Black and White 600 ppi would be better from my experience and you can certainly do what you describe with success but maybe resample the image as you scan that is scan at say 600ppi at 24x36 and rather than resample up down.
I know that might not be the file size you want but you might experiment and see if there is a difference in your eyes. But your own suggestion is a good one.
No one can correctly answer your question without more info:
What is the hardware resolution of the scanner?
What kind of original?
What size is the original?
Will the scan be used in the final, or will it merely be used as an intermediate step for tracing as vector artwork?
If printed, will the line art raster image be reproduced in a single ink or in a process color?
If printed, what kind of device will it be printed by?
I scan at either 300 dpi or 400dpi for the black and white line drawings I create...
What does that mean? Are the "line drawings you create" the originals that you scan, the scanning results (raster images) that will be used as part of the final Illustrator print-ready file, or vector line art you will trace to replace the scan?
, which result in smaller file sizes than gray scale scanning.
Because only 1 bit of data is used for each pixel, rather than 8 bits in grayscale.
However, if the purpose of the scan is to provide a raster image for you to trace in Illustrator, grayscale is often better because you can see more of the intent of your original sketch. For example: a pencil sketch is not black; it's several tones of gray. When you scan it as 1-bit ("bitmap"), a threshold grayscale value is used to demark which pixels go white and which pixels go black. Scanning in grayscale, you don't have to worry about that; the image looks like your pencil sketch as you trace it.
If the purpose is to actually use the scan as the final art, scanning in grayscale is still often advantageous, because in your raster imaging program (Photoshop or whatever), you can then adjust the gamma to control the white/black threshold before converting to 1-bit.
SInce the scanned image is bitmapped, does the resolution you scan at matter?
"Resolution" only matters with bitmapped (raster) images, because only raster images have a PPI "resolution." If you are using the term "bitmapped" here to mean 1-bit color depth, then whether the "resolution" matters depends on the purpose, which you have not provided.
For clarity, it's best to avoid the term "bitmap" as a synonym for 1-bit color depth. All raster images are "bitmaps" (maps, or arrays, of pixels). Plus, many use the term "bitmapped" to mean "pixelated," which can apply to images of any color depth. The question is how much data (how many bits) is used to define the color of each pixel. So basically a raster image is either 1-bit, 8-bit grayscale, 24-bit RGB, or 32-bit CMYK. They are all "bitmaps."
If the purpose of the scan is for it to actually be used as part of the final design, then, yes, the scanning resolution certainly does matter. But how it matters is dependent upon the intended use. Is this for desktop printing? Commercial offset printing? Web images? Again, you haven't provided enough info for a meaningful answer.
Would I gain anything scanning at 600 dpi?...I then import image into Illustrator to enlarge to 24 by 36 inches.
You're essentially asking, 'If I scan something at 600 ppi, is that enough resolution to reproduce at 24 x 63 inches?' No one can answer that question without knowing the size of the original and how it's going to be reproduced.
Assuming you are scanning a hand-drawn sketch just to use as a tracing guide which will be deleted after tracing:
Scanning color depth and resolution don't matter, so long as you have the fidelity you need to work with while you trace. (I am NOT talking about auto-tracing here, which is almost always an amateurish practice yielding amateurish results.)
Assuming you intend for the actual scanned image to be used as part of the final, the line art will be printed in a single color, not a composite color, and the target output intent is commercial offset printing:
Understand: Assuming a PostScript workflow, 1-bit raster images do not get halftoned. So the imaging device can render the image pretty much printer-spot(s)-per-pixel. In other words, the imaging device will not be using its printer spots to "construct" larger halftone dots, which in turn become the "dots" that relate to the pixels in the image. In other words, a 2400 SPI imagesetter can faithfully reproduce a 2400 PPI 1-bit raster image.
Typically, 1200 ppi at the final reproduction size is suitable for quality commerical reproduction of 1-bit raster line art. The resolution of the imaging device (the imagesetter) is going to be at least twice that, so it is capable of reproducing it faithfully. In most applications, 1200 ppi will appear fairly razor-sharp to the human eye at normal viewing distance; higher PPI is practically undiscernable, therefore unnecessary.
Assuming you intend for the actual scanned image to be used as part of the final, the line art will be printed in a single color, not a composite color, and the target output intent is desktop printing:
Typically, even affordable desktop laser printers nowadays have a hardware resolution of 600 SPI. A few have a hardware resolution of 1200 SPI. But laser printers, by definition, print "fuzzy" piles of toner, not sharp-edged round dots like an imagesetter (or direct-to-press plate). So 600 PPI (at, of course, final scale) is almost certainly sufficient for desktop laser printers.
Assuming you intend for the actual scanned image to be used as part of the final, the line art will be printed in a single color, not a process color, and the target output intent is a large-format composite printer:
Your mention of 24 x 36 inches raises the posibility of a one-off or short-run reproduction on a large format composite printer. There, two device-specific factors come into play: Is it a PostScript device, and if so, what is its hardware resolution? If it's not PostScript, the whole page gets rasterized and sent to the printer at once, regardless of individual object resolution. You can think of this as the computer "zooming" what you see on your monitor to something like 400% and then printing a "screenshot" of that virtual "monitor." The whole thing is processed as a single RGB or CMYK image anyway, so halftoning (probably stochastic) is applied to the whole thing. Therefore, PPI of 1-bit raster images may as well be the same as raster images of higher color-depth on the same page.
Assuming a pristine original that you want to reproduce as faithfully as possible:
Scan at the actual hardware resolution of the scanning device, or at a divisor of the hardware resolution closest to the desired effective PPI.
For example: A scanner marketed as having a 2400 PPI resolution may very well have an actual resolution of 600 PPI, and merely uses software pre-processing to upsample the scan by interpolation. If you need to do resampling, you can do it with more control in Photoshop.
Assuming an imperfect line art original that will be grossly enlarged, but will not be replaced with vector artwork:
Scan initially in grayscale. In Photoshop, adjust the gamma (to control the white/black threshold) and judiciously apply blur (to smooth out jaggies and erratic edges). Then convert to 1-bit at the final PPI required for the target reproduction. Careful use of that technique can yield 1-bit raster images that have a final smoothness that rivals that of a vector tracing.
Thank you kindly for your very useful answer. I forgot about gamma in PhotoShop. I am using a CanoScan 5600F with a 2006 MacBook Pro.
The goal is the have the line art be used as the final print.
I print on 24 by 36 paper then draw over the image.
So in a sense I'm tracing on paper.
I like the results I got from Fed Ex Office of reproducing my 6 inch by 5 inch drawing and enlarging it
to standard poster size.
JET has provided a ton of useful info, with all the technical backend, as always. (We used to have a lot of fun on the Freehand forum, way back in the day!)
I'll only add to this to say that I tend to use 1200ppi as my practical minimum for 1-bit images, but most often I scan/create them at output resolution (which for our film and platesetters is 2400).
You might see some pixelization or jaggy/"stairstepped" edges at resolutions below 1200, and definitely at the 3-400ppi you mention, but definitely not at 1200 or above. So if you're scanning a signature or other piece of b/w lineart as a high-res 1-bit it will look perfectly crisp when printed.