Numismatic and History Discussion Forums > Coin Photography, Conservation and Storage

Choice of exposure .. and what follows

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moonmoth:
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 ...

moonmoth:
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.

moonmoth:
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.

Bacchus:
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

moonmoth:
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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