Minimum EV for autofocus and f/ number

[milktree]

My D850 and D500 both claim autofocus functionality down to -4EV.

Is that independent of the largest f/ number of the lens?

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

I've never seen an autofocus spec that has any reference to the lens at all. I never see anything like "autofocus down to -4EV @f/1"

Is there some standard f-stop for this spec?

Is it a minimum of -4EV at f/1, but only -2EV at f/2? -1EV at f/4? -.5EV at f/8?

It feels like I'm missing something fundamental here.

The *real* question I want to answer (but this is a more interesting proxy) is about astrophotograpy: Is a 20mm f/2.8 with a hard stop at infinity better than a 20mm f/1.8 that depends on autofocus?

"hard stop at infinity" means autofocus is irrelevant.

But the 1.8 is a better lens and would allow for shorter exposures, at the expense of depending on autofocus.

 

[David Lal]

The *real* question I want to answer (but this is a more interesting proxy) is about astrophotograpy: Is a 20mm f/2.8 with a hard stop at infinity better than a 20mm f/1.8 that depends on autofocus?

You might get better and more informed responses in the DPR astrophotography forum:

https://www.dpreview.com/forums/1059

This isn't my type of photography at all but when I have photographed the moon I focussed manually.

If you have the lenses could you not experiment and find out yourself?

 

[WPMChan]

My D850 and D500 both claim autofocus functionality down to -4EV.

Is that independent of the largest f/ number of the lens?

No. Larger lens aperture will let more light onto the AF sensor to allow more responsive AF.

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

Yes, it will AF better at f/1.8.

I've never seen an autofocus spec that has any reference to the lens at all. I never see anything like "autofocus down to -4EV @f/1"

I have read on internet that the quoted low light AF limit by Canon is based on:

- f/1.2 lens
- Centre AF point
- ISO 100
- One Shot AF

I can't find any such reference for my Nikon Cameras. I'll take that as a common reference conditions in the camera industries.

Personally, such rating only serves as an indication, if my camera is good in low light situation. It is not a quantitative reference useful in my camera and lens settings.

 

[Leonard Shepherd]

For what it is worth the low light auto focus Nikon specification for the Z6 III of minus 10 EV is based on using an f1.2 lens.

I presume but cannot confirm the D850 lowest AF EV is based the fastest Nikon AF lenses available of f1.4 when the D850 was launched about 8 years ago.

 

[GeoffRG]

My D850 and D500 both claim autofocus functionality down to -4EV.

Is that independent of the largest f/ number of the lens?

No. Larger lens aperture will let more light onto the AF sensor to allow more responsive AF.

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

Yes, it will AF better at f/1.8.

I've never seen an autofocus spec that has any reference to the lens at all. I never see anything like "autofocus down to -4EV @f/1"

I have read on internet that the quoted low light AF limit by Canon is based on:

- f/1.2 lens
- Centre AF point
- ISO 100
- One Shot AF

I can't find any such reference for my Nikon Cameras. I'll take that as a common reference conditions in the camera industries.

Personally, such rating only serves as an indication, if my camera is good in low light situation. It is not a quantitative reference useful in my camera and lens settings.

From the Nikon web site:

AF range –4 to +20 EV (ISO 100, 20 °C/68 °F)

Metering range (ISO 100, f/1.4 lens, 20 °C/68 °F)Matrix or centre-weighted metering: –3 to +20 EV. Spot metering: 2 to 20 EV. Highlight-weighted metering: 0 to 20 EV

 

[Leonard Shepherd]

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

Yes, it will AF better at f/1.8.

Adding some detail - the F mount AF bodies focus with the lens aperture wide open - resulting in f1.8 letting in more light than f2.8.

However (often in photography there is a however) a few years ago there were many forum post about the actual AF module having what is a second aperture, limiting light reaching the AF sensor to about f5.6.

--
Leonard Shepherd
In lots of ways good photography is similar to learning to play a piano - it takes practice to develop skill in either activity.

 

[WPMChan]

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

Yes, it will AF better at f/1.8.

