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05-15-2011, 08:22 AM   #1
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focal length prime vs zooms

will there be any difference in (not comparing optically, IQ etc.,) a particular focal length in a prime compared to the same focal length in a zoom in terms of FOV coverage and the bokeh effect.

For example,the frame length and breadth covered by a 35mm prime lens and a 35mm focal set in a 18-200mm.

05-15-2011, 08:45 AM   #2
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Generally primes are sharper than zooms due to set focal lengths.

In some lenses, yes there is a difference. For example the 18-55mm kit lens, it is softer at both ends and gets sharper at the middle focal lengths. In your situation, it really depends on the lens.
05-15-2011, 08:45 AM   #3
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QuoteOriginally posted by sany Quote
will there be any difference in (not comparing optically, IQ etc.,) a particular focal length in a prime compared to the same focal length in a zoom in terms of FOV coverage and the bokeh effect.

For example,the frame length and breadth covered by a 35mm prime lens and a 35mm focal set in a 18-200mm.
No significant difference, as the "focal length" of a zoom is *determined* by FOV (or magnification, which is really the same thing) as the old "distance from film plane at infinity" simply doesn't work .

Bokeh is a different animal entirely though; it's very "individual" to lens designs, so I would expect it to be different between two different lens designs - particularly a prime and a zoom.
05-15-2011, 09:12 AM   #4
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QuoteOriginally posted by jstevewhite Quote
the "focal length" of a zoom is *determined* by FOV (or magnification, which is really the same thing)
?!?!
Since when is FOV and magnification the same thing? They are two VERY different things IMO

05-15-2011, 09:20 AM   #5
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QuoteOriginally posted by sany Quote
will there be any difference in (not comparing optically, IQ etc.,) a particular focal length in a prime compared to the same focal length in a zoom in terms of FOV coverage and the bokeh effect.

For example,the frame length and breadth covered by a 35mm prime lens and a 35mm focal set in a 18-200mm.
In theory, field of view is simply a function of focal length and image format so it would not change as a function of zoom over prime, however, and this is important to note. some zooms (specifically super zooms with high zoom ratios), and even some primes (most notibly ones with fixed rear element design but also some internally focused designs) only actually hold the true focal length at infinity focus. the reason is that focusing involves in these cases movement of the internal elements and changing to some extent the effective focal length of the lens when focusing closer than infinity.

Since by standards, the focal length is only tested for infinity, not many lenses have the change in focal length reported as a function of focus distance even in test results.

there have been some posts here in the past regarding this point.

Bokeh is a much different beast, and it changes from lens model to lens model even within primes, and is impacted by so many things, including the shape of the aperture blades when stopped down from wide open, and the placement of the blades within the lens grooup, as well as whether the lens is internally focused or not..... there is no simple rule
05-15-2011, 11:39 AM   #6
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QuoteOriginally posted by axl Quote
?!?!
Since when is FOV and magnification the same thing? They are two VERY different things IMO
Yeah, sorry, left out a few things. FOV is a function of magnification and distance. So if you don't change distance to subject, magnification and FOV are different numbers describing the same thing. I was talking about 'at infinity', which is where focal length is measured, as well.
05-15-2011, 01:04 PM   #7
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QuoteOriginally posted by jstevewhite Quote
Yeah, sorry, left out a few things. FOV is a function of magnification and distance. So if you don't change distance to subject, magnification and FOV are different numbers describing the same thing. I was talking about 'at infinity', which is where focal length is measured, as well.
At infinity FOV is , in linear dimensions infinite and magnification is zero. This is true for all lenses. You are better measuring the field of view in terms of an angle
05-15-2011, 01:14 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
At infinity FOV is , in linear dimensions infinite and magnification is zero. This is true for all lenses. You are better measuring the field of view in terms of an angle
Measuring, indeed, is better expressed as an angle. I was merely making the point that the FOV varies directly with the magnification at fixed distance.

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