I think we've been down this line before, but I can't recall ever getting to the bottom of it!

Redlion has joined the band of Nikon 8400 users and reports an issue with a purple fringe appearing around the edges of the the image:

On longish exposure shots of say 30 secs to a minute, I am getting a purple shading to the top right hand side of each photo, happens in raw & jpeg.

Trying to figure out if this camera is faulty, or if it is some anomoly with these 8400's, although on my existing 5400 the images are fine right up to 10 mins.

The effect seems similar to vignetting, in that it appears in the corners but rather than being black, it's purple.

I've occasionally seen this problem with the 8400, but haven't been able to tie down exactly the cause. Admittedly, the judicious application of Photoshop also solves the problem quite easily :confused:

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I think this will be what is known as "amp glow". The longer the exposure, the warmer various components in the camera get, and the more inteference is generated. Which is picked up on the sensor. This is why it's always in the same place in the frame.

The longer the exposure and the warmer the camera, the worse it will be. If it has a "long exposure noise reduction" option, this can deal with it fairly well. Does double the time of each exposure though.

Have a google on astronomy sites as it's a problem they suffer from as well! One other option apart from software correction is to frame the shot bearing the amp noise in mind. Then crop it out.

On my D70s it starts to become visible at around 2 mins.

One thing I've suspected is that it's affected by the gear used to light the image.

In particular, the Underwater Kinetics Light Cannon 100 seemed to cause more issues than some LED torches - it was noticeable that as the batteries ran down, the Light Cannon started to develop a purple tint.

One of the reasons I particularly like the new Stenlight S7+ is because it uses the Luxeon Rebel 100 emitters - I've had these in Fenix lights in the past and found them to produce a very white light, which helps with u/g photography.

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It seems to be more than just the edges in my case I would say it is across the top right hand side, seen in the enclosed sample.

🔗Personal-Album-347-Image-31043[linkphoto]Personal-Album-347-Image-31043[/linkphoto][/link]

On some images the purple appears almost as diagonal lines across the same area.

[tweak]Image tag fixed. You don't have to put {img} around the link: the {flink} tag will do the job. Vanoord[/tweak]

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Hmmmm... that's interesting!

Where was that taken and what did you use to light the pic?

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Hmmmm... that's interesting!

Where was that taken and what did you use to light the pic?

Lighting was that 6 Q5 led Romisen torch that you saw in Wales a couple of Sundays back.

As you can see from the image the purple is quite bad.

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Indeedy, although the purple seems very concentrated in that area rather than in the edges, which tends to be what you find with the 8400 and other Nikons.

What you could do is to take a few pics to get the sensor warmed up, then take one pic in the dark with a long, long exposure (perhaps even 10 minutes). Wouldn't have to be underground - and on reflection you could just take a pic with the lens cap on?

If that test photo has a purple patch in the same area, then you've probably got a camera fault, otherwise I'd be tempted to look elsewhere, eg the light source.

I'll give my 8400 a long exposure with the lens cap on and see what happens...

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Here's mine:

🔗Personal-Album-2-Image-31256[linkphoto]Personal-Album-2-Image-31256[/linkphoto][/link]

4 minutes 18 seconds

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Yeah Vanoord, we have been here before, i think the colonel came up with the explanation.
My camera does it on most things over 210 seconds, regardless of light source.

Yeah Vanoord, we have been here before, i think the colonel came up with the explanation.
My camera does it on most things over 210 seconds, regardless of light source.

I did search the forum for "purple" but drew a blank other than a reference to the (green tinted) Llangollen slate mines!

I suspect this may have been several discussions we had, so it seemed useful to re-open the debate and see if anyone else could throw any more light (as it were) on the subject.

As far as I recall, purple fringing is a relatively common complaint amongst digital camera users and an internet search reveals quite a bit, particularly about Nikons.

I've got a spare 8400 knocking around somewhere: I'll repeat the test above and see what the result is...

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Wasnt it all down to how the sensors store light, after a certain amount of time they 'flood' But why in the top corners, and in my case, a very straight line accross the top of the picture. Only really apparent in the left corner of this pic though ...

