[ducdao]

Guys,

A friend just gave me his problematic Pentax-M 85/2. Front element and aperture blades are in very good condition. The only problem is the rear element. It has a weird black discoloration all around the edge of the rear element. I've never seed this thing before. I've seen many lenses with fungus but this doesn't look like fungus to me. Anyone know what is this? Is it curable and how?





Thanks !

[Blue]

I think the rear group in this lens is cemented. It looks like a problem with the cement. You could check with Eric Hendrickson and see if he as that element on hand.



Diagram from Dimitrov's Site: Pentax K-Mount Lenses and Lens Accessories

[ducdao]

I think the rear group in this lens is cemented. It looks like a problem with the cement. You could check with Eric Hendrickson and see if he as that element on hand.



Diagram from Dimitrov's Site: Pentax K-Mount Lenses and Lens Accessories

Thanks for the lead, Blue. Sorry for my ignorance but how do I contact Eric Hendrickson?

[Blue]

Thanks for the lead, Blue. Sorry for my ignorance but how do I contact Eric Hendrickson?

You can contact him through his site be clicking on the link below. He worked for Honeywell and then a repair shop for years and has official test sets. He has worked on equipment for dozens and dozens of people here including me.

Eric> Home

[ducdao]

You can contact him through his site be clicking on the link below. He worked for Honeywell and then a repair shop for years and has official test sets. He has worked on equipment for dozens and dozens of people here including me.

Eric> Home

Thanks, Blue! Just e-mailed him.

[ducdao]

One more thing, guys.

After taking several test shots I don't see any problem with image IQ, contrast... so I guess I could just use it as is.

However, would the cement separation gets worse if I don't take care of it?

[Steinback]

Because the problem is at the outer edge of the element it isn't really in the path of the image reaching your sensor, especially on an APS-C camera. I would be tempted to use the lens as is and keep an eye out for a damaged M85 with a good rear element (or ask Eric to let you know if he comes across one). Seeing as the lens is already 25+ years old and still usable it could be years before you notice the problem in the corners of your photos.

In some cases it is possible to separate and then re-cement lens elements but from what I've read the process isn't easy or fun.

[ducdao]

Because the problem is at the outer edge of the element it isn't really in the path of the image reaching your sensor, especially on an APS-C camera. I would be tempted to use the lens as is and keep an eye out for a damaged M85 with a good rear element (or ask Eric to let you know if he comes across one). Seeing as the lens is already 25+ years old and still usable it could be years before you notice the problem in the corners of your photos.

In some cases it is possible to separate and then re-cement lens elements but from what I've read the process isn't easy or fun.

Contacted Eric and he said that if the elements are not damaged then he could un-glue the elements, clean them up then glue them back.

[drougge]

Because the problem is at the outer edge of the element it isn't really in the path of the image reaching your sensor, especially on an APS-C camera.

Is that really how it works? I've always been under the impression that everything passes through everywhere. I'm pretty sure that's how it works with simple lenses at least. So I'd expect that to be the case for at least for some elements in a complex lens as well.

[Blue]

One more thing, guys.

After taking several test shots I don't see any problem with image IQ, contrast... so I guess I could just use it as is.

However, would the cement separation gets worse if I don't take care of it?

If it were my lens, I would certainly use it as much as possible. Using it isn't going to hurt it anymore than letting it sit on a shelf. One factor that may have led to the rear elements condition is that it could have been left in a bag in a very hot car. Regardless, high heat will be its enemy and all of your gear for that matter. Using on the aps-c sensor is probably minimizing the effects of the separation. In the mean time, save up to let Erik have a go at it.

[Blue]

Is that really how it works? I've always been under the impression that everything passes through everywhere. I'm pretty sure that's how it works with simple lenses at least. So I'd expect that to be the case for at least for some elements in a complex lens as well.

They do but due to the sensor size of the APS-c compared to the 135 film, it can minimize the effects. Plus, not all the light from a lens contacts the film in those cameras either. The lens projects a circle and the film is rectangular. That is why rectangular hoods can work so effectively compared to their round siblings on some of those wideangle and normal lenses.

[drougge]

They do but due to the sensor size of the APS-c compared to the 135 film, it can minimize the effects. Plus, not all the light from a lens contacts the film in those cameras either. The lens projects a circle and the film is rectangular. That is why rectangular hoods can work so effectively compared to their round siblings on some of those wideangle and normal lenses.

Of course they project a circle, but that's irrelevant if my understanding is correct, and the sensor size should also be irrelevant. My understanding must be correct at the aperture, or stopping down would remove edges rather than light. I see no reason it would not be correct for at least some lens elements as well.

To clarify my understanding: The light hitting the very center pixel will have passed through all of some (or all?) elements in the lens, not just the middle. From this follows that a problem will affect all parts of the image equally.

Of course I don't know if this particular element has this property.

