Can the Metabones Speed Booster be used for Infrared?

Started Feb 6, 2013 | Discussions thread
Hye Regular Member • Posts: 405
Re: re: Speed Booster be used for Infrared?

jpr2 wrote:

FumbleDuck wrote:

I have a NEX-3 converted for infrared (well full-spectrum really but just used for IR) but one of the things I have noticed (and really didnt think about) was that using crop EF-S lenses on a crop sensor really does de-focus the borders of a photo. I am talking about wide-angle shots here so <24mm in FF format.
The last crop IR camera I had was a 400D but I used a full-frame Canon 17-40 on that and the borders were fine (although not that wide as it equated to 27mm at its widest)

I've also used the 17-40 on a FF IR camera - a converted 5D which introduced slight fuzziness at the edges but not overly noticeable.
So my experience so far:

Crop + FF lens = Good (although not that wide)
FF + FF lens = OK
Crop + Crop lens = Bad
Crops + FF lens + Speed booster = ????
I am wondering if using the 17-40 with the speed booster will give me decent wide-angle IR shots (in other words no de-focussing at the edges) because:

a) the lens seems to perform well for IR with very little border de-focussing
b) the inherent sharpness gain by compressing the full-frame image circle.
My main concern would be whether the speed booster will introduce hotspots because it is another glass element in the optical path?
Apologies for the brain-fart of a post but there a quite a few variables in my mind while considering this.

why don't you contact the designer, Bian C, himself? We already know that at least of UV and hence PF part of the problem is the use very high RI glass in SB - I don't know anything whether the whole reducer is thus shifted towards UV (as then it will be cutting off at IR, acting as a filter), but maybe no...

By the way (a bit OT), should you receive the instructions for SOT-23 surface-mounting by PF, could you pls. share them here on the forum?


Technically, the issue is not high refractive index, but the amount of dispersion in the material used for the optics:

My guess is that it will almost certainly be worse than using the same lens on an FF sensor. Basically, the speed booster is taking light that produces an FF-sized image on the sensor plane and bending it so that it hits an APS-sized region instead.

Since dispersion is going to be most evident where light bends the most (which is why purple fringing and other CA effects are strongest at the corners of an image), you're going to get *additional* dispersion at the image borders when using the speed booster, since the light has to be bent the most there, and not at all in the center. IIRC, this is reflected in several of the reviews of the speed booster, where the corners are not quite as sharp as they are for the same FF lens on an FF sensor.

How much more of an effect this will be in the IR range, I do not know, but the sharpness gain you speak of from compressing the image circle doesn't matter here, if the lens + SB combination can't combine the image circles from different wavelengths to the same position on the sensor.

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