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I have been researching the Pentax SMC DA* 300mm f/4 ED [IF] SDM as a possible wildlife lens to use with my K-5. There are only a limited number of reviews of the lens from reputable sources and even fewer with MTF data.

I have been checking the result of this lens in combination with the apsc K-5 at photozone, however my understanding of MTF data is limited. What also important is that I can compare this data to the Canon and Nikon 300/F4 equivalents, but it is simply to complex to get my head around it.

Photozone MTF for the Pentax

Your help in comparing and understanding the data is much appreciated.

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Note that the Photozone site says, several times, in big, highlighted areas: "Please note that the tests results are not comparable across the different systems!" –  mattdm Jan 21 '12 at 6:12
    
FWIW, I own this lens and it's outstanding. I think its optical performance is unlikely to be the limiting factor in your (or anyone's) success. –  coneslayer Jan 23 '12 at 14:16

3 Answers 3

up vote 2 down vote accepted

Well looking at the data for the Pentax lens, vignetting (very little and less than the Canon), distortion (virtually none) and chromatic aberration (minimal) are nothing to worry about.

The main thing to compare is the resolution. They are measuring this in Line Widths per Picture Height (LW/PH). Think of this as measuring the limit the lens can resolve (distinguish) very narrow lines close together before it just becomes a grey blur.

The Pentax lens maxes at around 2400 (LW/PH) in the center, the Canon at 3300. The figure of 2400 seems to be an very good result, as I see sharp macro lenses (Nikon 105mm for example) in that same range. The Pentax is almost as sharp in the corners, whereas the Canon is softer in the corners than in the center.

So the Canon lens appears to be able to resolve more detail (more distinct lines, 3300 vs 2400). However Photozone also say the "visually relevant" range is up to 2250 LW/PH . By that I take it that beyone 2250 the lens will resolve more than the sensor can record, or more than the human eye can detect in normal use. In fact the Nikon 300mm f/4 has a LW/PH value that maxes out around 2100, and it is called "very good to excellent"

I'm not an expert on MTF charts but it looks like optically the Pentax lens is very good. I guess you'd want to also evaluate the build quality, weather resistance, auto focus speed.

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photozone.de/lens-test-faq: It depends on the similarities between the image sensor system. A sensor SYSTEM contains the image sensor with or without micro-lenses, an IR filter, a low-pass (Moire) filter and the signal processing. As you can imagine the output quality is largely dependent on the whole chain on not just on the amount of megapixels. The different output quality between the Canon EOS 350D and the Olympus E-300 is a good example (despite a 8MP sensor). The tests are a good guidance for the lens quality as long as you compare the results WITHIN a test group (e.g. Canon). –  mattdm Jan 21 '12 at 6:33
    
Yes, the only fair way to compare Canon vs Pentax would be to mount them on the same body I guess. –  MikeW Jan 21 '12 at 7:00

You can't compare resolution numbers directly between aps-c and FF sensors. The same lens will give you a higer number in FF than in aps-c, usually by a factor of 1.45 to 1.5. So a non-scientific way to compare resolution numbers in photozone site if the tests are in different sensor sizes is to multiply number in aps-c by 1.45 factor (or divide in FF).

Don't believe me, look for a the same lens test in aps-c and FF and do the math (canon 50mm f/1.4 for example), you'll see a correlation between 1.45 and 1.5 factor for all diaphragms.

From what I see in photozone the Pentax SMC DA* 300mm is a very good lens in therms of resolution. Even better than Canon 300mm f/4L IS (aps-c): http://www.photozone.de/canon-eos/172-canon-ef-300mm-f4-usm-l-is-test-report--review?start=1

In digital, besides resolution you should see: - construction quality - bokeh - flare resistance - price

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Everything you need to know to understand MTF charts:

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

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That's a good page, but I don't see anything about the issue of comparing MTF charts made under different circumstances. –  mattdm Jan 21 '12 at 15:31

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