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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 11 Mar 2016 (Friday) 17:18
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How accurate should auto focus perform compared to manual?

 
Archibald
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Mar 15, 2016 18:07 |  #61

Jon wrote in post #17936529 (external link)
It's independent of the focal length but strictly dependent on the absolute diameter of the aperture. So f/16 on a 200 mm lens isn't the same as f/16 on a 20 mm lens, like I said before. So the ratio of perimeter to circumference of the 200 is 10x that of the 20 at the same f/number. So much more light that's not subject to diffraction (by passing next to an edge) can get to the sensor with the 200 than with the 20.

Makes sense - however, I think it's misleading.


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Mar 15, 2016 18:13 |  #62

Jon wrote in post #17936529 (external link)
It's independent of the focal length but strictly dependent on the absolute diameter of the aperture. So f/16 on a 200 mm lens isn't the same as f/16 on a 20 mm lens, like I said before. So the ratio of perimeter to circumference of the 200 is 10x that of the 20 at the same f/number. So much more light that's not subject to diffraction (by passing next to an edge) can get to the sensor with the 200 than with the 20.

What you are saying is correct, but it implies that one can stop down to a smaller f-stop with a tele lens than with a wide angle lens without being diffraction-limited. And that is not correct. However, maybe I misunderstand your meaning.

Check out http://www.cambridgein​colour.com …ffraction-photography.htm (external link)

They state, "Technical Note: Independence of Focal Length Since the physical size of an aperture is larger for telephoto lenses (f/4 has a 50 mm diameter at 200 mm, but only a 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel farther before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size)."


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John ­ Sheehy
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Mar 15, 2016 18:19 |  #63

Jon wrote in post #17936529 (external link)
It's independent of the focal length but strictly dependent on the absolute diameter of the aperture. So f/16 on a 200 mm lens isn't the same as f/16 on a 20 mm lens, like I said before. So the ratio of perimeter to circumference of the 200 is 10x that of the 20 at the same f/number. So much more light that's not subject to diffraction (by passing next to an edge) can get to the sensor with the 200 than with the 20.

The blur radius relative to the subject size is determined by the aperture size. The size of the blur radius on the sensor in microns depends on the f-number (but I believe it might get bigger for macro).




  
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Mar 15, 2016 18:21 |  #64

John Sheehy wrote in post #17936544 (external link)
The blur radius relative to the subject size is determined by the aperture size. The size of the blur radius on the sensor in microns depends on the f-number (but I believe it might get bigger for macro).

... because the effective aperture becomes smaller.


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Mar 25, 2016 17:50 |  #65

This seems to be another instance where ther is much angst generated over spec comparisons.

Go out and take real life photos. Are your manually focusd images better or worse than the camera AF photos. That provides your answer.




  
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John ­ Sheehy
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Mar 26, 2016 12:38 |  #66

kjonnnn wrote in post #17948622 (external link)
This seems to be another instance where ther is much angst generated over spec comparisons.

Go out and take real life photos. Are your manually focusd images better or worse than the camera AF photos. That provides your answer.

Maybe not. A good AF system can achieve near-perfect focus more often than manual focus, especially for things like action. However, when you're on the fringes of the AF system's ability, manual focus may allow you to be close to perfect focus almost every shot (as you don't take the shot unless you think you are in focus, where dodgy AF may keep the system from being anywhere near focus, causing you to waste tne switching to manual over-ride.

IME, good AF can get you to within 0.1% of the actual distance, and either does this or misses completely. With a large aperture, with manual, an experienced person can get almost 100% of shots within 0.5 to 1% of the needed distance (numbers fabricated for illustrative purposes).

AF can help a lot, and it can also get in the way, a lot, too, especially when you've been shooting automatic and are not in the zone for fast manual focus, or override is too cumbersome with your system.




  
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airfrogusmc
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Mar 26, 2016 19:12 |  #67

Not true. I shoot action all the time and use DoF scales. No auto focus in the world is faster than being pre focused and with a little discipline and practice you can be a lot more reliable than autofocus with the right equipment.




  
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Mar 26, 2016 20:05 |  #68

airfrogusmc wrote in post #17949907 (external link)
Not true. I shoot action all the time and use DoF scales. No auto focus in the world is faster than being pre focused and with a little discipline and practice you can be a lot more reliable than autofocus with the right equipment.

Hmm, I doubt you would be able to prove this in a shootout.


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airfrogusmc
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Mar 26, 2016 20:51 |  #69

Believe what you want. I know plenty of film and manual focus photographers that can with me being one. My street work is very fast paced and the moment is over if you have to what for focus. I know first hand.There's a real reason I sold all my DSLRs and went Leica M.




  
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John ­ Sheehy
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Mar 26, 2016 21:58 |  #70

airfrogusmc wrote in post #17949907 (external link)
Not true. I shoot action all the time and use DoF scales. No auto focus in the world is faster than being pre focused and with a little discipline and practice you can be a lot more reliable than autofocus with the right equipment.

Who are you replying to?

I don't understand how people can reply without leaving a clue as to who or what they are replying to.




  
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airfrogusmc
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Post edited over 3 years ago by airfrogusmc.
     
Mar 26, 2016 22:06 |  #71

That was you. Your post was the one above mine.




  
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How accurate should auto focus perform compared to manual?
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