Modding a A75 (Canon) - Expert advice needed

[Qazwer]

Dear all,

For those how know me ... I was busy for the pass few months due to work commitment and was unable to keep in touch with you guys.

Anyway, I've was trying to mod a A75 by myself and managed to took out the hot mirror (I suppose). It was a piece of blue coloured element with coating on it -- I hope I'm right.

However, the problem comes when I've completely assembled everything together - the focusing for infinity seems out. It is only able to capture pics that was close (like macro) by manual focus. I tried zooming in and out the lens, and the zooming looks fine.

Question 1:
Is the "hot mirror" that I've removed is the correct one?

Question 2:
Does anyone know if the hot mirror of A75 has any optics that attached to it to compensate the infinity? If yes, what can I use to replace it?

Question 3:
Or did I unpluged something by accident? Or didn't replace the parts properly?

Expert please help.

[ullyss]

I modded my G5,
how is your A75 photos, like to compare.
A75 should have about the same setup as G5

The blue glass is the hot mirror
measure the thickness of the mirror and replace it with the same thickness. you can use cheap slide glasses.
I replaced mine with IR filter

also, i place some flat plastic at corners of where you place the mirror, think you call it spacers, it helps with the focusing apparently.
you have to measure your IR filter and the hot mirror, compensate the difference of
thickness with the plastic

i followed the steps from lifepixel.com

[ndroo]

Originally Posted by ullyss I modded my G5,

Which IR filter did you use?

[Qazwer]

Million thanks bro,

I didn't replace anything with the hot mirror - guess this is where the fault is.

Anyway, before I start off, I've tried looking thru what was shown as the example of G3 & G5 but the casing and internal hardware configurations were all different.

By the way, is there any where that sells cut R72 or glass or plastics of that size (9x8mm)?

[ndroo]

Originally Posted by Qazwer By the way, is there any where that sells cut R72 or glass or plastics of that size (9x8mm)?

Try cutting them yourself. Just get an oil cutter and some sandpaper ... takes a bit of effort but works well

[Qazwer]

Originally Posted by ndroo Try cutting them yourself. Just get an oil cutter and some sandpaper ... takes a bit of effort but works well

Is the thickness the same? Please pardon me for the dumb question.

[Velectron]

You would need to measure the thickness of the hot mirror. R72 glass is 2.5 mm thick if I am not wrong. Microscope slides are around 1.0 mm to 1.2 mm depending on brand. But you may be prone to hotspot if you use uncoated glass slides...haven't seen any MC glass slides yet. Alternatives is to use MC UV filter.

But first check your hot mirror thickness lah..

[Qazwer]

Originally Posted by Velectron You would need to measure the thickness of the hot mirror. R72 glass is 2.5 mm thick if I am not wrong. Microscope slides are around 1.0 mm to 1.2 mm depending on brand. But you may be prone to hotspot if you use uncoated glass slides...haven't seen any MC glass slides yet. Alternatives is to use MC UV filter. But first check your hot mirror thickness lah..

Thanks Velectron.

I've checked the thickness of the hot mirror and it seems like 2.5mm or slightly thinner. Thanks to Ark19 on the sticky thread on P007, I've tried taken out my R72 (58mm dia) and compared both side by side and R72 seems slightly thicker. Is it okay if I use the R72 as replacement for hot mirror?

[Jemapela]

Originally Posted by Qazwer Dear all, For those how know me ... I was busy for the pass few months due to work commitment and was unable to keep in touch with you guys. Anyway, I've was trying to mod a A75 by myself and managed to took out the hot mirror (I suppose). It was a piece of blue coloured element with coating on it -- I hope I'm right. However, the problem comes when I've completely assembled everything together - the focusing for infinity seems out. It is only able to capture pics that was close (like macro) by manual focus. I tried zooming in and out the lens, and the zooming looks fine. Question 1: Is the "hot mirror" that I've removed is the correct one? Question 2: Does anyone know if the hot mirror of A75 has any optics that attached to it to compensate the infinity? If yes, what can I use to replace it? Question 3: Or did I unpluged something by accident? Or didn't replace the parts properly? Expert please help.

I have done "surgery" on a few compact cameras (not for IR modification), but with any digital camera, the general principle is the same.

Answer to Q1:

The hot mirror is also more often or more correctly called the band-pass filter. It used to be called anti-aliasing filter until this function was done by in-camera software. It is a piece of optical multi-coated glass in front of the imaging sensor and behind the imaging lens. Under light, the coating usually reflects as pinkish or purplish, although bluish may be possible. You probably removed the hot mirror (or BPF) correctly.

However, any element removed from an optical system will affect focus. You should ideally replace this hot mirror (BPF) with a glass of similar optical characteristics. The replacement glass should:

1) be of the same thickness as the original filter (fraction of a millimeter can affect focus but on a compact camera, DOF with the low focal length can save you from OOF images.)

2) be multi coated to increase contrast and reduce flare but coated to allow IR transmission (you could buy IR-pass or all-pass filter but all-pass lets in all visible light.)

