Low-Light Autofocus Performance

[BadHorse]

I was at a wedding with an old 70D and a new 35mm F/1.4L II and noticed when the lights dimmed for dancing I had a real tough time locking focus for stills (I had a Speedlite so the actual shots were properly lit).

As I process my results I'm left wondering how modern cameras would handle that situation.

My first thought was to assume that a full-frame body would be better equipped to focus in low-light but then I realized that would only be true if you were shooting in Live View (which I don't use). Can anyone please confirm my theory that the sensor size does not directly effect auto-focus?

My second thought was to look for specifications on various bodies to see if there was some real way to compare but all the specs seem to talk about Dual Pixel CMOS AF (which I believe also only applies to Live View?). Can anyone please confirm that DP CMOS AF is irrelevant for SLR stills since it focuses while the mirror is down and the sensor is blind?

I assume there's improvements with each generation -- some software, some hardware -- but I just wish there was some sort of standard measurement I could use to guide upgrade purchases.

I concluded that there were only two technical solutions I could rely on:

1) Faster glass since even though I wouldn't be likely to shoot at 1.2 the AF works with the lens wide-open.
2) A low-intensity ring-light on the lens for AF assist (blasted out by the speedlight)

Have I missed anything?

 

[PCM-madison]

Ability to focus in low light is part of camera autofocus sensitivity rating listed as an EV range. A lower bottom number indicates a lower amount of light needed for autofocus. Recently released RP has a -5 EV rating. 70D has a -0.5 EV rating for the center point so it needs much more light.

 

[BadHorse]

Ability to focus in low light is part of camera autofocus sensitivity rating listed as an EV range. A lower bottom number indicates a lower amount of light needed for autofocus. Recently released RP has a -5 EV rating. 70D has a -0.5 EV rating for the center point so it needs much more light.

Hmm, it seems that Canon's official specs often omit this number; I can see it specified for the RP but corresponding pages for the 1DXII, 5DIV and 80D don't have it. Even the DXOMark pages are missing this information. Where did you see the -0.5 value you quoted?

 

[PCM-madison]

Hmm, it seems that Canon's official specs often omit this number; I can see it specified for the RP but corresponding pages for the 1DXII, 5DIV and 80D don't have it. Even the DXOMark pages are missing this information. Where did you see the -0.5 value you quoted?

I got that specification from the Canon USA website.

 

[BadHorse]

I got that specification from the Canon USA website.

Ahh, I see -- the 70D wasn't listed any more but google found the old page and it has it listed; strangely none of the modern bodies I checked did though (80D, 77D, 5DIV, 1DXII). The only other one I found with a value was the 5DS with a AF Working Range of -2 to 18 which I found surprising.

 

[PCM-madison]

Ahh, I see -- the 70D wasn't listed any more but google found the old page and it has it listed; strangely none of the modern bodies I checked did though (80D, 77D, 5DIV, 1DXII). The only other one I found with a value was the 5DS with a AF Working Range of -2 to 18 which I found surprising.

I only checked the 5DIV, but if you click the "view full specifications" button you get the numbers. In this case, -3 to 18 EV for the center point at F2.8.

 

[BadHorse]

I only checked the 5DIV, but if you click the "view full specifications" button you get the numbers. In this case, -3 to 18 EV for the center point at F2.8.

My god -- the PDF has more information than the web page! Thank you!

 

[Valvebounce]

Hi Folks.
I think you will find this info in the spec section at the back of each manual, manuals downloadable from Canon websites.
I picked some of the bodies mentioned above.

Page 528 of the English 1DXII m@nual.
Focusing brightness range: EV -3 - 18 (Conditions: f/2.8-sensitive center AF point, One-Shot AF, room temperature, ISO 100)

Page 576 of the English 5DIV manual.
Focusing brightness range: EV -3 - 18 (Conditions: f/2.8-sensitive center AF point, One-Shot AF, room temperature, ISO 100)

Page 498 of the English 80D manual.
Focusing brightness range: EV -3 - 18 (with the center AF point supporting f/2.8 One-Shot AF, at room temperature, ISO 100)

My 7DII has the same range, seems they might all have the same centre point technology!

Cheers, Graham.

 

[Don Haines]

Keep in mind that F1.4 lenses are slow to focus

 

[BadHorse]

Hi Folks.
I think you will find this info in the spec section at the back of each manual, manuals downloadable from Canon websites.
I picked some of the bodies mentioned above.

Page 528 of the English 1DXII m@nual.
Focusing brightness range: EV -3 - 18 (Conditions: f/2.8-sensitive center AF point, One-Shot AF, room temperature, ISO 100)

Page 576 of the English 5DIV manual.
Focusing brightness range: EV -3 - 18 (Conditions: f/2.8-sensitive center AF point, One-Shot AF, room temperature, ISO 100)

Page 498 of the English 80D manual.
Focusing brightness range: EV -3 - 18 (with the center AF point supporting f/2.8 One-Shot AF, at room temperature, ISO 100)

My 7DII has the same range, seems they might all have the same centre point technology!

Cheers, Graham.

Thank you very much -- I sat down to make this table this evening and found that you've done it already! And you're right, I found my 70D manual and it's EV -0.5 - 18 as mentioned earlier.

The fact that the there doesn't seem to be any difference between Full Frame and Cropped Sensor seems to confirm my theory that the main sensor isn't involved at all.

Now time to speculate wildly about the next crop of cameras and if they'll be even better!

 

[BadHorse]

Keep in mind that F1.4 lenses are slow to focus

Actually my 35mm F/1.4 is pretty snappy when there's light but I've read that big F/1.2 glass can be slow so I get your point.

 

[Valvebounce]

Hi BadHorse.
You are correct, the only time the sensor gets involved in AF is when in live view, (you can find the EV values for this a couple of pages further back in each manual) the rest of the time it is done with some of the light siphoned off through the semitransparent mirror and reflected by the secondary mirror down to the AF sensor in the bottom of the mirror box.
Link to a graphic showing the basic principle.

Cheers, Graham.

Thank you very much -- I sat down to make this table this evening and found that you've done it already! And you're right, I found my 70D manual and it's EV -0.5 - 18 as mentioned earlier.

The fact that the there doesn't seem to be any difference between Full Frame and Cropped Sensor seems to confirm my theory that the main sensor isn't involved at all.

Now time to speculate wildly about the next crop of cameras and if they'll be even better!