[Ayoh]

The problem is that fine spot AF (which I am using all the time) is lost in very low light. In this case the camera is expanding the AF field to look at the "grand picture".

 

That is not really surprising - the sensor noise is very high in such a situation and this has impacts. The same is true for DSLRs but the AF photodiodes are MUCH bigger thus not quite as prone to noise.

 

I haven't used the A7S (II) - it's probably much better than on the A7R II - but I still doubt that it goes down to DSLR levels here based on spot AF. Feel free to correct me though.

 

As mentioned we would have to define what "low light" means. I am talking about -say- 10 candles in the living room here and focusing on a person with fine spot AF.

 

Thanks for the reply. I understand your point about the fine point AF and have also found that the AF sensitive reduces with the smaller sampling window on the A7s. However, even with that setting, the light level in which AF can be acquired accurately on the a7s is still substantially lower than in DSLRs I have tried. I am basing my opinion here on a Pentax k-3 (which despite some other AF shortcomings has a class leading low-light AF sensitivity, plausibly rated to -3EV by the manufacturer ) and a Nikon D800. Even in very low light levels where you are not typically able to discern the scene in a DSLR viewfinder, let alone focus at all, the A7S AF works well albeit slowly - the DSLR need substantially more light to get any sort of lock. I have not used the A7rII so can't compare to the A7s directly, however I have used the A7ii and the low light AF sensitivity of the A7s is substantially better. I suspect the A7Rii performance must be more similar to the A7ii and not benefit from full sensor readout and higher S/N ratio as the A7s does. 

 

So in short I would say that in my experience the a7s not only goes down to DSLR levels but complete surpasses them in senstitivity and accuracy in low light - this was my main motivation for buying the camera as I was sick of inaccurate DSLR AF with fast lenses in low light.

[Brightcolours]

Thanks for the reply. I understand your point about the fine point AF and have also found that the AF sensitive reduces with the smaller sampling window on the A7s. However, even with that setting, the light level in which AF can be acquired accurately on the a7s is still substantially lower than in DSLRs I have tried. I am basing my opinion here on a Pentax k-3 (which despite some other AF shortcomings has a class leading low-light AF sensitivity, plausibly rated to -3EV by the manufacturer ) and a Nikon D800. Even in very low light levels where you are not typically able to discern the scene in a DSLR viewfinder, let alone focus at all, the A7S AF works well albeit slowly - the DSLR need substantially more light to get any sort of lock. I have not used the A7rII so can't compare to the A7s directly, however I have used the A7ii and the low light AF sensitivity of the A7s is substantially better. I suspect the A7Rii performance must be more similar to the A7ii and not benefit from full sensor readout and higher S/N ratio as the A7s does. 

 

So in short I would say that in my experience the a7s not only goes down to DSLR levels but complete surpasses them in senstitivity and accuracy in low light - this was my main motivation for buying the camera as I was sick of inaccurate DSLR AF with fast lenses in low light.

Not every manufacturer's spec sheet is created equal then?

I got a 9 stop ND filter for my bday from a friend, and just now tried a little experiment before sleeping. It is night, and I only have a dim light on. EF 35mm f2 (old DC motor version) on my 6D, Hoya HMC ND400 mounted on it. Camera locks focus, at ISO 1600, f2.8 setting, 2.5 second metered exposure time in evaluative metering mode (kinda similar to matrix metering from Nikon).

Or ISO 12800, f2.8, 0.3 sec exposure time.

 

Indeed difficult to see anything through the OVF, only the lightest parts stand out from the dark. The 6D's live view with CD AF at ISO 12800 can't get that old CD motor lens to lock on focus. The 6D's PD AF option with live view clears the focus lock task with relative ease at around a second.

[JJ_SO]

Dave, since you also use a D750: Try to focus with PDAF at night and in some dark corners (but don't get lost there  :blink: ) If you manage to take some shots and see the AF is clearly confused, switch over to LiveView. Stunning. I always thought PDAF on D750 is better than with D810 and that remains true, but D810 with LiveView beats D750 PDAF in those situations. I just don't use it normally because if it's no static object it will be gone before this mirror ballet comes to an end. Can I please get a D750 without mirror?  :ph34r:

[davidmanze]

:wub: Dave, since you also use a D750: Try to focus with PDAF at night and in some dark corners (but don't get lost there  :blink: ) If you manage to take some shots and see the AF is clearly confused, switch over to LiveView. Stunning. I always thought PDAF on D750 is better than with D810 and that remains true, but D810 with LiveView beats D750 PDAF in those situations. I just don't use it normally because if it's no static object it will be gone before this mirror ballet comes to an end. Can I please get a D750 without mirror?  :ph34r:

 I'm off to Montpellier today to take some Christmas lights city shots where I know it will be thronging with people, I will try some low light stuff with the AF85 F1.8D  just for the "crack", but it will mostly be my wide primes.(low profile).

