Nikon D500 Auto Focus Spec for Low Light

<p>When reading the specs. for the new D500 I noticed that the AF works down to EV -4 at ISO 100 which is equivalent to LV -4. It seems odd that there is no lens aperture mentioned since the light reaching the AF sensor is dependant on aperture and surely the autofocus works better when there is more light on the sensor. This is not unique to the D500; my D40 and D7000 both quote the spec. in the same way though of course the limit is higher (-1 EV at ISO 100).<br /> The spec. for the metering quotes an aperture of f/1.4 which is what I'd expect so why isn't it similar for autofocus?<br /> Does anyone have an explanation? All I can think of is that AF will work less well with slow lenses in poor light but that larger errors can be tolerated due to the larger DOF and the effects cancel out.<br>

By the way. Is there much value in an AF system that works in lower light than the meter?!!</p>

<p>Take a look at Nikon's spec for the D750: http://imaging.nikon.com/lineup/dslr/d750/spec.htm<br /> The AF "Detection Range" section:</p>

<blockquote>-3 to +19 EV (ISO 100, 20°C/68°F)</blockquote>

<p>There is also no mention of any lens aperture.</p>

<p>The matrix meter doesn't just suddenly stop working below a certain threshold, it just becomes less accurate below a certain light level. Also you can just click and see on the image playback how the exposure was; metering is convenient but not strictly necessary to make images. In low light AF performance is often limiting the ability to capture images successfully.</p>

<p>Shun,<br>

Yes, none of the Nikon specs. I've seen mention lens aperture in connection with AF but they all mention it in connection with metering. I just wonder why that is so.<br>

I'll look and see what Canon say about it ....</p>

<p>Ilkka,<br>

Not just the matrix meter - they quote limits (D7000 matrix and CW down to 0 EV, spot down to +2 EV). Yes they will sort of work down to lower limits but how well is anybody's guess. Yes of course you can get by without a meter - I was playing devils advocate and also pointing what does seem to me to be a little bizarre.</p>

 

<p>I just looked at the specs for Canon EOS 70D and EOS 7D mk II.<br>

They are quoted the same way as Nikon; ISO 100, f/1.4 for the meter but no aperture quoted for the AF system.<br>

So they both seem to be saying that AF in dim light is just as good with a slow f/5.6 zoom as with a fast prime.</p>

 

<blockquote>

<p>So they both seem to be saying that AF in dim light is just as good with a slow f/5.6 zoom as with a fast prime.</p>

</blockquote>

<p>Definitely not.<br>

<br>

It is quite obvious that under indoor dim light, a 70-200mm/f2.8 AF-S VR II focuses faster and more accurately than a 70-200mm/f4 AF-S VR, and an f5.6 lens will be further inferior. On the other hand, outdoors in a sunny day, the difference maybe completely negligible.<br>

<br>

However, AF also highly depends on the subject matter. A subject with patterns and contrast is a lot easier to focus, auto or manual. A subject without much contrast such as a plain wall, a sofa without patterns ... will be difficult.<br>

<br>

In other words, there are many factors. I would merely use those AF down to -1 EV, -4 EV figures for comparison purposes among different cameras. They don't really describe absolute situations.</p>

<p>I'm not sure that phase detect AF works proportionally with aperture. As I understand it; some of the exit rays from the lens pass through a phase-splitter (i.e. a lens/prism system similar to the old split-image rangefinder) to be re-combined when the image is in focus. There must be both a maximum and minimum working diameter to the bundle of rays accepted by the phase-splitter system.</p>

<p>We know that the lower diameter limit given by Nikon is equivalent to f/5.6 ~ f/8, but the upper diameter limit isn't specified. I should think - given the size and position of the AF module - that it's quite a bit less than f/1.4 though. So maybe it doesn't matter to the AF system too much what the lens's maximum aperture is, as long as it's greater than f/5.6.</p>

<p>As evening draws in I've just tested what limit my "cheap" D7200 will focus to. The exposure reading I got was 2 seconds @ f/1.4 with 400 ISO. Equivalent to EV -2/100 ISO. Yet the AF was still (just) able to work with an f/5.6 lens attached. Incidentally it was far too dark to see the subject properly through the f/5.6 lens given the dim DX viewfinder.</p>

