Focal Length and moving manually

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vijay venkat
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p.1 #1 · Focal Length and moving manually

I am a dslr beginner using a digital rebel xt camera. I really want to get the tamron 28-75mm f/2.8 lens based on the reviews (in addition to the canon 50mm f/1.4). However on a 1.6x crop camera I really miss the 17-28mm range. So I stuck deciding between the tamron 17-50mm f/2.8 and tamron 28-75mm f/2.8 fixed aperture lenses. Some reviews have suggested that the 17-50mm is fully sharp only at f/5.6 with vignetting and barrel distortion at the wide angle which defeats the purpose of choosing this lens. The tamron 28-75mm doesn't seem to have any of these complaints.

Here is my question. Will I be able to make up for the 17-28 mm range by moving myself backwards a short distance? I did a very rough test at home (I have a tamron 17-35mm f/2.8-4 that I am in the process of disposing) and I was able to make up the the gap by moving backwards about 4 feet, which is not very bad unless I am in a really crammed space. Is there something more that I should be aware of besides this? Is there a calculator available that could give me the actual measurements I need to move backward to make up for this loss of focal range?

Edited on Dec 10, 2007 at 01:26 PM

Dec 09, 2007 at 03:18 PM

Doug Pardee
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p.1 #2 · Focal Length and moving manually

If you're shooting landscapes, you won't be able to back up enough.

The math is dead simple. With your 28mm lens you'd need to be 28 feet away from a subject that you could have shot at 17 feet with a 17mm lens. For landscapes, you'd need to be 28 miles away from something that you could have shot from 17 miles away with your 17mm lens.

So in the first case you'd have to back up 11 feet (often doable), while in the second case you'd have to back up 11 miles. I'm assuming a flat Earth here.

Edited on Dec 10, 2007 at 01:26 PM

Dec 10, 2007 at 04:15 AM

vijay venkat
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p.1 #3 · Focal Length and moving manually

Thanks! That decided it. I am going with the Tamron 17-50mm f/2.8. I hope it is a good choice. I am also using 77mm filters (B+W) with a step up adapter, hopefully the vignetting won't be too bad.

Dec 10, 2007 at 01:26 PM


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