[RobG]

A dark circle appears in some photos from the Tamron 17-50 f2.8. Viz:

Mount Stuart Panorama by RobGeraghty, on Flickr

This crop is out of a stitch. The full image is linked below, and the dark circle appears in at least two of the images that make up the panorama. It's quite visible on the left side and right side.


Mount Stuart Panorama by RobGeraghty, on Flickr

The dark spot appears very seldom. I have a lot of photos from this lens where there's no sign of it. It doesn't appear in other photos taken with different lenses on the same K3, so I don't think it's the camera, and it doesn't appear in many photos at all. Being a dark spot it's annoying and difficult to remove. Has anyone seen the same thing with other copies of this lens? Does anyone have any idea what it could be? If it was a spot on a lens element, I'd expect to see it in every photo of far more often. I've never heard of lens flare which causes a dark spot.

Here's another image where I'd expect to see the spot and there's no sign.

The light of Fuji by RobGeraghty, on Flickr

Or this one:

A Broad View of Politics by RobGeraghty, on Flickr

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Last edited by RobG; 05-16-2018 at 09:01 PM. Reason: more info

[Digitalis]

It could possibly be an oil spot on the sensor itself. Particulate matter inside lenses has to be either at a critical point in the optical path or of an extraordinary size to be visible in photos depending on focal length and aperture.

Does the spot appear at apertures f/8>= ? If the spot appears at wider apertures, than this it is probably the sensor that is at fault: rather than the lens. The reason for this is the sensor stack is closer to the sensor than the rear element of the lens will ever get. Thus anything on the sensor stack will have a visible effect long before anything in the lens shows up. It is just a simple process of elimination.

I've never heard of lens flare which causes a dark spot.

By definition: Flare is non-image forming light, a dark spot indicates an absence of light. So calling it flare is a bit of a misnomer.

[RobG]

It could possibly be an oil spot on the sensor itself. Particulate matter inside lenses has to be either at a critical point in the optical path or of an extraordinary size to be visible in photos depending on focal length and aperture.

Sure. Logically though, if it was on the sensor it would appear on photos with other lenses, but it never has.

Does the spot appear at apertures f/8>= ? If the spot appears at wider apertures, than this it is probably the sensor that is at fault: rather than the lens. The reason for this is the sensor stack is closer to the sensor than the rear element of the lens will ever get. Thus anything on the sensor stack will have a visible effect long before anything in the lens shows up. It is just a simple process of elimination.

I just took a couple of images of the sky, deliberately overexposed by about 3EV, at maximum and minimum f-stop and minimum focal length with the Tamron lens and my Pentax HD DA 55-300 PLM. No sign of the dark spot. If it was dust on the sensor, I'd expect it to be visible clearly at the highest f-stop. I've seen this when there has been dust on the sensor.

By definition: Flare is non-image forming light, a dark spot indicates an absence of light. So calling it flare is a bit of a misnomer.

I agree to an extent, although logically a lens behaviour could equally cause bright and dark spots relative to the image of the scene - for example if the lens system brightened everything except a certain spot, you could end up with a dark spot. I was simply looking for a word to describe a variation which is apparently cause by the lens rather than the camera.

Thanks for the suggestions. It's odd, because 99% of the time the lens captures great images (hende why I posted a couple of other examples of good images), and is certainly better than the Pentax DA 16-45 which it replaced. The Tamron does have a fault, which is that sometimes it ceases to function when zoomed to 50mm but readjusting the zoom causes this to go away. There must be a faulty contact inside the lens affected by the zoom. However, the electronic misbehaviour doesn't explain an odd optical behaviour.

[Digitalis]

the electronic misbehaviour doesn't explain an odd optical behaviour.

No, it doesn't. However, I would get the lens checked - of only to remedy the issues when it is zoomed to 50mm.

logically a lens behaviour could equally cause bright and dark spots relative to the image of the scene

One could argue this is largely semantics, but If a lens is projecting an image with such inhomogenities in its illumination pattern, there is clearly something rotten in the state of denmark. Retrofocus wide angle lenses at their widest focal length can extend inside the mirror box of an APS-C camera to the point where small spots on the sensor can become visually more apparent than they ordinarily would be. The Sigma 8-16mm f/4.5-5.6 is a lens that Is annoyingly good at highlighting this.

I just took a couple of images of the sky, deliberately overexposed by about 3EV, at maximum and minimum f-stop and minimum focal length with the Tamron lens and my Pentax HD DA 55-300 PLM. No sign of the dark spot.

Is the position of the spot consistent? does is appear under controllable conditions? Have you tried using the Pentax lens in a situation where the spot does appear with the tamron lens?

[Paul the Sunman]

Rob, your Mount Stuart photo doesn't display aperture. If it's something on the sensor, that should be more obvious at high f-numbers. But also, contrast and other adjustments in pp would bring it up. Your other two shots are f/8, where such spots would not normally be visible.

Have you tried shooting a uniformly illuminated subject, eg sky, fully stopped down?

[RobG]

Is the position of the spot consistent? does is appear under controllable conditions? Have you tried using the Pentax lens in a situation where the spot does appear with the tamron lens?

Yes, the blob is always in the exact centre of the image. I've never noticed the blob at the time when I have taken a photo where it has appeared, so it's a bit difficult to try an equivalent Pentax lens in exactly the same circumstances. Of the lenses I have, the only one which is close to equivalent is the old DA 16-45 f4. All the situations where I've noticed the blob have happened when I have been travelling so it's not practical to carry two similar lenses.

