Baffling

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elf
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Baffling

Post by elf »

I've seen several reports on other forums that claim that baffling a lens designed for a larger format will improve the image quality by blocking extraneous light reflections.

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baf·fle (bfl)
tr.v. baf·fled, baf·fling, baf·fles
1. To frustrate or check (a person) as by confusing or perplexing; stymie.
2. To impede the force or movement of.
n.
1. A usually static device that regulates the flow of a fluid or light.
2. A partition that prevents interference between sound waves in a loudspeaker.

[Perhaps blend of Scottish Gaelic bauchle, to denounce, revile publicly, and French bafouer, to ridicule.]
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I would like to find out if this is true or not, but don't want to build many baffles of different sizes to find the best size and shape. Is there a mathematical way to determine the optimum baffle size given the following information?

Example 1:
Olympus OM 50mm f/1.8 lens.
Reversed, the exit pupil appears to be 30mm from the (original) front of the lens
Mounted normally, the exit pupil appears to be 20 mm from the flange mount.
The flange focal distance is 46mm
The flange focal distance on my 4/3rds camera is 38.67mm

Example 2:
El Nikkor 50mm f/2.8
(technical data for this lens can be found in this thread: http://www.photomacrography.net/forum/v ... t=elnikkor)

Harold Gough
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Post by Harold Gough »

elf,

You seem to have some ideas of what you need to do but I cannot beggin to visualise what you have in mind. I suppose I am baffled! :?

"Baffle" is also another name for a motor vehicle's exhaust system silencer. Could that be why we have heard no details from you? :D

Harold
Last edited by Harold Gough on Sun Jun 07, 2009 4:03 am, edited 1 time in total.
My images are a medium for sharing some of my experiences: they are not me.

Planapo
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Post by Planapo »

From experience of others and my own it's true that such a 'baffle', which I understand here is a 'flare cut diaphragm', can improve image quality. Also inner lining with black flocking material or coating with really matte enamel will avoid/reduce flare caused by light being reflected from otherwise shiny inner parts. You can find some information here.

I made such a diaphragm from black carton and inserted it between lens and bellows somewhere between the stepping adapter rings. I just used a little bit larger diameter than that of the cut-out hole of a factory-made, glass-less Minolta MD to EOS adapter, which seems to have this diaphragm/baffle purpose. And that diameter seems to cause no vingnetting, even if it is located in the position further away from the camera.

But yes, it would be nice to have a way to calculate that diameter more exactly.

--Betty

Harold Gough
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Re: Baffling

Post by Harold Gough »

elf wrote:I've seen several reports on other forums that claim that baffling a lens designed for a larger format will improve the image quality by blocking extraneous light reflections.
Surely you are using the more telecentric, inner part of the lens, such that stray light would be less?

Harold
My images are a medium for sharing some of my experiences: they are not me.

rjlittlefield
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Post by rjlittlefield »

To get a hint of the problem, see http://www.photomacrography.net/forum/v ... .php?t=424, first panel, bottom half.

The difference in flare is due entirely to the lens shade cutting out extraneous light.

Without the shade, all that extraneous light bounces around inside the lens, inside the bellows, inside the camera, and back and forth between those surfaces, until it either gets absorbed harmlessly or hits the sensor as a defocused fog that degrades contrast.

Degradation is especially bad in the shadow regions. In the example shown, contrast in the darkest shadows was reduced by at least 10X in the no-shade case. That reduction in contrast produces also a reduction in resolution, though fortunately not by the full 10X.

The job of any behind-the-lens baffle system is to cause extraneous light to be harmlessly absorbed instead of striking the sensor. I say "cause" because the baffle system doesn't necessarily need to absorb the light itself. Even shiny metal baffles can be effective if they reflect the light to a suitably absorbing surface. In fact, shiny baffles can be more effective than diffusive baffles, if the baffles can be shaped to direct the reflections in particularly harmless directions. [ref]

Unfortunately, it is very difficult to accurately calculate the effect of any specified baffle system. By "very difficult", I'm talking about minutes of calculation on a fast scientific workstation using sophisticated software.

elf asks specifically "Is there a mathematical way to determine the optimum baffle size...?" I think the answer is no, even if we're pretty liberal about what "optimum" means. See HERE for the mathematics of designing a baffle system for a telescope to look through. Notice that this is an outside-the-lens system, designed only to meet the requirement that "no optical components are allowed to view any sunlit wall or vane edge directly, that is, at least two reflections from blackened surfaces are required between the stray light source and the optical element."

I'm sorry to sound so pessimistic, but I think this is one of those problems for which math is not a very good tool.

I suggest instead to haul out a piece of blank paper and make yourself a scale drawing that incorporates the measurements you've given. Then start sketching in baffles and drawing a few rays like the ones that are shown in the telescope paper. I suspect you'll converge pretty quickly on an arrangement that both blocks the stray light and can also be physically constructed in the bellows system or whatever else you're using.

By the way, while I'm thinking about bellows, it's worth noting that elf's big-black-bag approach to bellows design is probably pretty good even with no baffles at all. That's because almost all the stray light will be striking the bag at relatively steep angles, where the fabric doesn't reflect much. The biggest problem with bellows and extension tubes is with glancing rays, where most surfaces become pretty shiny and reflective even when they look dull black at steeper angles.

Before you spend much time designing baffles, it's probably worth a quick test using one of those outside-the-lens black cone shades. If the shade makes a big improvement, then you'll know there's an issue to be worked somehow or other. If it doesn't, there's not, and you can spend your time elsewhere.

