Optimizing Sharpness

By Michel Gilbert & Danielle Alary

Nobody likes fuzzy images—unless they are intentional. In this world of auto-everything, people take the performance of their cameras for granted. Fuzzy or out-of-focus images may result from a mechanical or electronics problem—or you may be the culprit.

Most of us buy the best lenses we can afford. Some spend hours looking at lens reviews, analyzing MTF charts, and reading forum postings on the subject. The exercise is not useless but remember that, short of a manufacturing or camera-induced problem, virtually all reputable brand lenses perform very well.

A YouTuber may say that such glass does not exhibit perfect corner sharpness at f/1.8—but who cares? He found that while pixel-peeping on a 32-inch 8K IMAX-size Retina display; your pictures will most likely end up on Facebook or on an iPad somewhere. Also, who is concerned about the slightly less-sharp corner of an image, especially if it happens to be bluewater?

Any lens is a compromise, which means that its performance varies depending on the settings and the shooting conditions. Also remember that there are manufacturing tolerances and, on top of that, these optical marvels are mechanical tools and they can go out of adjustment. Before registering a lens or discarding warranty cards, test it at different focusing distances, use any suitable target similar to your typical subject. If it consistently fails to produce acceptable focus, exchange it.

For minute adjustments, you can perform a lens calibration. Most DSLRs offer autofocus fine-tuning in their menus. Various providers sell special targets used to accomplish such calibration. You can search the internet for these tools, but here are a few options:

•  Focus Pyramid (US$25), a very simple tool.
•  Datacolor SpyderLENSCAL (US68$), a more complete option.
•  Vello LENS-2020 Lens Calibration tool (US$35), a less expensive option similar to the LENSCAL.
•  LensAlign (US$85) is more expensive. It has been around for eons. The company also offers a software called FocusTune that brings more automation to the process. 

Optimal aperture

No lens is perfect corner-to-corner at all apertures. There is always a sweet spot where most of them provide optimal results.

Often, the best settings are between f/5.6 and f/8. These apertures are great topside. When the idiosyncrasies of underwater photography come into play you must compromise—most of the time.

In macro, to maximize depth-of-field, stop down to f/16, f/22. Remember that diffraction reduces sharpness above f/11. Having said that, we use apertures of f/16 and above all the time. Run tests—on land—and look at your images at different apertures. Repeat it in a pool. Then you be the judge.

In wide-angle, the inevitable dome port requires its own compromise. Without delving into dome port optics let’s simply say that corner sharpness is significantly reduced by a dome port. Generally speaking, the smaller the dome the softer the corners.

This requires stopping down your aperture around f/16. You may go for f/8 or f/11 but the results may not be optimal. Jump in a pool, select a target with corner details and make test exposures at all apertures. This should indicate the optimal choice.

Shutter speed

Shutter speed makes a big difference in sharpness. Lower shutter speeds may induce blur, so keep that in mind and go for the faster speed suitable for your image.

Autofocus type

Most cameras offer a choice between continuous or single autofocus. Our personal preference goes for continuous autofocus in macro (unless we use manual focusing), and single autofocus for wide-angle, depending on the subject. In continuous autofocus the camera constantly adjusts focusing as you move or if the subject moves. Experience and testing will tell you what is best for your style of photography.

Refocusing

Cameras are far from perfect and they can make mistakes. If possible, perform a first focus on your subject either by using a dedicated focusing button or the shutter release. Then let go and refocus by pushing the control once again. This may alleviate some focusing problems.

You may also use focus bracketing, slightly varying your point of focus over a series of images.

Also remember, 1/3 to 1/2 of your depth of field lays before the focusing point. This means that 2/3 to 1/2 of the depth of field are found beyond the latter.

Go VR!

Vibration reduction (VR) either in camera or in lenses is a godsend. Some macro lenses offer VR (also dubbed IS) and this comes in handy in terms of sharpness.

Contrast

There is also perceived sharpness. If your image lacks contrast, then you may think the image is not in perfect focus. Remember it if you have just a slight focusing error. Increasing contrast may bring some improvement.

Old eyes don’t focus well

As we age our eyes experience more trouble in helping us focus an image. A friend who worked at Kodak told us about an experiment where they set up a camera on a tripod and asked people, from various age groups, to perform only one task: manually focus the image in the viewfinder. They found a direct correlation between the age of the person and the drop in focusing accuracy. Keep that in mind at your birthday!

After the facts

Making sure you take sharp pictures is paramount. However, sometimes we may not be able to optimize focus, and, to some extent, post-processing may help.

The infamous sharpening tool present in various software is not meant to correct complete lack of focus but used with caution it may help correct minor errors. It actually works on acutance (a subjective perception of sharpness that is related to the edge contrast of an image).

We recently found software that performs fairly well to correct focusing or blurring errors in images. As a last resort, this may be helpful for the unique picture of a subject shot during the trip of a lifetime. This is a last resort solution, but we found that it may be used to some benefits. The product is called Topaz Sharpen AI.

It’s not the end of the world

In the end, absolute perfect laser-sharp pictures are not necessarily the best ones. Light, composition and subject matter a lot. A slight focusing error may not cause the next world war and many viewers may not even realize it.

Simply make sure the odds are on your side and go shoot images.