[Blue]

Of course they project a circle, but that's irrelevant if my understanding is correct, and the sensor size should also be irrelevant. My understanding must be correct at the aperture, or stopping down would remove edges rather than light. I see no reason it would not be correct for at least some lens elements as well.

Actually it is very relevant, especially given the circle of the M 85mm is going to project the same diameter circle regardless of whether it is on a 24X36mm film or 15.7x23.6mm aps-c sensor. However, the rectangle is smaller with the latter. See added Image where red is for the 135 film and the blue rectangle for the aps-c sensor.

To clarify my understanding: The light hitting the very center pixel will have passed through all of some (or all?) elements in the lens, not just the middle. From this follows that a problem will affect all parts of the image equally.

Of course I don't know if this particular element has this property.

Of course the light will pass through every element. However, the elements don't act like a blender and homogenize the light. Otherwise, a slide projector would not be able to work. Take a look at this article regarding optical vignetting and the one on the properties of hoods. Remember optical vignetting is very different than mechanical vignetting.

Vignetting

Lens hoods

Edit: Here is an image

[drougge]

Of course the light will pass through every element. However, the elements don't act like a blender and homogenize the light. Otherwise, a slide projector would not be able to work.

Of course they don't homogenize it to a blur, but that doesn't stop all light from passing through all parts of it. (Except as limited by vignetting.)

You are talking about things outside the lens, and I'm talking about things inside the lens. These are related, but not the same. I'll admit to not reading all through your links, because I think they're mostly unrelated to what I'm talking about.

Here is a central part of my argument, please understand why it's relevant before answering: Stopping down does not reduce the image circle.

The only problem I can see with my way of looking at it is that camera lenses are not simple lenses, so this will not apply to all elements in them. But if you think it applies to no elements, please make an argument for that that addresses why stopping down reduces light everywhere instead of making the image circle smaller.

I've taken a typical lens illustration image from wikipedia and added blue lines to (hopefully) make what I mean clearer.

[Blue]

Of course they don't homogenize it to a blur, but that doesn't stop all light from passing through all parts of it. (Except as limited by vignetting.)

It sure sounds like that's what you are saying in that original post

You are talking about things outside the lens, and I'm talking about things inside the lens. These are related, but not the same. I'll admit to not reading all through your links, because I think they're mostly unrelated to what I'm talking about.

Actually, I don't know why you say I'm talking about is outside the lens when I'm talking about light being transferred through the lens. The light waves pass through the lens. They are from light reflected from the subject and the image reproduced on the film/sensor surface. Light coming through the center of the lens, perpendicular to the film isn't the same as light coming in at an angle, etc.

Here is a central part of my argument, please understand why it's relevant before answering: Stopping down does not reduce the image circle.

No one said stopping down reduces the image circle. See the excerpt from the article below. The image circle isn't completely recorded by the film/sensor because they are rectangular. Actually, the vignetting article discusses optical, natural (aka light falloff) and mechanical vignetting in regards to light transmitted through the lens so it is most certainly related.

The only problem I can see with my way of looking at it is that camera lenses are not simple lenses, so this will not apply to all elements in them. But if you think it applies to no elements, please make an argument for that that addresses why stopping down reduces light everywhere instead of making the image circle smaller.

I've taken a typical lens illustration image from wikipedia and added blue lines to (hopefully) make what I mean clearer.

That image isn't showing anything like what you are describing. However, the green line corresponds to the image circle in the previous linked image circle diagram with the red and blue rectangles inside that circle.

Remedies

Three causes of dark image corners have passed in review. Optical vignetting is due to the dimensions of the lens: off-axis object points are confronted with a smaller aperture than a point on the optical axis. The remedy is to stop down the lens. The darkening noticed at full aperture already improves greatly when the lens opening is decreased by one stop. A complete cure often requires two or three stops—depending on the design. The darkening that remains at small apertures is due to natural vignetting. Natural vignetting is not cured by stopping down the lens. Lenses which strongly suffer from natural vignetting benefit from a gradual gray filter which is dark in the center and brighter towards the corner. Finally, mechanical vignetting is due to extensions attached to a lens. The image corners may become completely black and the photographer can only blame himself as he should use proper accessories.
All types of vignetting are at their worst with the lens focussed at infinity. At close focus the field of view decreases and the size of the image circle increases. The vignetted area is pushed outward with the image circle and when the focus is close enough the optical or mechanical vignetting will be outside the film frame (or digital sensor). As a matter of fact, it is optical vignetting that determines the size of the image circle in the first place. Natural vignetting improves too, since the exit pupil moves away from the film (at least, with most lens designs).

It should be mentioned that vignetting is not always a bad thing. A lens designer can deliberately introduce vignetting in favor of a better control of aberrations, sacrificing field coverage for overall contrast and sharpness. Moreover, the vignetting effect can be used artistically to draw the attention away from the image periphery and to emphasize the subject.

© Paul van Walree 2002–2010