Just remember that the lens (and its associated barrel) can make a big difference in whether flare is an issue. As shown the earlier posting that I linked above, flare was a huge issue with two of my tested lenses. However, those lenses are also not the best for resolution. With the Olympus bellows lenses that I use most often, on the bellows that I use them with, I've never encountered enough flare to be concerned.

--Rik

Charles Krebs
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Post by Charles Krebs »

A very useful "accessory" (for a variety of issues) is an extension equal to the camera body depth ("register" or "flange focal distance"). At the front of this extension is the camera female mount, and at the rear is a flat rectangular cutout that has the same dimensions as the image sensor. An extension tube makes a good basis for this attachment. (If the digital camera has the same mount and flange focal distance as a film camera, you can use a film body set to "B" with the film aperture masked to the appropriate sensor size).

With this attached instead of the camera body you can easily look through the rectangular opening toward the lens and observe what the light is "doing" inside your setup. It makes it very easy to spot potential flare problems and vignetting issues. It is also extremely helpful in determining the best size, shape and placement of internal flare-cut diaphragms.

Sometimes flare will be serious only if the light enters from a specific direction. If you have a small bright flashlight you can move it around in front of the lens and find any specific angles where problems might occur.

While this is obviously most helpful in tracking down issues in the various adapters and bellows/extension we use, it can also point out lenses that have internal problems. Sometimes there is an element edge or an internal lens barrel surface that reflects much more than desired. If so, an external lens shade is most helpful, but sometimes an internal baffle can also be of some use. Worst situation are lenses where the shape and/or reflectivity of the lens element surfaces themselves cause a problem. Not much you can do here, but at least you will be able to assess the problem.

elf
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Post by elf »

It will take me a while to create the ray tracing diagram, but I did try a 14mm baffle on the reversed OM 50mm. The image was sharper at f/1.8 but also slightly darker, so I suspect the baffle was acting as diaphram. The size was just picked because it was 50% of the diameter of the (normally) front element.

Harold Gough
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Post by Harold Gough »

elf wrote:It will take me a while to create the ray tracing diagram
Rays? Rik will be after you! :shock:

Harold
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Harold Gough
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Post by Harold Gough »

rjlittlefield wrote:To The difference in flare is due entirely to the lens shade cutting out extraneous light.
My assumption is always that, where the option is available, the shade/hood would be used.

Of course, they are impractical with macro bellows lenses.

Harold
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elf
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Post by elf »

rjlittlefield wrote: Before you spend much time designing baffles, it's probably worth a quick test using one of those outside-the-lens black cone shades. If the shade makes a big improvement, then you'll know there's an issue to be worked somehow or other. If it doesn't, there's not, and you can spend your time elsewhere.
--Rik
Harold Gough wrote: Of course, they are impractical with macro bellows lenses.
Harold
Well, I failed miserably at making a multiple ray tracing diagram that resembled reality, so I made a hood instead:

Image

It's turned from a piece of Yew and is a press fit on the locking ring of the El-Nikkor 50mm:

Image

I left the inside surface rough but will probably need to paint it flat black or add some flocking to totally block light reflections. The outside is just to pretty to paint :) There is a small, but noticeable, improvement in contrast while using it.

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Post by rjlittlefield »

Oh, now this is fun!

My dad used to make little wooden bells. Purely decorative; we didn't have any woods that were hard enough to do much more than thud pleasantly. But the appearance of this hood sure brings back memories!

Yeah, dull black on the inside would be good. Just please don't get any on that lovely outside. :D

--Rik

Harold Gough
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Post by Harold Gough »

Harold Gough wrote:My assumption is always that, where the option is available, the shade/hood would be used.

Of course, they are impractical with macro bellows lenses.

Harold
I refered, by implication, to the high magnification end of macro, where there is a difficulty in lighting the subject if any kind of hood is used.

I note that the set-up used is for a flower scale, not that of a pollen beetle.

Of course, there are no rules carved in stone but some solutions may be carved in wood! :D

Of couse any black paint used should be matt black.

Harold
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NikonUser
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Post by NikonUser »

Nice looking hood but why so complex?

I use a tube made of black flocking (on the inside); fits over the end of a reversed El-Nikkor enlarging lens. Different lengths for different focal lengths. Quick, cheap, functional - parsimonius. Or am I missing something here?
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Harold Gough
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Post by Harold Gough »

NikonUser wrote:Quick, cheap, functional - parsimonius.
Inspired by this comment, I just checked: the cardboard inner tube from a toilet roll requires only a small reduction in circumference to fit my OM bellows lenses and the caps would fit the tube. :wink:

Harold
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elf
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Post by elf »

Harold Gough wrote: I refered, by implication, to the high magnification end of macro, where there is a difficulty in lighting the subject if any kind of hood is used.
I note that the set-up used is for a flower scale, not that of a pollen beetle.
Of course, there are no rules carved in stone but some solutions may be carved in wood! :D
Harold
I thought that's what you meant, but just couldn't resist the out-of-context quote :)

The length on this one allows focus at full extension plus a little room for light. I don't think the lens is fast enough to use with 4/3rds sensor at 4X (maximum extension for my bellows), so at 2.5X or lower there is plenty of room for lighting.
NikonUser wrote: Nice looking hood but why so complex?
I didn't have anything laying about the house that would fit. I've been turning wood for quite a few years and this was really a very simple project.

I think the opening could be made even smaller since I don't see any vignetting at any magnification with this one.

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