Dust Protector / IR filter

[kwaller]

Just a quick question as was just talking to someone on the AmateurPhotographer forum as he was asking about IR modded cameras ( you know the one, the UK mag that gave the SD14 such a bad review) i tried to tell him that the SD14 was ideal for IR photography as you just have to remove the dust protector and use the correct filter and away you go
he came back with the following

quote no 1

The "dust cover" is probably the IR blocking filter! Also it has optical thickness, unless you replace it with a plain glass slip the focal plane will be moved forward by about 1/3 of the thickness of the missing "cover" and you may have difficulty reaching infinity focus with some lenses.

Having said that, for all I know (nothing), it may be easier to remove the IR blocking filter from a Sigma SD14 than from most other makes and models. All the same I'd be surprised if you weren't trashing your warranty by doing so.

told him that removing the dust protector would in noway "trash the warranty" and that if he looked at the SD14 site he would see that it is designed to come out tried to get him to visit the SD14 web site and here to get the correct info but he came back with the following quote

If that's the case then clearly there is no issue with warranty. But unless the dust protector is infinitely thin or is made from a material with a refractive index of 1.0 (and I never heard of one) then removing it will change the optical distance between the mount flange and the focal plane. You might not notice this if your lenses have sufficient travel past "infinity" to be able to compensate, but if you focus using the distance scale (as is almost inevitable when using a "black" IR filter) you may still fall foul of this.

Thanks for enlightening me.

and then this

No, I haven't seen a SD14. And the web site requires Flash, which I will not have on my computer as I believe the Adobe binary version creates serious security loopholes. There is an open source version under development but it doesn't work well enough to be useful yet.

If there is a piece of glass or other transparent material in the optical path, it refracts (shortens the wavelength of) the light passing through it The wavelength of the light is restored when it leaves the material, as is its direction if the material is flat - simple lenses work by having surfaces which are not parallel to each other! Light passing at right angles through a flat plate of material of thickness "X" effectively travels a distance of "X" times the refractive index. The refractive index for most optical glass and plastic is somewhere close to 1.5 i.e. the optical thickness of the plate is about 1.5 times its thickness.

Put a 1mm thick plate between the lens and the sensor and you need to move the lens forward about 0.5mm to compensate. Conversely, if there should be one there and you remove it, you need to move the lens back about 0.5mm; the only way you can do that it to focus "past infinity". Not all lenses will let you do that. That is why the modified Canon and Fuji cameras have a clear (transparent to IR) replacement "front filter" of the same optical thickness.

This is a straightforward piece of optics, not related to the Sigma or any other camera.

so is he correct that when shooting the sd14 you wont be able to focus correctly unless the lens lets you focus past infinity?
ap forum here

http://www.amateurphotographer.co.uk/forums/showflat.php/Cat/0/Number/577174/an/0/page/0#577174

and you might also be interested in this one as well

http://www.amateurphotographer.co.uk/forums/showflat.php/Cat/0/Number/555488/an/0/page/0#555488

thanks

keith

 

[TJ Hanlon]

I'm not so sure about what he is saying. The proof is in Seng's, Kendall's, Lazlo's, Rick's, Laurence's, etc's IR images.

They are all in focus as far as I can tell. And that dust protector is amazingly thin. You are correct that the dust protector is designed to be removed both for cleaning and for IR photography.

Sounds like he's enamored with theory, and has insufficient experience with reality.

tjh
--
'To me, photography is an art of observation. It's about finding
something interesting in an ordinary place... I've found it has little to
do with the things you see and everything to do with the way you
see them.' - Elliot Erwitt

pbase gallery: http://www.pbase.com/tjhanlon/
Sigma Users Group: http://www.pbase.com/sigmasd9/tj_hanlon

 

[spm]

Hello Keith,

I am too much of short attention span and long winded mumbo jumbo completely lost me. I really don't want to grow hair on my chest from such conversation.
All I know from on hand experience is this:
Here are a few of ir in this gallery:
http://www.flickr.com/photos/seng_merrill/sets/72157600724666686/

With exception to an oof here and there and disregard the artistic merit, do you see I have problem with focus?

Whoever you discussed it with just need a conversation. I have done perhalp more ir with SDxx than most people that I know, with and without ir filters, most with auto focus, a lot with handheld. Whatever the point this person was try to make, the fact remains that SD10 and SD14 remains the most versatile from normal to ir for less than 3 minutes and back to normal shooting without permanently convert the camera. Not to mention the sensitive that put the shutter speed around 1/160s f8 iso100 in a sunny day with BW093 (blocking up to and around 830nm which is stronger than R72) as oppose to seconds or longer of others cam.
Have fun conversing...
S

Just a quick question as was just talking to someone on the
AmateurPhotographer forum as he was asking about IR modded cameras (
you know the one, the UK mag that gave the SD14 such a bad review) i
tried to tell him that the SD14 was ideal for IR photography as you
just have to remove the dust protector and use the correct filter
and away you go
he came back with the following

quote no 1
The "dust cover" is probably the IR blocking filter! Also it has
optical thickness, unless you replace it with a plain glass slip the
focal plane will be moved forward by about 1/3 of the thickness of
the missing "cover" and you may have difficulty reaching infinity
focus with some lenses.

Having said that, for all I know (nothing), it may be easier to
remove the IR blocking filter from a Sigma SD14 than from most other
makes and models. All the same I'd be surprised if you weren't
trashing your warranty by doing so.

told him that removing the dust protector would in noway "trash the
warranty" and that if he looked at the SD14 site he would see that it
is designed to come out tried to get him to visit the SD14 web site
and here to get the correct info but he came back with the following
quote

If that's the case then clearly there is no issue with warranty. But
unless the dust protector is infinitely thin or is made from a
material with a refractive index of 1.0 (and I never heard of one)
then removing it will change the optical distance between the mount
flange and the focal plane. You might not notice this if your lenses
have sufficient travel past "infinity" to be able to compensate, but
if you focus using the distance scale (as is almost inevitable when
using a "black" IR filter) you may still fall foul of this.

