AF accuracy with 85/1.8 in low light?

[jonass]

I'm currently using a D750 with a 85/1.8G. The lens has recently been serviced.

What do you think of the accuracy in low light with the 85/1.8G? Im struggling quite a bit to get photos in focus

- Im using single shot with the centre AF-point selected. Light levels is around ISO3200, 1/125, f/1.8. I've

tried it on a D610 aswell - same results.

 

Objects with lots of contrast is not a problem but focusing on my daughters / wifes eye pose a real problem. And

no, they are not moving and I've tried different focus modes. I'm just curious if the lens is not better than

this or if I have a bad copy.

[joseph_smith3]

It may not be your lens. Cameras with larger FX sensors require almost perfect shooting techniques.

 

It could be due to back focusing or front focusing because you have not done AF Fine Tuning for that lens and your D 750. It could be

caused by camera shake too or internal vibration.

 

If you are not using a tripod do some test shots with a tripod and trigger the shutter with a release or the self timer. Focus normally and

see what happens. Then use Live View and look at the pictures.

 

If the lens has VR, take some images with VR off and then with VR on.

 

Joe

[jonass]

Thanks, but AF-tuning has been done to death :) It works just fine in daylight.

[robert_bouknight1]

<p>My 85/1.8G seems to do pretty well with my D810, as it did with the D600 and and D800 bodies previously. However, The DOF at close focus and f/1.8 is razor thin, its hard to achieve "<em>perfect"</em> focus every time with any system. Even small movements by the subject or photog can result in a slightly out of focus image.</p>

[DWScott]

Does the Nikon D750 compensate for different autofocus results under different wavelengths of light? I know with early Pentaxes I would

often get perfect focus in daylight, but in low light tungsten, the results would be off. Pentax actually changed the cameras to adapt under

different wavelengths.

[Gary Naka]

<p>JS<br>

You hit the problem on the head.<br>

"Objects with lots of contrast is not a problem but focusing on my daughters / wifes eye pose a real problem."<br>

In fact if you tried to focus on a solid color shirt, even with GOOD light, the camera won't focus.<br /> I ran into this exact problem, which is why I know about it. <br>

So the subject or a substitute subject at the same distance needs to have enough contrast for the AF to work. That is just a limitation of the AF design.<br>

I would just manually focus the lens.</p>

 

[ilkka_nissila]

<p>I would use AF-C (continuous AF) and select the AF point closest to the eye that you want in focus as the single active point. Then fire a few frames (3-4) and select the best one. If your equipment is working correctly, this should yield some keepers. If I can use flash to augment the existing light, I will typically stop down the lens to f/3.5 or so for face + shoulders shots. In the studio I do this kind of shots at f/11, so there is enough depth of field to cover most of the head. If I cannot use flash (for the reason that it may be distracting or if there is no good clean bounce surface), I may shoot wide open but then fire a few frames to be sure to get one that has a good expression and is in focus. And remember the shot may still be ok at the final presentation size even if the right eye isn't quite the one in best focus. Usually I don't print event or portrait images so large that the difference would be perceptible in the print. Occasionally it may be, and for that reason I get a sequence of shots.</p>

<p>David, I don't know about the D750 but in the D810, Nikon corrected the wavelength dependent autofocus error in hardware (or so they said in an interview at the time the camera was introduced). I had this problem of color dependent focus error with the D800 (blue spot light on stage => always out of focus, daylight colored spot => correct focus) but the D810 works better in this respect. The D750 is a newer camera so perhaps this correction was made also for it. In my use of the D750, I haven't noticed this problem. </p>

[elliot1]

<p>Not all AF points are created equal. Only the 15 in the center 3 columns are cross-type points and only these will give you good results (the best you can expect) in less than ideal shooting conditions.</p>

[ronald_duren]

<p>The depth of field for an 85mm, f1.8 at 5 feet is only 1 inch. Your ISO of 3200 and shutter speed indicates a low EV which would challenge the auto focus of most cameras. If you must use those exposure values i would 2nd Ilkka's suggestion of shooting a burst and picking the best photo.</p>

[keith_burdett]

Focus and recompose using centre focus point at such shallow dof will alter the subject distance on recomposition. If this is your problem, your subject's ear or hair may be more in focus than the eye...

I agree- afc, burst, no recomposition.

[photo_galleries]

<p>It's not your copy of the lens. Others (online) have reported the same problem. I experienced it as well with my copy until I changed settings -- see Ilkka's recommendation.</p>

<p> </p>

[jonass]

I had high expectations with the D750 with its AF specified to handle -3EV. I tried my friend 70-200/2.8VR (@ 200mm f/2.8) - it performed way better. I guess not all lenses are created equal.

 

I will give AF-C a go.

[ilkka_nissila]

I use AF-ON to autofocus (or AE-L/AF-L reprogrammed to AF-ON); this allows me to access manual focus without

interference from AF when pressing the shutter button - or continuous AF by pressing and holding AF-ON, and finally AF

+ recompose is also possible, all without having to make changes to the mode or settings in use.

 

The 70-200/2.8II autofocuses exceptionally well in my experience.

[ian_white2]

Check out the very interesting piece on Bob Atkins’ site about focus errors due to varying amounts of IR in the ambient light.

 

I was having problems with the 50 1.8G and the 85 1.8G on my D800. AF was very inconsistent wide open until I started using a B+W 486 filter which cuts off all IR (and UV). The thing which connects these two lenses is that they both have significant chromatic aberration wide open.

 

As I understand it, the AF module in the bottom of the mirror box does not cut off IR because Nikon speedlights use IR in the AF assist light. The old F4 gave you the option of switching on/off an IR filter to avoid this problem.

[Brian1664876441]

Ian- very interesting, I need to try this. I have several 62mm and 52mm Tiffen Hot Mirror filters, left over from Kodak

DCS200 days..

 

Older "Hot Mirror" filters are often available inexpensively on the used market- not quite as strong as the B&W 486, but

should be good enough.

 

If the subject is stationary, you might try focusing using liveview, zoom in for critical focus, then recompose. Posting some

sample shots might also allow more insight into the problem.