dpi and print

flicker · 6 May 2010

When I open one of my photos in photoshop, it defaults to 72dpi (or pixels/inch). I know that to have things professionally printed, you usually have to provide a 300dpi product. Let's say the image is 3000px X 2500px and is 72dpi. If I go to image size and change it to 300dpi I can either have it resampled to some massive size like 15555px X 9000px or I can choose not to resample and have it the original pixel dimensions but at 300dpi.

If I click on print size in 'view', the resampled image shows the same as actual pixel size, but the none resampled image shows as a little 24% postcard size.

I am confused!!! What if I have taken a photo and want it on an A0 poster? Can anyone see where I am coming from?

flicker · 7 May 2010

*cough*

Craikeybaby · 7 May 2010

This has been covered loads before, maybe why there hasn't been any posts...

Your digital files have a certain amount of pixels in your case 3000 x 2500, in print terms each pixel is a dot, dpi stands for dots per inch. So if you print 3000 dots at 300 dpi your image will be 10 inches long, but if you are only printing 72 dots per inch it will be 41+ inches long, because the dots are more spread out.

In your second example, you know the length you need (however long A0 is) and that you want to print at 300dpi you will need to multiply the length by the dpi to work out how many pixels your image needs, if it doesn't have enough you will need to create more using something like Genuine Fractals.

Chappers · 7 May 2010

Looking at dpi figures can be confusing, especially when you want an image printed.

Whilst file size is a contributing factor to print size, so is the image quality. Poor optics and over processing can seriously effect the final print quality.

I have some A1 prints produced from a 6Mpixel camera that look superb. These were shot using a decent lens and not over processed. The resizing was handled totally by the printer software itself, I just supplied the the processed file.

hollis_f · 7 May 2010

Ignore dpi until you come to print an image - then ignore dpi.

Before printing dpi is totally irrelevant. A 3000x2000 pixel image at 72 dpi is identical (except for a couple of bytes in the exif) to a 3000x2000 pixel image at 1000 dpi.

If you want to print at the highest possible quality then ignore dpi. Crop your image to the correct aspect ratio then use the 'print to fit' (or equivalent) option. Or in PS Resize Image - untick the 'Resample Image' and set the image size in inches to the correct value. The dpi value will change but it will give you the highest value you can get from that image without inventing data.

flicker · 8 May 2010

I understand the pixel part as I have always worked in pixels for web. I also understand the dpi part. The bit that confuses me is the fact that I am using a 12 megapixel camera, changing the image to 300dpi without resampling (upscaling) and it seems a 100% print size is slightly larger than a postcard (going off what photoshop tells me). Should I be allowing it to resample into the larger pixel version for massive prints like A0? (lewis - A3 is double A4, A2 is double A3, A1 is double A2 and A0 is double A1) - or should I leave it as the original pixel dimensions, set to 300dpi and let the printer worry about it? Which will produce a better quality print?

hollis_f · 8 May 2010

flicker said:
I understand the pixel part as I have always worked in pixels for web. I also understand the dpi part. The bit that confuses me is the fact that I am using a 12 megapixel camera, changing the image to 300dpi without resampling (upscaling) and it seems a 100% print size is slightly larger than a postcard (going off what photoshop tells me). Should I be allowing it to resample into the larger pixel version for massive prints like A0? (lewis - A3 is double A4, A2 is double A3, A1 is double A2 and A0 is double A1) - or should I leave it as the original pixel dimensions, set to 300dpi and let the printer worry about it? Which will produce a better quality print?

A 12 MP camera will be shooting something like 4000x3000 pixels. If you print at 300 dpi then that will give you bigger than 13x10 inches (about 5 times bigger than a postcard).

If you want to print bigger than that then you use less than 300 dpi. You're not going to be looking at an A1 print from the same distance as you'd look at a postcard-sized print, so the dpi can be a lot lower.

I have several A2 or larger prints on my wall taken with a 10MP camera.

flicker · 8 May 2010

Ah! ok. So If I want to print my photo out about a metre wide, I will be setting it at 100dpi? BUT...why do so many printers insist that "all artwork must be at least 200dpi and 300dpi is preferred". I have had this from places that print 7 foot tall banners. This is where I was finding it confusing. I would be looking at something like 55dpi on my best quality photo to make it 7 feet. If they insist it must be at least 200dpi, should I be resampling it? What affect will it have on the quality? I was assuming since it would be printed so big, a reduction in quality would be even more noticeable.

blue max · 8 May 2010

For standard four-colour print work, 300dpi is the norm. It gives about 30mb of data for a full A4 sheet.

Obviously subject matter plays a huge part in whether all that data is necessary. A solid colour over the whole thing would need very little, but an image with lots of fine detail would use every bit of it.

Large format printing is slightly different as it is meant to be viewed from further away. These huge billboards have dots so big you can barely tell what they are from close proximity.

Making file sizes much larger than originally captured (assuming you used the full capabilities of the camera), is probably not worth while. It will just increase the file size and make distribution awkward. Obviously that is if you haven't gone to great lengths to retouch and sharpen to suit.

Enlarging it yourself allows you to get an idea of what will happen to it when enlarged, but I'm sure your printer will be able to do the same thing themselves. It may actually be better to let them handle the enlargement as they have more experience of their processes.

Graham