CI Photocommunity

Register a free account now!

If you are registered, you get access to the members only section, can participate in the buy & sell second hand forum and last but not least you can reserve your preferred username before someone else takes it.

SD14 Infrared Success, sorta, well closer

Steaphany

Well-Known Member
Question: How do you shoot IR photos with a SD14 ?

Answer: Put a Hot Mirror Filter over the lens.

As counter intuitive as this sounds, it's turning out to be what it takes. The reason is the Foveon imager is just too sensitive to the IR spectrum. Shooting with out the SD14's internal Hot Mirror filter opens up the IR spectrum, but the sensitivity over whelms anything recorded by the Green and Blue channels.

Prior to purchasing a LDP X-Nite CC1 filter to easily switch between IR and natural color photography, I tried a Tiffen Standard Hot Mirror. It turns out that the Tiffen filter is not strong enough to put the SD14 back into normal color balance, but IR is attenuated sufficiently so not to over whelm the imager. The result being images with the expected visible light colors plus an IR bias to the Red.

Here are some X-Rite Color Checker images demonstrating:

This is an image shot with the SD14's internal Hot Mirror filter installed, (that I shot prior to breaking it):

SDIM1079s.jpg

When the internal Hot Mirror filter is removed, this is the result:

SDIM1058X3F.jpg

Everything is some shade of Red and there is little Blue and no Green. This image is so far from the ideal that you can't correct the color by referencing a white or neutral grey.

Now, with the internal Hot Mirror filter removed and a Tiffen Standard Hot Mirror on the lens, here is the result:

SDIM1072X3Fs.jpg

and when color corrected:

SDIM1072ColorCorrs.jpg

Note that only some of the hues towards the Red end of the spectrum are off from their expected values.

When shooting a photo with this configuration, the resulting image is:

SDIM0001X3Fs.jpg

Color corrected:

SDIM0001s.jpg

Note that this is far from being a perfect photo, my goal was to see what I could get with a scene which included live plants which reflect a lot of IR along with soil and inorganic materials.

This highlights another issue, as the angle through the Tiffen Standard Hot Mirror deviates from perpendicular, more IR becomes attenuated leaving the IR effect greatest in the center and it diminishes towards the edges of the frame. This explains why the Sigma internal How Mirror is so thin and fragile. They need to minimize the effect of edge of frame light being color shifted.

I still have to run more tests and see about achieving a uniform IR effect across a whole frame, but at least I have things a lot closer.
 

foveonfan

Well-Known Member
Greetings from Wodonga ...

... and thank you for the heads up, Steaphany. I'm really looking forward to following your IR adventure.

Sincere regards, JR

You are all welcome to visit my humble gallery @ pbase.com/jrdigitalart
 

tc95

Well-Known Member
Steaphany,

Just looking at the picture of the shed....if you increase your f-stop to around 6 to 11 on a sunny day it should take away the hotspot in the center of the screen....you can correct the darkness by turning up the ISO...

The SD14 does OK with IR w/o the filter in place....but you still have a piece of glass in front of the sensor...which will inhibit some of the light that passes outside of normal spectrum...Sigma has confirmed that this piece of glass does have uv/ir blocking characteristics...not heavy but enough to change your picture.....but unmodified...the Sigma line of camera's do a better job of capturing other light wavelengths...

My own experiments lead me to believe that the filter....helps correct the red shift that the foven sensor actually has....(tendencies towards the red color when taking a picture...[I know, you know this...just explaining for others!!!])

If you used a 720nm filter in front of the lens you should be able to produce some spectacular pictures with your SD14.....that being said...without the filter the camera does a heck of a job with photo's if you convert them to B&W..and then mess with them...

Here are some test shots of my Cat....w/o the 720nm filter

First shot:
Standard AWB mode with-out the UV/IR filter at lens mount...
1.jpg

Converted image to B&W
2.jpg


Second Shot:
WB to Bulb with-out the UV/IR filter at lens mount....
3.jpg

Converted to B&W
6.jpg


Third Shot:
Custom WB Mode with-out UV/IR filter at lens mount....
5.jpg

Converted to B&W
4.jpg


I think the custom WB is the way to go when shooting the SD14 like this...(just a warning the filter is very fragile....make sure you are very careful when removing or inserting it back in...the glass is very thin and breaks easy...!!!)

Hope this helps out...

Oh...all images were shot with ISO 400 f-stop 4.....

Tony C. :z04_cowboy:
 

Steaphany

Well-Known Member
Here is related data which should be very useful to anyone looking to shoot IR, Exposure Offsets.

My experience has been that the SD14 internal light meter has a peak sensitivity in the Green region of the spectrum. This is great for visible light photography, but throws things off when using filters which pass IR and block visible light.

I have compiled the following list of exposure compensations for various filters all with the internal filter removed. The compensation listed is to achieve ideal exposures, not shifted left or right:

Code:
Please, Log in or Register to view codes content!
 
Top