Answer to Q2:

The hot mirror (BPF) is itself an element in an optical system. If you take it out, you have to replace it with something very similar except for the coating that allows IR transmission. Refer to the above answer. This would be the main reason why images shot are obviously OOF.

Also, because IR does not focus on the exact same plane as visible light (recall that red, green and blue light don't focus on the same plane), the optical system has to be adjusted to compensate for IR focus. However, again, due the deep DOF with the low focal length, you should get away with it on a compact camera, but not so forgiving on a DSLR camera.

Answer to Q3:

I haven't performed "surgery" on this Canon compact camera so I don't know if you have removed/separated the lens assembly from the imaging sensor. However, to remove the hot mirror, there is an extremely high chance that you did. This changes the distance between lens and imager (very slightly but you may escape OOF due to the deep DOF with a low focal length).

Everytime I remove and replace an imaging sensor from the lens, I have to calibrate the focus using proprietary camera servicing software and a machine called a focus collimator that simulates an infinity distance target.

Note:

There is a company in Singapore that manufactures optical components including optical mult-coated glass suitable for IR but their products aren't cheap. The company is called Edmund Optics.

[Qazwer]

Wow ... Jemapela, thanks for the detail explanation.



By the way, where do I get those IR pass or all pass filter? Do they come in different thickness?

[Hosea]

Welcome back bro.. finally u appear again.

[Jemapela]

Originally Posted by Qazwer Wow ... Jemapela, thanks for the detail explanation. By the way, where do I get those IR pass or all pass filter? Do they come in different thickness?

You probably can get them from Edmund Optics. I once had their product catalogue, and was very overwhelmed and impressed by the wide range of optical glass and lenses they design, make and sell. However, you got to understand the technical specifications printed in those pages before you buy, and honestly speaking, it can be brain-cramping to most people.

[Qazwer]

Originally Posted by Hosea Welcome back bro.. finally u appear again.

Thanks bro.

[Qazwer]

Originally Posted by Jemapela You probably can get them from Edmund Optics. I once had their product catalogue, and was very overwhelmed and impressed by the wide range of optical glass and lenses they design, make and sell. However, you got to understand the technical specifications printed in those pages before you buy, and honestly speaking, it can be brain-cramping to most people.

Thanks Jemapela. Is there any website for its products?

[deadpixel]

Originally Posted by Qazwer Thanks Jemapela. Is there any website for its products?

http://www.edmundoptics.com/onlineca...productid=1918

This is for their IR Longpass filter. It's the one that's most reasonable and suitable for your purpose. Don't bother with purchasing the R72 from them, their prices for the R72 are way above what you'd pay for one you get at Cathay.

_

P.S. Their delivery charges are quite reasonable as they'll deliver your order right to your doorstep.

_

[Jemapela]

Originally Posted by deadpixel http://www.edmundoptics.com/onlineca...productid=1918 This is for their IR Longpass filter. It's the one that's most reasonable and suitable for your purpose. Don't bother with purchasing the R72 from them, their prices for the R72 are way above what you'd pay for one you get at Cathay. _ P.S. Their delivery charges are quite reasonable as they'll deliver your order right to your doorstep. _

Thanks for providing that information.

With regards to the filter's thickness, I guess some compromise may be expected of the thread-starter.

[ullyss]

Hey Jemapela
can you explain about calibrating the focus using proprietary camera servicing software and focus collimator. How abouts do you do that with the camera? how exactly do you do that
is it about updating the firmware to your camera?
that is interesting coz i know canon service center does that and charge quite a bit on calibrating lens and camera

[Jemapela]

Originally Posted by ullyss Hey Jemapela can you explain about calibrating the focus using proprietary camera servicing software and focus collimator. How abouts do you do that with the camera? how exactly do you do that is it about updating the firmware to your camera? that is interesting coz i know canon service center does that and charge quite a bit on calibrating lens and camera

I don't have all the technical details of how a collimator works, and I won't flex any muscles to pretend I'm an expert on it.

As best I know, seen, and used, a focus collimator is used to calibrate focus in optical imaging systems which could include cameras. It is a closed/shielded device with an illuminated patterned target (such as a Siemens star chart) inside that simulates infinity distance. It is essentially a targeting device to determine an infinity focus reference point in cameras.

Used together with service software, (usually compact) cameras are calibrated using infinity as a reference point. The software checks that the camera is able to focus to infinity, and if not, electronically corrects the errors of the focusing system to achieve it.

This focus calibration is done when a camera lens or any part of the imaging system has been disassembled or displaced. A very small displacement in a fraction of a millimeter can affect focus, and since it is humanly impossible to reassemble to such an accuracy, this error is compensated electronically using service software and a collimator.

[Qazwer]

So many thanks all of you for the valuable information and suggestions.

[Qazwer]

Hi guys,

I've tried with 2 pieces of clear transparent plastics to replace the hot mirror closer to its thickness. At some distance, the camera still don't seems to focus well.

Anyway, this is one of the shots I tried out in my office.



It don't seems like coloured IR. Anyone can identify why?

Thanks for viewing and your help, though.