The 50,000 lux light compromise

Luminys Systems Corp. Editorial for Crash Testing Technology magazine

Photography requires light to capture images. ….the more light you have, the more of the hidden image you can see. Professional photographers struggle with balancing numerous considerations related to the quantity and positioning of light to obtain meaningful photographic images. Whatever limitations a photographic system has, light is one of the most critical issues. The quantity of light, the quality of light, and the positioning of light can make a significant difference in the usefulness of the details from the resulting images. 100,000 Lux is roughly equivalent to the level of sunlight on a sunny day. So 50,000 lux would be a not so sunny overcast day. But bright sunlight alone does not always produce good images. Moreover, if the position of the sun relative to the subject and the camera is inappropriate, resulting bright highlights next to deep shadows can render the images of little value. For the moment, let’s just talk about basic light level.

Today’s modern cameras can produce a high speed image with just 50,000 lux, but there is a price to pay for a low light level image. Although there are camera sensors available featuring advertised ISO (ASA) ratings even far beyond 1,000, a keen photographic eye will opt to de-rate the sensitivity (ISO) to 200 or below to allow the sensor to produce much better images. Some will even de-rate the sensor to ISO 30. Assuming ISO of 200 with 250 microsecond exposure, 50,000 lux will just give enough light for an f2.8 lens opening. Even at 250 microseconds, you often get blurred images. So you go to a faster shutter speed of at least 125 microseconds. However, if you do not have more light or a faster lens, you must choose a compromise between a 250 microsecond shutter or ISO 400. You can either have an image that is slightly blurred or picture quality slightly diminished. Unfortunately you are dealing with compromises, and not getting the picture you had hoped for from your new camera.

So let’s look at the lens performance and do an experiment. Keeping 200 ISO and 50,000 lux, first decrease the shutter speed to a full 1,000 microseconds. Now we can stop down the lens to just over an f5.6. Traditional lens design teaches that a lens performs best between an f 5.6 and an f8, which is the sweet spot for optimal lens definition and contrast. Capture an image at this setting. Being careful not to move the camera, set the lens at f2.8 and reduce the light by two stops, ideally by placing an ND 6 filter on the lens. Again capture an image. Compare the two images. The image shot at f 5.6 immediately looks better, and wow, look at the sharp focus on the objects in the front and all the way to the rear. The focus range (lens depth of field) has increased dramatically. Even distant objects are in focus since a higher f stop automatically gives you more depth of field for focus.

The ideal camera settings you need to achieve the best possible image are an f8 with a 125 micro second shutter speed with an ISO of 200. But how much light will that take? The answer is 640,000 lux – almost four “stops” above the 50,000 level Yeah, that is a lot of light that you don’t have and probably can’t afford. So you have to compromise.

Potential compromises –

  • Slow shutter speed = blurred fast moving objects
  • More gain (increased ISO) = degraded picture with “video noise”
  • Lower lens f stop = degraded image quality and less of the picture in focus

OR –

Accept a middle of the range increase in light level of 200,000 lux and you can eliminate at least two of the above and even improve on the third under ideal conditions. Often there are older cameras mixed in with new ones. The added light will breathe new life into the tired images of your old cameras — guaranteed!! Take heart! It is possible to raise your light levels to 200,000 lux with a limited number of small high intensity “green” fixtures that demand very little power.

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