Scanning slides using 600DPI gives better results then 2400 dpi ... Why ?

[emptyNL]

Hi

Using the Canon Canoscan 8000F I scanned a slide using 2400dpi (the Maximum
optical resolution) and also using 600DPI
I was very surprised to see that the 600DPI image was sharer than the 2400
Dpi

I've tried all kinds of settings but never got better than the 600dpi scan

I my scanner broken or is there something about it i do not understand ??

I have taken a small part of both the 600 and 2400 scan and placed it here
: http://maatwerkpc.nl/sample/

 

[CSM1]

Hi

Using the Canon Canoscan 8000F I scanned a slide using 2400dpi (the
Maximum optical resolution) and also using 600DPI
I was very surprised to see that the 600DPI image was sharer than the 2400
Dpi

I've tried all kinds of settings but never got better than the 600dpi scan

I my scanner broken or is there something about it i do not understand ??

I have taken a small part of both the 600 and 2400 scan and placed it
here : http://maatwerkpc.nl/sample/

The 2400 dpi image is not in focus. You could try to find the optimal focus
point by raising the image a little from the glass. Put small strips of
paper between the image and the glass until you find where your scanner is
focused.

You may be seeing the fact that flatbed sensors are not as sharp as the
manufacturer claims. A lot of people have posted on that very fact in this
and other newsgroups.

 

[Wayne Fulton]

Using the Canon Canoscan 8000F I scanned a slide using 2400dpi (the Maximum
optical resolution) and also using 600DPI
I was very surprised to see that the 600DPI image was sharer than the 2400
Dpi

I've tried all kinds of settings but never got better than the 600dpi scan

I my scanner broken or is there something about it i do not understand ??

I have taken a small part of both the 600 and 2400 scan and placed it here
: http://maatwerkpc.nl/sample/

Your 2400 dpi image size is 2316x1837 pixels. (0.96x0.76 inches)

Your 600 dpid image size is 565x406 pixels. (0.94 x 0.67 inches)

My guess is that you are viewing these on the video screen. Note that the
larger 2316x1837 pixel image is very much larger than your video screen.
Therefore your photo editor (viewing program) probably resamples it smaller,
to fit on your screen, to maybe 1/3 size, and shows you that temporary copy.
Such temporary resampled copies are "quick and dirty", inferior sampling job
as compared to the job that the slower resample menu can do.

Probably the point you dont understand is that the entire point of higher
resolution is to create a larger image, for a larger purpose. For example,
to print 8x10 inches at 300 dpi, you need 2400x3000 pixels. But to view it on
a 1024x768 pixel video screen, you dont need more than 1024x768 pixels.

The other factor here is that 2400 dpi images on a flatbed are never very
sharp.

 

[Kennedy McEwen]

Hi

Using the Canon Canoscan 8000F I scanned a slide using 2400dpi (the Maximum
optical resolution) and also using 600DPI
I was very surprised to see that the 600DPI image was sharer than the 2400
Dpi

I've tried all kinds of settings but never got better than the 600dpi scan

I my scanner broken
Unlikely.

or is there something about it i do not understand ??

I would be very surprised if you understand everything about your
scanner (what value is C12 on the circuit? for example) so this is the
most likely explanation. ;-)

Seriously though, there are several perfectly valid reasons why this
happens. A lot depends on how you actually view the images for
comparison. Wayne explained that if you view the images at the same
size on screen, the viewing software can make a mess of decimating the
amount of data in the 2400ppi image, resulting in a poorer image than
the 600ppi version.

However, viewing each image at 100% scaling (so that each pixel in the
image is displayed on a single pixel on the screen) you will still find
the 600ppi image looks sharper than the 2400ppi one - and in this case
no resampling of the large image is occurring at all. What is different
though is the magnification of the final image from that original slide
- so you are seeing the limitations of the slide and the scanner
magnified by a factor of x4 on the 2400ppi image as you are on the
600ppi one. Another way of considering this same effect is to remenber
that no scanner is perfect, the optics and the CCD elements add to any
blur that is present on the original slide. When you scan at 2400ppi,
the distance between the pixels is only 1/2400th of an inch, and so any
blur can be seen between adjacent pixels in the image. However, when
you scan at 600ppi, only every 4th pixel in the CCD is actually used so,
although all of the blurring is still the same, the distance between
adjacent pixels in the image is 4x greater, and this distance may be too
great to see the effect of any blur in the image.

Better quality scanners actually produce the 600ppi image by averaging
4x4 blocks of the 2400ppi optical resolution image. This has the effect
of increasing the effective size of the CCD sampling element, and there
are good reasons for doing this such as improving the signal to noise
and preventing aliasing etc., but the overall effect is to increase the
blur on the 600ppi image. In other words, the defect on your scanner is
that the 600ppi image is actually a little sharper than it really should
be - although it is likely that the 2400ppi image is also a little
softer than it ideally would be as well.

 

[Andy Salnikov]

Hi

Using the Canon Canoscan 8000F I scanned a slide using 2400dpi (the Maximum
optical resolution) and also using 600DPI
I was very surprised to see that the 600DPI image was sharer than the 2400
Dpi

I've tried all kinds of settings but never got better than the 600dpi scan

I my scanner broken or is there something about it i do not understand ??

I have taken a small part of both the 600 and 2400 scan and placed it here
: http://maatwerkpc.nl/sample/

Sharpness on 2400dpi image is probably all you can get from your scanner.
For many (or maybe all) flatbeds optical resolution is significantly lower
than "dot pitch" advertised as resolution. 600dpi image looks very
over-sharpened to me, the artifacts are terribly over-amplified. You
probably
can do a simple exercise with your favorite editor by reducing the 2400
image
size to 600 dpi and applying insane amount of shaprening - you should get
the
result very similar to your 600dpi image.

Andy.

 

[David Springthorpe]

Sharpness on 2400dpi image is probably all you can get from your scanner.
For many (or maybe all) flatbeds optical resolution is significantly lower
than "dot pitch" advertised as resolution. 600dpi image looks very
over-sharpened to me, the artifacts are terribly over-amplified. You
probably
can do a simple exercise with your favorite editor by reducing the 2400
image
size to 600 dpi and applying insane amount of shaprening - you should get
the
result very similar to your 600dpi image.

I'm a tyro at scanning (slides on a flatbed) also, and the replies are
interesting as I've noticed these outcomes.....I've tried scanning at 2400DPI,
sharpening several times (Irfanview) and then reducing to 600 DPI.....

DS