Norm's tips for low-light auto focus performance

Started Sep 5, 2008 | Discussions thread
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Norm1234
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Norm's tips for low-light auto focus performance
Sep 5, 2008

Pentaxians posting here have reported difficulties in achieving autofocus in dim or low contrast lighting situations, and in cases where they attempt to follow-focus fast moving subjects coming directly toward, or heading directly away from the camera location.

In some cases, the focus efficiency is a factor of the camera system – Pentax has a good autofocus system, not the fastest in the industry, but not the slowest. From my experience, it falls between the best of the pro-level DSLRs and the majority of the prosumer competitors… in most cases, the prosumer models that seem to focus faster, don’t necessarily focus accurately – read the other forums, and you may be quite surprised at the frequency and the intensity of the complaints – even with the top line professional camera systems!

The key to faster focus in dim or low contrast situations is to adjust the camera to the optimum settings for focus performance, even if it means compromising your favourite auto programs.

Bear with me for a few minutes, and I will explain.

We have several Pentax DSLR systems in use on a regular basis.

We shoot mainly in reasonably well-lit environments, but even at trade shows and conferences where lighting is not so good, we don’t seem to have the degree of difficulties that have been reported on this forum – and that pretty much spans the *istD, DS, K100, K10D and K20D bodies, and a range of Pentax lenses.

Now we do match the equipment we use to the needs of the assignment, and we may set-up the cameras differently from the average photographer… but nothing we do is “special” or in any way beyond the reach of any of the Pentaxians… we just set a few of the options on the bodies so that we can retain a bit more control over the automated processes.

There is no question that some lenses focus faster, and lock focus better in dim light – not necessarily depending on the maximum aperture, the age, or the cost of the lens. I have an old smc Pentax FA 28-70mm f/4 AL lens, left over from one of our MZ-S systems, that focuses with no difficulty at all in very dim light on an *istD. I have a stunning FA* 28-70mm f/2.8 lens that does not focus as fast – even though it is sharper and probably worth five times as much. I believe that for most lenses, the simpler the lens construction, and the lighter the lens components, the faster the focus will be… it would seem to be a matter of physics.

You would think that the newer lenses would outperform the old workhorses, but when I tried the new DA 55-200 lens on a K20D in a camera store, it hunted all over the place, unable to focus at all. I was quite baffled by this performance, which was not at all impressive. Then I looked at the settings on the K20D – it was on auto everything – ISO, focus point, white balance, AF-C – everything that could possibly cause the camera to slow down. There was a battery in the camera, but who knows when it was charged, and a “garden variety” SD card in the slot. Under the watchful eye of the sales clerk, who was concerned that we couldn’t get the lens to focus on an illuminated sign about 30 feet away – inside the brightly lit store, I made a few changes, and the lens snapped into focus with a barely noticeable delay… I tried focusing on a variety of targets within the store – high contrast bright product displays, low contrast dark coloured products, the lens had no trouble at all - quite acceptable performance for an inexpensive prosumer optic.

Now I don’t claim to be a magician, but I do have some experience in the field with the Pentax system. What I did to substantially improve the focus performance of that 50-200 lens is really quite simple.

Here are some of the tips that may help with your focus issues.

With all of the cameras from the *istD to the K20D, focus always seems faster with a fully charged set of batteries in the camera and the battery grip.

If your lens tends to “hunt” for focus in dim or low-contrast lighting conditions, you can use your Pentax external flash in “SB” (spot beam) mode to illuminate the scene for focus assist. Check the flash manual for details.

Remove any unnecessary filters from the lens – the glare between the filter and the front element can be just enough to lower contrast, making focus more difficult. The same goes for the lens hood – use it to help prevent extraneous light from entering the lens.

I have found that putting the camera on AWB (Auto White Balance) sometimes helps speed up focus.

Set a specific ISO rather than letting the camera determine the ISO automatically. Every process adds to the time it takes to capture the image.

Try setting the focus control (surrounding the four-way control and the OK button) to central spot position, rather than SEL or the multi-focus auto setting.

Try setting the autofocus system to AF-S rather than AF-C.

Make sure that your lens is set to “A”. The actual aperture you select will not affect the autofocus, because the lens meters and focuses with the aperture fully open, then closes down the aperture the instant before the shutter trips.

If there is a place in the frame where you can detect a bit of contrast, focus on this area, use the focus lock, and re-compose as required. You may be able to ask a model to hold a small piece of white paper for the focus setting, and then either put it down, or out of the frame for the photograph.

I have always carried a small flashlight in my camera bag – in very dark situations, I sometimes hold the flashlight parallel to the lens, and let the beam from the flashlight illuminate the scene just long enough to achieve focus. I then switch the AF control to manual, and take the picture.

I am sure that other Pentaxians have suggestions to contribute to this list. In the meantime, try some of the tips in a dimly lit environment, and see if they help.

Norm.

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