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Author Topic: How does max lens aperture affect low light autofocus?  (Read 5343 times)

drmikeinpdx

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How does max lens aperture affect low light autofocus?
« on: June 23, 2013, 12:53:05 AM »
I'm going to be doing some nighttime urban shots without a tripod using available light from streetlights and nightclubs.  I'm expecting to make use of the high ISO abilities of my 5D Mark 3.  A grainy, black and white motif is the plan.

I am wondering about which lenses to take along on this shoot, and here is my question:

How much does the maximum aperture of a lens affect autofocus performance in low light conditions?  I can select from lenses with max apertures ranging from F/1.4 to F/4.   Will the Mark 3 be able to focus better in marginal light with the larger aperture lenses?  Or does it depend more on the mysterious magic of the autofocus system in each particular lens and how it talks to the corresponding microprocessor in the body?

I've seen some discussion of the 6D being able to focus down to EV-1, or something like that, but nobody ever says if that varies with the lens that is attached to the camera.  It seems to me like it should.  Hopefully one of our resident experts can bring this issue into sharp focus!  LOL
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How does max lens aperture affect low light autofocus?
« on: June 23, 2013, 12:53:05 AM »

jabbott

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Re: How does max lens aperture affect low light autofocus?
« Reply #1 on: June 23, 2013, 02:53:23 AM »
From my own experience with the 5D3, it practically has night vision with an f/1.4 lens...  there were times during my visit to India when I tried shooting in available light at night with an f/4 max aperture lens and the camera would often struggle to focus, and it would auto select ISO 25,600 with an impractical shutter speed.  Switching over to the 50mm f/1.4 was a totally different experience - not only could it focus a lot better, I was also able to get some decent shots at ISO 6,400 - 10,000.
« Last Edit: June 23, 2013, 02:57:26 AM by jabbott »

WillThompson

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Re: How does max lens aperture affect low light autofocus?
« Reply #2 on: June 23, 2013, 03:18:19 AM »
High precision AF starts at f2.8, so use 2.8 or faster.

If AF speed is important use f2.8 for the best of both, if not go as wide as you can.
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Re: How does max lens aperture affect low light autofocus?
« Reply #3 on: June 23, 2013, 03:48:25 AM »
I call my 50L & 85L II "shining night" - always shoot wide open with center AF point, when there is almost no light ;)
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rs

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Re: How does max lens aperture affect low light autofocus?
« Reply #4 on: June 23, 2013, 07:28:19 AM »
AF speed is a combination of the AF system, what mode it operates in, and the lens.

The 5D3 has 5 of its central AF points which work as dual cross type AF points with lenses having an aperture of f2.8 or greater.  So I'd recommend staying well clear of anything slower than f2.8.

With my 5D2, 24-70 II and a Sigma 50/1.4, the results are about the same with either combo in low light. Even though the Sigma has much slower AF than the 24-70 II in good light, in very low light it seems about the same as the Canon when shooting stationary subjects. And when the light levels get so low that the Canon starts to hunt without being able to lock on, the Sigma gives up at that point too. However, to keep the ISO lower or gain faster shutter speeds, the Sigma 50/1.4 is my preferred low light choice.

What lenses can you choose from? You talk about an f4 lens being available, and I can't see an f4 lens in your equipment list - so while the Sigma 50/1.4 and 135L both look like the pick of the bunch, I'm guessing there are other fast lenses for you to choose from.

A bright lens with better AF than the Sigma (such as a 24 L II or even a Canon 50/1.4) should focus faster than both the Sigma 50/1.4 and 24-70 II in very low light. And as to how a 5D3, the lenses you've got available and the lighting/subjects all differ, that just all mixes it up and makes it next to impossible to give you a definitive answer. I'd suggest making an educated guess to pick a couple bright lenses which are known for fast AF from whatever selection you have available to you, and find out the answer to your question while you're there.
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bdunbar79

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Re: How does max lens aperture affect low light autofocus?
« Reply #5 on: June 23, 2013, 12:03:25 PM »
Well it's not always about AF speed.  It's about accuracy too.  My 70-200 focuses super fast, but if it can't find the object due to low light, it doesn't even focus.  In general yes, the wider the max aperture, the easier it is to auto focus.  In general.
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drmikeinpdx

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Re: How does max lens aperture affect low light autofocus?
« Reply #6 on: June 24, 2013, 09:09:44 PM »
Sounds like I should just do my own testing! :)

I recently got a 24-105, that's the F/4 lens I was thinking about.  I know the F/4 max aperture disables some of the focus sensors that need F/2.8, but the general performance of the autofocus in that lens is so impressive that I wonder if it will offset the small aperture limitations.  Guess I'll just have to find out.

