Is my D4 sensor heating up + Long Exposures?

[leon_pugh]

<p>I noticed some purple banding today in some of my images. I shot a series of 20, 30 45 second and 1 minute exposures at ISO 640. On one edge of 2 images purple blotch and a smaller blue blotch are visible.<br>

I was using a ND 10 stop filter to photograph water falls. <br>

I put the lens cap on over the ND and shot some dark frames at 15, 20, 30 and 57 seconds. The Banding became less and less visible, and the last 2 images look ok (JPG out of camera) but the in the raw image it is still seen if I tweak the exposure up 2 stops.</p>

<p>Do I have a problem with my D4 or is this normal?<br>

I posted a dark frame image. <br>

Here is a link...</p>

<p>http://www.flickr.com/photos/82664576@N03/9512495833/</p>

<p> </p>

[MichaelChang]

<p>Leon, I don't own a D4, but was Live View turned on during those exposure? The bright smudges on the top and bottom look like amp glow, and it's curious that you say it progressively goes away with longer exposures - sure it's not the other way around? </p>

[Wouter Willemse]

<p>It is quite normal, though some sensors are more sensitive to these effects than others. Indeed sensors do warm up, and start to generate noise at long exposures. It is not typical to the D4.<br>

One thing that helps is the Long Exposure NR (but this does double the time each exposure takes). But it helps resolve any issues in the output - it does not take away the root cause as it's behaviour by design. Indeed, though, as Michael said, the effect should get worse at longer exposures - unless you do use the Long Exposure NR already - its effectiveness might be better as the results go 'worse' (and note that this type of NR does affect RAW files!).</p>

[kari_oinonen]

<p>Leon, to make it sure:<br>

Let enough time pass after you use 10 stop filter. Repeat test exposures in darkness with the lens cap on.<br>

What Michael and Wouter wrote is true. One explanation might be that the sensor has got too much light within too long exposure resulting to "after image" or sensor blooming. This would fade away gradually by time.<br>

How much IR or UV leaks trough your filter and how does the leaked light effect on the sensor. </p>

[luke_kaven]

<p>Thermal noise affects different sensor designs differently. With the amp/a-d chips placed off the sensor on the D4, the effect is minimized, but still very much a factor. Try this same experiment with the D800 to see just how much thermal noise is produced by placing the amp/a-d circuits on the sensor. </p>

<p>Use long-exposure noise reduction in cases like this, especially at higher gain settings. Note also that the use of live-view will significantly add to the thermal noise. A short cool-down period between the use of live-view for framing and focus, and taking the actual exposure, will help. </p>

<p>It is hard to tell from your sample whether you got a true black frame or not, but either way, the aforementioned measures are prudent. </p>

[michael_bradtke]

<p>Was the viewfinder closed off? Im not sure if it would make a difference but it is something I always try to do on long exposures</p>

[leon_pugh]

<p>Thanks for the responses!<br>

To clarify.... <br>

I was not using Live View.<br>

I was not using Long Exposure Noise Reduction. <br>

I did not have the viewfinder covered (I thought it only makes a difference when metering, not while shooting as the flipped up mirror should prevent light leakage).<br>

I will have to look into the time the images were taken to see if the problem is due to continuous almost uninterrupted use of the sensor regardless of exposure and if a little time between shots improves things.<br>

I could also try covering the viewfinder with the built in blind. I can imagine light getting in through the viewfinder and leaking in around the mirror due to a non-100% light proof sealing of mirror when flipped up.<br>

As for Long Exposure Noise reduction.... I was under the impression this was for "noise" and that what I was experiencing might be quite different from noise which is more like banding. </p>

<p>It seems so far nobody has said they have experienced the same sort of issue (purple banding) as I have although it has been mentioned that this sort of blooming and hot amps thing happens.<br>

Again, thanks for the replies and suggestions.</p>

[leon_pugh]

<p>Thanks to Michael Bradtke for nudging me in the proper direction.<br>

I performed some tests with a lightbulb, the ND filter and lenscap on the lens and the viewfinder blind open and closed. <br>

BIG difference!!!!!<br>

Problem solved.<br>

Thanks to all who replied.</p>

<p> </p>

[kari_oinonen]

<p>Leon, nice to hear - good to know.</p>