Problems with enlarger-lens

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lolhonk
Posts: 39
Joined: Sun Jan 11, 2015 4:58 am

Problems with enlarger-lens

Post by lolhonk »

Hi,

i use some enlarger lens like the el-nikkor 50mm or componons 50mm with some extension tubes. Recently i regognized some bright spots in my imagesImage

What could that be? Is this a reflection or dust/ fungus in my lens?

enricosavazzi
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Post by enricosavazzi »

Looks like a typical example of central flare. It is caused by stray light inside the lens (or in the space between lens and sensor), and has nothing to do with dust or fungus (although extremely large amounts of dust and fungus in a lens can make central flare worse).

The most effective way to eliminate center flare is to use a lens shade just narrow enough to prevent peripheral light from entering the lens front element without vignetting or darkening the image corners. The lens shade should also be as long as practical in front of the lens. These requirements are of course difficult to achieve in photomacrography, since they reduce the already small working distance.

An additional thing to do is eliminate all internal reflections between the rear of the lens and the camera sensor. This usually means covering the interior of extension tubes (and sometimes the rear metal rings of the lens if shiny) with black flocking. This is usually easier to do in photomacrography than the first suggestion.

Ideally, you should do both things. In practice, the second thing (flocking) may be enough to reduce central flare to the point where it is no longer visible.

See the following link for a discussion specific to central flare with the Coastalopt Apo 60 mm, and its elimination with lens shades. The principles are applicable to most lenses.
http://savazzi.net/photography/coastalo ... tspot.html

Adding flocking to the interior of the equipment is more common in photomacrography and astrophotography than in general photography. There are several excellent discussions in this bulletin board, which no doubt other users can point to.
--ES

ray_parkhurst
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Post by ray_parkhurst »

You can quickly determine if reflections off inner surfaces of extensions are a problem by removing the camera and looking down toward the lens. If you see light reflecting off the inner surfaces, your sensor will also "see" them and it will cause reduced contrast or hot-spotting like you're seeing.

lolhonk
Posts: 39
Joined: Sun Jan 11, 2015 4:58 am

Post by lolhonk »

Thank you very much enricosavazzi and ray-parkhurst for the great answers!

I will test this out and will write my results here.

jjphoto
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Post by jjphoto »

It's historically called a flare spot, these days more commonly called a hotspot. I suspect it's exacerbated by digital sensors where the flat glass on the sensor reflects light back into the lens. In any case. it's very common and TBH I see this quite a lot with various enlarging lenses and some taking lenses, and not just old lenses from the film era. Even my Sony RX100M2 compact camera, a modern lens, has a serious flare spot problem and is my biggest complaint with this camera. I didn't even have to look far to find an example, this is an example from just something I was processing just last night.

Image

As stated by others, reducing stray light entering the lens and reducing internal reflections (including your choice of adapters, bellows etc) is all you can do aside from using a different lens.

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