Today’s cameras are capable of capturing quality images at high ISO (6400 and above). Further, with image stabalization technology (like IBIS on the SONY cameras), it is realistic to take pictures at much slower shutter speeds, at least for static images. As a result, it is now possible to take great pictures in very low light conditions without using additional flash lighting. To take advantage of this image capture flexibility the performance of autofocus in low light becomes much more important than in previous generations of cameras. I was reminded of this at an event recently where I wanted to take pictures, indoors, in low light, and had a devil of a job getting my A7R II to autofocus when I wanted it to. All I got was focus hunting and the occasional grab at correct focus. Either the camera was way over-hyped in it’s AF performance, or I didn’t have the right technique. Further, I realized that I had never really tried to photograph in such low light conditions before (I don’t like using flash), even with my D810. I was curious which camera had better low light AF. So I set about to understand more about how to focus in low light and get good results.
First, some theory.
By “low light" I generally mean indoors, probably after dark, under artificial light. The normal way of measuring the impact of light on camera exposure is through EV, or exposure value (https://en.wikipedia.org/wiki/Exposure_value). This is a normalized measure where an exposure of 1sec, f/1.0, ISO-100 = 0EV. Since all light entering the camera is determined by f-stop and shutter speed only, EV is defined as a measure at ISO=100. According to Wikipedia, Indoor, artificial light, ranges from 4-8EV with office spaces at the high end and home interiors at the low end. However, there are plenty of circumstances where you’d want to shoot at 0EV or even lower. For example, 1/30sec, f/1.8, ISO-6400 is an equivalent of .6EV. Some cameras claim to be able to autofocus down to -2EV (f/1.4, 1/15sec, ISO 12800) so there shouldn’t be a problem, right?
One assumption that I am making is that the AF ability of a specific camera / lens combination depends on the amount of light falling on the focus sensor during the AF operation. Most cameras open up the lens to the widest aperture during AF (mirrorless included) so the faster the lens, the better the AF (independent of shutter speed and ISO). When claims are made of the minimum light for focussing I’m guessing this is made with the fastest lenses available (generally 1.2 or 1.4)
Factors which affect AF performance include:
1. Absolute light levels
2. Contrast ratio of the subject
3. Maximum aperture of the lens
4. AF mode of the camera (AF-S, AF-C, Center, etc.)
5. Other illumination (AF-assist)
6. Camera electronics and lens interface (speed)
7. AF-type - CDAF, PDAF
Note 1:
With the A7R II, best settings for low light AF are Flexible Spot (M), AF-S. Flexible Spot (S) is significantly worse. AF-C falls apart because, while the first AF step is with the lens wide open, subsequent steps set the lens at the target aperture (this was my problem at the event). IBIS on or off didn’t seem to make any difference. I didn’t try center focus or moving the focus points in flexible spot. Will try another time. (remember to put these settings into my spreadsheet for the low-light, auto-ISO setting)
A7R ii versus D810:
Both set to AF-S. See above for A7R II
A7R II focussed to -2EV (13sec, f/1.8). D810 focussed to -1EV (~6sec, f/1.8)
Note 2: At -1EV it was pretty hard to see the subject through the optical viewfinder. On the other hand, the SONY EVF fully illuminated the scene making it easier to compose. Further, the SONY was easy to manual focus whereas it was impossible withthe D810 (except in LiveView on a tripod, which defeats the purpose)
Note 3: I took the shots on a tripod but not with a remote(SONY) or MUp(D810). I noticed quite a bit of motion blur at 100%. Reminder to test for maximum sharpness.