Constant aperture vs variable aperture zooms.

Started Mar 6, 2013 | Discussions thread
Adrian Tung
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Re: Constant aperture vs variable aperture zooms.
In reply to jvkelley, Mar 13, 2013

jvkelley wrote:

Great Bustard wrote:

What advantages, for example, does a constant 70-200 / 4 zoom have over a 70-200 / 2.8-4 zoom that produces the same IQ at a given aperture? That is, it's understood that the performance at 70mm f/2.8 may well be worse than the performance at 70mm f/4 on the variable aperture zoom, but let's assume that by f/4, the IQ is the same as the fixed aperture zoom.

Will the fixed aperture zoom be smaller, lighter, and/or cost less? If not, why not a faster (at the wide end) variable aperture zoom instead? If so, is there any reasonable estimate on by how much?

Here is Chuck Westfall's explanation. It doesn't go into much depth

http://digitaljournalist.org/issue0909/tech-tips.html

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J.V.

Surprised that no one else discussed this in further detail.

Correct me if I'm wrong, but from what I understand of the article, for the telezooms the physical aperture never changes in diameter. They just use optics in front of the aperture to increase the virtual aperture size to maintain F-ratio. That would explain why it isn't just a simple matter of assuming a constant f/4 could have been a 2.8-4.0.

For reasons I don't fully understand, this also applies to telewides, only that the optics are behind the aperture blades instead.

However, the part where variable aperture actually makes better (economic? value?) sense are the standard zooms where the aperture size is actually regulated to maintain the constant F-ratio.

Did I get it right?

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