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Old Jan 3, 2007, 8:55 PM   #21
NHL
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Mark1616 wrote:
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Now call me thick (really please do) what actually are the graphs showing, I understand when the data is presented by people like Photozone but have looked at the Sigma site where these are and drew a blank. How do the two relate?
They both show the MTF of a lens, but in different ways:
o The Photozone gives you the MTF reading at the center, border, and sometime extreme border at various apertures
o The Sigma site show the classical MTF curves for the entire field of view of a lens. Sigma only plots theses curve @ wide open which is the worst case (the lens only get sharper when closed down). Others like Canon (or the respected Photodo) also plot a 2nd set when closed down to f/8 - See here: http://sigmaphoto.com/lenses/popup.asp

On a digital only lens - the plot stop @ <13mm because the rest of the curve is not seen by the sensor:




On the full-frame 120-300mm plots above, even @ 13mm (the edge of the 30D), the 30lp/mm plots exceeds that of the 300mm f/2.8 prime
-> A cropped camera is blind to anything on the right side of the 13-24mm curve for a full-frame lens
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Old Jan 4, 2007, 3:54 AM   #22
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That's cool, makes a bit more sense now, just need to find some spare cash to get one.
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Old Jan 4, 2007, 11:01 AM   #23
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BTW this apply to many modern zooms (especially the "digital only" ones)

-> Just don't automatically assume that a prime is better than a zoom... This may be true decade ago, but with newer CAD and manufacturing techniques this 'old' thinking no longer applies... Remember that most if not all primes are designed in the film day (i.e. at least a decade old) as compared to newer released zooms which benefited from the knowledge gained:

A cropped camera only use the best part of the image hence the digital lens designers concentrate their attention to the center thus the newer lenses are now both smaller and lighter as well as guiding all the light hitting the sensor straight on - This mere fact increases the contrast as opposed to full-frame lens which tend to hit the sensor from the sides.

Folks who pick primes have the intention of using them on their full-frame camera, because this is where they hold the edge. The prime are faster as well, but this is a compromise too since they are not neccessarilly sharper at those apertures...
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Old Jan 4, 2007, 11:45 AM   #24
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NHL wrote:
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BTW this apply to many modern zooms (especially the "digital only" ones)

-> Just don't automatically assume that a prime is better than a zoom... This may be true decade ago, but with newer CAD and manufacturing techniques this 'old' thinking no longer applies...
Great point and certainly true as far as optical quality goes. But there are also other factors to consider in any lens purchase:

Focus speed: as an example many birders prefer the 400mm 5.6 to the 100-400 for in-flight shots because the focus is better/faster. Sports shooters choose the Canon 24-70 over the Sigma again because low light focus speed is faster

Build quality - unfortunately, many of the new digital only lenses are plastic and don't have the same build quality as the pro grade lenses which are still designed for full frame. My 100-400 got knocked off a bench a year ago onto concrete - it's got a little dent in it. If it had been a plastic ef-s lens I have little confidence it would have survived.

So while I agree with you on the optical quality front I would just add that selecting a lens solely on that basis may be a bad idea. Sometimes other issues can be very important as well. And in those issues, the manufacturers may not be giving the digital lenses as much attention (focus speed, build quality). Also unfortunatelythings like focus speed aren't necessarily tested and charted so you don't have handy reference points like an MTF chart - you have to rely on actual usage reports - preferably from experienced photogs who have something to compare it to (i.e. fast is a relative term).
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