Incense

by Bart Arondson

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My Nikon D90 dumps out photographs that are 4288 x 2848 @ 300 DPI. They're about ~3 MB in size.

I'm an amateur in every sense of the word. I don't know what 75% of the camera settings do.

I'm wondering if there any rules of thumb in the industry when it comes to compressing these images down. I've got about 100 GB at this point and really need to start compressing them to save space. The storage service I'm using can't really go any higher than that without costing large fees.

At the same time, I don't want to lose a lot of quality. Once I compress them, and lose quality, there's no way of going back.

I was hoping this community of experts could offer some advice as to what is usually tolerated in terms of lost quality. Any software apps that deals with this concern? I found a couple of one-off apps that optimize (like this one), but no recommendations on how low to go.

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DPI is meaningless for general purpose camera since the ratio of subject size to sensor size can vary widely. If you are talking about the raw sensor, then it would have to be over 14 inches wide to have 4288 pixels accross at 300 DPI, which I happen to know a Nikon D90 is not. –  Olin Lathrop May 20 '12 at 22:48
    
I only know what I see come out the other end. The files written to disk (either on the SD card or through the USB cord) are 300 DPI at that size. –  phtimsp1970 May 20 '12 at 23:11
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What Olin's getting at is: Just say they're 4288 x 2848. The "300 DPI" is a meaningless, made-up number in this context. –  coneslayer May 21 '12 at 1:31
    
Look at different storage services, if you want cloud storage and fee hiking is a concern. At Amazon S3, for example, you pay mostly a flat fee per GB per month. I'm sure there are other reasonable alternatives. Especially do not blanket recompress existing JPEGs. –  Michael Kjörling May 21 '12 at 11:40

2 Answers 2

You are correct that once you compress, there is no going back. The best way I would consider compression is to actually delete images which are low quality, uninteresting and near duplicates, rather than systematically reducing quality globally. This is probably not the question you were asking though :)

The level of tolerable compression is mostly relative to print size and secondly to one's own pickiness :) Generally though, lossy image compression is designed to compress away the least noticeable details of images. This is why images compress and lower quality without a reduction in resolution.

Your camera's JPEG files at maximum quality should be around 3 MB in size, so if you go down one quality level you they will be 1.5 MB and most people would be hard pressed to see the difference. This is the difference between JPEG quality of 95 and 90 on must cameras.

To see what the original compression of your images is, you can use JPGQ which estimates the amount of lossyness of JPEG compression. You can then recompress your images a little more and see if you can spot the difference. I use nconvert which takes a quality parameter from 0 to 100 where 100 is the maximum quality but but values above 90 save plenty of space with little noticeable artifacts.

Keep in mind that JPEG compression cannot guess what you will actually notice, it is based on much broader studies of the human visual system and the same amount of compression can be noticeable in one image and not in another by the same person.

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Your first paragraph is exactly the answer I was going to leave. :) –  Eric May 20 '12 at 23:09
    
Hey, great ideas here. I hadn't considered that first para, but you're right - there are semi-duplicate prints in there. I could probably purge a gig or two that way. I'll look into some of the other techniques you offered as well. –  phtimsp1970 May 20 '12 at 23:15

Keep all the Pixels - You may want them in the Future

You may feel that you are "an amateur in every sense of the word" now, but in the future, you will hopefully become more knowledgable and have more appreciation for what you can do with all those pixels.

Disk space really is relatively cheap these days, so I suggest that you are better off (i) deleting images you really don't like and (ii) keeping the highest possible resolution images that you do like. This will give you the maximum flexibility in the future.

As an aside, you may like to investigate using RAW format for your images: this gives you absolutely all the information your camera can squeeze into the file.

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