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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 09 Oct 2013 (Wednesday) 14:24
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AF in low light: 2.8 vs 1.8

 
D ­ 550D
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Oct 09, 2013 14:24 |  #1

The answer to this question might be obvious but I still want check with you guys.

I sometimes run into situations where my 550D is unable to acquire focus lock with central point and a 2.8 lens because of the dark conditions.

Would using a faster lens (such as an f1.8) help me with this problem?


http://domenulbl.blogs​pot.com/ (external link)
550D|Sigma 18-35 1.8 Art|Sigma EX 70-200 OS|Nifty Fifty|Samyang 85 1.4|430 EX II|

  
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lovemyram4x4
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Oct 09, 2013 14:34 |  #2

In theory yes as there's more light hitting the AF senor, however some lenses don't focus well even in good light.




  
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D ­ 550D
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Oct 09, 2013 14:38 |  #3

lovemyram4x4 wrote in post #16358712 (external link)
In theory yes as there's more light hitting the AF senor, however some lenses don't focus well even in good light.

Learned this the hard way with my nifty fifty. But if both lenses were equiped with a USM/HSM motor the 1.8 should be better.


http://domenulbl.blogs​pot.com/ (external link)
550D|Sigma 18-35 1.8 Art|Sigma EX 70-200 OS|Nifty Fifty|Samyang 85 1.4|430 EX II|

  
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apersson850
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Oct 09, 2013 15:37 as a reply to  @ D 550D's post |  #4

Since the camera measures where to drive the lens to get it into focus, it's hardly the drive motor type in the lens which determines how successful the camera is in calculating the proper focus setting.

There's also the confusion about f/2.8 high-precision and f/5.6 standard precison focusing. But that has nothing to do with how much light comes through the lens. It's instead a question about how far apart the rays are at one and the other side of the lens. High precision focusing requires a longer baseline for the AF sensor pair, and thus a wider opening in the lens for these rays to hit the AF sensor.

Furthermore, if you read the camera's specification, you'll notice that Canon specifies which EV range the AF system works with. But the EV value is the same, regardless of which lens is mounted on the camera! Thus the camera's AF sensor is capable of working with the rays it gets through the lens, down to a certain ambient light level, and that's not dependent on the total light coming into the lens. The AF system's secondary image forming optics pick out two rays from the lens, one from each side, and it will do so regardless of the max aperture of the lens (with the exception of the high-precision focus mentioned above - but there too it's a question about where the rays come in, not how much light enters the whole lens).

On the other hand, lower cost lenses typically have inferior image quality compared to the more expensive ones. Chromatic abberation and various other imperfections in the lens will not only distort the final image. They will also distort the image the AF sensor has to evaluate.

If you want to see a test which proves that the reasoning above in this thread is at fault, you are welcome over to me. It's pretty dark in Sweden this time of the year, so well have no problem finding low light situations. Then I can show you that a 400D or a 7D equipped with an f/1.8 lens will sometimes fail focusing on something a 1DX can handle with f/4 or f/5.6.

Finally, there are indeed lenses which have problems setting focus at the right distance. Sometimes, like with the EF 50 mm f/1.4 USM, it's due to a rather coarse stepping of the focusing motor. Sometimes it's due to a fault in the calibration of the lens, not the lens technology itself.

Thus the answer to the original question would be yes, it can make a difference, but not for the perceived reason.


Anders

  
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gonzogolf
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Oct 09, 2013 15:40 |  #5

One option would be to get a YN622c flash trigger. The focus assist beam on that device is very helpful. Moreso than any of the other focus assist devices I've tried (built in on 580exII or STE2.




  
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apersson850
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Oct 09, 2013 15:52 as a reply to  @ gonzogolf's post |  #6

And if you don't want to buy/bring anything like that, then a small LED torch may be all you need. There are such things you can clip onto a 9 V battery, and they'll usually assist AF pretty well.


Anders

  
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Gobeatty
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Oct 09, 2013 16:09 |  #7

I've been pondering related questions lately. I returned a 50 1.4 because the dang thing was just too inconsistent in focusing and required +13 or so of adjustment to get it in the ballpark.

I have a 35 f2 and an 85 1.8 and been wondering if the AF accuracy with these lenses on my 6D is up to snuff when shot wide open. I understand DOF at closer distances is nill but if I shoot three shots in succession and get three differently focused shots, I wonder. I'm a leftover from the split image film days - I'm shocked in 2013 with the tech on offer that AF can be off, even when given an unambiguous target (high contrast, flat at same distance from the camera).

