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11-13-2010, 06:33 PM   #16
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Oh now you've wandered into technical turf where I'm totally lost. Mind you, I use TC's regularly and think a good TC on a good lens does well--especially if the TC and lens are from the same manufacturer and the TC was pretty much intended for the lens.

11-14-2010, 03:59 AM   #17
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either way, I think we agree that the image with the TC is not as nice as the crop. I'd be more concerned in the gross resolution looking poor, than in being able to nit pick details...so my conclusion is that this TC is not for me! I hardly use the 200 anyway, so turning it into a 400 is unnecessary.
11-14-2010, 05:44 AM   #18
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QuoteOriginally posted by hoojammyflip Quote
either way, I think we agree that the image with the TC is not as nice as the crop.
We do.
11-14-2010, 07:02 AM   #19
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A great illustration of the difference between image quality and resolution!

How could the TC's overall IQ be so poor? Flare, white balance, chromatic aberration, what else?

Can the TC's IQ be fixed in post processing? I think some of it can be repaired.

Dave

PS I looked at the individual color channels; Green seems flared a lot (red and blue channels are much crisper than green); there might be flare in all channels (areas that should be dark are maybe 50% exposed in green & 33% in blue).


The red channel is excellent I think. I wonder why?

What light source did you use?

Also, maybe there's a strange MTF for the TC where the contrast is low at low at low frequencies, but stays more or less constant at higher frequencies?

Local contrast tools and simply subtracting some flare can help the TC image.


Last edited by newarts; 11-14-2010 at 07:57 AM.
11-14-2010, 11:45 AM   #20
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QuoteOriginally posted by newarts Quote
Also, maybe there's a strange MTF for the TC where the contrast is low at low at low frequencies, but stays more or less constant at higher frequencies
wow, the comparison for the difference between RG and B is very very interesting indeed. I was just using normal daylight around 3pm UK time as the light source: overcast UK autumn day. I think I still have the raw file if thats useful. I shoot BW film, which means I can use filtration to good effect, so the TC is useful to me with filtration. I like the way you broke it into the individual channels. I cant believe the TC could maintain contrast for higher frequencies....contrast drops linearly with frequency due to the linear increase in the overlap of circles of confusion from negating each other. The net resolution on the sensor is the convolution of MTF's of the lens, TC and sensor....any one of these 3 could play havoc with the others and may take an age to work out. Maybe a red 52mm filter is the solution

Last edited by whojammyflip; 11-14-2010 at 11:59 AM.
11-14-2010, 02:47 PM   #21
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QuoteOriginally posted by hoojammyflip Quote
wow, the comparison for the difference between RG and B is very very interesting indeed. I was just using normal daylight around 3pm UK time as the light source: overcast UK autumn day. I think I still have the raw file if thats useful. I shoot BW film, which means I can use filtration to good effect, so the TC is useful to me with filtration. I like the way you broke it into the individual channels. I cant believe the TC could maintain contrast for higher frequencies....contrast drops linearly with frequency due to the linear increase in the overlap of circles of confusion from negating each other. The net resolution on the sensor is the convolution of MTF's of the lens, TC and sensor....any one of these 3 could play havoc with the others and may take an age to work out. Maybe a red 52mm filter is the solution
I used ImageJ free software to separate the channels. It is easy to use and easy to understand.

I'm thinking a red filter might help BW film shots.

The most obvious explanation for the red image's looking so good is there was very little uniform red flare adding to the image brightness over the black bars of the target. The blue image can be corrected to a great extent by just subtracting about 1/3 background flare (this makes the bars black).

The green image has even more uniform green flare and it is out of focus compared to the red & blue images. look at the bar edges in the green channel above -there's a ghost about equal to the bar width.

I suspect the lens simply does not focus the green image at the same plane as the red & blue images. Maybe your focus wasn't optimum; red is in focus - maybe if green were in focus things would have looked better?

Dave

Last edited by newarts; 11-14-2010 at 02:58 PM.
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