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-   -   Focal length (https://forums.steves-digicams.com/general-discussion/164852-focal-length.html)

fbords Jan 17, 2010 10:50 PM

Focal length
 
This may be a dumb question, but I'm gonna ask it anyway. When I got my camera, it came via a bundle with an 18-55mm 3.5-5.6 lens and a 55-250mm 4-5.6 lens. I was curious as to why I could focus on an object about 2ft away with the short lens at 55mm, but not with the longer lens at 55mm? Is it because the max aperture is different at those focal lengths on the 2 lenses (5.6 on the short lens at 55mm and 4 on the long lens at 55mm)?

Hards80 Jan 17, 2010 10:57 PM

it is not because of the max aperture difference.

it is because of the minimal focal distance, or the shortest distance the lens can focus. the 55-250's minimum focus distance is 3.5'

fbords Jan 17, 2010 11:01 PM

ahh...that makes sense...

shoturtle Jan 17, 2010 11:01 PM

It has nothing to do with the aperture, more with the design of the lens. With the longer zoom, you need to be 3.6 ft away for the AF to lock, while he short one you need to be .8ft away. It is mark in the lens the minimal distance the subject is away. You will not be able to focus the long lens at 5.6 either. Every lens has the minimal distance printed on them. The 50mm 1.8 you need to be 1.5ft away.

rfortson Jan 18, 2010 3:03 PM

Some zooms are designed to allow you to focus closer. Tamron calls that a "Macro" function, but it's not really following the strict definition. However, it's still very useful. My 18-250 zoom has this "macro" feature. Zoom all the way out, focus closely and you can get shots like these:

http://farm4.static.flickr.com/3510/...39b3f9f6_b.jpg

http://farm1.static.flickr.com/224/5...61a3e8bd_b.jpg

By contrast, I have a super-telephoto prime lens and the minimum focus distance on that lens is 30 meters. :O


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