by Steve Balcombe

Here's a little treatise on resolution.

Apologies to those who will find this a bit beginnerish.

Apologies to those ;-) who find it quite the opposite...

I think I'll assume everyone knows what a pixel is! - although if I were writing a book on the subject I'd explain that too.

The actual amount of detail in any digital image depends on the number of pixels it contains - that's why digital cameras are given a rating in megapixels. So to give a common example, a camera which can record a 2,000 x 1,500 pixel image is 3,000,000 pixels or '3 megapixels'. A 6 megapixel camera can capture twice as much data, simple as that. Notice there is nothing in there about 'dpi'.

Since resolution is always discussed as a linear quantity we will consider the 2,000 pixels across rather than the 3 megapixels area. If you spread those 2,000 pixels across a 10" wide print, it doesn't take much mental arithmetic to work out that there are 200 pixels for each inch of image, or '200 dpi'. (You will occasionally see references to 'ppi', by the way - 'dots per inch' and pixels per inch' mean exactly the same thing.) If you print the same image 5" wide it will be 2,000/5 = 400 dpi. And so on. Note that this is what I referred to earlier as 'output resolution' - the resolution of the printed photograph.

And what's the point of knowing the resolution? Simply that a higher dpi means more fine detail in the printed picture. So more is always better, right? Wrong. Publishers know from experience that 300 dpi is a good rule-of-thumb value for high quality reproduction in a magazine, so that's what they will usually ask for if you submit photographs for publication. A 300 dpi printed image will have enough fine detail without requiring an unnecessarily big file.In fact I will go further than that: if the resolution is higher than 300 dpi you will not see any significant improvement in the printed image, because any extra detail which might have been there is lost in the printing process.

You can do the division the other way round - instead of calculating the resolution by dividing pixels by inches, we can get the size at a particular resolution. So using our hypothetical 3 megapixel camera, the image can be printed 2,000 pixels / 300 dpi = 6.66" wide, at the publisher's required resolution.

Of course it may be that you want to print the photograph on your inkjet printer at home, and for this you can get away with a lower resolution. A reasonable figure for this is 150 dpi (based on experience not science), so you can print your masterpiece at 2,000/150 = over 13" wide or slightly bigger than A4.

A very, very important point to understand from the above is that a digital image *doesn't have an intrinsic resolution*. It depends on how big you print it.


Right then, let's put digital cameras to one side for a moment and look at scanning prints and trannies.

If you make a scan from any kind of original, you end up with a digital image file which is for all practical purposes the same as one from a digital camera. You've just had to go through an extra stage to get it. And unlike the digital camera which has a fixed image size (or rather, in most cases, a choice of two or three fixed sizes), with a scanner you have to specify the size you want.

Some scanning software has an idiot mode which hides all the scary details from you and just has a 'Scan Now' button. My advice is don't use it. Not only will you get poor results but you won't know why. The so-called 'advanced mode' simply requires that you learn a few basic techniques like those discussed here, and with practice you may still get poor results but at least you'll know why.

All modern scanning software (that I've seen) allows you to specify the size at which you want to print the scanned image, and the resolution required at that size. If you've been paying attention you'll recall that this is the output resolution. So, put your original in the scanner, tell the software you want the scan 6" wide and 300 dpi, and it'll adjust the scanning resolution (yep, the input resolution, I knew you'd been paying attention) to produce a perfect scan.

So do we even need to know the input resolution, or can we leave that entirely to the software? In most cases, yes we can leave it to the software, especially when scanning prints. This is because we will be working well within the capabilities of the scanner so the simple process outlined above will produce exactly the same result as if you'd done the calculations yourself. If you make a scan which is twice as big as the original, you will get a print in which all the blemishes are also twice as big as the original - but it will be the best achievable and that's all we can ask.

