How many times you see practically new close up lenses being sold on eBay after sitting for years in a cupboard? Have you ever experienced the frustration of buying a close-up lens to find out it did not add anything to your set up?
There is a lot of confusion in terms of what is that you are really getting when you buy an underwater close-up lens. In most cases lenses are specified in diopters, in some other cases with a magnification factor, in few cases with the focal length.
The focal length is probably the most useful of those specification because in a traditional diopter the following formula is valid if you approximate the lens to converge in the rear element:
Working distance ≤ Focal length – physical length of the close-up lens
For example for a Subsee +5 diopter the focal length is 1000/5=200mm the lens is 32mm therefore the working distance is less than 168mm.
Now on its own this means very little. Depending on the camera we use a distance of 168mm may be a great improvement or no improvement at all.
To understand why we need to look on our camera and find the little symbol that indicates the focus distance reference mark.
While for DSLR the shooting distance is typically calculated from the front on the lens for micro four third you need to use this obscure symbol.
Once we locate the reference mark we need to identify the corresponding reference on the housing we can use the other dials as starting point. In this case the mark is approximately in the middle of the cold shoe.
When we put the camera in the housing we can then look up the specifications of the lens and calculate the minimum shooting distance. For example for an Olympus 60mm this value is 19cm.
We take note of this point and then measure the distance between the glass and the minimum working distance on land.
This works out to be around 9cm. We now need to consider that once immersed in water this distance will be multiplied by 1.33 to become 12cm.
It follows that any diopter with a focal length close to 12cm or longer is not going to be effective for our camera and lens combination. More likely we need something that can focus substantially closer or the benefit will be minimal. Assuming we half the physical working distance between the port and the subject we need something around 60-80mm focal length which considering the length of the lens itself could mean getting 2cm away from the subject.Continue reading How to choose the best close-up lens for your underwater MFT rig