From: "Joseph Tainter" <[EMAIL PROTECTED]>
Hi Joe -
Question for Mark Cassino:
Re:
"My conclusion was that a low resolving lens with low CA and high edge sharpness - which is what my tests showed the ATX 400 to be - will create an image with clean edges and a high degree of _apparent_ detail."
Mark, I have this lens and am interested in your observations. Would you please define your terms, though? What do you mean by low resolving? I am having trouble understanding how a lens with "high edge sharpness" can be "low resolving."
Here's an example: a long time ago I did a comparison of the Tokina ATX 400 f 5.6 vs the Pentax 500 f4.5 (screwmount version.) I set up an "eye chart" of ever diminishing numbers, letters, punctuation marks etc. Then I set up the lenses so the magnification was the same, shot the chart, and compared the results, I was using Kodachrome 25. When I looked at a large simple letter, like a 16 pt. capital 'I' at high in a high res scan, I was disappointed to see noticeable chromatic aberrations with the Pentax 500mm. There was a clear magenta blur to one side and a clear yellow blur to the other. The same character with the Tokina had virtually no CA - it came very close to going from black to white (I think there was a very little bit of yellow fringing.)
But then I looked at smaller characters, and found that the Tokina did not resolve them as well as the 500mm. So, for example, an small '@' sign was just a black circle with the Tokina, albeit with crisp edges. The same figure was still discernable as an '@' sign with the 500mm, even though it was fringed with magenta.
Similarly, when you evaluate film (or digital sensors) you look at acuatance (edge sharpens / edge definition), resolution (the ability to display fine detail) and grain / noise (I'd refer you Ansel Adams' "The Negative" for more on that.) I think that in evaluating lenses, the same concepts of acuatance and resolution come into play. (Obviously, grain or noise is not an issue with lenses.) I don't know why, but when speaking of lenses 'sharpness' seems to be used as opposed to 'acuatance.'
Anyhow, sharpness looks at the ability to render a change in contrast abruptly - the quicker, the better. Resolution measures the ability to capture fine detail - you can almost think of it as the 'amount' of info captured. The two are clearly related in that high sharpness is needed for high resolution, but high sharpness does not guarantee high resolution. The same is true of film, and there are films with lower resolving power but higher acuatance that can produce a sharp image, but one lacking in fine detail.
According to Photodo, at f8 the Tokina scores the same as the FA 400 f5.6. It is weaker than the Pentax wide open.
I had the Sigma for a short period of time, and traded it for the Tokina. To be honest, I'm basing my perceptions on the Sigma more on what I've heard and read about it, than based on my actual (and very limited) use of it. So, the two lenses may be closer in performance than I think.
But, looking at Photodo, the Sigma actually does somewhat better in the overall weighted average at f8 (0.77 vs 0.73). There are some real limits on the usefulness of the weighted average info on Photodo. It's a shame they pulled the raw MTF charts from their site. But if you compare the weighted averages at 10, 20, and 40 lpm, the Sigma does do better in these. You'd expect a lens that has low resolution and high sharpness to do well at the low lpm figure, and then drop off quickly at the higher lpm figures. The Tokina does that, though not radically worse than the Sigma.
- MCC
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Mark Cassino Photography
Kalamazoo, MI
www.markcassino.com
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