Thanks for enlightening me.

and then this

No, I haven't seen a SD14. And the web site requires Flash, which I
will not have on my computer as I believe the Adobe binary version
creates serious security loopholes. There is an open source version
under development but it doesn't work well enough to be useful yet.

If there is a piece of glass or other transparent material in the
optical path, it refracts (shortens the wavelength of) the light
passing through it The wavelength of the light is restored when it
leaves the material, as is its direction if the material is flat -
simple lenses work by having surfaces which are not parallel to each
other! Light passing at right angles through a flat plate of material
of thickness "X" effectively travels a distance of "X" times the
refractive index. The refractive index for most optical glass and
plastic is somewhere close to 1.5 i.e. the optical thickness of the
plate is about 1.5 times its thickness.

Put a 1mm thick plate between the lens and the sensor and you need to
move the lens forward about 0.5mm to compensate. Conversely, if there
should be one there and you remove it, you need to move the lens back
about 0.5mm; the only way you can do that it to focus "past
infinity". Not all lenses will let you do that. That is why the
modified Canon and Fuji cameras have a clear (transparent to IR)
replacement "front filter" of the same optical thickness.

This is a straightforward piece of optics, not related to the Sigma
or any other camera.

so is he correct that when shooting the sd14 you wont be able to
focus correctly unless the lens lets you focus past infinity?
ap forum here

http://www.amateurphotographer.co.uk/forums/showflat.php/Cat/0/Number/577174/an/0/page/0#577174

and you might also be interested in this one as well

http://www.amateurphotographer.co.uk/forums/showflat.php/Cat/0/Number/555488/an/0/page/0#555488

thanks

keith

--

'Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great.'

• Mark Twain

 

[mike earussi]

I do know what he's talking about but he doesn't fully. A refractive index of one would be the same as air. And yes theoritically when you do add or subtract another piece of glass in the optical path it does change focus point. But the IR filter is so thin that it would hardly affect it at all. Ask him if compensates every time he adds a filter to the front of his lens. Of course he doesn't because his eye or the camera's autofocus does automatically. Same with the SD14.

 

[Kendall Helmstetter Gelner]

Since this is a person obviously not moved by physical or observable proof, here are two thoughts to run past him:

1) If removing the SD-14 dust protector (which yes is designed to be user serviceable without tools) leads to focusing issues, then why is it that cameras that have the AA/IR cut filter removed for IR work also work just fine? Those layers have some thickness as well, after all.

2) Focus (in theory) shifts with pure IR work anyway. Look at an older lens with an IR indicator - the way you work it is you set the focal distance you like first as normal, then shift to the focus point you set to the IR mark instead which moves the focal point AWAY from infinity (by a fair distance).

If you want to give him some possible reason it all works out, mention that the microlenses in front of the sensor act as negative-index refractive material when the IR-Cut filter is removed, or that it reverses photonic polarity.

If you want to poke him a bit just for fun, ask why he doesn't understand computer security well enough to enable flash in a way that will not compromise his system.

--
---> Kendall
http://InsideAperture.com
http://www.pbase.com/kgelner
http://www.pbase.com/sigmadslr/user_home

 

[docmaas]

I'm not an expert either but --

The focus takes place at the film plane and accounts for all the glass between there and the image. It has to or Sigma would have to program in information for every possible lens DSG might come up with in the future.

If you add or remove the IR filter, a UV filter in front of the lens, the rear element of a schneider LF combo lens or if you take all your pictures through a piece of plate glass you carry around it doesn't matter. Either autofocus or your own focusing without auto assistance will accomodate whatever layers of glass are between you and the image.

Punkt!

Mike
--
'America is not at war,
The Marine Corps is at war;
America is at the mall.'

 

[kwaller]

thanks guys thats what i thought, don't think he'll take much notice though, get the impression he's one of these people who even though he's never seen a SDxx or looked at the images he already knows that its not a good camera.

perhaps 'll just let Alf explain to him in his usual style

 

[spm]

Hi Mike,

I do know what he's talking about but he doesn't fully. A refractive
index of one would be the same as air. And yes theoritically when you
do add or subtract another piece of glass in the optical path it does
change focus point. But the IR filter is so thin that it would hardly
affect it at all. Ask him if compensates every time he adds a filter
to the front of his lens. Of course he doesn't because his eye or the
camera's autofocus does automatically. Same with the SD14.

I too know exactly what he is talking about, I was not entirely napping in those physics classes in college and I know a lot of people in this forum knows, and I know exactly what he was doing.

I should had been clear that he was not famaliar with SD14 ir capability or does not want to acknowledge the SD14 ir capability. In such that he created a Y intersection of the subject to diverge the attention from the fact that:

Sigma SD14 is quick and easy to modify from normal to ir use just remove dust protector and the modify is not permanent.

He did it by takes the subject to direction that he is comfortable. I think some called this tactic a smoke screen or diversion. As my father would said "take the battle to the ground that you are famaliar"

S

--

'Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great.'

• Mark Twain

 

[docmaas]

LOL great idea!

Mike

thanks guys thats what i thought, don't think he'll take much notice
though, get the impression he's one of these people who even though
he's never seen a SDxx or looked at the images he already knows that
its not a good camera.

perhaps 'll just let Alf explain to him in his usual style

--
'America is not at war,
The Marine Corps is at war;
America is at the mall.'