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06-30-2014, 02:22 PM   #1
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f stops, sharpness and teleconverters

Can someone clear this up for me, just can't to seem to get it clear in my head. If I had a lens say a 300 f2.8 that was soft wide open, now as I stepped it down till the sensor sees f8 the image is now razor sharp, all is good. What happens when I take the same lens and add a 2X teleconverter, I set the lenses aperture to f4 and with a 2 stop loss from the teleconverter the sensor sees f8, will the sharpness be the same?

Dave

06-30-2014, 02:30 PM - 1 Like   #2
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If your lens sweet spot is f/8, it'll stay at f/8 (on the lens), TC or not. So to keep the same sharpness, with a 2xTC, your lens will still need to be at f/8 but you will expose at f/16.

So if your lens is soft by itself when at f/4, the TC will only make the softness worse.
06-30-2014, 02:33 PM   #3
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QuoteOriginally posted by originalwinger Quote
with a 2 stop loss from the teleconverter the sensor sees f8, will the sharpness be the same?
No. It will AT BEST be the same as you would see at f/4. But unfortunately the TC will also introduce softness so the actual image will be worse. Possibly much worse depending on the quality of the TC.

Most studies I have seen indicate that using a 2x TC produces an image no better, and usually worse, than the image from the lens alone cropped to the same subject size.
06-30-2014, 02:43 PM   #4
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Also with the converter keep in mind you will loose light also. So you will need to take that into account.

Jim

06-30-2014, 03:10 PM   #5
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QuoteOriginally posted by fgaudet Quote
If your lens sweet spot is f/8, it'll stay at f/8 (on the lens), TC or not. So to keep the same sharpness, with a 2xTC, your lens will still need to be at f/8 but you will expose at f/16.
I have to disagree a bit.

In my experience with, e.g., a 2x teleconverter, stopping down another stop will generally be slightly more sharp, assuming a completely rigid setup (hence no shutter-speed complications). So if the base lens is best at F/8, generally you could pick up a bit of resolution by going to F/11.

It's usually not much though, but with an 2x T/C, every tiny bit really helps.
06-30-2014, 03:44 PM   #6
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Thanks so far as we have several views. I know in the real world most 2X telconverters will degrade an image but if it was of high quality and just used 2 stops of light will the sensor see the same thing?
Maybe I should explain what prompted this question. I had previously gone through my lenses and determined at which aperture they became sharp at. Then I purchased a AFA 1.7 which steals some light so it got me to thinking does the sweet spot ( sharpness) f stop change?

Dave

Last edited by originalwinger; 06-30-2014 at 03:55 PM.
06-30-2014, 04:38 PM   #7
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I wrote this review last year while playing with tc's. The links I give are interesting to refer to; the tc comparison thread from mflenses discusses similar points to what you are asking.
Having since had more experience with decent tp's like tamron 300/2.8, I tend to agree that tc's these days are more often a detrimental add on, the IQ is degraded more than would occur enlarging the image. The exception is when a high quality tc is correctly matched with a best quality lens, typically a prime. Lately I have been trying to use a tamron 014F 1.4x with my 200-500mm 31A. although both lens and tc are well rated here in the reviews the results so far have been disappointing.

But to quote from my review:
"One point of view is that the goalposts have moved since vintage TC's like these were made. Back then they were competing with an enlarger, these days the benchmark is the quality obtained from resizing the image by resampling on the PC. Enlarging is the equivalent of clicking the zoom-in icon. Resampling uses mathematical algorithms to create new closely matching pixels to make the image larger. If the TC doesn't make that cut (of at least matching the IQ of a resampled image) there isn't really much point to it.

Another point of view is that the previous point of view ignores practical considerations of utility, circumstance and cost.

The most subjective POV says that the only POV that matters is whether the photographer is happy with the results."


06-30-2014, 08:49 PM   #8
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It really depends on the lens combination. My 300 2.8 tamron 107b (iirc) is pretty soft alone, but with the 1.4 tamron tc it is better. With the 1.7 pentax, it simply magnified the lens flaws.

07-01-2014, 06:17 AM   #9
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Thanks derekkite ,marcusBMG (links were interesting reading) fgaudet, jatrax. jbrowning. What I've learned so far is when your dealing with teleconverters, the results are not easily predictable, you need to test on a per lens bases.
Thanks for helping me reach this conclusion.
Dave.
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