10 Tips for better construction photography

03 - Focus, Focus, Focus

Even the simplest of today's digital cameras provide the user much more functionality than the most sophisticated professional cameras provided 20 years ago.  Much of the increased functionality has come in the area of exposure control and auto-focus lenses.  When the first lightweight Single Lens Reflex (SLR) film cameras appeared on the market in the early 1970s they came with automatic exposure control but focusing had to be done manually with a split screen viewfinder or a separate range finder.  Every image had to be focused manually and if you forgot to focus the lens you got a fuzzy image that no amount of retouching could make sharp.  

Most modern cameras, both film and digital, have auto-focus lenses.  For the most part these systems do an excellent job of focusing images under standard lighting conditions.  However, there are three main ways in which the auto-focus mechanism on modern cameras can be fooled: low light, low detail, and objects at widely varying distances from the camera.  To get the best results from your digital camera it helps to know the limitations of auto-focusing in general and of quirks related to your camera in particular.

When taking construction images, getting the image properly focused is second only to composition in determining the quality of the result.  Lets say you have taken a picture of a construction detail, perhaps a water pipe or electrical wire that you need to find after sheet rock has been installed.  PhotoShop can do wonders to recover lost detail out of shadows and correct the color balance of your image but if the image is out of focus there is nothing you can do in PhotoShop to improve the sharpness of the image.  If the image is just a bit blurry because the focus wasn't tack sharp you will still be able to use the image for most purposes but if it is severely out of focus it is equivalent to not having an image at all.  The same comment applies to images that have motion blur because they were shot at a slow shutter speed and the camera moved during the time the picture was being taken.  Severe focus or motion blur can render an image useless.

The first situation that can fool most auto-focus mechanisms is low light.  Interior bathrooms and closets often have no windows.  If you need to photograph details in these rooms during construction you may find that it is hard to see details with the naked eye.  Most digital cameras can get acceptable results with shutter speeds up to two seconds.  By leaving the lens open for several seconds the digital sensor can pull detail out of rooms that are almost too dark to walk around in without a flashlight.  Although the camera may be capable of taking an excellent photograph in low light, the auto-focus mechanism may not work in those same conditions.  To get around this problem you may need to bring in additional light like a flash or a portable work light.  That may allow the auto-focus mechanism on your camera to work properly but can cause other problems with non-uniform lighting of the scene.  The other thing you can do is to set the focus manually if your camera allows you to do so.

The second situation that can fool you’re auto-focus mechanism is pictures that have little detail.  Shooting sheet rock before a nailing inspection is one such situation.  If your local jurisdiction’s building inspectors know that you do high quality work they may allow you to substitute some photographs for an actual inspection when you are pushing schedule and need to tape some sheet rock before the nailing inspection.  In this case you are shooting a uniform field of gray (the sheet rock) with a few tiny screw-heads sprinkled throughout the scene.  If you’re camera's auto-focus mechanism uses one or more small areas of the image to determine focus it will have trouble focusing on the uniform gray area.  You can help the camera by putting something with more detail like a tape measure in the scene but if that doesn't solve the problem you may need to resort to manual focusing.

Even if there is plenty of light and plenty of detail an auto-focus system can give poor results if there are objects at widely different distances within the scene.  This can happen if you are holding your camera at an angle to a wall or you are shooting framing before sheet rock.  If the camera focuses on the nearest object in the scene, the farthest objects may be out of focus.  On the other hand if the camera focuses on the farthest object in the scene the nearest objects may be fuzzy beyond recognition.  To solve this problem you may again need to switch to manual focus and set the focus point to somewhere between the nearest and farthest objects in order for the entire scene to be in focus.  Photographers have solved this problem for years by using the concept of a hyper focal distance.  The hyper focal distance is the distance at which you should focus your camera in order to achieve maximum sharpness of the image of all objects located between 1/2 the focal distance and infinity.  For example, the hyper focal distance of a pro-sumer digital camera using a 24mm wide-angle lens set at f8 is 11.8 feet.  With this lens and f-stop combination, all objects between 11.8/2 = 5.6 feet and infinity should be acceptably sharp.  This means that if you are standing in the middle of a room that is 12 ft square with 12 ft ceilings you can set your camera at f/8, manually focus your lens at 11.2 feet, and shoot in any direction and your photos will be well focused.  Of course you need to hold the camera steady during the exposure or you will create motion blur.  A tripod is a great help when trying to maximize your depth of field by focusing at hyper focal distances.

	Hyper focal distance in feet from digital camera for various focal length lenses
		14mm	16mm	18mm	24mm	35mm	50mm
F-stop	f/2.8	11.48	15.00	18.98	33.75	71.77	146.47
	f/5.6	5.74	7.50	9.49	16.87	35.88	73.23
	f/8	4.02	5.25	6.64	11.81	25.12	51.26
	f/11	2.92	3.82	4.83	8.59	18.27	37.28

 

The nearby table shows the hyper focal distance for digital cameras for various f-stop and lens combinations.  Notice that the use of wide-angle lenses at small apertures (i.e. large f-stop numbers like f/8 or f/11) produces the shortest hyper focal distances.  The image should be acceptably sharp at these settings from 1/2 of the distance listed in the table to infinity.

To summarize, don't assume you’re camera's auto-focus mechanism will work in all situations.  Watch out for scenes with low light, low detail, or objects at varying distances from the camera.

 

Ken VanBree is the owner of Imaging Perspective, a Bay Area firm that specializes in as-built construction photography.  Questions or comments are welcome: ken@imagingperspective.com