Auto-focus problem in long exposure shots

[batmura]

Hello,

My D3100 camera with a 10-stop ND filter wouldn't autofocus yesterday when I was trying 20-plus-second exposures. Mind you, I had it pointed at the sea. with some buildings and islands in the very distant, but when I tried shooting normally at, say, 1/100 it worked well. The sun was right behind me at the time. Whenever I partially included the sun setting on the right or allow its light to enter the frame, the long exposure (with a remote control) worked. The result wasn't that good, though.

Here's the picture I'm talking about. I find it's blown out on the right side.

Do you think it's a problem with compostion or lighting? And what could I do to get the camera to auto-focus correctly with the sun behind me?

Here's another shot I tried with the camera really lowered to eliminate the direct sunlight, but this time I lost the background with the islands.

Therefore, I finally recomposed, turning the camera away from the sunlight as much as it would let me, and I got a terribly crooked picture still somewhat overexposed.

These are my first-ever long exposure shots using an ND filter, so I've got a lot more to learn. If you have any recommendations, especially with regard to the auto-focus problem and composition, I'd appreciate your assistance.

 

[Buckster]

AF relies on the active focus point(s) to be positioned on an edge where contrast is available to figure it out. If the point is set on sky or sea with no edge to detect, it will fail. If the edge is 'fuzzy' because of fog or mist or something, it's not going to be able to figure it out very well either.

When that doesn't work out for the composition, you need to position to find the focus, then lock it down, recompose, then take the shot. Most of us use the back focus button, rather than the "half-pressed shutter button" to make that easier.

Exposure is the same problem. Depending on the setting on the camera, it may only be looking at one spot or area for setting the exposure. That being the case, you can either point at an area you definitely want to be exposed properly, note the settings and lock them down or dial them in manually when you recompose, or learn to work with +/- EV to compensate, which is usually easier, especially with digital cameras, where chimping and histograms help a lot.

 

[KmH]

Auto focus won't work with a 10-stop ND filter on the lens. The ND blocks to much light for the auto focus module in the bottom of the camera to work properly, even if there is edge contrast at the focus point.

Focus with the filter off, lock focus, put the filter back on, take your shot. If your point of focus is close enough, you might be able to use a strong external light as AF assist light with the filter on the lens

I recommend you re-read and study pages 55-63 in your D3100 User's Manual and in particular note on page 56 those situations Nikon illustrates when AF won't work very well.

 

[batmura]

Auto focus won't work with a 10-stop ND filter on the lens. The ND blocks to much light for the auto focus module in the bottom of the camera to work properly, even if there is edge contrast at the focus point.

Thanks for this piece of information. I thought it had to do with my wanting to do a long exposure, as it seemed to work fine when I tried the same shot at 1/100. Do you think that was purely a coincidence?

 

[TCampbell]

The shutter speed doesn't matter -- it was coincidence that the camera happened to catch a glimpse of something bright and contrasty. The shutter speed is only used to determine how long to leave the shutter open when you take a shot. When you're still focusing the shot, the shutter speed can be anything -- it wont affect auto-focus performance.

When using the 10-stop filter, follow Keith's advice above. Basically you have leave the filter off to focus, then disable auto-focus and put the filter on to shot -- while being careful not to disturb the focus, composition or zoom when you're putting the filter on.

The camera will simply not have enough light to focus if the filter is on. It'd be like trying to focus in the dark -- there's just not enough contrast.

 

[ph0enix]

Auto focus won't work with a 10-stop ND filter on the lens. The ND blocks to much light for the auto focus module in the bottom of the camera to work properly, even if there is edge contrast at the focus point.

Thanks for this piece of information. I thought it had to do with my wanting to do a long exposure, as it seemed to work fine when I tried the same shot at 1/100. Do you think that was purely a coincidence?

Was the ND filter on the lens when yo shot at 1/100?

 

[batmura]

Auto focus won't work with a 10-stop ND filter on the lens. The ND blocks to much light for the auto focus module in the bottom of the camera to work properly, even if there is edge contrast at the focus point.

Thanks for this piece of information. I thought it had to do with my wanting to do a long exposure, as it seemed to work fine when I tried the same shot at 1/100. Do you think that was purely a coincidence?

Was the ND filter on the lens when yo shot at 1/100?

Yes, it was. I guess I should have focused and locked in without the filter, and then go to manual focus before putting the filter back on and taking the shot?

 

[ph0enix]

Thanks for this piece of information. I thought it had to do with my wanting to do a long exposure, as it seemed to work fine when I tried the same shot at 1/100. Do you think that was purely a coincidence?

Was the ND filter on the lens when yo shot at 1/100?

Yes, it was. I guess I should have focused and locked in without the filter, and then go to manual focus before putting the filter back on and taking the shot?

That should work as long as you don't move the camera too much while re-installing the filter.