CONTRAST radically effects test results with scanners and lenses:
If one tests a device with a 1:1000 contrast test negative; one might get results of = X line pairs per mm
If one tests a device with a 1:8 contrast test negative; one might get results of = 0.8 of X above line pairs per mm
If one tests a device with a 1:1000 contrast test negative; one might get results of =0.45 of X above line pairs per mm
Thus if one uses super high contrast targets; one gets high numbers. It is sort of like warming up your car driving it around; adding 50 psi to the tires; driving up to interstate topping of the gas tank and driving on a trip at 55mph non stop and getting fantastic gas mileage. You best cased the test and got numbers folks normally never get in real life.
What you really want to use is test targets that represent what YOU are trying to compare; so one has a more realistic test. Thus if your gig is 35mm Kodachromes; use some sample 35mm Kodachromes. If it is microfilm; use microfilm. If it is C41 35mm negatives; use some of our own C41 negatives.
In reality the public does not shoot 1:1000 test targets; they shoot low and medium contrast things. Using only 1:1000 test targets really gives one the limiting best case resolution of scanners 1,2,3, 4 etc. Scanner 2 and 3 might *test* the same numbers with 1:1000 references; but #3 model can be alot better when a 1:10 test target is used. The lay public ignores this and often cannot fathom a scanner has a transfer function of resolution that drops when the test target drops in contrast.
Here we got our first 35mm slide scanner back in 1989; ie 21 years ago. I use for testing some lens test panatomic-x negatives that were of 1:1000 USAF charts were a Konica Auto S2 at F5.6 recorded about 92 line pairs per mm; one of the best tests I have ever shot. I also use a similar set where I used USAF charts that were grey at 1:10 bars; and a Kodak reference glass slide that has vertical and horizontal progressively smaller bars.
What I do is I jsut test out each new flatbed; 35mm film scanner we buy with our own references. Thus if say our Canon FS4000 35mm film scanner pulls out alot of the details at 4000 dpi; and our latest 4800 or 6400 dpi flatbed pulls out say one half; I know that our flatbed is not really a 4800 or 6400 dpi device; maybe they only test in at 1800 to 2400dpi. I down sample the great drum or 4000 dpi film scanners image to appear to look like the ACME6400 flatebeds image. Thus experimentally I TEST for each devices *real world* specs. It really doesnt matter if another drinks the Koolaid and believes an Acme 10000 flatbed scanner is a 10,000 dpi device. In Engineering if I cannot measure it it is crowned marketing BS. Others/Lemings will jump in and say an Acme 10000 flatbed has an optical resolution of 10000; and thus I am in error.
If you scan for the public like I do; nobody brings in test targets to scan. They bring in shoe boxes of moldy gobs of Instamatic slides; faded 620 Kodacolors shot with box cameras; messes of 35mm slides; gobs of C41 35mm stuff. *ALOT* is not this best case stuff that taxes a scanner if it is the common Joe Six packs stuff.
I have some "old GIANT super sharp negatives to scan" is what a lady mentioned on the telephone last week. I am thinking 4x5" or 616/116 stuff. It turns out to be 6x6cm stuff; old 120/620 Kodacolors shot with a box camera. They are not even very sharp; our Epson 1200U flatbed from 10 years ago is a vast overkill; I scanned them at 1200 dpi; they really have only about 600 dpi worth of info since they are so overexposed; portraits shot too close with a fixed focus box camera. The background clutter is way sharper than the persons face.
You really want test targets that have mythical best case lab/bragging conditions; and some test targets with that are not the amateur/goosing to look good too; ie real world targets. Having some test targets with practical real world moderate contrasts like 1:10 or 1:5; 1:2 is what really makes scannera be more comparable; since it represents 99.999 percent of the publics "stuff". The publics stuff is not shot all at F5.6 with panatomic-X with Summicrons on granite blocks with timed lights. The public stuff is shot alot with iso 800 stuff; with kit zooms; with autofocus that grabbed the wrong kid in soccer.
Testing scanners for "resolution" is very old; it is old than photo.net It was being down before Photoshop or even digital cameras. Newcomers tend to fall into the best case trap; they use 1:1000 targets and crown their scanner a 2345 dpi device. Some of us who have the same scanner will look at these claims as being totally crazy/insane; they just make bloaded files which are useless.
In working on check scanners; we did use some best case references sometimes; but using actual checks written by humans is where the tire hits the road. One has weird ink colors that do not scan as well; one has faint lettering; pencil too.
The whole transfer function of a scanner varies by color too; faint 1:2 or 1:5 contast yellow or blue lines on whiten are not going to give the same resolutions as 1:1000 contrast black targets folks worship as gospel.
If you want goosed pumped up numbers to brag about; then use only 1:1000 targets. If want reality; use actual targets/samples like you are going to scan with your scanners; and you will get way lower numbers.
Here I really *want* to know the transfer function of my scanners; ie best case 1:1000 stuff and real world 1:10 and 1:15 contrast stuff of black and white and weird colors too. Knowing 1:1000 data is a best case number; 1:2 contrast stuff might be only 25 to 50 percent as good as resolution. It actually represents alot more valid to real world originals; but strangely few understand this. It is sort of like wanting real world gas mileage going to the store; work versus a super best case conditon that has high goosed numbers.
, Mar 21, 2010; 12:36 a.m.
