ZPS

UPDATED: Monday 20 January, 2014 15:09

CAMERA FOCUS ADJUSTMENT (Mechanical correction of back/front focusing problems)

Relevant to most film and digital changeable lens cameras (SLR and DSLR as examples)

One of the reasons I used the Sigma SD9 more than the later SD14 was because my perception "told" me the images taken with the SD9 were sharper. It generally went no further than that. However, a recent closer look at one specific image showed the focus point was slightly behind where I thought it should have been. I do, of course as taught on workshops, always focus on the Nearest point and then the Furthest point and set the lens focus midway on a scale (or midway relative to the movement) which of course means it is not set midway in distance but rather midway on the lens mount movement = one third into the Near/Far distances. All this of course to ensure adequate Depth of Field for the subject.

When I decided to check back through a number of images, I found that the actual focus did not quite cover the required sharpness in respect of the Nearest point of the subject. So I set about checking whether the lens was focusing on the actual points that I perceived as being sharp in the camera viewfinder. As I was using the same lenses on the Sigma SD9 and SD14, it clearly was not the lenses that were to blame but rather that, perhaps at that stage, the SD14 had some problem with its in-camera focusing mechanism.

The first image shows one shot that made me question the focusing. The image should have been sharp, using the Near/Far focusing method and an appropriate f-stop (aperture) to suit, being f11 in that shot. As should be reasonably clear, the Nearest is not quite sharp enough - the point of focus (meaning where it first became sharp) was slightly behind the nearest point, meaning the DOF had not quite covered from Near to Far and as can be seen, the distance is sharp, yet in real terms, could have been slightly softer in definition, as the main subject would have benefited being sharp throughout its depth. Here's that image. Just see how sharp the distance is (well, for that matter, from the seated "figure" to the far distance) yet the immediate foreground is just off full sharpness. To keep the image a sensible size for this TIL, it is shown smaller than perhaps ideal to show what I mean. You can see a larger size by CLICKING HERE

So, to check how accurate the in-camera focusing is - I always use manual focusing - BUT there is information and an illustration at bottom to show how to adjust both for Manual and Auto focusing, should the latter also be inaccurate. As will be seen, in my SD14, there was a problem in that when the in-viewfinder image looked sharp, in reality, the lens was slightly focusing further away = Back Focusing. A simple adjustment (most SLR and DSLR cameras, for film and digital, have similar adjustments) soon corrected the problem, which probably means I will use the SD14 with more confidence. Here are the shots, the first showing the actual in-camera focus behind the in-viewfinder focus point of the "20". Second shows after one adjustment and last is the final. In each case I took shots to see what was happening. I used the maximum aperture of f2.8 and the lens (24-70EX) at maximum 70mm. To test it is ESSENTIAL to copy the method shown using a tape measure set at a slight angle to the shooting position!

The FINAL one below shows actual focus point on 20 in viewfinder now agrees with actual focus point in the camera. I only did the Manual Focus because I never use Auto-Focusing.

The picture below, which I borrowed from somewhere (sorry, forget where as long ago) gives an idea of how most reflex cameras can be adjusted - think it was related to a Nikon D70 film SLR - shows the location of in-camera adjustments for both the Manual-Focus and Auto-Focus. Much later DSLRs (well, some) have internal digital adjustments for correcting for such focusing errors. Taping up the mirror does not actually apply to many cameras as most have a specific way to raise the mirror for such events like this and, as another example, sensor cleaning - so do read your camera Manual! If you should have mislaid yours, try the manufacturer's online website as most manuals can be downloaded. Anyone unsure can mail me but of course, I can only make one reply to non-members of the Zone 8 Photo Society as members subscribe to obtain such help on many photographic and computing topics related to same. The "wrench" is actually a 1.5 HEX (Allan Key) which does not just fit in the end but actually can go through the adjustment head. To correct for the amount shown in my examples, I had to make approx two small clockwise turns (meaning about equal to 10-15% of a full circle) to attain the final adjustments per the images above. Clockwise to correct for Back Focus and Anti-Clockwise for correcting Front-Focus. That applies to BOTH adjusters should you need to adjust both of course. They are independent, so a camera could have correct focusing for one or both and thus need no adjustment or different focus adjustments necessary for both Auto and Manual of course. Please note the adjusters are CAMS - which means one full turn simply returns to the starting position. As a rough guide, turn CLOCKWISE if back focusing needs adjusting and if your HEX key has a long side of, say, around 2½"-3" move so end only moves around ½" to ¾" maximum for each trial adjustment before checking as shown above.

Here's a local view shot on 15th June 2010 to prove the adjustment worked

For a larger size click on link below

http://www.photosnowdonia.co.uk/ZPS/localview150510.jpg

CLICK HERE to visit a small gallery of additional pictures taken with the corrected SD14

Villanueva del Trabuco

Andalucia - Spain