Focus Stacking

Regardless of how you captured it, it is lovely clear image.
I have never tried focus stacking, although I recognise that because much of my macro is with extension tubes you end up with such a tiny depth of field it is something I need to experiment with, so it is great to see how well it can work.
To help the discussion, on an EOS 7D at f/4.5 assuming a distance to the object of 0.35m (the minimum focusing distance for your 17-55, which I'm guessing you used - imgbb doesn't tell me) the depth of field works out at only 0.58cm and I'm guessing the shell is a couple of cm deep? If so, you will have needed at least 4 images to get it in, although 11 is perhaps a tad excessive! Of course, you are balancing depth of field with your lens's 'sweet spot' aperture, but by comparison, f/8.0 would give a depth of field of 1.04cm and f/22 would give 2.94cm. So in theory going to f/22 negates the need for stacking in this example, but actually is it any better? More experimentation needed...

Thanks, Alex. I tend to not get too involved in the technical part of things and am just happy if the end results are what I was after. However, I am glad to see that the discussions following my post have added info that is helpful to others.
For me the big takeaway was the reminder to use Live View that I gleaned from Judy's article.

For anyone interested in getting a feel for what the DOF (depth of field) is for a particular situation and camera setup when photographing close subjects with or without a macro lens, this is one practical exercise you could try.

1.  Mount your camera on a tripod or hand held is still ok since this exercise is just to get an idea of the DOF.

2.  Set up a long or short ruler or tape measure horizontally so that the start of the ruler/tape is nearest the camera and the end farthest from the camera. Make sure the numbers graduations on the ruler/tape are legible in your viewfinder or lcd screen on your camera.

DOF is determined by the aperture, focal length and subject distance from the camera. 

1.  For a given focal length and subject distance from the camera the smaller the aperture (increasing fstop number) the larger the DOF.

2.  For a given aperture and focal length, the greater the subject distance from the camera the larger the DOF.

3.  For a given aperture and distance from the camera, the longer the focal length the shorter the DOF.

Using the above info, take several photos at various configurations of the above to get a feel for what the actual DOF is for your particular setup.  You can then use some of these photos to practise focus stacking post processing to produce an image that is in focus/sharp from a starting point on the ruler/tape to an end point that could not be captured in a single shot. 

This exercise should also give you an idea of how many photos you will need to successfully focus stack a particular situation in the future.

Focus stacking is never something I've tried so I applaud you for taking the time to learn and practice Helen. I like the result!

Thanks, Andy. I tried this one other time and in both instances I felt the images were much sharper/clearer than other work that I have done, although it could be a case of seeing what you want to see. I have a couple of more things I would like to try so
I may post additional images during the next week.