Adding some detail - the F mount AF bodies focus with the lens aperture wide open - resulting in f1.8 letting in more light than f2.8.

However (often in photography there is a however) a few years ago there were many forum post about the actual AF module having what is a second aperture, limiting light reaching the AF sensor to about f5.6.

I am sure those learned advices on this forum were well informed advices.

However, yes, there is a however in photography, I have just done a test with my D5600 with my Sigma 30 F1.4 lens on it. In a particular low light situation, single point and spot metering, the D5600 was hunting for AF at f 5.6, but could find it at f 1.4 to 1.8.

I did another test with the Sigma on my D780, which has much better low light AF than the D5600. I chose a much darker object, the D780 was hunting for AF at f 6.3 but got it at f2.

I am convinced that I will have better low light AF when using larger aperture.

 

[WPMChan]

. . . . .about the actual AF module having what is a second aperture, limiting light reaching the AF sensor to about f5.6.

I am sure those learned advices on this forum were well informed advices.

However, yes, there is a however in photography, I have just done a test with my D5600 with my Sigma 30 F1.4 lens on it. In a particular low light situation, single point and spot metering, the D5600 was hunting for AF at f 5.6, but could find it at f 1.4 to 1.8.

I did another test with the Sigma on my D780, which has much better low light AF than the D5600. I chose a much darker object, the D780 was hunting for AF at f 6.3 but got it at f2.

I am convinced that I will have better low light AF when using larger aperture.

I posted the above after I had done a 5 minutes demonstration of low light focus before I went to bed.

I can explain a bit more now:

DSLR/SLRs use a focus module to determine the correct focus adjustment of the lens to obtain best focus of the object as selected by the photographer or by the camera with subject detection.

The focus module has two holes "apertures", working like two pin-hole cameras collecting and projecting two images on a focus sensor. The image is from the lens on the camera, reflected from a secondary mirror behind the translucent part of the main mirror.

These two apertures are located at the oppose ends of the reflection from the secondary mirror, as such, the light intensity profiles of these two images on the focus sensor are generally out of alignment, or "out of phase".

The AF mechanism, PDAF "phase detected auto focus" is to adjust the focusing mechanism of the lens, so that the light intensities on the focus sensor, from these two images, are the same, at the point of focus.

PDAF is performed by calculation of the amount of "out of phase" light intensity at the point of focus, and to move the lens focusing mechanism, based on an algorithm, to achieve the light intensity alignment.

The "second aperture" as pointed out by Leonard are in fact the apertures at the focus module, which are of fixed diameter, "about f5.6" as quoted by Leonard.

These two apertures project reflected light from the secondary mirror onto the focus sensor.

Lens on camera with larger aperture will project stronger light on the secondary mirror, creating stronger and clearer light intensity profile on the focus sensor for PDAF.

Lens with smaller aperture will project less light on the secondary mirror, in marginal situation, not enough light on the focus sensor for PDAF calculation, hence, AF hunting.

 

[Tord S Eriksson]

The *real* question I want to answer (but this is a more interesting proxy) is about astrophotograpy: Is a 20mm f/2.8 with a hard stop at infinity better than a 20mm f/1.8 that depends on autofocus?

You might get better and more informed responses in the DPR astrophotography forum:

https://www.dpreview.com/forums/1059

This isn't my type of photography at all but when I have photographed the moon I focussed manually.

My best shots of the moon have all been shot with AF, but in bad light situations, like lots of clouds, I sometimes switch to manual (when the AF isn't reliable).

If you have the lenses could you not experiment and find out yourself?

Indeed, it is the only way to find out!

 

[Tord S Eriksson]

i.e.: Will my D850 focus in lower light with a 20mm f/1.8 than a 20mm f/2.8?

Yes, it will AF better at f/1.8.

Adding some detail - the F mount AF bodies focus with the lens aperture wide open - resulting in f1.8 letting in more light than f2.8.

My Nikon cameras focus much better with fast lenses!

However (often in photography there is a however) a few years ago there were many forum post about the actual AF module having what is a second aperture, limiting light reaching the AF sensor to about f5.6.

That was news to me! Any links?