Book Scanning, Lens Math, and DPI
I haven’t posted one of these in a while. However, every time I run into a problem were I find myself scribbling page after page of equations and numbers I think – maybe this is a blog entry someone might find useful!
At Case Western Reserve University we have (hidden away) an Atiz BookDrive Mark 2 book scanner.
It really isn’t a scanner but a very nice dual camera system, with really good lights, a great book cradle, and a fabulous platen which hold the pages down without any reflection of the lights. It even has a hardware switch where you can shoot just by lifting up and down the platen (after of course you turn the page). I can scan and 300 page book in about 30 minutes! This is a very good tool.
We purchased our unit with twin Canon T5i cameras (these are 18MP each) with 50mm prime lenses for regular books and 35mm prime lenses for larger books.
Operating the unit is pretty straight forward you just need to make sure that you have everything setup properly first (lot’s of things get locked down: exposure, shutter speed, aperture, white balance, sensitivity etc.)
I do NOT lock down the focus because the book will move slightly as the pages turn and also the book gets thicker and thinner depending on where you are in the book.
So why this post?
Well the boss turned to me the other day and asked me to scan a very rare book and asked me specifically for an output of 600DPI.
The specifications were set by the organization that we plan on sending the scans to.
Normally when I work with rare books I am asked to just shoot a page or two and it is done from a photographic point of view i.e. shoot the object as an artifact.
Or, if I want a really good scan of a book I often use our Plustek A300 book edge scanner – that will give me 600-800 DPI without a problem and also give me VERY flat pages. However it involves a lot of handling of the book and, while the scans are great, it is less than ideal for something very delicate unless you want to move in slow motion.
So back to the boss. Can I scan the book at 600DPI?
Well that’s not really the question. Since I am not scanning the book but photographing it – the question is – will the resulting digital image amount to 600DPI? And if not – what do I have?
I could run fancy math (and will in a moment) but since I have the scanner I figured it would just be easier to actually run a test. I photographed a target I made of 1″ squares.
No – I am not going to show you a picture the target – because it is just 1″ squares.
The T5i is an 18MP camera with a resolution of 5184 x 3456.
With my “default” setup the 50mm lens could see about 14.9″ x 9.9″ so
5184/14.9 = 348
3456/9.9 = 349
or about 350DPI.
Not even close.
Now don’t get me wrong – for most of the jobs that I do this is fine. At 300+ DPI the text is perfect and I can see enough of the halftone pattern to correct the images. I would love more but it hasn’t been pressing. That is until today!
What can I do?
- Change my lenses
- Change my sensor
- Change my camera type altogether!
Change my Lenses:
If I get longer lenses (that zoom in more) I would increase my DPI but also decrease my effective scanning area.
The 60mm is the largest PRIME lens I can get which would only get me to 420DPI, a 15-85mm zoom (locked at 85mm) would give me 595DPI (and let’s just call that 600DPI) but my problem now is that the size that it can photograph is MUCH smaller than most books.
Change my Sensor:
Right now I am using a Canon T5i if I upgraded to a Canon T6i (for $800 each) I would get a resolution of 24MP (6000 x 4000) instead – meaning:
Closer but still not really getting to 600DPI at a decent scan area. Assuming the 60mm lens would work it could cost $2,400 just to get to a little under 500DPI (and the scanning area might still not be right).
Change my Camera Type:
Both the T5i and the T6i are crop sensor DSLRs. What about going full frame?
Now I will work backwards. The question I can ask is:
Assuming I like the scan area of 14.9″ x 9.9″ – what resolution camera would I need to get 600DPI?
Well that’s easy. 15×600=9,000 10×600=6,000. 9,000 x 6,000 = 54MP. I
would need a 54MP camera! Oy! Well at least I can jump to the top of the line and skip the 30MP cameras.
The Canon EOS 5DS is a 50MP camera with a resolution of 8688 x 5792. Interestingly enough it is the same price ($3,500 each) as the 5D Mark IV which is only 30MP.
But with a full frame (not cropped sensor) all my math changes again – and not for the better as the lenses become wider angle – in fact the 50mm lens now covers 24.1″ x 16.1″ at 360DPI.
The trick here is that once you start getting into this range of camera the lenses also get expensive. You can get an 85mm lens for $400 or the fancy “L” series for $1,800 – reading through reviews I picked something in between! The 75mm lens above is a zoom lens that can lock at 75mm. Also I checked and the good news is that our prime 50mm and 35mm are compatible with the full frame sensor (though I would clearly never use the 35mm lens for anything).
So for right around $8,500 I can upgrade my kit to allow for 600DPI scanning… considering WHAT I am scanning this is actually reasonable.
Now if only the boss will approve it!