Flocking in extension tubes

[canonian]

Rik's post made me think and investigate. I use one extention tubes on top of the bellows to get the Canon body fitted on an Exacta bellows and one at the bottom of the bellows to get a match on the thread of the lens.
I have the tubes flocked. When I checked I see a lot of light reflection from the flocking and the lens ring at the bottom of the bellows.
Does it influence the shot when only a small FOV is seen by the sensor?
I try to illustrate this by taking two shots, a zoomed in and zoomed out photo taken with a compact, resting on the EOS-bajonet of the bellows.
Allthough I see no fogging on the results, it still got me a bit worried, to say the least.


Zoomed out


Zoomed in, I think a lot has to be improved here.

[rjlittlefield]

Does it influence the shot when only a small FOV is seen by the sensor?

Yes. What happens is that light from the FOV is sent straight to the sensor as a focused image, while light reflected from the tube walls gets to the sensor as unfocused glare that adds more or less uniformly to the focused image. The effect of adding unfocused light is mainly to brighten the dark areas. This can be substantially corrected by curves adjustment in post-processing, although as DQE points out, the glare will inevitably degrade signal-to-noise ratio.

To check how bad the problem is, one good method is to photograph a subject that should have some pure black, and see where that black ends up on the histogram. A high quality black spot for testing can be constructed by making a "light trap" that consists of a black-lined box with a small hole facing the camera. The trap is even better if the back of the box is a black cone with its wide end toward the camera, and perhaps counter to intuition, in that case the black should be shiny so that any light inside the box keeps getting reflected away from the camera instead of bounced back toward it.

--Rik

[ChrisR]

Fred - ring(s) of black paper woud stop much of the greenish reflection. Like the one below arrowed blue.

Your second picture I'm not sure about. The sensor doesn't see what a lens/your eye would, I think.
You couldn't eg make a rectangular mask inside the lens that's subject fov size. You CAN though make a conical hood which goes to a rectangle, and slide it on the lens so it's just out of shot. I use silver tape to hold the black paper together, so the outside reflects light onto the subject. More useful with enlarger lenses.

[elf]

That's baffling

[Charles Krebs]

Fred,
As Chris mentioned, a good lens shade, or blocking direct light from hitting the front of the lens will also help a great deal. But when you are using something with a very short working distance that is not always easy to do, so a careful "treatment" of the tube/bellows internals is key.

I also strongly recommend an internal "flare-cut" aperture like Chris has shown. I always use one with enlarging lenses and the aperture opening is even smaller than Chris shows. Just large enough to let the lens illuminate to the corners of the frame.

[canonian]

Thanks you Rik, Chris and Charles.
I will definitely try Chris' internal aperture. Hope to post some results soon.

That's baffling

?? sorry elf, I missed the pun. My English is not that good.

Your second picture I'm not sure about. The sensor doesn't see what a lens/your eye would, I think.

I kind of hoped the 2nd picture is what lands on the sensor, the first is what I see when I remove the camera body and look into the bellows.
The greenish light is reflected from the background I used, it is its color cast.

[ChrisR]

It's only lightly baffling ?

Charles is right of course - the picture was from an old post. That baffle was to cover the shiny rear end of the adaptor shown on the right.
The hole in the middle should only need to be a little bigger than the red arrow lens diameter.
That baffle lives in the bellows pernanently - some "bellows lenses" actually protrude back, through it.
I have a smaller ones glued permanently to the step up rings I use with enlarger lenses, but the camera I use to snap pictures of duch things just died .
This is at the front of the bellows, ie lens end.


At the rear you would need something more like you find in some extension tubes - a rectangle near the size of the sensor - think about the angles..
I reckon it's a good idea to have both baffles though - kill it wherever you can!
Ah - I found one for the back ! :

[canonian]

I'm baffled how easy it is to modify...
I will try one later on the lens side as well (ran out of black paper)


Focussed on both ant and on EOS bajonet, 2 images combined. cut-out is as big as the size of the sensor.


Formica rufa, quick stack from 20 frames, cropped

[elf]

I'm baffled how easy it is to modify...
I will try one later on the lens side as well (ran out of black paper)

English can be quite baffling when discussing baffles

I've added them to most of my lens. The cone shaped hoods work well for the front of the lens if the working space is large enough.

[ichbtm]

Hi! first post here
Really great forum and community. I've learned so much by travelling here

I was wondering if the use of an iris would do the same result.
The advantage would be that the opening could be adapted.
The disaventage is that it cost a million more that a paper sheet
(but i have a jinfinance 58mm iris in a box)

thank you to all

[rjlittlefield]

ichbtm, welcome aboard!