To complicate matters, I sent my Sigma 50 in for a tune up last week and won't have it back in time for my shoot. 

I do have a 28mm F/1.8 and an 85mm F/1.8 that could be good performers. I am reluctant to try hand holding my 135 L lens, but that could be an option.

I'll just have to set up a low light focus test and see what happens. 

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Re: How does max lens aperture affect low light autofocus?
« Reply #6 on: June 24, 2013, 09:09:44 PM »

bholliman

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Re: How does max lens aperture affect low light autofocus?
« Reply #7 on: June 25, 2013, 06:33:31 AM »
Well it's not always about AF speed.  It's about accuracy too.  My 70-200 focuses super fast, but if it can't find the object due to low light, it doesn't even focus.  In general yes, the wider the max aperture, the easier it is to auto focus.  In general.

Which 70-200 variant do you have?  I have a 70-200 2.8 II and its AF accuracy is pretty good in low light.
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Lawliet

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Re: How does max lens aperture affect low light autofocus?
« Reply #8 on: June 25, 2013, 09:11:28 AM »
I've seen some discussion of the 6D being able to focus down to EV-1, or something like that, but nobody ever says if that varies with the lens that is attached to the camera.  It seems to me like it should.  Hopefully one of our resident experts can bring this issue into sharp focus!  LOL

The important part about the aperture: the AF-system has its own, and that is always stopped down quite a bit, so anything wider them f/16 or so has no effect on the amount of light the AF gets. Thats necessary to get sharp images of OOF areas that the AF system can compare to each other and calculate the misfocus from the difference. To compensate for the loss of light the sensor cells are extremely large in comparison, and the exposure times tend to be quite generous as well. The AF gets slower at low light? Sure, the system gets less updates when each sampling takes longer.
The aperture of the lens is only important in relation to the spacing between the pairs of AF sensors, when trying to use an f/2.8-sensor on a f/5.6 lens the AF strips don't see the world out there but the rear side of the aperture blades or the inside of the lens barrel. Place the pair closer together and they have an unobstructed view through the window, but also less stereoscopic seperation and thus lower precision. Thats one reason why some AF points of the 1Dx/5d3 work with certain lenses of a specfic aperture but not with others, somethimes the rays hit an obstruction, sometimes they just skirt them, depending on their actual positions of the AF sensor strips involved.

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Re: How does max lens aperture affect low light autofocus?
« Reply #9 on: June 25, 2013, 09:28:17 AM »
I've seen some discussion of the 6D being able to focus down to EV-1, or something like that, but nobody ever says if that varies with the lens that is attached to the camera.  It seems to me like it should.  Hopefully one of our resident experts can bring this issue into sharp focus!  LOL

The important part about the aperture: the AF-system has its own, and that is always stopped down quite a bit, so anything wider them f/16 or so has no effect on the amount of light the AF gets.

Um? The AF system gets its light through the lens just like anything else... and the camera AFs with the lens wide open. The reason the AF system is dealing with so much less light is that it only gets the fraction of the light that comes through the mirror; anything reflected to the viewfinder can't be used for AF. So, a wider max aperture does provide more light- both to the viewfinder and the AF system...
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drmikeinpdx

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Re: How does max lens aperture affect low light autofocus?
« Reply #10 on: June 25, 2013, 09:30:41 AM »

The important part about the aperture: the AF-system has its own, and that is always stopped down quite a bit, so anything wider them f/16 or so has no effect on the amount of light the AF gets.

Thanks Lawliet, that's very interesting - just what I was wondering about. That would appear to suggest that selecting a lens with an aperture larger than F/2.8 will not provide any additional advantage for autofocus speed/accuracy.

Can you tell me more about where this autofocus-specific aperture is located?  Does each focus sensor have its own?

