scan resolution...again!

[psilosimon]

My understanding of scan resolution is a bit vague.
I have just sold a Primefilm 3600U and purchased a Plustec 7400, reason being that the 3600 did not yeild enough resolution or dmax as the latter. I am hoping to supply my images to Photo Libraries, Magazines and such, who require high resolution scans with file sizes above 50MB. Alamy for instance, state they require 4000ppi 70MB scans.

My new scanner will go up to 7200 dpi. I guess this is the max optical resolution?
This gives a massive file size. 175MB tiff is about 9000 x 7000 pixels

where does 300dpi come in to it?

Is that what I then change in the Image size box (from 7200 to 300dpi)with constrain proportions no resample???

I'm a bit lost in all this. Need some laymans terms.

[donny]

As far as I can understand the Plustek has an optical resolution of 2,900 and the Primefilm has an optical resolution of 3,600. I stress that I am referring to the optical resolution , anything else that is claimed is being done digitally and probably being done worse than you could digitally expand yourself in your own software.
I think therefore that you have in fact reduced your resolution and that scanning on such a basic machine will not give the results that you are looking for.Magazines , libraries etc. are notoriously picky even with the best of files and to get any thing like their standards would be well beyond these machines capabilities.
Don
PS having reread my reply I think I may sound rather elitist over your machines capabilities and I am sure that it will produce adequate results for most uses but not for the very demanding uses that you quote.

[GlennH]

As far as I can see the Plustek does have a 7200 dpi resolution (described as hardware resolution on the Plustek site). If that's true, it's OTT for Alamy purposes and will vastly increase your scanning times - not to mention your HDD requirements.

Last part of your question, looking at the Silverfast interface on the internet (if Silverfast is what you're using?) - yes, set the dpi to 300, and double-check the file size to make sure you're complying with Alamy's requirements (ie 100MB in 48-bit, or 50MB in 24-bit).

[donny]

The Plustek has an optical resolution of approx 2,900 boosted to its claimed resolution by software and I seriously doubt having seen some scans done with it that it would ever meet the sharpness that most libraries require. If one looks at the the density range in both the higlights and shadows then the lack of detail is apparent, the loss of detail from the original to the scan is so very apparent.
Any scan can of course be increased to a given file size by interpolation but at the expense of quality of detail and sharpness
Don

[GlennH]

PS - just to clarify, 300dpi is the printing resolution that Alamy require. It's not the same as scanning resolution, so 7200 dpi and 300 dpi are unrelated.

Essentially, you just need a file size of around 50MB (8-bit/24-bit), with a longest edge of approx 5000 pixels (roughly equivalent to an A3 print at 300 dpi).

[GlennH]

I'd be willing to believe you Donny if I could find a shred of evidence. Plustek claim a hardware resolution of 7200dpi - that means it's not interpolated. Various reputable vendors sell it as such.

I can't account for the quality of the scans of the Plustek - but many libraries don't accept scans regardless, and especially from consumer-level scanners. Alamy, however, sell mostly editorially and at typical reproduction sizes such scans will be fine.

[GlennH]

Okay, this link gives an indication as to the limitations of the Plustek. Whilst the resolution is not interpolated, the article questions the effective resolution - and claims it to be 3800 ppi.

This is not such a shock - as there comes a point where the scanner can extract no further useful detail from the film, even if this is determined partially by the hardware. It might perhaps be interesting to see a scan at around 4000 ppi - which is nearer the resolution you'd need.

If you're submitting to Alamy - they will soon let you know if the scans are sub-standard.

[donny]

Hello Glenn,
The reason I picked up on this theme is an acquaintance of mine bought one of these machines to scan his old slides, he has now gone digital, and he was disappointed with the results. I went to the same review site that you quoted in your last post and found the same results, I mistakenly quoted 2,900 when I meant 3,800.I subsequently lent my acquaintance my Nikon Coolscan IV and the results were so much better.This is from a piece of kit that is over five years old.
My main thought was not to get bogged down too much in technical issues but to suggest that practically any "enthusiast" scanner would probably prove to be unacceptable to the libraries.
Don

[GlennH]

Aye, well you're right Don. A library that I submit to was using a Hasselblad drum scanner I think last time I enquired - somewhat beyond my budget!

How much that level of quality is appreciable on a quarter-page reproduction is debatable - that's what Alamy exploit.

But I agree that you should buy the best hardware you can afford when aiming to sell pics.

[mark_jacobs]

Firstly welcome to the AP forum psilosimon

DPI? SPI? PPI? LPI? It's a nightmare isn't it?

Hard/software manufacturers, magazines and websites haven't made it any easier by mixing and matching the terms as if they were interchangeable, they're not.

DPI - Dots Per Inch:
In this discussion is best used when describing the output from a desktop printer, and therefore not relevant.

SPI - Samples Per Inch:
Rarely used unfortunately but it would be better if used more often when describing the number of input samples per inch of the source material made by a scanner.

PPI - Pixels Per Inch:
Best used when describing the digital output of a source, be it a camera or software.

LPI - Lines Per Inch: Although often also referred to as DPI
Describes the halftone* frequency of a printing press used for newspapers, magazines and the like, often the intended clients of photo libraries.

