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09-12-2007, 09:20 AM - 1 Like   #1
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Just for fun - Takumar 28mm vs Pentax-F 28mm vs Pentax-FA 31mm ltd

Had 'em, so I used 'em. Have a look at some (totally non-scientific) comparisons - feel free to comment

28mm - 31mm Wide Angle shoot-out - a photoset on Flickr

09-12-2007, 10:59 AM   #2
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That's pretty cool, good job.

Of course, one of the benefits to having the wider-aperture lenses is that when you shoot them at f/2.8 (or f/3.5) they are already stopped down a bit. I'm not surprised one bit, though, since my S-M-C Tak 28/3.5 shares almost equal time with the 31mm Limited.

--Sean
09-12-2007, 10:00 PM   #3
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Thanks Sean. I kinda wonder about that... could one theorhetically simply add a stop to an inferior glass and have perform poorly, yet leave one the option? I've noted that the larger apertured lenses aperturers are, in fact, phyiscally larger. Is that a necessary rule to a larger aperture rating?
09-13-2007, 05:24 AM   #4
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QuoteOriginally posted by thePiRaTE!! Quote
Thanks Sean. I kinda wonder about that... could one theorhetically simply add a stop to an inferior glass and have perform poorly, yet leave one the option? I've noted that the larger apertured lenses aperturers are, in fact, phyiscally larger. Is that a necessary rule to a larger aperture rating?
I'm not an expert in optics (although it fascinates me) but I have a guess about that.

For two lenses of the same focal length, the slower lens has a smaller opening. (Aperture is nothing more than the ratio of focal length to diameter of the opening.) Therefore the faster lens has a bigger opening, and will need to gather more light to cover the full circle of that opening. Given the typical construction of camera lenses, this means a bigger front piece of glass, and a bigger lens overall.

09-13-2007, 07:26 AM   #5
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QuoteOriginally posted by thePiRaTE!! Quote
Thanks Sean. I kinda wonder about that... could one theorhetically simply add a stop to an inferior glass and have perform poorly, yet leave one the option? I've noted that the larger apertured lenses aperturers are, in fact, phyiscally larger. Is that a necessary rule to a larger aperture rating?
You are right on the track: Aperture is defined as Focal length of the lens divided with the open diameter. So, the bigger the diameter, the smaller the f-number gets, the faster the lens.

In reality the "open diameter" is only a rough estimate, the entrance pupil is the really defining factor. But that is only to be scientifically correct.

Example: Pentax Macro (old) 4.0/100 gives a diameter of 25 mm (roughly 1 inch)
Pentax macro (new) 2.8/100 gives a diameter of 35,7 mm (roughly 1.4 inches)

Lenses often get better, when you step the aperture down app. 2 steps. (There are only very, very few lenses, which have top-quality fully open). If you mask down the diameter of the front lens (cut a mask out of black board) you have basically the same effect. Your open diameter gets smaller, the f-number gets bigger and the quality is improved. This is, because you cut out those rays of light, that go through the outer peripherie of the lens - and this is the area of the lens, where most aberrations are produced. Also, cutting out the peripheral rays means, that you loose those rays which are bundled under very oblique angels, which als reduces aberrations (especially coma and colour fringes).

regards
Ben
09-13-2007, 10:41 AM   #6
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Thanks for taking the time to explain Ben. I was just thinking, in answer to my own question then (being that physical size is a factor in aperture rating) that if one were to mismatch the maximum size of the aperture in relation to the front lens to get an extra stop, you would then simply cut off the picture circle around the edges (massive vignetting). So it all makes sense to me then I think - especially for extreme focal lengths with high aperture values why their physical properties increase and why you couldn't simply 'add a stop' just for kicks.

Here's another question then, is it fairly common to use all the glass on a lens? (ie. are the max aperture and front glass usually perfectly matched, or does a good lens have 'extra' diameter to prevent distortion?)

Thanks,


Kelly.
09-16-2007, 01:35 AM   #7
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Hi,..I'm not that Surprised Either,..as I have owned a Super Takumar f3.5 28mm M42, for Quite sometime and have over the years been Very satisfied with the Overall Image Sharpness of this lens.

One Only has to stop down a Bit and this lens is Superb!

Mine still (Almost), looks and feels like new !

A Great lens.
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