peter_wagemans

Thanks Bob. I also realised last night that the diagram from Canon is

much too coarse. It got me thinking in the wrong direction. I suppose

that in fact it is a very fine grating.

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Rather loosely speaking, I suppose the steps are angled so that each

individual small step bends the light in approximately the right

direction to a beam with a broad peak (diffraction), and the

interference between those beams narrows it down to a single small

spot.

Why do they use the term "diffractive"? Canon's own footnote describes

what they mean by this:

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> Diffraction refers to the tendency of light to spread around the

edge of an opaque obstruction.

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Given this description, does anyone understand why Canon uses this

term to describe the new optical element? To me the picture looks like

a double Fresnel lens construction, i.e., refractive like normal

lenses: bending the light by passing it through slanted surfaces.

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Also I would be worried about light scattering on the ridges,

especially for slanted light rays (at an angle to the optical axis),

to which the ridges represent a larger part of the total area. So will

the flare indeed be minimal if you have strong lights off center? I

suppose we'll have to wait for actual tests. Or can any optics experts

shed an early light on this?

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For the same reason I suspect it will be difficult to use these new

elements with shorter focal lengths: much more light at large angles

to the optical axis.

Here's a report on my experiences so far.

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I found a cheap small tripod with a relatively sturdy top assembly,

with small braces from the legs to the guide tube of the center

column, for extra stability. The center column was removed (too

wobbly) and the legs replaced by three Leki avalanche probe tubes. An

aluminium plate was screwed to the top, with a 3/8 inch bolt for a

ball head.

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After some searching for a suitable ball head, good enough for a 2kg

load, but not too heavy, I decided on a Gitzo 1175 (175 grams). It is

not very user friendly, but sufficiently sturdy for this setup.

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Including ball head the resulting hiking pole tripod weighs 1140

grams. If you include two grips for normal use of two hiking poles and

a monopod attachment then the total is almost 1400 grams.

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Height range including ball head (8 cm) is from 85 to 150 cm (a bit

low for me, but not too bad), with leg end points 60 to 113 cm

apart. Stability suffers below 94 cm height due to the tapering ends

of the lower hiking pole sections (they no longer fit tight in the

second section at those settings).

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Limiting factor for stability is the flexibility of the hiking pole

legs. When shooting with a tele lens I try to keep the tripod at the

lowest stable point (94 cm height) and lower myself behind it. When

there is not too much wind this works fine up to my maximum focal

length of 400mm (with mirror pre-fire on an EOS 5).

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It is a pretty light setup and not too stable, but certainly better

than no tripod at all. It has allowed me to take pictures that would

otherwise have been impossible, e.g., in the forest. And the hiking

poles have already come in handy for companions with knee trouble.

Have you checked whether it is actually a circular polarizer, or

whether it was accidentally put in its mount with the back in front?

If you hold the polariser in front of a mirror and look through it so

that the light passes twice through the polariser, it should give a

darker image when the front is to the mirror but it should be (almost)

completely dark if the back of the filter (which is mounted onto the

lens) is to the mirror. A linear polariser doesn't show a difference

when flipped front to back and gives the same darker image in both

cases.

Eric, how did you determine that the hyperfocal distance is something

like 8000mm for the 85mm lens? The formula you used is based on

general optical principles and some usually reasonable approximations.

Do you have any evidence that is not correct for your 85?

I've done some flower close-ups with 100-400 plus 500D (near the long

end, and I think mostly at F11) and was favorably impressed with the

projected slides. I have not done a direct comparison with the 100/2.8

macro on the same subject (which is almost certainly better optically), nor do I have suitable resolution targets, so I can't produce any resolution figures or other objective measures of image quality. Advantages of the combo are increased working distance, the ability to zoom, and the 500D is a bit lighter and smaller than the macro (backpacking with 100-400).

This question was asked some time ago on the general forum.

Apologies to people who are seeing the same question again.

So far there are two reactions that don't answer the question. The

thought occurred to me that it might have escaped the attention of

people on the Nature forum that could contribute useful ideas (past

replies have shown that plenty of backpacking experience is around).

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Original question:

<p>

Leki makes ski poles with detachable grips. Two poles without grips

can be joined back to back with a short tube to make a longer stick

for use as an avalanche probe.

<p>

Leki also makes a small and lightweight monopod attachment for those

poles. It is a short tube with a 1/4 inch screw on top that replaces

the grip.

<p>

The next idea is to mount three of the avalanche probe tubes together

in a tripod head such that you can attach three poles to make a

tripod. Leki does not make anything like that. I have not been able to

find anyone else who does.

<p>

Taking along three poles with two grips, a monopod attachment and the

special tripod head would provide a monopod, a tripod and two hiking

poles to help carry the backpack with the rest of the photo equipment.

The short tripod head without poles can also serve as a tabletop

tripod.

<p>

My current plan is to make a first attempt at this by modifying a

rather cheap short tripod. (Maybe it works good enough for my

purposes, maybe I'll learn how to do it properly.)

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Has anyone tried something like this before? Are there any reasons why

it would not work? Which pitfalls to avoid? Useful tips and tricks?

<p>

I'm not expecting a miracle of stability, just something useful for

35mm camera-lens combinations of up to about 2kg (4 pounds) and up to

200-400mm (experiment will have to determine the limits).

<p>

Worries:

<p>

Too low for comfortable use.

The poles may be too flexible when fully extended.

The tripod head construction is not stable enough.

My back will complain anyway...