What is causing this purple spot?

Nikon D70, Sigma EX DG 24-70 2.8. I have never had this issue before with

this set up. The only new thing I have changed was that I added a Hoya Super

HMC UV filter to the lens. Is the filter the culprit? Photo was taken around

1pm on a really bright day.<div></div>

Does the 'spot' show up on all your images? Or just one image?

It is a large area for a dead pixel. You may have over-exposed the sensor's ability to capture bright highlights?

Looks like either a reflection from the UV filter, or stray light from your viewfinder..

Yaron, how would stray light from the viewfinder get onto an image shot with a D70? When the image was captured, the mirror should be up, blocking the light path from the viewfinder.

 

If only this one image has this problem, it is unlikely to be a problem on your camera body.

 

It may be hard to pin point the cause. If you see it again, try to reproduce it w/ and w/out the filter, add a lens hood or use your hand to shield the front of your lens from stray light. That may give you the clue to the problem.

Can you show the full frame image? I get that kind of purple spots on the diagonal through the image when a bright light source is in the frame when using zooms. I would guess that this could be your problem also.

 

Purple is a favorite colour of digital cameras. :-)

Try shooting without the filter. Look at the rear element of your lens. You might have a water spot or something similar. You might also have something on your sensor.

I used the lens hood that came with the lens. It is not on any of the other images from the day. I have never had this issue on this camera or ever on my F5. I suspect that it from the new filter. I will attach the fullframe now. The spot is about half way up.<div></div>

Filter flare.

It reminds me of the line from Lady MacBeth where she says "Out damned spot!" I could never understand why she was mad at the dog.

It looks like the sun is just outside your frame. Maybe the filter extends just far enough inside the hood to allow the sun to strike it, but not the lens elements, so you only get one circle as opposed to the cascading repetition flare pattern (one "circle" for each glass/air surface) usually seen caused by the light source shining directly into the actual lens elements... t

<i>"Yaron, how would stray light from the viewfinder get onto an image shot with a D70?"</i> -- Who knows ;) it's not a real prism is it? ;) .. you're obviously right. Still looks like a filter-caused glitch to me.

David,<br>

<br>

The purple spot is ghost, a type of flare. Its caused by

internal reflections in the lens. Sometimes the edges are soft,

sometimes ghost take on the shape of iris or aperture of the lens.

It occurs when a bright light source is in or just outside the

image and strikes the front of the lens. Lenses with large

bulbous front elements can suffer from ghost when the light

source is quite outside the frame. With troublesome lenses you

can have a string of ghost extending from the light source

through the center axis of the lens. For example if the sun is

in or near the upper right corner the ghost can extend all the

way to the lower left corner.<br>

<br>

To control ghost...<br>

<br>

Always use a lens hood to keep strong non-image forming light

from striking the front element or filter.<br>

<br>

Shade the lens with a hand or hat. When this works it is often

more useful than removing a filter. Lens hoods for many wide

angle lenses are useful but limited by the angle of view of the

lens. Take care not to photograph your hand or hat.<br>

<br>

Keep the front element and or filter immaculately clean. Dust and

especially fine water spray can cause significant problems with

ghost.<br>

<br>

Remove the filter if necessary by day (or just dont use one)

and always by night. Sometimes removing the filter subtracts only

one or two ghost patterns from a group of many. A classic example

is a 24/2.8 AI, AIS or AF-D Nikkor. Shading the lens with a hand

or hat is far more useful than not using a filter with this lens.<br>

<br>

Use lenses with multi or super multi-coating for troublesome

situations. If you must use single coated lenses get really good

at using your hand and hat for shading the lens.<br>

<br>

Regards,<br>

<br>

Dave Hartman.

This is caused by reflections between the glass filter on the sensor and the rear element on the lens expecially when the lens is stopped down. My macro and 50mm 1.8 both do this when stopped down past f16 or so. In theory the DG lens shouldn't do this but that is what it looks like.

<em>Still looks like a filter-caused glitch to me.

-- Yaron Kidron<br>

</em><br>

To those who do not use a filter most or all of the time, these

patterns look like a filter-caused glitch. To those who do use a

filter most or all of the time the ghost pattern would be there

anyway...<br>

<br>

The truth is using a hand or hat (or black card) is more

effective either way. For a view camera or maybe even a 6x6

camera a compendium lens shade is the ticket (so why dont I

own one). I guess thats because a black card is cheaper and

not easily damaged in the field.<br>

<br>

<em>This is caused by reflections between the glass filter

on the sensor and the rear element on the lens expecially when

the lens is stopped down. -- Jonathan Ratzlaff<br>

</em><br>

Come on guys, dont be so sure (though a digital sensor can

add more problems). Removing or not using a filter is no

guarantee. One needs to be observant and whip out that hand when

ever its needed. Those who own the 15/5.6 QD or AI Nikkor

learn to be most proficient at this sleight of hand.<br>

<br>

---<br>

<br>

Some lenses come with built in lens hoods. The built in hood may

not be as effective as the older designs accessory lens

hood. Examples are the 105/2.5 and 135/2.8 AIS Nikkors. When

using these lenses with the HS-8 or HS-14 lens hood, flare and

ghost are almost never a problem. Even with a deep lens hood a

lens like the 75~150/3.5 Series-E can have problems with ghost if

the sun is included.<br>

<br>

A good argument for prime lenses is they are often far less

troubled by flare and ghost. Some are particularly free of flare

and ghost. Some examples are the 16/3.5 and 2.8 full frame

fisheyes, 20/3.5 AI & AIS, 28/2.0 AIS, AI and IC.<br>

<br>

Some zooms are much better than others, for example the 25~50/4.0

AI and AIS. Prime or zoom, if you like to point your lens into

the sun it pays to check <a href="http://www.naturfotograf.com"

target="_new"><u>Bjorn Rorsletts reviews</u></a>.<br>

<br>

Best,<br>

<br>

Dave Hartman.

You're right Dave, I don't use filters :)

There appear to be a few other spots in this photo, though I'm not quite sure if the middle one is a jpeg artifact or not. They are on a diagonal as is typical of internal reflections. And the direction is consistent with the sun being in the upper right as appears to be the case in this photo.<div></div>

I am sorry to say this but flare is the least of the worries here.

 

There is a ton of color fringing (blue, etc)in this image.

Yes, I know. This is obviously not one for the portfolio. My question though was about the purple spot. I am glad that some of you found the diagonal in relation to the sun. It is very interesting though that so many of the experts have such different opinions in regards to the question. From front filter to rear element to sensor you all have your own take on what the purple spot could be.

If it is on just one or a few images, it is definitely not the sensor.

 

Tom has good eyes, though, spotting flare on several spots in a small image. When they line up diagonally, it is pretty clear that it is some kind of flare. If you can get into a similar backlit shooting situation, put your camera on a tripod and move your hands around to shield the front of the lens. You should be able to see the flare disappear (and reappear) from your viewfinder.

 

Again, remove any filter and then put it back on so that you can determine whether it plays a role in this problem.

David just from looking at the image one can not tell for certain what the cause is. The answers you got are the most likely explanations (well some are - the contribution to the prism is a bit less likely ^^ sorry Yaron its just one of those days I cant resist:-).

 

To find the weakest part in the chain take similar images and test (exchange) filter, lens and body.