What is EV -4?

The excitement about the autofocus module in the D5 and D500 partly accrues to this specification: -4EV. 

What does that actually mean?

Okay, if you’ve got a well-exposed shot at f/1.4, 1/15 second, and ISO 51200, you’re shooting in -4EV light. Note that the light meter stops at -3EV ;~). 

But let’s be clear about a few things. -4EV is only for the center autofocus point. So you’d have to be in Single Point autofocus using the central sensor to get that ability. As you move outward from the center of the frame, the mirror-based phase detection systems all lose low light discretion, and the D5/D500 are no different in that. 

But what I’m finding is that a lot of people just assume if there’s the appropriate amount of light (e.g. -4EV) the focus system will work just fine no matter what they’ve set and are shooting. No, that’s not true. 

The focus system still needs contrast in the focus sensor area in order to work. In low light with low contrast subjects—especially dark subjects that aren’t reflecting much light—you’re still going to find the D5/D500 might be struggling to find or keep focus. The more you use a range of autofocus sensors, the more likely that is (e.g. Single Point will be more reliable than 23-Point which will be more reliable than 153-Point). 

So the question becomes this: how low a light can you shoot with the new cameras and get reliable autofocus?

The answer is pretty much what Nikon tells you—with my qualifications—at least in my initial testing. Yes, you can focus reliably at -4EV with the central sensor. But only if you have clear contrast in your subject at that focus sensor position. 

Even in lowish light—tested at EV1.5 in my office at edge of day with no artificial light—I can make the autofocus system fail by trying to focus on something with virtually no contrast. Give it a clean contrasty edge or pattern, and everything tends to be okay. Give it a swath of plain fabric, a wall, or any plain low texture surface (includes the side of many faces) and the focus system will hunt or worse.  

Trying to quantify how much better the D5/D500 are over previous Nikons—the D750 is specified to -3EV, for example—is difficult because of the need for contrast. The lower the light, the more contrast you probably need, and really dark tones are more difficult than brighter ones.

For example, I have a black mesh copy holder and bright silver mesh on the front of my MacPro. While there is “contrast” in the meshes, the black mesh is tougher to get focus on with my office at 1.5EV than is the silver mesh. Sometimes I get a hunt or miss with the black mesh, rarely do I get that with the silver mesh. Nevertheless, the performance with the same lens and settings on the D500 is better than on the D750.

So, yes, the D5/D500 will allow you to shoot deeper into low light and still retain autofocus. But you still need subject contrast to get fast, precise autofocus. This can be troublesome in some situations. This week my shooting partner and I are covering a theatrical production. In rehearsals we’re not having any focus issues, but I’ll bet you that we’ll see some in the final production tomorrow when the stage lighting is done. 

Some of the costumes are dark, and I know the lighting direction of the group to tend towards narrow spots with low light levels (and often with colored components), and the actors don’t always hit their marks. If I’m on the non-key side of any lighting effect, the face side I’m shooting will go into shadow and often be blocked partly by wig, hairpiece, or other costume component. In one case, we have a character who will be wearing all black, including a hood that covers most of the face. I anticipate a lot of low contrast situations for the autofocus sensor. 

This is one reason why pros use back button focus and then also tend to give themselves a bit extra depth of field by using a smaller aperture. If I can get focus on the subject and my distance-to-subject isn’t changing by more than a few inches, I can let go the AF-On button once focus is obtained and just shoot. I’m also known to not jump back to autofocus in such situations but instead to use the focus ring on the lens if the subject moves. So a relevant question: if the subject moves a foot towards you which way did you move the focus ring and by how much? 

Yes, you should know that. Even on a camera that focuses at -4EV.

Still, the D5/D500 are better at low light autofocus than previous Nikon DSLRs. Just make sure you know what the system is doing and what it needs.

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