<p>The conclusion I draw is that up to a point the lens aperture doesn't matter too much, and that the D500 will therefore focus in light that's far too dim to frame a shot in. I can think of very few practical situations where such AF sensitivity would be useful, unless the camera was set up before the light became too low to frame the subject. Maybe paparazzi shooting "blind" with flash might find a use for it.</p>

<p>On the Nikons I used the meter certainly stops working all together when it's too dark. It just start flashing in the viewfinder.</p>

<p>Regarding the AF sensitivity the AF sensors are like small image sensors but like Rodeo Joe said there are beam splitters in front of them so they are basically just taking a small sample of rays from the entire image. With smaller apertures the incoming rays are more parallel so it's harder for the AF to determine how far apart they are. That's why there is a limit. With larger apertures you get a better reading but also lens aberrations can mess with your reading. Canon specifies that they have extra sensitive AF sensors that work at f2.8 so it would make sense if Nikon has something similar. What this means is that you wont get better AF even if you have a lens with larger than f2.8.</p>

<p>When the AF sensors are reading the incoming light they are comparing dark with light patterns. So it's easier for the AF to determine where it is if the contrast is high. When the light levels are low the gain is increased and there is more noise when the from the AF sensors which makes it harder to determine changes in contrast. The light level where AF is said to work is at maximum contrast, white and black. The actual numbers and the actual measuring method is mentioned in the CIPA and corresponding ISO standards.</p>

<p>Anyway as the contrast becomes less, the camera needs more light (less noise in the readings) to determine the difference between light and dark. So a lower EV capability for the AF means both better AF in very low light with high contrast and better AF with less contrast needed in a little more light.</p>

<p> </p>

<p>RJ & Pete S,<br>

Thanks for that - I hadn't thought of the beam splitter (rather like the wedge prisms on a reflex screen perhaps?) and that is likely the explanation. Even though the AF sensor is in the equivalent of the sensor plane the rays from the cone edges of a wide aperture lens will be too divergent to be of any use unless the splitter is designed to make use of them as is the case for the ones designed for f/2.8 min. rather than f/5.6 min. That is presumably the explanation of why there is no aperture mentioned in the AF specs. for Nikon, Canon (and Pentax too I notice).<br>

<br />RJ,<br>

Excuse me asking but presumably you did have the AF assist lamp off in your experiment? Mine is normally on.<br>

<br />It is frequently the case that the faster lenses have faster AF motors so they may perhaps focus faster due to the faster motor rather than better AF measurement.</p>

 

<p>Richard, I have the AF assist permanently disabled in the menu. I generally have no use for it, because if it's too dark for the camera to AF, then my aging eyes can't compose properly in that level of light.</p>

<p>Besides which I don't want to burn out the (stupid choice of) filament bulb. Doubtless the replacement cost would be close to what the camera is worth! Have Nikon never heard of miniature white LEDs? Such as are fitted to every smartphone on the market.</p>

<p>Whoa! That raises a potential issue in my mind. If the choice of filament bulb is a deliberate one, chosen for its high red/IR content, then that means the AF system is IR sensitive. And that to me is a <strong>bad thing</strong>. IR has a different focus to visible light, which could account for some AF anomalies.</p>

<p>Thanks again - I just this moment disabled the AF lamp on my D7000. The camera does indeed work very well in the gloomy UK indoor light of today even with the cheapo 18-55 mm zoomed in to 55 mm f/5.6. I'll leave it off and see how it goes. Actually the life is probably pretty good since the light looks yellow and under-run for AF purposes; it's brighter for red eye reduction I think.</p>

 

<p>Please keep in mind that the D7000 uses the Multi-CAM 4800 AF module. The center 3x3 AF points (i.e. total 9) are cross-type AF points. I would try to use those 9 as much as possible, especially indoors. Those 9 should much better than the remaining line type AF points.</p>

<p>Just did a size comparison of the D300s, D7000 and D500.<br>

<img src="http://www.kohanmike.com/uploads/D500%20size%20compare.jpg" alt="" /></p>

<p>Oops.</p>

<p>Hi,<br>

newest AF adjustment takes into account the distance and the focal length?</p>