Camera Controls (Part 2, F-Stop)

May 182010
 
 May 18, 2010  Posted by Habenero at 10:10 pm How To, Photography , , , , ,  No Responses »

Aperture or F-stop is the lens opening that controls how much light will pass through the lens. Regardless of the size of your lens, sensor, or film plane, a given f-stop will allow the same amount of light per square centimeter (or inch) to go past. The number selected for an f-stop is actually the denominator of the fraction 1/f-stop. The higher the f-stop number, the smaller the opening and the less light that will be admitted for your exposure. The f-stop controls your depth of field. A low number ensures that from near to far focus points a shallower area of clarity will exist. At higher numbers, the area that is in focus increases.  Part of what makes an image interesting is having your eyes drawn to certain areas within the image.  It is not important to have everything in the image tack sharp.

The danger in using small apertures is that they also increase diffraction of the light through the lens. On film, diffraction is not as much a problem as it is on a digital sensor because color spilling over onto another grain doesn’t cause the entire grain to react (individual atoms within the grain do react, but not the entire grain), but pixels react under a different set of rules and any light that spills onto even part of a pixel will trigger the entire pixel to react which softens the overall sharpness of the image. That is one of the drawbacks to using a digital camera in a situation that calls for keeping as much area in focus as possible. When in doubt, take the picture anyway and as your skills in processing your images increase, you may find a work around to the problem.

Selective focus is the principle technique that separates good photographers from the larger set of people that take pictures. Selective focus requires using your depth of field to keep the area of interest clear while letting the rest of the image blur. Sometimes, the blur will take on an appealing characteristic called bokeh. Bokeh is caused by the shape of the iris in the lens and the light pattern of the scene being photographed.  Not all lenses create good bokeh, which is one more reason testing a lens on your camera prior to buying it is important.

Fast Lens or High ISO

Apr 232010
 
 April 23, 2010  Posted by Habenero at 5:32 pm How To, Photography , , , , , , , , , ,  No Responses »

When you use some of the newer digital cameras, you can shoot with very little noise at some ISOs that were unheard of in the days of film.  This is due to the new sensors and better noise reduction algorithms.  The question that pops up in my mind is do these cameras negate the need for high cost fast lenses?   The answer to that question is no.

Regardless of the improvements in camera technology, it is the lens that does the majority of the work in producing a clear image.  Think back 20 years ago, and what was the problem with the Hubble Space Telescope, a key lens was not correctly shaped which made it impossible to focus properly.  All the electronic and programming tricks that were tried couldn’t get around the problem.  When correct lenses were put in place, the images forever changed astronomy.  Quality glass is far more important in producing your image than any of the electronics.

Every lens has a sweet spot.  That is the f-stop at which it produces the sharpest images.  Here’s a hint, on a fast lens, it is not often at its widest opening nor its smallest.  You can find it by focusing on a grid and adjusting the aperture until you get the clearest result.  That is the ideal lens opening for the lens and most of your shots should be done at that setting.

It is the lens that has to create the sharp edges that the sensor has to interpret.  The better the lens, the easier the sensor has to work.  Most of the fast glass used in the older cameras can produce images that make the sensor’s job a breeze.  The only drawback to using some of the older glass is that it decouples your metering system.   If your metering doesn’t work well with the glass, try to remember that you are shooting digitally and can see instant results.  Take your camera off of the Program mode and shoot manually.  Adjust the exposure until the image looks great to you.

With high ISO, your images are more susceptible to degradation due to the higher voltages applied to the sensor.  Higher voltages lead to higher heat values when the image is snapped.  Higher heat means more electronic noise.  Of course the same kind of problem occurs with long exposures, because the sensor is held at charge for a longer time.

In order to have the best of both worlds, couple your great old glass to your camera, shoot manually, and adjust the ISO and exposure to take advantage of the sweet spot of your lens.