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B&H Photo Video Pro Audio - Funny, They Don't Look f/2-ish
 
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Funny, They Don't Look f/2-ish
Depth of field in a sub-35mm world

Text and Images by Allan Weitz

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Not too long ago a friend of mine purchased a digital point-and-shoot that boasted a zoom lens with a maximum aperture of f/2 at the short end of the zoom range. Being a longtime Leica aficionado who often found himself shooting under less-than-desirable lighting situations, he figured his new digicam would be a neat electronic alternative to his traditional camera of choice. He wasn't disappointed. The camera delivered wonderful photographs under lighting conditions that would have given his Leica a good run for the money. The ability to push the ISO sensitivity as needed and an auto white balance feature that corrected most any light source made him a happy camper. There was one thing however that drove him absolutely batty.

It seemed that when shooting at maximum aperture, just about everything was in focus. It made him nuts. Even though he was shooting at the widest aperture with a lens that, according to the specs, was the focal length and maximum aperture equivalent of a 35mm f/2 lens, his pictures didn't look like he expected them to look.



Canon PowerShot G6
Nikon D100
Canon EOS 5D


What escaped my friend, at least for a short time, was that even though he was shooting with a lens that covered the image sensor with the same field-of view of his beloved 35-Summcron, he was in fact shooting with a 7.2mm lens. The CCD sensor in his camera, being far smaller than a frame of 24 x 36mm film, requires a lens about 1/5th the focal length of the equivalent 35mm lens. Now if you were to put a 7.2mm lens on a full-frame 35mm camera you'd have a 180-degree fisheye lens, and every school kid knows when you take pictures with a fisheye lens, everything is in focus. Have a nice day.

To illustrate the point, I tried a little test of my own using a trio of digital cameras, each with a lens of "equivalent" focal length and a maximum aperture of f/2. The cameras I chose included a Canon PowerShot G6, one of a few digicams with a lens that opens up to f/2, a Nikon D100 with a 35/2, and a Canon 5D with a 50/1.8 lens, which was stopped down a half-click to f/2.

The Canon 5D with a 50mm was my standard. The 35mm lens on my Nikon D100 worked out to 52.5mm (close enough!), and I zoomed the Canon G6's 7.2 - 28.8mm (35 - 140mm equivalent) lens to a bit past 9mm to match the ‘50mm' image coverage of the other two cameras.

The resulting images tell the story. With the full-frame Canon 5D and the 50mm lens, the background was way out of focus. With the Nikon D100 and 35mm lens, the background was somewhat, yet noticeably sharper, and with the Canon G6 at 9+ mm, the background was quite clear.

Canon PowerShot G6
(enlarge image)
Nikon D100
(enlarge image)
Canon EOS 5D
(enlarge image)

So what's a mother to do? Well for starters, move further away from your subject and get used to using longer focal lengths. While the theory of ‘equivalent focal-lengths' works as far as composing the look of your photo, depth-of-field is and will always be a fixed commodity. If you liked the narrow depth-of-field of an image taken with a 35mm lens at f/2, then that's the lens you should be shooting with. You're just going to have to move further away from your subject than you're used to. If you are shooting with an APS-sized imaging sensor such as those found in most entry-level DSLRs, this will amount to mounting whichever wide-aperture lens you fancy, and taking a few additional steps backward. If you plan on using a wide-aperture digicam, your solution will not be as simple.

While there are a few fast (f/2) digicams available today, most will be limiting on the long end of the zoom range. As an example, Canon's PowerShot G6 tops out at 28.8mm, which while passing in this smaller format as a "140mm equivalent telephoto lens" is still a 28.8mm lens. While you can somewhat isolate your subject at this focal length, it will never emulate the look of your 35/2 Summicron, or any other long, wide-aperture lenses.

So what can you do if want to retain the look and feel of shallow-focus imaging when shooting to a smaller sized sensor? The answer lies in Photoshop... but this is a story for another day.

Happy shooting!

Please email feedback on this article, or suggestions for future topics, to emailfeedback@bhphotovideo.com.