How to set focus and make images sharp?January 7, 2011 on 5:30 pm | In General | No Comments
Use Manual Focus
In the past, we have recommended customers to use AF (Auto Focus mode), but actually it’s better that you scan in MF (Manual Focus mode) so that the sharpness is consistent on every page, and you don’t have to re-shoot an image, or have focus failure when dealing with blank pages.
The depth of field that we normally use (around F5.6) actually give a depth of wield wide enough to cover the varying distance of camera to book.
As long as book thickness (or more precisely the varying of distance between camera and book) doesn’t go above the depth of field (the calculation here shows about 3.3 inches), the image should still be in focus and considered sharp from first to last page.
1. We only need to set the proper focus at first. Let’s set the camera lens to AF temporarily. AF will help us lock focus properly.
2. Then open the book and turn the pages to somewhere in the middle. (because for the setting we normally used, the depth of field will cover around 3.3 inches. Around 1.6 in front of subject and 1.7 behind subject.)
Half press the shutter button so you hear the beep of camera, which means it can now lock focus.
3. Switch camera lens back to MF.
4. Now, try shooting the cover page (check the preview if you are happy with the result or not. The magnifying area will greatly facilitate this task).
If the test shot looks good, then you can proceed with scanning the entire book.
As we are relying on MF mode, whether a page to scan is blank or not, no longer affects us.
If the user is meticulous, he may want to re-adjust focus again every 300 pages (as it approaches the edge of depth of field sharpness).
(In our case, a 2000 page-phonebook, we only set focus at first and never readjust, and all the pages still turns out sharp)
5. You may consider putting some kind of scotch tape and stick it to the lens itself to make sure that the lens focus ring not become loose and change the focus by itself gradually during the scanning without us knowing.
Sometimes a lens’ focus ring can become loose and move itself a little bit as we lift the glass up and down and there’s lots of vibration and impact on the camera. So locking it with the scotch tape is a good practice of ensuring same focus throughout the session.
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