The forums are awash in long diatribes and discussions about underwater optics. Most of the discussion revolves around corner sharpness, which becomes an issue with smaller domes and those made of acrylic instead of glass. In general, the larger the domes and the better the material, the sharper the corners.
If that is the case, then why do we use the newer line of “mini” domes, a term generally used for those measuring less than 8” (200mm) in diameter. These domes do cause a loss of corner sharpness, that is a fact. The trade off for this corner sharpness comes in our ability to get even closer to the main subject, therefore increasing its size relative to the surroundings, in most cases reducing the need to have the corners sharp. In much the same way as bokeh (created using shallow depth of field on macro lenses) can make a subject stand out, using a mini-dome to enhance the dominance and also the curvature of the image can make the foreground subject stand out in a wide angle image.
That’s not to say that mini-domes are a one-trick pony. When used appropriately with attention giving to settings such as aperture and focus they can produce corners of acceptable sharpness for reef scenic or larger pelagic images. By getting closer to the subject, lighting becomes simpler and more effective.
While the domes do enhance the size of the main subject, they also have the reverse effect for the middle and background, increasing the curvature away from the main subject and making those objects seem farther away. This again will help highlight the main subject but can make scenic images like the one below difficult.
How to get around the sharpness issue?
The 140mm dome is already a step ahead in that it uses optical glass instead of acrylic or lower quality glass and generates by far the best corner sharpness of any mini-dome we’ve used. The dome is purpose built to accommodate wide angle lenses.
Aperture, in addition to controlling overall exposure also controls the depth of field. By maximizing aperture and using higher ISO to compensate for shutter speed (going below 1/60th can cause motion blur and with most modern FF SLR cameras going to ISO 400 is not a problem) you can help sharpen up the edges by sharpening the overall image. You can also shoot at extremely low ISO (64) and use a shallow depth of field F2.8 to intentionally blur the croners making edge-sharpness irrelevant.
Making sure that your focus point is properly chosen is critical. Remembering that depth of field covers 1/3rd in front and 2/3rds behind the point of focus means that focusing on the closest part of the foreground subject will unnecessarily move the depth of field too far forward. Using a higher aperture and focusing correctly, you can maximize the sharpness of the corners.
With rectilinear lenses the corner sharpness issue will be more accentuated and in these cases it is critical to follow the above guidelines to enhance sharpness.
There is no silver bullet in photography. The right tool must be used for the job but the 140mm Nauticam mini-dome does allow for a wide range of photo styles in a compact and lightweight dome. For the weight and size conscious crowd this is a great all around tool when used with its strengths in mind.