[mee]

Today I went out to a lake and played around with two 10-stop NDFs stacked one on top of the other in the middle of the day.

This is 20-stops of filter in front of my camera lens. That's HUGE.

I was ending up with 7 minutes or so long exposures. At that length, when I got home and looked at the images 1:1 on the PC monitor, I noticed the images were horribly ruined by hot pixels. And not just one or two.. but gazillions of them peppered throughout the images.

Subsequent exposures (30 seconds or less) show NO hot pixels. Just the ultra long ones.

So my question is how long is max for this sensor in broad daylight?

[Adam]

The camera should be able to handle more than 7 minutes, but I think DFS will be almost essential at that point.

[interested_observer]

Were they actually hot pixels or white dots?

• https://www.pentaxforums.com/forums/190-pentax-k-1/327326-k-1-white-dot-issue.html

In the discussion in the above thread, they have found that different RAW processors handle them differently....

[mee]

Yes, I'm sure.

I will say I was shooting at ISO 400 in the long shots. But you can still obviously see the hot pixels (red, blue, and green).

Here is a crop of a 3 minute and 45 second exposure followed right behind it with a 10 second exposure. The 7+ minute ones are hugely worse than the 'bad' case shown here though. But I wanted to show a back to back since it obviously isn't the sensor being faulty (I don't see a single hot pixel until I extend the exposure over a minute or so, then they slowly grow in number the longer the exposure).

It was hot out though.. and I was shooting in broad daylight. So maybe the camera was just toasty..

Attached Images

   

[mee]

Oh, and here is a 1:1 crop of 447 seconds at ISO 100:

I should point out I was 3 stops under exposed still.. so I had to bring it up that amount in LR. That probably has something to do with it.. either way it looks rather gnarly (in a bad way).

[UncleVanya]

This is what Dark Frame Subtraction is made for.

[SKYGZR]

Yup, DFS would help in this situation...that would mean twice your exposure time to allow for DFS to function,,,so ..it would mean about a 15 min exp ...

[mee]

Thanks, guys. I didn't realize extra techniques were used in long exposure shots like this. I'll take a look into it.

Extra cookie to UncleVanya for typing out the whole term too. hehe

[Tan68]

I would add a recommendation to use either ISO 100 or 80

You want a long exposure so little reason to brighten things digitally with ISO adjustment. Extend shutter to brighten things. I admit that your exposure time would be about 30 minutes and maybe you were trying to avoid that :^)

You mention you took a picture at ISO 100 and later lifted it 3 stops. I would try to have exposure set more correctly in camera. Lifting the images a couple stops shouldn't be a trouble but it couldn't hurt to let more light in by extending the shutter.

I believe there is no longer any way for a thing to be gnarly in a good way. :^|

---------- Post added 10-02-16 at 07:24 AM ----------

I took a look and the longest exposures I have made were ISO 80 for 3-5 minutes, the air was cool, and exposure was reasonably good in-camera. I really don't remember if I used the DFS. I suspect so. As far as longest exposure time suitable for the camera, I might hunt threads related to star pictures / star trails.. there will be some long exposures there.

I have used long exposures during the day to blur people movement to the point they are invisible in the picture. This has worked better at some times than others. The best result had one man sitting on steps and one bird on the roof remaining while all other people in the courtyard were gone. The bird and the man didn't move.. but I like them in the picture.

[mee]

That's exactly why I wasn't at 100 or lower. Exposure time was too long with 20 stops.

I ordered a 3 stop filter though when I got back home.. 13 is probably way more useful than 20. And Adorama has them for super cheap (36 dollars vs 97 at BH)

I had a similar shot at the lake.. older couple were sitting on the dock fishing and were fairly motionless. At the time I was hoping they'd move so they'd disappear from the scene. But when I finally saw the final exposure I was happy they hadn't -- Water looked soft, clouds stretched, but there were these statue like people in the shot. Its nice. Helps when they're old and move slower :cP (that's a joke people)

[UncleVanya]

Thanks, guys. I didn't realize extra techniques were used in long exposure shots like this. I'll take a look into it.

Extra cookie to UncleVanya for typing out the whole term too. hehe

You are welcome. By the way as I understand it you can do this in camera (which is supported but means each shot will be twice as long when they exceed the threshold time (I can't recall 30 seconds?) or you can take a single shot of your own with the lenscap on and the finder blocked in the same conditions for the same basic length of time and then use lightroom or other tools to subtract that frame from the one's you captured. The DFS in camera is probably more accurate since with repeated shots you may have changes in the sensor output the longer it goes.

But I am no expert. I am so happy this can be done in camera so I don't have to mess with it. But if I were doing 30 mins exposures (multiples) I would consider the options and I might be shooting with two bodies to reduce the lag time between shots!