Old DSLR vs Newer Compact?

[Crispy]

Hi

I'm currently sourcing cameras for my Archivist book scanner, and am trying to decide between going for two older dslrs or a pair of newer compacts runing CHDK.

I have an old Canon 350d (8mp); I understand that the number of megapixels this camera has falls below the recommended standard, but it's tempting to buy another 350d because they're cheap and they have the advantage of a better lens than a compact, and I wouldn't need to use CHDK (not that it would be a huge problem to use this).

I've done some (very crude) scanning using the 350d and the rig created by Daniel years ago in the Instructables video, and the output was useable but not brilliant, a consequence of poor lighting and the fact that I had to capture a double spread rather than single pages.

In this instance, does the benefit of a dslr lens outweigh the 8mp offered by the 350d, or should I bite the bullet and go for a canon compact as recommended? What kind of image file size do people normally achieve with a 12mp camera?

[qv_]

I have not try to check "how good" the mpix are in the compact.
From a Canon 550D with the kit lens (IS) I can "collect" 15 mpix of the 18 that the sensor have.

The quality of light and lens is the major factors (i think) to get all possibole mega pixel (information) on the picture.
The file will "always" be 18 mpix but what you want is the information in the file and that is not the same thing. But that you already know.

The problem with my setup for testing is that I test from 2 meters (~7 feet) distance because I don´t have a very good ISO chart to test with.
The best test is to have a chart in the scanner but I do not like to pay for that.
People tell me that compact lens is "better" in short distance and DSLR lens is better on "long" distance.
But I don´t know what to think. I will keep the 550D because I can control the camera automatic without programs and computer and that will simplyfi the use of the scanner (max speed is 1500 page/hour). The DSLR also have the good think that the lens stay in position when you turn off the camera No need for rezoom when you start working after lunch.

The light need to be in 45 degree to get maximum (information) pixels.
And I need to build the scanner into a working state! But soon I will have time for that.

/Jan

[duerig]

qv_, can you go into more detail about 'collecting' pixels? Thus far when I have scanned, my camera (either cheap Canon point and shoot or a nicer DSLR) has always been in jpeg mode and so I end up with a fixed resolution photo that is already processed somewhat by the camera hardware. Are you processing raw image files instead? Is this giving you better quality? What tools are you using to do this?

-D

[qv_]

If I use my 550D (in max resolution) and take a photo of (thight) lines I will se more or less lines on the picture (in JPG) depending on the light and lens quality.

If I use the internal flash I can see 70 lines/mm
If a change to a better light sourse I see 85 lines/mm

The camera have the same maximum resolution (18 megapixel) but the file get better (sharper) by just changing the light.
If I change the lens to I prime like 50mm f1.4 I get even better lines/mm.

So if you read about ISO test charts you will se that they are talking about lines/mm, not "megapixel"
to convert megapixel to lines/mm you can do a simpel math.
The lens/optics/sensor "lose" 10% of it´s maximum megapixel (lines/mm) so a 18Mpix camera can only fill the picture with lines/mm thats equels 18-10% = 16.2 megapixel

With my 550D and the kit lens (IS) I think I can hit the "15 Mpix information" in the JPG file. Thats my goal.

I like to have minimum 300 dpi on my scan. Sofar I have just test the lines/mm in the center of the piqture.
When you look at the couner of the piqture the lines/mm go down. the piqtuer is not that sharp there.
My gool is the have 300 dpi minimum even in the couners, but I have not had the time to build every thing ready yet.

In the programs the do not see the "information" pixels. The program see the "sensors pixels"
The program just hope that the light and lens is so good that all information is there. The program do not know if the pixel should be black or white. You do.

So I will use JPG (simple) and put some energy to make the best light sours.
The 550D will be in the "best" ISO (100 or 200) and the lens is sharpest at f5,6 -f8.
If the lens is not sharp enought when I zoom to the book I have to change the distance between book - camera so I can use a different focal lenght that is sharper.

If I use a "bad" light sourse I get 10 mpix of information in the piqture.

/Jan

Hope that you can easly read my english.