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Charles Krebs
Joined: 01 Aug 2006
Posts: 2362
Location: Issaquah, WA USA
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dmillard
Joined: 24 Oct 2006
Posts: 245
Location: Austin, Texas
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 Posted: Tue Nov 25, 2008 5:13 pm Post subject: |
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| Charles Krebs wrote: | dmillard,
Thanks for the link. . |
And thanks very much for your additional links Charles. The MTF graphs on the link I provided indicate that the maximum aperture of f/2.8 primarily provided ease of focusing, and that the lens performance improved greatly when it was stopped down.
David |
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rjlittlefield
Site Admin
Joined: 01 Aug 2006
Posts: 7323
Location: Richland, Washington State, USA
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| Posted: Wed Nov 26, 2008 1:41 am Post subject: |
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| dmillard wrote: | | The MTF graphs on the link I provided indicate that the maximum aperture of f/2.8 primarily provided ease of focusing, and that the lens performance improved greatly when it was stopped down. |
That's the way it looks at first, but the story looks quite a bit different when you specifically consider photomacrography applications.
A lens this short (28 mm) will most likely be used at high magnifications, such as 5X and above. At 5X, on an APS-C size sensor (27mm diagonal), the reversed lens will only be looking at a 5.4 mm field diameter. The diagonal coverage of that lens is specified as 30 mm, so the entire field of interest fits within the central 18% of the nominal lens coverage.
What this means is that most of the MTF chart doesn't matter. The graphs that are most relevant are the center row (magnification 10X), and only the portions of those graphs below 20% on the x-axis.
In those areas, the graphs look quite similar at all f-stops.
However, the data that is graphed is still not very relevant. That's because with the lens reversed, the graphs are now specifying MTF in terms of resolution on the subject. The lines that are graphed are only for 10, 20, and 40 lp per mm, and that's at the subject. At the sensor, at 5X, those lines on the graph correspond to 2, 4, and 8 lp per mm. Since even a 6 MP APS-C sensor can resolve up to 66 lp per mm, the graphs are showing MTF values only for spatial frequencies that would correspond to extremely blurred images.
What is much more relevant is the MTF of the lens at much higher spatial frequencies, say around 30-60 lp per mm at the sensor, or 150-300 lp per mm at the subject.
At those frequencies, diffraction plays a much larger role. If the graphs were extended down to those frequencies, then the faster falloff at smaller apertures would become obvious.
It would be interesting to test one of those Componons, but I don't have one. However, I would naively expect its behavior at high magnification to be similar to my Olympus 38 mm f/2.8 bellows macro lens. As shown at http://www.photomacrography.net/forum/viewtopic.php?t=424, that lens has significantly higher resolution at f/2.8 and f/4 than it does at f/5.6.
The bottom line is to beware reading too much into manufacturers' MTF charts. They're very accurate and useful for certain purposes, but photomacrography is typically not one of those purposes.
--Rik |
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dmillard
Joined: 24 Oct 2006
Posts: 245
Location: Austin, Texas
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| Posted: Wed Nov 26, 2008 4:32 am Post subject: |
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Excellent points Rik.
I have a 28mm Componon with a fixed f/4 aperture that I picked up for $15 on eBay a couple of years ago, but I never got around to reverse mounting, let alone testing it. I just picked up a 30.5mm filter from Best Buy that I'll use to secure it, and I'll run some tests against some other
lenses at 4X this weekend.
David |
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Charles Krebs
Joined: 01 Aug 2006
Posts: 2362
Location: Issaquah, WA USA
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| Posted: Wed Nov 26, 2008 9:08 pm Post subject: |
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David,
You should give it a try. It's actually a lens I really like using. I've posted a couple pictures taken with it here, and here.
Charlie
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