Is it just me? My kit? Am I expecting too much?


6D | 35 f2 | 50 1.8 | 85 1.8 | 28 - 135 f3.5 - 5.6 | 70-210 f4

  
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uOpt
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Oct 09, 2013 16:23 |  #8

D 550D wrote in post #16358684 (external link)
The answer to this question might be obvious but I still want check with you guys.

I sometimes run into situations where my 550D is unable to acquire focus lock with central point and a 2.8 lens because of the dark conditions.

Would using a faster lens (such as an f1.8) help me with this problem?

The light that you have, what kind is it?


My imagine composition sucks. I need a heavier lens.

  
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D ­ 550D
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Oct 09, 2013 16:24 |  #9

I have 430ex ii and its af assist is often helpful.


http://domenulbl.blogs​pot.com/ (external link)
550D|Sigma 18-35 1.8 Art|Sigma EX 70-200 OS|Nifty Fifty|Samyang 85 1.4|430 EX II|

  
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D ­ 550D
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Oct 09, 2013 16:26 |  #10

uOpt wrote in post #16358980 (external link)
The light that you have, what kind is it?

It depends on the venue. I like to shoot agains a source of ambient light because it gives me nice rim light but the faces are than in shadow wich makes the focusing more difficult.


http://domenulbl.blogs​pot.com/ (external link)
550D|Sigma 18-35 1.8 Art|Sigma EX 70-200 OS|Nifty Fifty|Samyang 85 1.4|430 EX II|

  
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PH68
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Oct 09, 2013 17:02 |  #11

I have the new EF 28 IS USM.
It might be "only" f/2.8, but on my 60D it will auto focus is some ridiculously low light.


Fuji XE1 ~ XF18 ~ XF27 ~ XF60 ~ XC50-230

  
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uOpt
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Oct 09, 2013 17:33 |  #12

D 550D wrote in post #16358985 (external link)
It depends on the venue. I like to shoot agains a source of ambient light because it gives me nice rim light but the faces are than in shadow wich makes the focusing more difficult.

My question was more what kind of light it is. Fluorescent? There are some forms of artificial light that pulsate and can make things very difficult for AF.


My imagine composition sucks. I need a heavier lens.

  
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tat3406
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Oct 10, 2013 00:43 |  #13

Gobeatty wrote in post #16358957 (external link)
I've been pondering related questions lately. I returned a 50 1.4 because the dang thing was just too inconsistent in focusing and required +13 or so of adjustment to get it in the ballpark.

I have a 35 f2 and an 85 1.8 and been wondering if the AF accuracy with these lenses on my 6D is up to snuff when shot wide open. I understand DOF at closer distances is nill but if I shoot three shots in succession and get three differently focused shots, I wonder. I'm a leftover from the split image film days - I'm shocked in 2013 with the tech on offer that AF can be off, even when given an unambiguous target (high contrast, flat at same distance from the camera).

Is it just me? My kit? Am I expecting too much?

Buy Better L lens, u will get more faster and accurate AF. The old 35 f2 and 50 f1.4 are not good at AF.


6D, 100L,24-70 F4L, 40mm pancake, 70-300L
Carl Zeiss MP 50
http://www.flickr.com/​photos/tat3406/ (external link)

  
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D ­ 550D
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Oct 10, 2013 01:06 |  #14

uOpt wrote in post #16359115 (external link)
My question was more what kind of light it is. Fluorescent? There are some forms of artificial light that pulsate and can make things very difficult for AF.

When there are flourescent lights involved there is usually enough light to focus
I have heard about pulsating issues but I have nevr experienced it.


http://domenulbl.blogs​pot.com/ (external link)
550D|Sigma 18-35 1.8 Art|Sigma EX 70-200 OS|Nifty Fifty|Samyang 85 1.4|430 EX II|

  
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sharraleigh
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Oct 10, 2013 01:34 |  #15

The nifty 50, despite being 1.8 is horrible, horrible, HORRIBLE at focusing in low light.


Canon 70D, 550D + Tamron 24-70 f2.8 VC, 28mm f1.8, 85mm f1.8, 50mm f1.8
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AF in low light: 2.8 vs 1.8
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