Dark spot from some lenses

[Walker]

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Did a sensor cleaning yesterday, and noticed some faint but dark spot in pictures from some of the lenses. Testing for dust, I took some pictures of a (relatively) clear sky (this is Bergen, Norway) at smallest apterture opening. Later, at the screen, I noticed a faint, hard to see, dark spot in in the right hand of the pictures taken. I thought it was some smudge on the sensor, but tried another lens for testing this hypothesis. It turned out that my 28 (2.0) and 35 (1.4) create this effect, but not the 75 (2.0), 35 (2.0) or the Noctilux. The spot turns up, by the way, in different places for each of the two lenses affected. I also tried this on two different cameras - same effect. Also with/without filters and so on. The spot dissapears when opening up the aperture - from about 11 downwards. When sharpening contrast, they become more visible.

 

Is this just an imperfection in some lenses that one has to accept, or may there be an issue to be concerned about? Have anybody got an idea what may be causing this at all?

 

Willly

[carstenw]

Stopping down means that the light rays used for a given area of the picture are taken from a smaller area of the lens. I would look for spots of dust inside the lens. If you hold the lenses wide open in front of a bright lamp, can you see anything? Since it is only visible below f/11, the spots must be very small, so look carefully.

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[stunsworth]

Hi Willy, I can't see it being dust in the lens that's causing the problem, there just isn't the depth of focus to bring the point of focus someehere inside the lens. It's more likely to be dust on the sensor. Perhaps it's moving as you mount a new lens?

 

Besides, all lenses have dust inside them - if you ever want to frighten yourself shine a torch through a lens and look the dust revealed inside.

[rwfreund]

Check for dust specs on the surface of rear elements, especially the concave rear elements of lenses such as the 35's.

-bob

[Bob Ross]

Check for dust specs on the surface of rear elements, especially the concave rear elements of lenses such as the 35's.

-bob

I'll second this advice and add that rear lens caps also be cleaned. Jono told me about this and while I was skeptical, there was an improvement, even with a brand new 28 Elmarit.

Bob

[rpierce]

I see what may be the same thing on my 28mm f2.8; I need to check other lenses. The spot is strongest at f22 and is hard to see at f11 and below. At f22 the outline is larger than any dust I have seen and the edges are indistinct. In contrast, the dust particles are sharp. I think the spot might be internal reflections; it reminds me of the reflections I have seen at small apertures when I used IR-pass filters with the 28 f2.8 lens.

 

Bob Pierce

[adan]

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I see what may be the same thing on my 28mm f2.8; I need to check other lenses. The spot is strongest at f22 and is hard to see at f11 and below. At f22 the outline is larger than any dust I have seen and the edges are indistinct. In contrast, the dust particles are sharp. I think the spot might be internal reflections; it reminds me of the reflections I have seen at small apertures when I used IR-pass filters with the 28 f2.8 lens.

 

Bob Pierce

 

Dust spots are always more visible the smaller the aperture used. the smaller the aperture, the more the lens becomes a point light source, casting very sharp shadows like a spotlight. The more open and larger the aperture, the more diffuse the light it casts is.

 

What we see as "dust" is actually just the shadow of the dust speck on the silicon (separated by a thickness of cover glass) - the wider in area the source of light, the less distinct the dust speck's shadow will be.

 

In general, wider lenses will reveal dust more than longer ones, because, again - f/8 on a 21 is a much more pinpoint light source than f/8 on a 90.

[Walker]

Thanks everyone for many helpful thoughts and good advice. Since the problem had to be with the lenses, not the camera/sensor, I gave the back of the two lenses afflicted a good cleaning, as adviced here. Now the spot has gone for the 28, and hopefully on the 35 (which I have not tested yet). Again, many thanks!

 

Willy