The 23a red filter is offered by Tiffen to increase contrast in B & W photography.
Because it let pass infrared, while cutting UV and visible light up to orange (~590nm), it is also a great tool for infrared photography with a full-spectrum modified camera.
The sample pictures provided in this listing has been taken with the Tiffen 23a filter on a full-spectrum Sony Nex camera, with a Nikkor AFS 18-55mm G lens.
These results CANNOT be obtained with a standard camera.
With this filter a "golden" IR effect can be obtained by swapping the red and blue channels.
I will combine shipping for filters at no additional charge.
Feel free to ask any questions.