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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Coin Photography, Conservation and Storage  |  Topic: Choice of exposure .. and what follows 0 Members and 1 Guest are viewing this topic. « previous next »
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moonmoth
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« on: August 20, 2006, 11:09:22 am »

When taking a coin photo, it's essential to get a decent depth of field, which means setting the aperture correctly.  I find that f16 works best.  The next question is how to deal with exposure time.

It's quite possible to let the camera calculate this.  But care is needed.  Left unadjusted, a camera will try to balance out the picture so that if colour tones were removed, it would average a neutral grey.  But what we want is for the coin to be correctly exposed, regardless of the state of the background, which is due to be edited out later anyway.  Shiny highlights and dark areas can both fool the camera.

Here is a set of 5 shots of a Sasanian drachm.  (Ignore the other coin for the moment.)  These are Photoshop thumbnails, and the exposure compensation ranges from one stop under to one stop over at half stop intervals.  The best of these is half a stop overexposed.

That other coin is a small bronze of Pergamon.  The next image is a similar range of thumbnails of that coin, and you can see that in this case the best shot is half a stop underexposed.

As a rule of thumb, this works for me.  Bronzes, half a stop under; silvers, half a stop over; neutral tones, no adjustment.

More to come ...
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moonmoth
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« Reply #1 on: August 20, 2006, 11:42:19 am »

These photos need some processing.  As they are, they look a little washed out, because the camera sensor hasn't behaved perfectly as wanted, and the range of tones on the coins don't cover the whole range of white to black.  This can be adjusted by tweaking the levels.  With the Pergamon coin, this produces the first image here.  This is the image from the first set above, and the background tone has been removed with the paint bucket.

But .. adjusting the tonal range has unnaturally emphasised the colours.  To compensate for this it may be necessary to desaturate the image a little, as shown in the second image, which some may think is a better approximation to reality.

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« Reply #2 on: August 20, 2006, 11:47:54 am »

In the process of paint bucketing, you might come across the problem shown on the first image here.  The tones in this coin are close to some of the background tones, and to paint bucket the background successfully you would need to protect the coin itself.  Photoshop's quick mask does this well enough.

The second image shows another constant bugbear - tiny hairs and flakes of skin.  It's best to brush coins before you photograph them.  I use a stencil brush, which is ideal.  The bristles are strong yet soft, and you can punch them onto the surface of a coin if necessary.  Some coins seem to hang on to skin flakes unreasonably well.
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« Reply #3 on: August 20, 2006, 12:25:02 pm »

Hi,

'Photoshop's quick mask does this well enough'

Could you please tell me a bit more about this - I actually use paint shop pro but I think the general processes are the same if not the actual 'buttons' you need to click?

Many thanks
MAlcolm
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moonmoth
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« Reply #4 on: August 20, 2006, 12:36:47 pm »

I'm not sure if Paint Shop Pro will do this, but in Photoshop you need to select quick mask mode - the button shows in red about the middle far right of the screen in the first image.  Then select the eraser and use a huge brush with an edge curve that matches the edge of the coin, and go round and make the coin pink, being careful at the edges.  (Second image.)

 
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« Reply #5 on: August 20, 2006, 12:38:44 pm »

Make sure the brush you select has a hard edge, not a fuzzy one.   Then when you re-select the normal mode (the square to the left of the one to select quick mask mode) you will find that all of the coin is selected out (last image), and then you don't even need the paint bucket.  You can just wave a huge paintbrush over the whole picture and it will only paint the background.

To quick mask with white as the paint colour, use the eraser; if black is the paint colour, use the paint brush.
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Scotvs Capitis
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« Reply #6 on: August 20, 2006, 01:07:09 pm »

Make sure the brush you select has a hard edge, not a fuzzy one. 

