Why does "Despeckle" even out the histogram?

Ok guys and gals, help me out here. I am new to Photoshop (fairly)

and my current equipment situation leaves me without a printer. When

I take a B&W scan straight from the scanner and look at the histogram

I see a nice full graph with no gaps or spikes. I work on the photo

with Levels and Curves and then take a look at the histogram again

and I have gaps and most of the time spikes. I understand what the

gaps represent but I'm not sure about the spikes. If I then apply

the Despeckle filter to the scan I notice that the spikes seem to all

but disappear. What exactly is the despeckle filter doing to the

scan and is this a good thing when working toward the final print?

If I had a printer I could compare results but as it is I don't. Any

ideas on this?

The gaps are missing areas of data. If you scan at 8bits and do heavy levels or curves work you'll easily produce gaps. Spikes usually come hand in hand with this. It's possible, and not too uncommon, that the despeckle will smooth out your spikes.

 

If you have erroneous bits of information here and there, and despeckly catches it, it will clean that up and this will produce a visibly smoother image. This will translate to your histogram which should now show fewer spikes. If your gaps appear to have disappeared, photoshop has "filled in" the histogram but you've still lost signifigant data.

 

I don't particularly suggest the use of despeckle though.

Scan in 12 bit or higher mode if your scanner supports it, then edit in 16 bit/channel (48 bit Mode) in PS. Do all your curves, levels, and whatever tonal or colour adjustment, and only then convert to 24 bit mode to allow the use of the filters etc.<p>Spikes in the histogram mean that you're partially posterising the image. They shouldn't be there, EVER.

Thanks for the advice. I will try that and see.

If you enlarge your image to say 500%, then use despeckle, you will see that it is a kind of blurring tool. It fills in, much the same way as interpolating. It smoothes, yes, but it introduces information that is not in the original. It's a trade off. Norm La Coe