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Technology March 2015

Fritz on Photography

Depth of Focus

By Fritz Penning

Using a feature known as focus stacking it is possible to have all flowers in focus by taking a series of pictures with the focus point changing from front to back and combining the shots to make a single image.

When we take pictures at fairly close distances we may find the front part of the picture to be in focus while the back part is out of focus. This can happen when subjects are fairly deep from front to back. An example would be a bouquet of flowers where the closer flowers are sharp and the back ones are out of focus. Three factors interact to determine the amount of acceptable focus, known as depth of field. These are; distance to subject, lens size, and lens aperture (opening).

The closer we are to the subject the smaller the depth of field. A few inches compared to many feet will greatly increase the amount that is in focus, other settings remaining the same. A wide-angle lens has much greater depth of field compared to a telephoto lens. With wide angle lens virtually everything is sharp however each object is much smaller when compared to the same object captured with a telephoto. A common method of increasing depth of field is to shoot at small apertures such as f/22 and f/32. Bigger numbers correspond to smaller apertures. Shooting at small apertures has its problems in low lighting because of camera shake during slow shutter speeds.

Two of the photos of the orchids show how depth of field is affected by being close (about 18”), with an 85 mm lens, and a f/4.5 aperture. One shot focused on the front flower, the other on the back flower with obvious out of focus effects. However because we have digital technology it is possible to have sharp focus from front to back.

Using a feature known as focus stacking it is possible to have all flowers in focus by taking a series of pictures with the focus point changing from front to back and combining the shots to make a single image.  The procedure is to open all the shots as layers using Bridge>Tools>Photoshop>Load Files into Photoshop Layers. When in Photoshop first select all layers, then go to Edit>Align-Layers>Auto>OK. Then go back to Edit> Auto-Blend Layers>Auto>OK. It does it all by itself. Depending on how many shots it may take several minutes for each step to complete. The picture with all in focus was done with 28 shots each at an increasing focal distance, going from front to back. How many are needed for any situation? Two is the least and the “right” number is based on your patience and the amount of detail you find acceptable. Programs other than Photoshop will do focus stacking as shown by a Google search. Flowers were illustrated but subjects such as landscapes can be stacked to have sharp focus from foreground to background. It’s fun and may give you pictures you really like.

 

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