Guide / Tutorial Focus Stacking a large coin

[Charliemouse]

1980 Quin

About 40 photos per stacked image, which is a lot.

I was close to minimum focus, which means the coin was larger than the height of the frame.  Therefore, I needed to do two passes, one for the bottom 2/3rds and one with the top 2/3rds.  I think the large number of images caused some of the odd artefacts, like bloom and ghosting, visible in the final images.  I will continue to experiment

[Charliemouse]

So I think I figured out what caused the ghosting - the double image most obvious around the TH of Elizabeth.

The problem

This is going to get quite detailed.  Sorry, but that's the nature of the beast.

As I said above, I was very close to the coin, almost minimum distance for the lens, in order to get maximum detail.  At minimum distance, objects appear 1:1 on the sensor.  The sensor is 24mm high.  But a quin sovereign is 39mm in diameter.  So I needed to do two passes, first across the bottom of the coins and second across the top, varying the focus between each shot.

Here's a photo from each of the two passes.  You can see only a narrow vertical stripe is in focus, maybe a quarter of the width of the coin on the left.  That's how focus stacking works - taking multiple images slowly moving the focus across the coin until you've covered the whole thing.

After taking the shots of the bottom of the coin, I adjusted the camera slightly, tilting it upwards, and then repeated the process with the top.

Once I've photographed each pass, I use Photoshop to stitch everything together.  It will create a panorama of the two rows, and then take the sharpest parts of each image to make a single, super sharp image.  Very clever.

This normally works fine, but the problem here is with the stitching of the two halves.  Because I was so close to the coin, tilting the camera from bottom to top of the coin meant that the coin was at a significantly different angle.  That meant that Photoshop couldn't stitch them together to create a perfect join.

Photoshop is capable of adjusting for angle when working with a few images.  It will actually account for the tilt by distorting the images to fit together.  Also very clever.

But when it has to blend 40 images, 20 from each row, I think it gets overwhelmed.  And with glare from light spots in the mix, it struggles.

The solution

The easiest solution is to just move the camera further back, so I can fit the whole coin into the frame.  That's what I've done most other times.

However, sometimes I just want to get into the tiniest details of the surface, but just can't fit the whole subject in one image.  So I have to use a panorama.

To make job easy for Photoshop, I needed to move within the plane of the image.  In other words, instead of tilting the lens I needed to move upwards without changing the angle.  The tripod has a vertical adjustment, but at macro scale it's not practical - any movement would be huge and with multiple kg of kit, pretty awkward to achieve accurately.  The macro mount allows me to precisely move in two axes, left-right and forward-backward, but not up-down.

However, to paraphrase, "if the camera won't come to the coin, the coin can come to the camera."

I can move the coin, much more easily and precisely than the camera.  Put the coin, cloth and background onto a small 'lab lifter'.  That way I can keep the camera locked down and move the coin up and down, and no angles change.  This provides movement in the third axis.

Results to follow...

Edited May 11, 2023 by Charliemouse

[Charliemouse]

I did think of another (cheaper) solution.

Since the problem is caused by being unable to move the camera along the short axis of the image, i.e. up/down, I could mount the camera portrait-wise (vertically).  Then instead of photographing the top then bottom of the coin, I could photograph the left side of the coin then the right, using my existing macro rail to move 'sideways'.  This would translate to a top-to-bottom movement on the frame.

It would definitely be more cumbersome having the camera mounted that way.

[Aldebaran]

On 10/05/2023 at 19:32, Charliemouse said:

1980 Quin

About 40 photos per stacked image, which is a lot.

I was close to minimum focus, which means the coin was larger than the height of the frame.  Therefore, I needed to do two passes, one for the bottom 2/3rds and one with the top 2/3rds.  I think the large number of images caused some of the odd artefacts, like bloom and ghosting, visible in the final images.  I will continue to experiment

What lens are you using?🌞

[Charliemouse]

19 hours ago, Aldebaran said:

What lens are you using?🌞

Sigma 150mm f/2.8 EX DG APO HSM.

But as I state in the Choosing a Macro Lens tutorial, I wouldn't recommend it just for coin photography because it's too long.

Edited May 12, 2023 by Charliemouse

[Aldebaran]

42 minutes ago, Charliemouse said:

Sigma 150mm f/2.8 EX DG APO HSM.

But as I state in the Choosing a Macro Lens tutorial, I wouldn't recommend it just for coin photography because it's too long.

Thank you, that’s great.

It’s Canon in our house and we have a 100mm Macro. 

Coin photography is an art form and you are good!!!!!

[Charliemouse]

17 hours ago, Aldebaran said:

Thank you, that’s great.

It’s Canon in our house and we have a 100mm Macro. 

Coin photography is an art form and you are good!!!!!

Thank you.

100mm is a better focal length for coins.  It's a bit more convenient, because you can get closer to the subject.

I use Nikon cameras.  Canon vs Nikon vs Sony after a while, doesn't matter.  You pick one to start with, and because lenses last 10 times longer than camera bodies, you are stuck with that choice forever.

But I thoroughly recommend third-party lenses, like Sigma.  Saved me a lot of money over the years.

[Charliemouse]

First use of the lab lifter was with the Coronation £5.

Certainly improved, although not perfect.  I need to work on the technique, since I think the coin slipped when I moved the platform for the first time.

You can see a slight mis-join where I've highlighted.