*Halftone being the small dots that make up the image, when looked at closely, in a newspaper, magazine or book.

Now…

where does 300dpi come in to it?

Firstly it would be better to ask 'Where does 300ppi come in to it?'

In relation to submissions to libraries, magazines etc for final output on a halftone press it comes in to it in so far as… stay with me… the accepted rule is that the ppi resolution of an image should be double the lpi halftone frequency of the final printed output at 1:1.

Now… newspapers usually have an lpi halftone frequency of 85-100lpi, magazines 150-185lpi. The 'rule' (which can be broken) says that an image appearing in a magazine that uses a halftone frequency of 150lpi should have a resolution of 300ppi at 1:1.

In other words the 'rule' says that at 1:1 the input resolution should be double the output resolution (for halftone printing)**.

**The obvious issue being that when submitting images it is unlikely you will know at what size they will finally be used. It is for this reason that libraries usually ask for a minimum file size. For example a file in the region of 4960x3508 pixels or 48MB RGB would allow for an A3 DPS (Double Page Spread) halftoned reproduction using a 150lpi halftone frequency at 300ppi and as such cover most of the bases.

Is that what I then change in the Image size box (from 7200 to 300dpi)with constrain proportions no resample???

Don't forget ppi not dpi in this case
Yes. If you were to resize a 1"x1" digital file with a resolution of 7200ppi with constrained proportions and no resampling to 300ppi nothing would change except the image dimensions.

The 1"x1" 7200ppi image would now be a 24"x24" 300ppi image. The number of pixels it contains would remain the same.

[psilosimon]

I'm sorry guys I'm still none the wiser really. The information you have given me is a bit conflicted.
I would like to see how much difference there really is between a 4000 and a 3800 dpi scan.
It would also be nice to prove that an 'enthusiast' scanner can compete with higher priced scanners. Otherwise Photography is becoming just about how much money you can spend.
Before digital infiltrated our lives a photographer could take some excellent shots on a cheapo camera with a reasonable lens maybe a filter or two and a roll of film and as long as the exposure was ok you could yield some pretty good shots for probably a fraction of the cost of equivalent gear nowadays. I have books filled with pictures like this.
Sorry rant over!

I did a scan of a slide at 7200 dpi and ended up with a Tiff 6416 x 9578 that was about 175MB, I changed to 300dpi in image size (This is what is required is it not - the output size???)
I saved as a jpeg the pixels remained the same but the MB shrank to 28MB

The quality of the scan is instantly superior to the scan on the old 3600U

Still confused!

[GlennH]

Mark's right and my statement was wrong - for which I apologise (thanks for the clarification and education Mark!). Because 7200ppi gives you a file which is too large, however, you should scan at something like 4000ppi.

The quality of the scan from your scanner may be better at 4000dpi than it is at full resolution - which would be okay as you don't really need 7200ppi scans.

[Fen]

I did a scan of a slide at 7200 dpi and ended up with a Tiff 6416 x 9578 that was about 175MB, I changed to 300dpi in image size (This is what is required is it not - the output size???)
I saved as a jpeg the pixels remained the same but the MB shrank to 28MB

JPEG is a compressed file format which makes it smaller (in filesize) than a TIFF file which isn't compressed.

[Norman]

OK, lets do some maths.

Alamy state that they require 4000dpi 70mb scans.

Taking a 35mm film frame of 1" X 1.5" scanned at 4000dpi (or ppi) gives you a image size of 4000 X 6000 pixels or 24 million pixels. A TIFF file is usually about 3 times (*) the number of pixels so will be about 72mb in size, as they require.

The same 35mm frame scanned at 3800dpi (ppi) will give a 3800 X 5700 image or 21,660,000 pixels which will produce a TIF file of about 65mb.

* Each pixel requires 3 bytes to store the amount of red, green and blue information.

[GlennH]

Alamy require 48MB files, Norman, but I'm not questioning the maths (seemingly it's not my strong point!).

[Norman]

I don't use an agency so I was quoting what the OP put in his original post.

[psilosimon]

Mark's right and my statement was wrong - for which I apologise (thanks for the clarification and education Mark!). Because 300ppi gives you a file which is too large, however, you should scan at something like 4000ppi.

That's all I was really saying. The quality of the scan from your scanner may be better at 4000dpi than it is at full resolution - which would be okay as you don't really need 7200ppi scans.

Ok, please clarify diference between 4000dpi and 7200ppi.

[psilosimon]

I'm looking at the scan display right now, I can do a 48>24bit scan
the output resolution ranges from 150 dpi to 7200 dpi or 59 to 2835 dpcm.
The highest dpi yeilds a 194mb file

If the highest optical res is 3800dpi - is that what dpi I should scan at by typing that figure into the output resolution box - this will yield a 55MB file.

I think I'm beginning to understand. I am crap at maths!

[GlennH]

A 55MB file would be fine for Alamy, so yes - scan at 3800ppi (or dpi as it may appear in the software).

The 3800ppi effective resolution is just the figure arrived at by one review site after testing, but I suspect that you'll get a better quality scan at that size than you would at 7200ppi.

[mark_jacobs]

I think I'm beginning to understand.

I think you are

I am crap at maths!

Me too