Where we are more likely to run into trouble is scanning transparencies (or negs). In theory you can get better scans from these, because they haven't been subjected to the loss of quality that printing entails, but there is one problem and that is their small size. Here's a calculation of the scanning (input) resolution required to print a 35 mm tranny at A4 size, 300 dpi.

We have to use the short (210 mm) dimension of the paper for the calculation because of the slightly different proportions of the paper and film. We need a scan which is slightly larger than the page so it can run edge to edge - publishers call this 'bleed'; 3 mm all round is usual. A 35 mm film frame actually measures 36 mm x 24 mm, and you have to lose a very small amount off this because the edges are never truly straight - let's assume a usable width of 22 mm. So:

Scan width is 3 + 210 + 3 = 216 mm = 8.5 in. almost exactly.

Width in pixels at 300 dpi is therefore 8.5 x 300 = 2,550 px.

Usable width of film is 22 mm (0.866 in) so input resolution = 2,550/0.866 = 2,944 dpi.

The more mathematically agile amongst you will have realised that there's a shortcut. The printed image is near enough ten times the size of the original so the scanning resolution must be ten times the printing resolution = 3,000 dpi, near enough.

As I've already admitted, we could have left that calculation to the software, but doing it ourselves has highlighted a major issue - most of us don't have a scanner which can resolve 3,000 dpi. (This must be the 'optical' resolution of the scanner to get good results - not the 'interpolated' or 'software' resolution.) Even if you do, it will probably be working right at the limit of its technology and the quality may be disappointing. And bear in mind also that we used the full size of the tranny - if we need to crop it at all, it'll be enlarged even more to fill the page.

[example - if you've been scanning trannies at 1,200 dpi that's only enough for about 4x enlargement or about 5.5" wide in a magazine. Assuming of course that your scanner can manage 1,200 dpi without a trade off in quality.]

The best desktop scanners for this kind of work are dedicated film scanners, which are designed to work at these very fine resolutions. Even better is a 'drum scanner', if you have 50k to spare.


Somewhere way back, I said that digital images don't have an intrinsic resolution, and indeed they don't, yet somehow the computer seems to know what size to print them. This is because most image file formats (major subject which I won't even begin to go into) include the ability to store 'header' information including the intended resolution. This is only a suggested value, put there by the scanning software or the digital camera 'firmware', or by an image manipulation package like Photoshop. Every page layout or word processing package I've ever used reads this value and uses it by default, but most allow you to stretch the image which means you have chosen to override the suggested size - and therefore resolution. Stretch it to be twice as big and it will be half the resolution - not good.

If you use your scanning software correctly and specify an output resolution of 300 dpi, then a publisher can drop your image on the page and it will be the size you intended and the resolution he (or she in the case of certain paddling mags) needs. Similarly, if you're scanning pictures to include in your laser printed club newsletter, specify 150 dpi in your scanning software and you'll get a much smaller file and adequate quality for most purposes.

But what do you do if it's a digital camera image? The camera puts a resolution value in the file header but what if it's not the one you want? This is a very common problem, and in fact my Fuji S602Z uses a value of 72 dpi - useless for most purposes. The best answer is to use one of Photoshop's features to change it. Open the file, go to the Image menu and click Image Size. Make sure Resample Image is *off* then change the resolution to 300 dpi or whatever you need. You'll see the dimensions change as Photoshop recalculates them. It's useful to understand exactly what is happening here: Photoshop is changing *only* the header information, it does nothing whatsoever to the image itself. This exactly the same as the manual calculation we did above, working out the image size for a chosen resolution.

Then save a copy of your photograph. This is a good time to mention file formats: basically, always use either TIFF or Photoshop's own PSD format. If you know enough to know when to use the other formats, you're working beyond the scope of what I'm trying to cover here so go ahead. Never, ever save in JPEG format, it's a 'lossy' format and the image will degrade if you do this multiple times as you edit the image. If you specifically need a JPEG - for a web page or to send by email - then make a JPEG copy for this specific purpose but keep the TIFF/PSD as a master copy.


