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06-08-2011, 12:59 AM   #16
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QuoteOriginally posted by 1marek Quote
MTF50 is about sharpness, MTF20 about resolution. I never bothered to find out more about these indicators because this is all I actually need to know, but if someone has more theoretical knowledge on this, feel free to share... for forum's sake
Well, I'll give you my take on this, not as an optics expert, but as an engineer who has an interest in photography:

The MTF test involves a test chart with alternating equal-width black and white bands or lines. The line width varies, and in any given locality is usually implied as a (spatial) frequency expressed in line pairs per millimetre.

MTF50 is the frequency at which the lens's response has caused the contrast between black and white to drop to 50% of the low-frequency (i.e. large line width) value. MTF20 likewise, but to 20% of the low-frequency value. The contrast drop is cause by blurring, whether due to lens imperfection or diffraction (at smaller apertures).

Now, why is it that MTF50 is linked to sharpness, and MTF20 to resolution? Well, I suspect it's because "sharpness" is generally associated with the edges of large objects, whereas resolution is more to do with close examination of tiny details.

To draw an electronics analogy, "sharpness" could be linked to the statement "flat to 20kHz", whereas "resolution" would be linked more to "useful response to 20kHz". The statements are not just different ways of expressing the same thing, because this would be rather dependent upon the slope around 20kHz.

I hope the above makes sense (and I hope I haven't made any major boobs in my analysis!).

06-08-2011, 03:25 AM   #17
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30+ years ago, I used to eagerly await the USAF resolution targets in Modern Photography before salivating over a lens. Now, not so much. First, it seems to me, there used to be a lot of really bad lenses out there years ago. You would find cheap zooms that basically rendered shots that looked like you were shooting into the light or through fog.

Now, just about any lens, even the super-cheap kit lens, will give you a nice center image, and a good image across the field at F8. The DA 18-135 which Photozone basically panned has numerous fans among actual users here. What reviews often seem to treasure are differences in corner resolution. Perhaps that is because that is where the differences are, but in many of our shots, the trick is actually to blur the corners. I find that even with landscapes, it is a minority of my shots where I want everything in the frame in perfect, sharp focus or could make it so with depth of field anyway.

In addition, 30 years ago, I always had Kodachrome in one of my bodies. What you saw (at amazingly high resolution) was exactly what you exposed. Not so, anymore. There is no "slide" which records exactly what the camera exposed--not even a raw file is without processing by the time we see it. A lot of the very subtle differences between lenses can be approximated (though not totally duplicated) with a bit of post processing.

In part, these points are one reason why I still shoot a good deal of film and use a lot of old "film era" lenses. I like to know what I am really shooting. It is also a reason why differences in lens performance are not as crucial to me as they once were. Now, I am likely to be considering whether the lens has features that I need at an acceptable level of performance. Is it fast enough, small enough, and does it offer the right range to help me get the shot? Is the result pleasing? That is more how I buy a DSLR lens.
06-08-2011, 04:30 AM   #18
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Everybody thanks for the input!

But now I want to ad another ingrediŽnt to my confusion.

I want to compare the photozone results of the DA35ltd and the DA18-135WR:

DA18-135WR:
Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis

And the DA35ltd:
Pentax SMC DA 35mm f/2.8 Limited macro - Review / Test Report - Analysis

Photozone was very disappointed with the resolution of the 18-135, but very impressed with the da35ltd. But their charts clearly shows that the 18-135 outresolves the da35ltd at ALL apertures.

...My confusion is still growing...
06-08-2011, 05:58 AM   #19
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QuoteOriginally posted by Clavius Quote
I want to compare the photozone results of the DA35ltd and the DA18-135WR:

DA18-135WR:
Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis

And the DA35ltd:
Pentax SMC DA 35mm f/2.8 Limited macro - Review / Test Report - Analysis

Photozone was very disappointed with the resolution of the 18-135, but very impressed with the da35ltd. But their charts clearly shows that the 18-135 outresolves the da35ltd at ALL apertures.
Photozone tested the DA35 on a K10D (10 megapixel), while the 18-138 zoom was tested on a K5 (16 megapixel). Measured resolution depends on the lens resolution and on the sensor resolution as well, hence the caution note posted on the Photozone website: "Please note that the tests results are not comparable across the different systems!"

Cheers!

Abbazz

06-08-2011, 06:00 AM   #20
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The other thing to remember is that Photozone tests the mtf on the infinity focus setting. Plus, the just switched over from the K10d as there bench body to the K-5. Lastly, they don't really like Pentax all that much over there. I look at 4 or 5 different places including this site and still try to keep an open approach to these things.
06-08-2011, 06:32 AM   #21
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QuoteOriginally posted by Abbazz Quote
Photozone tested the DA35 on a K10D (10 megapixel), while the 18-138 zoom was tested on a K5 (16 megapixel). Measured resolution depends on the lens resolution and on the sensor resolution as well, hence the caution note posted on the Photozone website: "Please note that the tests results are not comparable across the different systems!"

Cheers!

Abbazz
I assumed that meant that the figures can't be compared between brands. If they also can't be compared between bodies... Then the whole use of producing those MTF figures eludes me.
06-08-2011, 07:14 AM   #22
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QuoteOriginally posted by Clavius Quote
I assumed that meant that the figures can't be compared between brands. If they also can't be compared between bodies... Then the whole use of producing those MTF figures eludes me.
Those bodies have 2 different sensors that are not only different in mp (10 and 16), they also have different types of sensors, ccd (k10d) and cmos (K-5). Ironically certain Nikon bodies have the same sensors as some Pentax bodies, BUT the way they are controlled etc is somewhat different.
06-08-2011, 09:51 AM   #23
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It probably isn't what you want to hear but I say, Go with your own impressions and forget about Photozone. I actually tend to trust User reviews more than these 'pro' review sites. Especially from those willing to back up their claims. YMMV.



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