I was wondering if the use of an iris would do the same result.
The advantage would be that the opening could be adapted.

Best way to tell is try it and see.

Perhaps it will help to quickly review how baffles provide benefit. In general, the goal of baffling is to block all light paths except straight lines from the lens pupil to the sensor. Those straight line paths occupy a sort of truncated cone whose cross section matches the lens pupil at one end and the sensor rectangle at the other end. In between, the cross section is a hybrid between the shapes at each end. Baffles must stay outside the cone or they will cause vignetting. But the tighter they fit to the cone, the more likely they are to actually do some good.

So the iris has both pluses and minuses. On the plus side, it's adjustable so it can be sized easily to match the light cone. On the minus side, the shape is wrong to fit the sensor end. Best results might come from placing it near the middle of the extensions.

--Rik

[ichbtm]

Thank you rik,
I understand the goal to block light paths for deviant rays but i'll have to draw some line in autocad to understand why you tell me that the iris should be better positionned in the middle of the extension...

In fact my reasonning was the following :
With an ideally adapted rectangular baffle, the corners of the baffle rectangle should be in line with corners of the field of view rectangle and the corners of the sensor rectangle.
So, if i look at the first picture of this post the rays coming from the FOV rectangle drawed should be the only ones which arrives on the sensor. The adaptation is perfect.

Now if I insert an iris instead of a rectangle, the optimal opening for the iris would be the one which fits the FOV rectangle (not sure in english but google tells me circumscribing circle).
Look at the red circle in the following picture :


With a iris opened like that, i believe that only straights rays arrive to the sensor. And whatever the position of the iris.

Do I miss something?
(sorry for my english )

[rjlittlefield]

Do I miss something?

Yes. You are not thinking correctly about where the light goes, and the pictures you're looking at are not helping you to understand what's going on.

Let's look at a couple of ray diagrams instead. Here is a 2X setup with two different apertures: f/4 and f/20, object on the left, image on the right.



The black lines represent light rays from one point in the center of the field. The pink lines represent light rays from another point on the edge of field.

Please notice:
1. Light from every single point in the field fills the aperture of the lens.
2. There is no relationship between the size or shape of the aperture and the size or shape of the field of view. It makes no sense to draw the field of view at the location of the lens.

Earlier, I wrote that: "Those straight line paths occupy a sort of truncated cone whose cross section matches the lens pupil at one end and the sensor rectangle at the other end. In between, the cross section is a hybrid between the shapes at each end."

Perhaps now it is more clear what's going on. At the lens, the light rays are all mixed together and fill the aperture of the lens, whatever size and shape that happens to be. There is no image at that place. In the plane of the sensor, the light rays are separated and focused to form an image. The ones we care about are the ones that hit the sensor. Between the lens and the sensor, the light goes in straight lines. All the light rays we care about, taken together, form a complicated shape that I've called "a sort of truncated cone". At the lens, the shape fills the aperture. At the sensor, the shape fills the sensor. In between it is a strange shape with relatively more light rays near the optical axis than near the edge.

What a baffle needs to do is block stray light rays without blocking any of the rays that go straight from lens to sensor. The optimum shape varies depending on where it is placed. If placed near the sensor, it is more like the sensor. If placed near the aperture, it is more like the aperture.

--Rik

[canonian]

....the pictures you're looking at are not helping you to understand what's going on.

--Rik

Thanks Rik, I wrote this post a while ago but together with the diagrams it makes things a lot clearer.
Drawing a sensorshape at the position of the frontlens or the subject indeed makes no sense.

[ichbtm]

Rik,
Your explanation was great and clarifies a lot of things in my mind.
But after thinking during long hours( ) i still find it compatible with my previous reasonning...
I really regrets that english is not my native language as i fear that what i will try to explain will seem confuse.

Let's forget the FOV as you demonstrated that it's misleading.
What I understand, now, is that every pixel on the sensor is the result of the focusing rays comming from everywhere on the lens.
I also understand that, if i'm near the lens the shape of the important rays is a circle and if i'm near the sensor it's a rectangle.

In the following modification of your diagram, i did the following modification :
- reduced the size of the sensor to the red line
- drawed in green the cone of rays containing the famous sort of truncated cone made of the rays hitting the sensor.
On the sensor plane, the green cone is a circle. And this circle is circumscribing the sensor rectangle. It's why i represented its profile larger than the sensor (no scale respected).



Now, in blue, i drawed 2 positions for a circle baffle.
I believe that, to avoid vignetting, the openning of the baffle must be adapted to the green cone, and this, whatever it's position on the lens-sensor axis.

Am I correct?

If this is the case, then i see absolutely no difference, in term of passing rays , between the left and the right baffle.