Bizarrely, the blob isn't always dark. One of the first times I spotted it, the blob is actually pale green in a dark image. Like this:


Higashi Hongan-ji Gate by RobGeraghty, on Flickr

In this case, it does seem to be lens flare. This photo was taken with my previous camera, the K5iis.

---------- Post added 17-05-18 at 09:11 PM ----------

Rob, your Mount Stuart photo doesn't display aperture. If it's something on the sensor, that should be more obvious at high f-numbers. But also, contrast and other adjustments in pp would bring it up. Your other two shots are f/8, where such spots would not normally be visible.

Have you tried shooting a uniformly illuminated subject, eg sky, fully stopped down?

Yes, I did exactly that this afternoon, deliberately overexposing the sky and trying both the highest and lowest f-stops. No blob. I checked one of the images of the panorama where the blob is most obvious and the photo was f11. The panorama has lost most of the image information because of the stitching software.

[KenBarley]

My Tamron 17-50 is very good at showing dust!

Back story. I normally have my DA 18-135 mounted as a walkabout lens, so this is normally on when I do a dust alert check.
I recently got a K-3ii as the K-3 is getting a bit long in the tooth, and the 18-135 was on that body.
The Tamron was on the K-3 and I did a dust alert check at 50mm. Several bunnies.
Put the DA 18-135 on and repeated at 135. No bunnies.
Back to Tamron. Bunnies still there. Tried with Sigma 17-70 at 70 and OK.
All dust alerts stated f16 as aperture.

No done any scientific tests, but I can presume is that the Tamron actually has a smaller aperture than is recorded, and that is more prone to showing dust.

Ken

[Digitalis]

Does the blob show up the same size no matter what focal length you are using? or does it vary?

the blob isn't always dark. One of the first times I spotted it, the blob is actually pale green in a dark image

In this case, it does seem to be lens flare.

That's because what you have shown us is flare, non-image forming light. The blob you have depicted is smaller than this, and would be hidden in the shadows of this overexposed image.

[Digitalis]

Does the blob show up the same size no matter what focal length you are using? or does it vary?

the blob isn't always dark. One of the first times I spotted it, the blob is actually pale green in a dark image

In this case, it does seem to be lens flare.

That's because what you have shown us is flare, non-image forming light. The blob you have depicted is smaller than this, and would be hidden in the shadows of this overexposed image.

[RobG]

My Tamron 17-50 is very good at showing dust! {snip}
No done any scientific tests, but I can presume is that the Tamron actually has a smaller aperture than is recorded, and that is more prone to showing dust.

Thanks Ken! I assume you mean dust on the sensor. As I mentioned, I have checked this by photographing the sky at the highest f-stop. I agree that it does seem to show up dust on the sensor if any is there.

---------- Post added 17-05-18 at 11:02 PM ----------

Does the blob show up the same size no matter what focal length you are using? or does it vary? {snip} That's because what you have shown us is flare, non-image forming light. The blob you have depicted is smaller than this, and would be hidden in the shadows of this overexposed image.

I've only seen the blob in images at 17mm. It always seems to be the same size and always in the exact centre of the frame. Yes, I can see that the green blob is lens flare from the backlit subject, but it also appears in several frames, and always in the exact centre of the image. I can't help feeling that there is a link between the two. I have never had lens flare with any other lens that always appears at the exact centre of the image as a fuzzy blob. Flare is usually one or more shapes leading from the light source (usually the sun). Flare is most often quite sharp edged, not fuzzy. There's two other lens flare spots in the image of the gate, up and to the left. Neither is nearly as indistinct as the blob in the centre.

I have a thought for a possible cause - the day in Kyoto was very cold and I think quite humid. The day in Townsville was hot and very humid - and I had not long before got out of an air-conditioned car. I'm wondering if it could be a circle of condensation somewhere in the lens. That could explain what is seen in both images. When I first arrived in Townsville, I tried to photograph a bird in the car park and had to wait for a long time for the condensation on the lens to go away (although in that case it was the HD 55-300). It was the front element which had fogged on the weather sealed lens, but the Tamron isn't weather sealed. In the case of the Tamron, the condensation could be on an interior element.

[RobG]

Does anyone have any thoughts on the possibility that condensation is the cause of the blobs?

[stevebrot]

Does anyone have any thoughts on the possibility that condensation is the cause of the blobs?

Out of curiosity, was a filter mounted?


Steve

[RobG]

Out of curiosity, was a filter mounted?

No. The Tamron was the first lens where I went with suggestions of other people now to use a protective filter - mostly because the lens was much cheaper than the Pentax equivalent.

[Digitalis]

Does anyone have any thoughts on the possibility that condensation is the cause of the blobs?

It isn't at all improbable that condensation formed inside the lens somewhere, and managed to pick up some grease/dust and when it dried up, left a deposit upon the surface of a lens element somewhere in the rear cell of the lens.

[RobG]

It isn't at all improbable that condensation formed inside the lens somewhere, and managed to pick up some grease/dust and when it dried up, left a deposit upon the surface of a lens element somewhere in the rear cell of the lens.

I agree - although I was thinking more along the lines of there being a little bit of condensation on those two occasions. However, the light coloured blob appeared in the centre of an image again today, so I'm going to experiment a bit more to see if I can reproduce both the light and dark blobs predictably.