resolution. Most windows software ignores/overrides it.



Jules - Irfanview will change the dpi setting without altering the rest of the image. If it gives you the option, make sure both horizontal and vertical values are set, cos they can be different (usually shouldn' be). Not sure if it's available for Mac, but this looks promising: http://www.brothersoft.com/downloads/irfanview-for-mac.html
 

Pete Thomas

Chief of Stuff
Commercial Supporter
Messages
14,007
Not sure if it's available for Mac, but this looks promising: http://www.brothersoft.com/downloads/irfanview-for-mac.html
From the Irfanview FAQs:

Q: Can I use IrfanView on Mac?
A: Yes, probably. There is no native-Mac version of IrfanView. However, you can use IrfanView in conjunction with Mac program like WineBottler/DARWINE. Take the ZIP version of IrfanView and unzip it or copy your existing Windows IrfanView folder to Mac. This is easier because the installer may need additional Windows DLLs to run.
 

spike

Old Indian
Messages
2,261
Tip 1.) for Mac users who don't have Photoshop.
Open an image in "Preview" (it comes with every OS) - in the tools menu open the Inspector - it will give the info on the image: Doc type, File size, image size, image DPI, colour model and colour sync. (if any). Can be used for simple editing but I personally wouldn't recommend it.
Tip 2.) Pixelmator is a very good app for Mac, can do an awful lot that photoshop does and only costs around $15. I use it extensively for image editing for internet work, it's about 60MB and as a result very fast. Doesn't have any CMYK facility but there are apps available that will do that job as well as P/shop for a fraction of the price.
Tip 3) Resampling - upsizing up to 200% is no problem in Photoshop and will not normally make an appreciable difference to quality. For busy colour images the "bicubic" interpolation method is recommended. For images with large areas of flat colour it's worth trying the linear method.
 
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