[photo]Minllyn-Slate-Mine-User-Album-Image-020[/photo]

Trial with a different 8400, 4 min 40 sec

🔗Personal-Album-2-Image-31307[linkphoto]Personal-Album-2-Image-31307[/linkphoto][/link]

Basically, it's the same pattern with the same artefacts, including the angled black stripe that runs across the top right corner - so I reckon this indicates the "standard" 8400 pattern (anyone else care to test please?!).

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canon 350d
4 min 25 sec
🔗Personal-Album-78-Image-31313[linkphoto]Personal-Album-78-Image-31313[/linkphoto][/link]
A few random bright pixels in there but no fringing

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Was out playing with the 8400 again last night at "my local" 😉

This time reduced the exposure time to 8 secs, ISO set to 100 instead of 50 that I would normally choose, f stops from 5.1 to 7.1, lighting using a Romisen 6 Q5 led torch, using this and some smaller led torches gave an overall purple hue to all the images with the white balance set at Auto & made the colour of everything look a little odd, I changed the white balance to daylight, this made the image appear more natural.

No sign of the dreaded purple fringing at all, although I feel the images lack some light, especially the large darker passageways, so I may try some 15 exposures next time out.

some sample images.
🔗Personal-Album-347-Image-31341[linkphoto]Personal-Album-347-Image-31341[/linkphoto][/link]

🔗Personal-Album-347-Image-31339[linkphoto]Personal-Album-347-Image-31339[/linkphoto][/link]



🔗Personal-Album-347-Image-31340[linkphoto]Personal-Album-347-Image-31340[/linkphoto][/link]

🔗Personal-Album-347-Image-31342[linkphoto]Personal-Album-347-Image-31342[/linkphoto][/link]

🔗Personal-Album-347-Image-31343[linkphoto]Personal-Album-347-Image-31343[/linkphoto][/link]

I guess you just have to get used to different camera characteristics even of the same manufacturers.

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Interesting - no sign of the purple.

I'm tempted to suggest it's a result of the lighting having purple in it, which has been picked up by the sensor - especially given that your camera doesn't seem to exhibit the purple fringing that mine do!

On a slightly different note, I ran your purple pic through Photoshop and desaturated the purple bits (with permission!)

Original:
🔗Personal-Album-347-Image-31043[linkphoto]Personal-Album-347-Image-31043[/linkphoto][/link]

Revised:
🔗Personal-Album-2-Image-31344[linkphoto]Personal-Album-2-Image-31344[/linkphoto][/link]
(Purple desaturated to grey, turquoise enhanced and a wee bit of shadow/highlight used. This is a pretty quick effort and I'm a bit heavy-handed, but it gives an indication of what can be done.)

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Hmmm, interesting what youv'e been able to do with photoshop, given it's not the original raw file.

Maybe I should look at getting a copy of photoshop & learn how to use it! 😉

I'm away this weekend so no mine exploring for me, but shall try some more settings next week.

At least I can say then that the camera is not faulty as such, it's some rather annoying anomoly with these Nikons, but given that they are supposed to be capable of long exposures a little puzzling.

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Maybe it's the sensor picking up UV somehow?

Chris.

Back-light or light pollution through the view finder?

In my case, no!

Firstly, it's not a DSLR, so no danger of back light.

Secondly, I covered the lens as well as relying on the lens cap - there's an earlier pic which has some yellowish areas where light got past the lens cap.

As rjw says above, this appears to be "amp glow":

The longer the exposure, the warmer various components in the camera get, and the more inteference is generated. Which is picked up on the sensor. This is why it's always in the same place in the frame.

The longer the exposure and the warmer the camera, the worse it will be.

What's interesting is that I've tried it with two 8400s and got broadly the same pattern: it would be interesting to see what the output from any other 8400s is (hint hint! 😉 )

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Yes, as you surmise there Vanoord, the purple will be in the same position for each make/model of camera. Linky for Nikon D80;

http://www.fotocanada.ca/Articles/D50%20vs%20D80%20amp%20glow%20images/D80%20vs%20D50%20amp%20glow.html 

One thing I've only just picked up on is that it can be reduced by leaving a longer time between shots to allow the sensor to cool down a bit.

I guess that Mr Nikon et al never really envisaged that the cameras would be used in this way. Not sure that excuse holds good for cameras like the D80 though. They must have been aware that they would be used by hobby astronomers etc. Maybe factory testing long exposures is a bit too time consuming 😉