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Old Jan 13, 2007, 7:01 PM   #1
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I've been looking on ebay a lot to see what's out there and I've got a couple of questions that I'm hoping someone can clear up.

1. Pentax F vs. FA: Are these two different lines of lenses? Although ebay picturees can sometimes be misleading, they seem to look the same to me. Is one Digital and one 35mm? Are both subject to the 1.5x DLSR sensor size magnification? IT's my assumption that everything that is not a DA lens is subjest to the 1.5 magnification.

2. What is a "mirror" lens? It seems like I see a couple lenses every time I look that are labled as being a mirror lens (I think they tend to be longer, rather than wider, lenses too).

Thanks for your help unscrambling my brain!
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Old Jan 13, 2007, 8:51 PM   #2
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Basement Shows wrote:
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I've been looking on ebay a lot to see what's out there and I've got a couple of questions that I'm hoping someone can clear up.

1. Pentax F vs. FA: Are these two different lines of lenses? Although ebay picturees can sometimes be misleading, they seem to look the same to me. Is one Digital and one 35mm? Are both subject to the 1.5x DLSR sensor size magnification? IT's my assumption that everything that is not a DA lens is subjest to the 1.5 magnification.

2. What is a "mirror" lens? It seems like I see a couple lenses every time I look that are labled as being a mirror lens (I think they tend to be longer, rather than wider, lenses too).

Thanks for your help unscrambling my brain!
#1 not sure of the difference between the two.. neither are digital optimized. ALL LENSES are subject to the crop factor-- ALL lenses.. even the DAs.

#2 a mirror lens hastwo mirrors inside the lense. light enters and is bounced off the rear mirror onto the front mirror and then to the film/sensor plane. thuus increasing the focal length in a shorter lens. they are a lot shorter than the equivalent non-mirror lens but they do have a large diameter to length ratio. they usually have a fixed aperture also. IQ varies greatly.
a 500mm f8 is around 3.5'' diameter and 6'' long.. my 50mmf4.5 has a front element about 5'' across and is over 19'' long. i doubt tho that there is a mirror lens at any aperture that has better IQ than my 500.

rroy
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Old Jan 13, 2007, 9:14 PM   #3
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Mirror lens?

Do they produce very sharp picture?
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Old Jan 13, 2007, 9:31 PM   #4
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Hi Basement,

1. Both the F and FA series lenses are auto focus. The F series are generally heavier in weight due to generally more metal being used in the barrel construction. The big difference in function is that the F series lenses just transmit focal length and aperture, while the FA series lenses also transmit MTF (not really the same, but you can consider MTF a measure of sharpness/resolution) information for the lens to the camera body. This information is considered by the camera's program to attempt to get the best performance from the lens in automatic metering (in the K10, you can actually specify optimizing MTF as the main consideration in the programed exposure line).

2. Mirror, or catadioptric lenses, as Roy stated, won't equal a great optical lens like the Pentax 500/4.5, but a good one, like the Tamron SP 500/8 Adaptall II can give you some good results, and have the advantages of of relatively small size, relatively light weight, no chromatic aberration or purple fringing, and being relatively inexpensive to acquire. The disadvantages are a bit less sharpness in the best models compared with the best optical designs, lousy quality in a lot of the off-brand models, the infamous "donut bokeh" where out of focus points of light appear as fuzzy O's (as opposed to sft circles) in the background, and relatively slow, fixed aperture compared to optical lenses.

I own a Tamron SP 500 f8 mirror, have used it quite a bit, and like it for what it is. I can deal with most of its IQ faults in post processing, and find it very convenient to carry in situations where I might find an unanticipated need for a very long lens.

Scott

Edit:

RP33 -- See my post from a while ago for a couple of examples:

http://www.stevesforums.com/forums/v...mp;forum_id=80

Scott






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Old Jan 13, 2007, 9:32 PM   #5
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some do but they have a weird bokeh.
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Old Jan 14, 2007, 4:06 PM   #6
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To summerize everyone:

From a user point of view, I'm getting that there isn't much difference between the F & FA lines. Mainly just a difference in info passed to the camera from the lens.

Even the digital only lenses are subject to the 1.5x magnification.

Mirror lenses work pretty much the way I thought they would. Stick to optical lenses unless you need a smaller sized long lens.


Thanks for all the info everyone!
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