Colour problem M8 with 28mm Cron ASPH lens?

[ptruman]

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I'm wondering if the problem I'm seeing is a result of the well known M8 IR issue? I have not yet received the IR filters so am using without. Two pictures attached, not fantastic for artistic content but both show an area in the centre of the image which has a cast. The bluebells image was shot on an overcast (but bright) day whereas the other just after sunset (it had now dipped below horizon). On the bluebells image I have increased saturation much more than I should to emphasise the problem.

 

Anyone else experienced this with the 28mm Summicron ASPH on the M8, repeat no IR filter? Both images were stopped down to f11, 160ASA, etc. My concern is will the IR filter prevent this or is there something else wrong? Views much appreciated.

[carstenw]

Have you checked the lens, front and back? That looks awful, and not at all normal.

[B&W]

Did you try with a different lens?

 

It doesn't look like IR problems to me, more like a lens or a CCD problem

[ptruman]

I've checked the lens front and back for dust, etc and my 50mm lens does not suffer as far as I can tell. The CCD appears OK (and tested by shooting clean piece of paper) Its only a very few images (6 in the last 500) that show this problem and difficult to reproduce. The vast majority of my others are perfectly OK apart from my goofs.

 

Agree though its awful and not what I would expect. Real difficult to eliminate with PS!

[gravastar]

I believe some lenses are known to be unsuitable for infra red photography because they produce an IR hot spot in the center of the image. I don't know if the 28 'Cron is amongst them.

 

I'd be interested to know which lenses have a hot spot since I'd like to try out the M8 for IR.

 

It's unfortunate you didn't have an IR filter to try which would confirm the problem.

 

Bob.

[tashley]

I believe some lenses are known to be unsuitable for infra red photography because they produce an IR hot spot in the center of the image. I don't know if the 28 'Cron is amongst them.

 

I'd be interested to know which lenses have a hot spot since I'd like to try out the M8 for IR.

 

It's unfortunate you didn't have an IR filter to try which would confirm the problem.

 

Bob.

 

As I understand it (and this might well be wrong) the IR hotspot is made worse by small apertures due to diffraction issues.

 

In any event, especially in shot 2, it's pretty nasty. I've shot a lot of frames with everything from CV 15 and WATE thru 24 f 2.8, 28 cron, 35 lux and cron and CV 35 f2.5 and not seen anything like this, but I do tend to avoid anything over F8 where possible.

 

Tim

[ptruman]

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Many thanks for your contributions - I will try again once my IR filters arrive (sometime...) My concern was exacerbated given the area is smack in the centre of the frame and consistent in size, also with colours that are prone to problems - bluebells often show up purple if in direct light (but they were not) and just after sunset transmits a wonderfully warm spectrum. Just the conditions that an IR hotspot, assuming such a thing exists with the M8, would pick up.

 

I must emphasize that the vast majority of my images show nothing in this zone and I am generally delighted (even amazed!) with the performance of the lens and camera.

[woodyspedden]

I believe some lenses are known to be unsuitable for infra red photography because they produce an IR hot spot in the center of the image. I don't know if the 28 'Cron is amongst them.

 

I'd be interested to know which lenses have a hot spot since I'd like to try out the M8 for IR.

 

It's unfortunate you didn't have an IR filter to try which would confirm the problem.

 

Bob.

 

The 28 cron is my "standard" lens for the M8 and I probably shoot 75% of my images with it. If this color was "normal" for the M8 28 cron I would have already sold both and gone with a Helga!

 

Woody

[jonoslack]

Hi Peter

It's the derbyshire lurgy. You should count yourself lucky that your bluebells came out blue!

 

More seriously, it couldn't be flare could it? Both shots seem to be taken pretty much into the light, I'm probably wrong, and you would normally expect a more defined 'edge', but it is a thought

[AlanG]

My guess (only a guess) is that it is flair from diffuse light hitting the sensor reflecting back to the lens and then back to the sensor. You may only notice it when the center area is dark and another area is substantially brighter.

 

This was a problem with the Kodak full frame digital cameras, some reported it with the D1X and perhaps it occurs with other cameras too. See this link:

 

14n - Dreaded Red Blob - Pro Photo HOME

[adan]

I did not see this particular central red spot when I had a 28 f/2 ASPH on my M8 - but I did occasionally get several other kinds of sensor reflections with that lens, and also with the 75 APO. Both of which have concave rear elements that can act as "mirror lenses" and reflect and focus light from other parts of the also-mirrorlike sensor surface back to the sensor.

 

[Edit: Do'h! Of course the 28 'cron does NOT have a concave rear element - I was thinking of the 35 ASPHs. The next-to-last element is concave, though.]

 

Personally, I DID get rid of the 28 'cron in part because of this, despite its overall excellence otherwise, and went back to earlier 28s that have convex rear elements. And I checked the 28 f/2.8 ASPH optical design (convex rear surface) before ordering one as my 30%-discount lens.

 

Technically, since it is due to reflections within the optical light path, I guess it counts as a kind of "flare" - although it is digital-specific.

 

The red coloring of the spot may well be an IR side issue.

[ptruman]

I thought I would report back having been out a couple more times to both locations to see if I could isolate the problem. I'm pleased to report I can reproduce the problem, but better still now know how to avoid it. In my original post the lens was stopped down to f16. I tried again with the 28mm cron asph lens, f16. Bingo - once again a reddish cast in the exact centre of the frame, maybe covering 5% of the image. At f11 this cast was not as bad and at f8 it was undetectable.

 

I tried the same with a 50mm f2 cron lens, but could not reproduce the problem.

 

My IR filters just arrived so tried the above with filter attached. Again, I couldn't reproduce the problem with the 50mm lens, and on the 28mm it significantly reduces the effect at f16, but it's still there and therefore an annoyance. At f11 I could not detect it.

 

Tim's advice not to go beyond f8 therefore seems quite reasonable and given the DOF available it is rare that anything smaller should be needed.

 

I now have lots of shots of bluebells, which are blue! Once again thanks for the suggestions.

 

Pete