  I have one of those cheap chinese "Hoodman" style viewers for the mirror-less Pentax K01(no OVF) which turns the screen into a huge optical viewfinder which is the only way I can use it, .... I would like to find one for the D750! (cheap of course), otherwise I need too many pairs of reading glasses, I just can't get on with looking at a screen when everything in town will be bustling.

 

  As for the mirror-less D750, I can lend  you a decent pair of dentists extraction pliers!   :wub:  :wacko: 

 

  Go well....

[JJ_SO]

I also have a loupe from Kinotehnik for D810 and with a self-made metalframe it works very well for the little Sigmas which really need it in bright sunlight. But haven't found one of the size of the D750's screen 52×68 mm is hard to get by. I doubt there's one because of the two little humps, but it would be cool to look into it from top.

 

Thanks for the pliers, I also have some but I doubt the camera would work after extraction. I was very close to pull the trigger on a X-E2, but I don't know enough about those types of mirrorless. Also, the duel of Canon G5X and Sony RX100III or IV ended up without any decision. The Canon is better ergonomically, dispalywise and range of zoom - which is also a weak point because the lens is worse than the Sony's and the "speed" when it comes to RAW brings it out of discussion. Pity, otherwise it would have been the right one.

[Brightcolours]

Interesting to read that the D750's -3EV focus ability claim is then apparently a lie, JoJu.

Another spec sheet "lie" apparently: the 100% view finder coverage. 

http://www.imaging-resource.com/PRODS/ni...d750A4.HTM

My 6D is rated as having 97% view finder coverage.

http://www.imaging-resource.com/PRODS/ca...n-6dA4.HTM

[davidmanze]

To be quite honest I'm very happy with 97% especially on FF, it gives that little margin of error. The first camera I had that had 100% coverage was the Nikon F, I wanted that because I was shooting Kodachrome at the time and the general OVF's had a lot less than that, Nikormat 93%, Pentax Sa1 in the eighties, Zenith was in the 70s. 97% is just about right for the AF-D lenses, you can crop out the extreme corners and get to the sharp meat without losing the composition....APSc I think you want 100% there as you don't want the frame going to waste. Again -3/-2 the problem I find is the exposure not the AF! I've just come back from shooting 250 night time shots in Montpellier mostly at 1600 ISO, the AF didn't even think of "not" locking on, I would have given it a slap if it hadn't......I've yet to look at the shots.....beers first photos later.....well it's Christmas!

 

OT.

  What a great city Montpellier is!..... and what a lovely day!.. 18/19° blue sky and sunny, siiting out drinking a couple of sherbets. Christmas decorations, entertainment, everybody was out, families..... not a hint of aggression, loads of buskers, two virtuoso violinists and a great pianist....  the 23rd of December??

 

  In England the councils have abandoned decorations at Christmas if you want them you have to put them up in the garden yourselves.....in the rain!........... I digress, where was I?    

 

                Oh yeah love the D750! 

[Brightcolours]

I too do not mind the 97% view finder of my camera. And my Nikkormat FTn I did like too, nice to know its coverage.

[JJ_SO]

I made that PDAF vs CAF game last night and this night, after seeing a nice movie, again. How can I say? In no case the LiveView AF gave me a less good sharpness and in about 80% it was clearly better than PDAF. I didn't use LiveView often, partly to save battery and the long delay between shutter button down and picture stored. That will change now.

 

'Course this makes me think. "If its' so easy to miss sharpness wide open with normal AF, no matter how much this fast lens costs, no matter how much time I spent with adjusting that thing and if the LiveView of a DSLR always will be a crippled mirrorless, then why bother?". One can say, what do you expect form a third party lens? Answer: Well, LV didn't care about genuine or Sigma lens, it just nailed the focus better. If I get in the mood I'll make a sample gallery. How about those nightshots, Dave - did you compare focus with PDAF and CAF?

[thxbb12]

Hi Joju,

The issue you're describing is exactly the reason why I switched to mirrorless.

I'd be more than happy to meet up with you and let you test my Fuji gear if you happen to be in Geneva.

Shooting at f1.2 with the confidence the AF is gonna be spot on is truly liberating 

Send me a PM if interested.