Thanks!
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drmikeinpdx

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Re: How does max lens aperture affect low light autofocus?
« Reply #11 on: June 25, 2013, 09:44:07 AM »

Um? The AF system gets its light through the lens just like anything else... and the camera AFs with the lens wide open. The reason the AF system is dealing with so much less light is that it only gets the fraction of the light that comes through the mirror; anything reflected to the viewfinder can't be used for AF. So, a wider max aperture does provide more light- both to the viewfinder and the AF system...

bseitz234, that doesn't sound like the system used on current Canon DSLRs as I understand it.  To clarify, I'm talking about the phase detection system that uses an array of sensors in the viewfinder area.  This system only does its job when the mirror is down and all the light coming through the lens is reflected up into the viewfinder.  You may be thinking of cameras that use a partially silvered mirror which splits the light between the viewfinder and the image sensor?

Or perhaps you are thinking about the contrast detection system that operates in live view mode?  Let's save that for another discussion. :)
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bseitz234

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Re: How does max lens aperture affect low light autofocus?
« Reply #12 on: June 25, 2013, 10:14:28 AM »

Um? The AF system gets its light through the lens just like anything else... and the camera AFs with the lens wide open. The reason the AF system is dealing with so much less light is that it only gets the fraction of the light that comes through the mirror; anything reflected to the viewfinder can't be used for AF. So, a wider max aperture does provide more light- both to the viewfinder and the AF system...

bseitz234, that doesn't sound like the system used on current Canon DSLRs as I understand it.  To clarify, I'm talking about the phase detection system that uses an array of sensors in the viewfinder area.  This system only does its job when the mirror is down and all the light coming through the lens is reflected up into the viewfinder.  You may be thinking of cameras that use a partially silvered mirror which splits the light between the viewfinder and the image sensor?

Or perhaps you are thinking about the contrast detection system that operates in live view mode?  Let's save that for another discussion. :)

From https://en.wikipedia.org/wiki/Autofocus#Phase_detection:
"The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera."

try this: unmount your lens, and look into the camera. start LV, to flip the mirror up. Look down. See those funny looking slots? That's where the AF system is. Those slots allow the light in, so if that light isn't coming through the lens, and through the mirror when it's down, where is it coming from? You can actually see the semi-transparent area of the mirror, too, if you get the angle right...
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Re: How does max lens aperture affect low light autofocus?
« Reply #12 on: June 25, 2013, 10:14:28 AM »

bseitz234

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Re: How does max lens aperture affect low light autofocus?
« Reply #13 on: June 25, 2013, 10:15:43 AM »
More reliable source:
http://www.the-digital-picture.com/photography-tips/Canon-EOS-DSLR-Autofocus-Explained.aspx

I knew I could do better than wikipedia ;-)
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privatebydesign

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Re: How does max lens aperture affect low light autofocus?
« Reply #14 on: June 25, 2013, 10:21:43 AM »

Um? The AF system gets its light through the lens just like anything else... and the camera AFs with the lens wide open. The reason the AF system is dealing with so much less light is that it only gets the fraction of the light that comes through the mirror; anything reflected to the viewfinder can't be used for AF. So, a wider max aperture does provide more light- both to the viewfinder and the AF system...

bseitz234, that doesn't sound like the system used on current Canon DSLRs as I understand it.  To clarify, I'm talking about the phase detection system that uses an array of sensors in the viewfinder area.  This system only does its job when the mirror is down and all the light coming through the lens is reflected up into the viewfinder.  You may be thinking of cameras that use a partially silvered mirror which splits the light between the viewfinder and the image sensor?

Or perhaps you are thinking about the contrast detection system that operates in live view mode?  Let's save that for another discussion. :)

bseitz234 is right Lawliet is wrong. The AF module is not in the viewfinder either, it is in the base of the camera and it gets its light after it has come through the lens and through the semi silvered portion of the main mirror, it is then reflected again via the secondary mirror and onto the AF module.

Aperture makes a huge difference to AF performance and apertures are cumulative. If your AF module has an aperture of f16 and your lens f1.4 it is getting eight times more light than if your lens is f4, that makes a difference. Not only is the amount of light important but so is the angle of the light, the smaller the aperture the narrower the angle and the less accurate the AF.

This diagram shows why.
« Last Edit: June 25, 2013, 08:24:06 PM by privatebydesign »
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Re: How does max lens aperture affect low light autofocus?
« Reply #14 on: June 25, 2013, 10:21:43 AM »