Just a small number of you - will be waiting, tongue lolling, for the answer to a point which I've avoided so far. Because there's a rather tricky question to answer if you're preparing photographs for publication in a magazine, and it goes like this:

"I've read and understood everything about resolution and I've learned how to scan correctly and fix my digital camera image files, but what if I don't know what size the editor of Drains Monthly intends to print my picture?"

Unfortunately there is no clear cut answer to this. You could ask him of course, but until he sees the pictures and knows how much room is available in the issue, he probably won't know. All I can do is to make some sensible suggestions based on experience and common sense:

- Scan prints at same size and 300 dpi. There is little point in enlarging them as they usually don't have the quality to stand enlargement. Of course I haven't seen your pics so I might be wrong. Might be. Bet I'm not, though :-)

- Scan trannies at 300 dpi and the greatest enlargement that your scanner's *optical* resolution will allow. Optical resolution of scanner divided by output dpi gives you the max. enlargement. Same for negs.

- If you have a reasonably new high-res flatbed scanner with a transparency adapter, and you have both negs and prints, try scanning both using the above recommendations and see which comes out best. It could be either.

- There is only one sensible thing to do with digital camera images, and that's to send a file as near as possible to the one that came off the camera - retouch and enhance it if necessary but otherwise leave it alone. Set the resolution as described above if you wish but *do not* resample the image.

If you follow these guidelines you'll have an image which is as large as your equipment can provide without a trade-off in quality. It's always possible to reduce the size of an image with only minimal loss of quality, so the magazine editor has that option if he needs it.

The editor will want to do his own cropping and he'll do it better than we can because he has the advantage of seeing it in the context of the printed page. So for a digital camera pic don't crop; for a scan you should generally scan the whole frame or at least leave a big margin round the intended 'cropped' image. There is nothing worse for an editor than needing a few mm wider photograph and you've chopped it off. If you've seen a great way of cropping and rotating your photograph for dramatic effect then by all means do that, but think about sending the original as well. How many times have you seen a photograph with 'too much sky' used to really good effect on a printed page, by using the sky as a backdrop for text or a title? [Cue Mark's photograph on the cover of the as-yet-unpublished English White Water book...]


Has it occurred to you that everything I've said assumes one pixel is just as good as another pixel? In fact that's far from true, and one aspect of this is especially worth highlighting: digital camera images are usually of noticeably higher quality than similarly sized scans. This is because they are first generation images, whereas a scanned print is (think about it) a third generation copy. This is compounded by the fact that most scans (outside the professional arena) are likely to be taken from poor quality prints.

A consequence of this is that you can enlarge a digital camera image quite a bit further than a typical scan, and in practice something like 225 dpi is perfectly adequate for magazine repro of a really nice digital photograph. However the world of publishing doesn't (yet?) take this into account and you will usually be asked for 300 dpi whatever the original.

Most digital cameras have a choice of image sizes and compression ratios. High compression ratios produce small files so you can fit more onto one memory card, but at the expense of quality and they can be downright awful. Use the best quality JPEG setting you can, given the limitations of how many you can fit on the card. If your camera has a TIFF or RAW option, they will give an even better quality image but the cost is a *HUGE* file. Only a trained eye would ever see the difference between these and a best quality JPEG, so don't use them. (This is different from what I said earlier about saving TIFF files - use JPEG on the camera and TIFF on the computer.)


That's it then. This has turned out even longer than I intended but I really don't want to spend the time it would take to trim out the unnecessary waffle and well, you've read it now so there's no point.

Anyone who really knows this subject will have seen countless places where I've over-simplified. Contribute the extra detail if you wish!

I have assumed that people have reasonably recent amateur or semi-pro ('prosumer') equipment. If you have professional equipment which is capable of much more than I've described above, then this is not the place to learn how to use it!

Anything I've said about Photoshop applies equally well to any other reasonably capable photo editing software package.

Steve Balcombe.