Focus problems, 7D, 60mm Macro+Aquatica Minidome

Hi everyone,

Here is a question for you techies out there!

After reading a few posts on shooting with the Canon 60mm Macro behind a minidome I decided to try it out today in the pool before heading to Australia to hopefully shoot some seadragons and other medium sized subjects.

I have a Canon 7D, Canon 60mm USM, AquaticaA7D housing, Aquatica 100mm minidome and Aquatica extension rings.

I tried it 3 different ways:

1: Without extension ring, it could hardly focus, it would only focus on subjects >50cm from the front of the lens but even then with a lot of "searching" for focus.

2: I tried it with the 18456 (16,5mm) extension ring, and that helped but I still did not get 1:1 magnification, minimum focus distance was still increased and the focus was still a bit slow

3: Finally I tried with the 18453 (28,5mm) extension ring and it worked well.

Can anyone explain to me why this is? It seems that other people have managed to use the 60mm without any extensions?

Best regards, Morten

Have you thought about trying a flat port?

Dear Doug,

Yes I use a flat port regularly for both my 60mm and my 100mm, the reason that I want to use the Minidome is that I want to shoot medium sized subjects, and since a flatport increases magnification with 33% through refraction I wont be able to fit "bigger" things in the frame without backing up too far

The point of using the 60mm under a dome is to be able to shoot bigger things than you can when shooting behind a flat port.

Best regards, Morten

Your minimum focus distance. For that lens is 7.8". It looks like if you are not using a diopter, the apparent image of the dome will be closer than 7.8 inches when using the shorter extension rings.

Hi Morten

I agree with Doug that the dome you have chosen is too small for that lens minimum focus distance.

You will need a 5" or larger dome and it would be best if you place it the proper distance away from the nodal point for that lens which is about 2" or 50mm. The front of the dome should be (the radius of the dome) + the nodal point from the front of the camera. So if you are using a 5" dome then the front of the dome would need to be 2.5 + 2 = 4.5" from the front of the camera. The virtual or apparent image is at 9.25" and the minimum focus distance for that lens is 7.8"

For the 100mm / 4" dome the virtual or apparent image is at 7.25" and the minimum focus distance for that lens is 7.8". So if you are using a 4" dome then the front of the dome would need to be 2 + 2 = 4" from the front of the camera and you will have to use a diopter,

You can read more about this here. http://scubageek.com/articles/wwwprim.html