IR conversion musings

[Lisa]

I'm planning to get a Sony NEX-7 converted to do infrared (IR) photography by lifepixel, who did the conversion on my Nikon DSLR several years ago. Now they have, in addition to the previous full-time-IR conversions, a "full spectrum conversion", in which they remove all filtering in front of the sensor and one can take either visible, IR, or UV by putting appropriate filters on the front of the lens.

It sounds like a real convenience to have to carry just a couple of extra filters instead of an entire extra body for IR (in addition to one for regular visible-light photography). However, I'm concerned there may be disadvantages I'm unaware of. The one I can think of offhand is that one would have an extra filter (possibly two) that would degrade the image to some extent. B&H carries the filters I would need, but they aren't listed as multi-coated. Does someone here have any objective feel for how much the image would be degraded? I'm not worried about pixel-peeping levels, but only a level of degradation that would be noticeable in 12"x18" prints. (I'm using the Sony 18-200mm lens for convenience, so I'm already not getting the very best sharpness out of the Sony images, and that's OK.)

If I'm stacking two filters on the 18-200mm lens (one to get the wavelengths I want, plus one polarizer), is it likely to experience undesirable levels of vignetting? Or is a polarizer inappropriate for IR photography anyway?

Might there be any other disadvantages to the full-spectrum conversion (compared with the IR-only conversion)?

Thank you,
Lisa

 

[Vivek]

When you have a "full spectrum" camera, how/what do you intend to make photos?

Unless you have an IR polarizer (a lot more expensive than your entire kit for a small filter), the regular variety has no use in the near IR region but it would have an effect (depending on the polarizer) in the far red (that a 720nm filter leaks, for example).

 

[Shashin]

That depends on the filter. Dichroic filters only filter based on an angle of incidence. When angles get wide, you start filtering different wavelengths giving concentric banding. And he other question is if the filters have the same response as those removed--you may not get the same results.

A Polarizer is rather a pointless item--your skies will be black and IR does not reflect very well off water.

How much IR do you do? It might be simpler to carry the filters to convert the camera to IR. It makes a tripod a requirement. I do this with an E-P1 and Pentax 645D. From my E-P1.

 

[Lisa]

I wasn't sure whether polarizers would do anything useful in IR, so just wanted to keep my options open.

I spent a couple of years using external filters with a previous camera, and don't ever want to go back. It was a pain - need long exposures, need a tripod, can't see through the viewfinder (at least with SLRs), etc. I've been using a dedicated-IR Nikon DSLR the past several years, and far prefer that.

I exchanged a couple of emails with someone at lifepixel who sounded quite knowledgeable, and he recommended a dedicated IR conversion for the NEX cameras rather than the full spectrum conversion because, given how the NEX focusses, there's better focus accuracy with the dedicated IR version. (Unlike things like the Panasonic m4/3 cameras, where the focus wouldn't be an issue.)

Lisa

 

[Vivek]

I am pretty sure there isn't any differences in AF (external/internal filters). Since all NEX lenses are IF, no worries about the weight of the filters either.

 

[GrahamWelland]

I wasn't sure whether polarizers would do anything useful in IR, so just wanted to keep my options open.

I spent a couple of years using external filters with a previous camera, and don't ever want to go back. It was a pain - need long exposures, need a tripod, can't see through the viewfinder (at least with SLRs), etc. I've been using a dedicated-IR Nikon DSLR the past several years, and far prefer that.

I exchanged a couple of emails with someone at lifepixel who sounded quite knowledgeable, and he recommended a dedicated IR conversion for the NEX cameras rather than the full spectrum conversion because, given how the NEX focusses, there's better focus accuracy with the dedicated IR version. (Unlike things like the Panasonic m4/3 cameras, where the focus wouldn't be an issue.)

Lisa

Lisa

If you go with the full spectrum conversion you'll be using filters either for IR only or visible only use. For IR you'll be back to viewing through a dark filter depending upon the filter cut off.