Why do you use a hard edge for masking, doesn't the resulting pixellated step effect occur? I almost always use Photoshop to mask out the dust and tonal variations of my scanned images, I scan with the flatbed open since I prefer black backgrounds, but get dust and light splotches sometimes. But I always use a selection that is feathered slightly (1 to 1.5 pixels) and cheat in on the coin's edge by 1 pixel to eliminate a halo.

Maybe its just personal taste but I can't stand a pixelated hard edge on coins.  Sad

Good thread by the way.

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« Reply #7 on: August 20, 2006, 01:37:36 pm »

I don't get a pixelated edge effect, not a visible one anyway.  Maybe that's because I am working on quite large image files.  Here's a section of coin edge that has been masked out, showing the crisp edge deliminated by the marquee, and the end result reduced to an image that would be 1,000 pixels wide at full size, showing both sides.  That's the image size I use as a keeper.
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« Reply #8 on: August 20, 2006, 01:50:30 pm »

... Though, I find that the tones on the coin that are affected by the paint bucket are the light areas just inside the very edge of the coin where the rim reflects the light, and sometimes areas further in, but not usually the delimiting edge itself, which is a bit darker.  You can see that darker edge quite clearly in the image attached to the previous post. 

So if you mask out almost, but not quite, to the very edge of the coin, the paint bucket can still do its work.  That way, masking still needs to be accurate, but does not need to be quite as precise at the edges, which helps speed things up.
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« Reply #9 on: August 20, 2006, 09:18:52 pm »

I use the lasso tool because I've never been able to get the hang of the paint bucket. Took a while to get used to, but now I can select a coin out of the background quite quickly (within 10 seconds). I use feathering (usually at 2px) to get rid of any resulting jagged edges.

Peter
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moonmoth
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« Reply #10 on: August 20, 2006, 11:19:37 pm »

10 seconds sounds very practical.  It would take me much longer to draw around a coin freehand.

Ancient coins aren't precise circles, but the edge is usually made up of a series of arcs or close approximations, and adjusting the brush size in quick mask mode makes selecting very quick and easy.

Also, if only part of the coin is affected by the paint bucket, you only need to protect that part.

The paint bucket covers the exact tone you select, and also any other tones quite like it that connect up on the image.  In Photoshop, you can set how close a tone needs to be for the bucket to paint it, and you have to strike a balance between too exact and too all-encompassing.  Maybe if you play around with that you might get a better result.

Although in Photoshop I don't need to feather to avoid jagged edges, I remember using the lasso in an early version of PSP back when it was shareware (it was version 4.12), and then I did the same as you - some feathering was essential.
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« Reply #11 on: August 21, 2006, 12:41:38 am »

Many thanks for those tips - I did this process manually for a few coins and was dreading having to do it for more.  I have downloaded the trial version of Photoshop and will experiment
All the best
Malcolm
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« Reply #12 on: August 21, 2006, 01:51:31 am »

reduced to an image that would be 1,000 pixels wide at full size, showing both sides.  That's the image size I use as a keeper.

Gotcha, thanks! I usually rudce to that size or similar, 1200 to 1000 px wide also, but I geberally start with just 2400-3000px raw image.

Again, thanks for the answer and this is a great thread! Cool
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« Reply #13 on: August 21, 2006, 06:05:22 am »

Photoshop is a big and complicated program, and not always easy to learn.  Here are a few more quick Photoshop tips.

1.  To see the brush size properly, make sure you have a "brush size" cursor selected under Edit/Preferences/Display & cursors. 

2.  To change brush size quickly, use the left and right square brackets.  I use fingers of the left hand on these, right hand on the mouse. 

3.  The paint bucket looks at the colour at the very tip of the stream of paint coming out of the bucket on the icon.  It paints this point, and anything connected to it that's the same or close to the same colour, with the current paint colour. You can select the start point quite accurately with this rather odd cursor.

Even if you select a "precise" cursor, you still get the paint bucket icon for this.

I'm using version 8, aka Photoshop CS.  There's at least one later version, but I don't think it differs by much.

I'm glad the thread is proving useful!
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