My experience with the D800 and fast primes

Started Dec 26, 2012 | Discussions thread
light_bulb Contributing Member • Posts: 835
Re: My experience with the D800 and fast primes

primeshooter wrote:

As some of you may be aware been having some difficulty with fast primes with regards to focus and the D800. I have been doing some basic tests to establish what I can do to improve accuracy.

1. So far I have concluded that most of Nikon's prime lens range (under 135mm anyway) are poor at obtaining focus using the PDAF system in the camera body in poor light / backlit situations - and actually sometimes there are times when the light is decent and they still have this problem - even if selecting a contrasty subject to place the AF sensor over. The G lenses I have tried are the 50mm f/1.4G and the 24mm f/1.4G. The D lenses I have tested are the 85mm f/1.4G, the 135mm f/2 DC and a few others which are all showing inaccuracy not to do with focus fine tuning or anything like this, as the tend to hit perfect AF in good light with good local contrast (most of the time).

2. Focusing in low / low-ish light is hit or miss literally with the PDAF (lets say anything around or lower than 1600, f/1.4, 1/160), and the system needs really good contrast to pull the lens to focus. This is tricky because if there isn't good focus and you focus on an eye from distance, it's likely the shot will be slightly or very OOF.

3. Focusing is at it's absolute worst when the subject is moderate to distant from the camera - the size of the AF point sensor along with the distance of the subject makes it hard for the D800 to actually know what you want to be in focus, plus the lack of contrast makes this even worse. In low light the D800 is able to pull most lenses to focus if the subject is close or filling the frame, infact even in terrible light it often works well using PDAF. Take a portrait situation for example, the PDAF picks up the eye easily as it nicely fills the sensor in the viewfinder, and the eye / eyelashes usually are good enough contrast to pull the lens to focus.

4. Switching to Liveview and focusing straight off the sensor (autofocus) gives great results - especially when zooming in, nearly every time if your subject is static. It seems to require alot less contrast to pull the lens to focus and amazingly although it's slow (I seem to be able to do the focus manually in LV as well a little quicker) the hit rate is much higher, I'd say over 80% of my shots with LV are in great focus with fast primes from 24mm 1.4-135mm f/2 lenses.

So there you have it, this is no doubt nothing new to many folks on here - but coming from a D700 I honestly didn't ever use LV focusing except for landscapes. I think though what is happening here is that at 100%, if the images on the D800 are even slightly out of focus the image can look terrible and the D700 was not really ever getting it 100%, because the resolution was so low it was never noticed which makes coming the D700 vs D800 focus systems an unfair comparison. In reality print sizes the same, there will be no difference. I would ask if those out there could take their prime lenses out and trial what I say - medium to far distances, wide open so f/1.4 or f/1.8 f/2 lenses and shoot in low to low-ish light ie something from say 1600 @ f/1.4 and 1/160 up to around ISO 6400 with similar parameters. Infact, with some lenses I found even in excellent light, at a distance they where less than satisfactory grabbing focus with PDAF.

Does anyone have any similar experience - or am I completely crazy? Luckily in the real world most of my shots are subjects filling the frame at close distance and wide-moderate apertures so the AF system (PDAF) seems fine for this, as like I said it seems to be medium to far distances that the real problems occur.

Can anybody tell me, would the SB-700 speedlight have a a similar function to my old and sold SB-900 where it shot a beam of red light onto subjects to help AF in low light when the central sensor is chosen to assist the PDAF? Can anyone comment if you can do this with and without actually firing the flash gun as such? I just like to have the option of available light to directional bounce flash snouted.

Am I loving the D800 still? Like you would not believe!

This is pretty much in tune with my experience. Thus this seems to be failure by design and not by individual copy of camera body.

The high resolution requires more accuracy and at the same time pretty well shows when accuracy was not achieved for per pixel sharpness.

I have done a side by side comparison wide open in studio with dim light (D800 85 1.8 G/Olympus OMD 45 1.8) and using the face recognition feature of the OMD I got many keepers while the Nikon combo struggled.

However if you shoot in studio or outdoors from a tripod at f8 you will hardly notice this.

Likewise I had some trouble with a backlit bird where the D800 with Sigma 120-300 OS had substantial problems to acquire precise focusing wide open. I have done similar shots with the Olympus E-5 150 2.0 with EC-20 and had much less problems.

One of the obvious issues is that the AF sensors are simply too big for these kinds of shots. They often cover too much of the subject matter which can be insufficient with subject matter that is not flat. The Olympus E-5 has got a feature that allows to change the size of the AF sensors which works pretty well for precise focusing. Something similar would have been a good thing to have with the D800 and its threefold pixel count.

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