Canon 7D AF Performance Under Low Light Shooting Conditions

[kenlow]

Hello,

I own a Canon 7D, overall its a pretty good camera, but i find its AF performance in low light shooting conditions unsatisfactory for a semi-pro camera that is spec up to ISO3200.

To understand the low light shooting performance of Canon 7D (or other Canon DSLRs) under 'AI Servo' and 'One Shot' mode better, i tried out the following AF sensitivity test at http://eosdoc.com/manuals/?q=AFSensTest , with a Canon 50mm F1.4 lens and single point focus, center AF point.

It would be much appreciated if fellow Canon 7D owners out there could try out the AF sensitivity test too and comment which patch number is your camera able to focus down to, consistently, without hunting/hesitation when in 'AI Servo' and 'One Shot' mode?

Do note your patch number, f-stop, shutter speed and ISO setting. I use spot metering (center AF point) to obtain exposure readings.

From my test, I find that:
(i) the center AF point is most sensitive and can achieve clean focus (no hunting/hesitation) in lower light conditions better than all the other AF points.
=> this is probably due to the higher sensitivity (2 times more sensitive?) center AF point.

(ii) With single center AF point, 'Single Shot' mode, I can achieve clean focus (no hunting/hesitation) down to about patch no. 8 @ 1/60s, f1.4, ISO 1600 (= EV 3).

(iii) With single center AF point, 'AI Servo' mode, I can achieve clean focus (no hunting/hesitation) down to patch no. 35 @ 1/60s, f1.4, ISO 800 (= EV 4).
=> Performance is much poorer  in 'AI Servo' mode compared to 'One Shot' mode. About 1 stop lower AF performance compared to 'Single Shot' mode in my test. 

Canon specifies EV -0.5–18 for the 7D AF sensitivity.
How is this derived?
My test result seems to indicate (my) 7D is not performing within this range.

I would prefer/like to use 'AI Servo' focus mode for low light shooting situations, especially for (slow) moving subjects, but the Canon 7D 'AI Servo' focus does not seem to be able to cope well with it at all, leaving me with no choice but to use 'One Shot' focus, hopefully, for with better results (very frustrating to get moving subjects in focus), although the camera's image sensor (ISO performance) can still cope with the available light level.

Have a go at the test, I look forward to hearing from you on your test results and comments.
Is your Canon 7D (or other Canon DSLRs) AF having similar (or better!) AF performance characteristics?

Thanks.
Ken

[Aglet]

I was pleasantly surprised to find my 7D with 100-400mm L, zoomed to 400m, was able to track birds in flight using center spot AF, servo mode, in very low light well after sunset where ISO 3200 wasn't adequate any more and 6400 wasn't what I wanted to use.
OTOH, I've had it fail in broad daylight.

[jfretless]

If you look hard enough, you can find faults in anything.  Instead of searching for the images that I can't capture, I focus on the images I can capture.

All I'm saying is photography is a art, not a science.  You'll drive yourself crazy trying to change that fact.

John

[bycostello]

in real dark conditions.. i just use a small led flash light to put enough light on the subject to focus

[AprilForever]

AI servo does much worse in Low Light than One Shot. I learned this the hard way... Use back button AutoFocus and one shot in low light and you will be amazed at the difference! I was when I discovered it... The caera hunts in AI servo in the dark because it is trying to figure out if the subject is moving or not.

[MK5GTI]

didn't know the 7D is rated at -0.5 EV only.
the 5D is rated at -1EV and i find it performs alright, sometimes i still can't focus.
the 6D is rated at -3EV and i wonder how much of a difference that can make?

the newest copy of pop photo mag just test the Nikon D600, it mentioned at -1EV, the camera took long to lock focus, i guess it has to pay a price for having more AF points ? just like the 5D3?

[neuroanatomist]

didn't know the 7D is rated at -0.5 EV only.
the 5D is rated at -1EV and i find it performs alright, sometimes i still can't focus.
the 6D is rated at -3EV and i wonder how much of a difference that can make?

The 5D and 5DII are rated at -0.5 EV just like the 7D and most other Canon bodies.  Older 1-series bodies like the 1DIV are rated to -1 EV.  The 5DIII and 1D X rated down to -2 EV.

[DCM1024]

I was extremely unhappy with my 7d when I first purchased it. I  opened a ticket with Canon and uploaded photos for them to review. They advised me that ai focus is only intended for fast moving subjects.  I found the 7d to require more tweaking in the menu than I haveexperienced with other bodies, but I do now get satifactory results.

[DCM1024]

I meant to say ai servo

[kenlow]

Hi MK, Neuro,

Although the 7D AF is rated EV -0.5–18, my center point AF 'One Shot' (this setting gives the best possible AF performance) test indicate it is already struggling at EV3.
If the spec is good, the AF should work well at EV3, as it is still very much within the working range. 
Do you know why is that so? Could it be my 7D's  AF sensor assembly is defective?       

Thanks.

Very best regards,
Ken

[neuroanatomist]

Although the 7D AF is rated EV -0.5–18, my center point AF 'One Shot' (this setting gives the best possible AF performance) test indicate it is already struggling at EV3.
If the spec is good, the AF should work well at EV3, as it is still very much within the working range. 
Do you know why is that so? Could it be my 7D's  AF sensor assembly is defective?   

First, the spec is for an f/2.8 lens.  So if your lens is slower, that affects EV (which is a 'stops' scale like aperture, so with an f/5.6 lens it should work in 1.5 EV.

But the other thing is that to achieve focus, you need both light and contrast.  I have no idea how Canon sets the spec, but I suspect (rather cynically, I know) that they aren't using a real world scene with -0.5 EV lighting, but rather a target with maximum contrast (black/white squares, a starburst, etc.).  If that's the case, performance in the 'real world' would not be up to the spec.

[kenlow]

Hi Neuro,

I am using a Canon 50mm f1.4 lens for the test, which should qualify the 7D AF for EV-0.5-18! but its not getting even close. It may be contrast, as you pointed out or my 7D AF sensor assembly is kaput!

Very best regards,
Ken
 

[CharlieB]

I bought a  7D for its "better focusing" but my 5DmarkII outperforms it easily.

Right now, the 7D is off to Canon for calibration.  None of the lenses would focus linearly.  You could AFMA far, or close, or middle, but thats it.   If you got the lens to focus ok at ten feet, it would be back focusing at 5 feet, and front focusing at infinity.... and not by a small amount.   Since this happened on all my lenses - L's or not - I just sent the body.    Hope they can fix it.

[kenlow]

Hi Charlie,
I had similar focusing issues with my 7D too earlier on. Took Canon support a number of attempts to finally resolve it for me. They did not disclose to me the cause of the problem, just that the AF needed recalibration. It focuses fine now, in well lit shooting conditions

Ken         

[Dianoda]

Instead of asking if it's defective (it's most likely not, more likely that you've hit the limit of the system's performance for the mode you've selected), how about asking how can I make better?

For that question I have a solution: turn on the expansion points (ie, one main + the four surrounding points) - you'll get better low light performance in both AI Servo and One Shot AF modes.  In one shot mode center point w/ expansion points enabled, my 7D can practically focus in the dark - not quickly (it certainly hunts some), but I've been impressed with the camera's ability to lock focus in very poor light.

And neuro is right, the AF system needs contrast to work as well.  Generally, the darker and more consistent the shade of the object/scene, the more likely the AF system is to have issues locking/tracking focus.