Autofocus

By Rishi Sanyal

The D750 inherits a version of the 51-point Multi-CAM 3500FX AF system that we've seen used in the D800 and D810 but its 'II' version is actually more sensitive - rated down to -3 EV on all of those points. The D750 thereby offers superior AF reliability in poor light compared to the D810 and D4S, which is quite something (and which might prompt more than a few D800 owners to 'upgrade' to the D750).

D610
D750
D810

As shown above, the D750's AF points are spread out about one point further than the D610's, while the opposite is true compared to the D810. The D810's wider focus area gives it an advantage when tracking a moving subject or when framing an off-center subject, such as in the photo above.

In other respects, the D750's autofocus specification is identical to that of the D810, and offers single-point AF, 9-, 21- or 51-point dynamic-area AF, 3D-tracking, auto-area AF (with face detection), and Group Area AF (introduced in the D4S and D810). In Live View, face-priority AF, wide-area AF, normal-area AF, and subject-tracking AF are available.

Something worth pointing out is the limited number of cross-type points on the D750. The 15 central points on the D750 are the only cross-type points, where on Canon's EOS 5D III 41 of the 61 total points are cross-type. The wide area of cross-type points on the 5D III reduces the risk of hunting as you leave the center of the focus area, increasing the chances of locking focus particularly on low contrast subjects in low light, where subject detail is often difficult to lock on to. We'd love to see Nikon expand on the capabilities of the Multi-CAM 3500FX AF module by increasing the number of cross-type AF points cross the frame, if not make all points cross-type as Canon has done in the 7D Mark II. The video below demonstrates how the lack of cross-type points may be limiting in low light situations where subject contrast is low.

Cross-type vs. non-cross-type AF points. This video illustrates how the D750 focuses in very low light (-2 to -3 EV) when using different focus points. We start with the center point, which is cross-type, and then switch to an outer point, which is not. The non-cross-type off-center AF point struggles to focus on a vertical line (it's horizontally sensitive); meanwhile, the off-center point placed over a cross-hair with both vertical and horizontal detail has no problem locking focus. Focus is intentionally thrown off between each shot.

Another thing to note is that the center AF point locks focus considerably faster than the off-center AF point, despite both points focusing on a cross-hair. This is likely due to the center point being cross-type: it's more definitive at locking on to focus as it has more detail to evaluate.

The video clip above shows the value of cross-type focus points. The cross-type points in the center allow the camera to focus quicker, and more definitively, than those on the outer areas that are only sensitive in the horizontal detail. In low light, these outer, non-cross-type points, fail at focusing on completely vertical detail, which we see as hunting. Do note, however, that this is a stress test - the light has been lowered to approximately the lowest level of light the AF points work at (-2 to -3 EV). In fact, turn up the lights, and the outer points focus on the vertical line just fine. It's when there's a lack of subject contrast (low light, or backlit situations) that non-cross-type points particularly struggle. Under these situations, the more axes along which an AF point is 'looking' for detail, the more likely focus will be acquired.

Low light focusing

In this test we're looking at the light levels at which four competitive DSLRs are able to focus, to put to test the claim that the Nikon D750's AF points all focus down to -3 EV (previously, the Multi-CAM 3500FX module claimed AF down to -2 EV). Briefly, we very slowly lowered light levels until the camera was no longer able to focus with the focus point under study. We then, in very minute increments, increased light levels until the camera was once again able to focus. This light level indicated the lowest level of light the AF system is able to focus in.

In the examples below, the darker the photo, the lower the measured light level, which we indicate*. We examine this performance on both a center and off-center subject. The four cameras in question are the D750 and D810 along with Canon's EOS 7D II and EOS 5D III. 35mm f/1.4 primes were used for the full-frame bodies, and a 24mm f/1.4 prime was used for the 7D Mark II. Photos were taken under tungsten lighting.

Centered subject

Let's start with our subject at the middle of the frame, with the focus point also placed in the center.

Nikon D810 (-1 EV) Nikon D750 (-2.3 EV, in some cases -2.7 EV)
Canon EOS 7D Mark II (-2.7 EV) Canon EOS 5D Mark III (-3 EV)

The Nikon D750 focuses at impressively low light levels. In this case, it consistently focused on Sam's eye under -2.3 EV conditions, and sometimes even at -2.7 EV. We note, but do not show, that the D750 focused down to -3 EV levels with a high-contrast cross-hair target.

Now, While the Nikon D750 focused on Sam's eye in light levels at least 1 EV lower than the D810 (rated down to -2 EV), as expected, both the Canon 5D Mark III and the 7D Mark II also do quite well - even a bit better in this test. The 5D Mark III came as a bit of a surprise, as its center point is only rated down to -2 EV. Furthermore, although we can't show it here, we generally felt that the 5D Mark III and 7D Mark II more confidently locked focus in our test above, likely a testament to their dual cross-type design that 'searches' for detail along four axes using f/2.8 and brighter lenses.

If you're wondering why the D810 didn't quite make it down to its rated -2 EV, or why the D750 and 7D Mark II didn't quite make it down to their rated -3 EV, it's likely because we're focusing on a lower contrast subject here. Ratings are typically based on idealized scenarios, such as high contrast targets - which doesn't always translate literally into practice.

Something to keep in mind is that AF systems rated to lower light levels don't only offer benefits at the lower threshold; rather, their increased sensitivity can come in handy even in slightly brighter, yet still low light, situations where subject contrast may be lacking. In such situations, expect the D750 to slightly outperform its siblings. Nice.

Off-center subject

In this example we've moved our subject to the left side of the frame and chosen an appropriate focus point. We didn't get the opportunity to test the 5D Mark III in this example, but we intend to follow up.

Nikon D810 (-1 EV)
Canon EOS 7D Mark II (-2EV)
Nikon D750 (-2.7 EV)

Here, the Nikon D750 pulls ahead of all the other cameras in this test, which is not unexpected given the -3 EV rating of all of the D750's AF points. The D810's off-center points perform much like its center AF point in this respect, as long as there's some horizontal detail for the AF system to lock on to. The 7D Mark II falls slightly behind the performance of its lower-light-rated center AF point, only focusing down to roughly -2 EV.

tl;dr

The Nikon inherits a pro-grade 51-point AF system from its higher-end siblings and, arguably, improves upon it. Although AF area coverage has been reduced a bit from the D800, D810, and D4S, the D750's AF system impressively focuses in light levels lower than any of its siblings, and we found it capable of focusing down to light levels between -2 and -3 EV. Although the center AF point didn't perform down to quite as low light levels as the Canon 5D Mark III or 7D Mark II on a real-world, human subject, the D750's outer AF points - all rated down to -3 EV just like the center point - pulled ahead of its competitors, whose off-center AF points typically don't perform down to as low light levels as the center point. Notably, Nikon's off-center non-cross-type AF points can be somewhat hesitant focusing on low contrast subjects in low light, and we'd love to see Nikon update its module with more, if not all, cross-type AF points.

Head on to the next page to learn about the D750's other autofocus trick: subject recognition and 3D tracking.

Our method of determining the light level was as follows (light meters generally don't work down to -3 EV): we sampled an area under Sam's eye that the focus point was placed over, to determine the LAB value there. We found the exposure at which L = 50, and then extrapolated the EV rating.