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Rear-mounted split-field diopters?


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I'm wondering if anyone's had any experience with mounting split-field diopters to the rear of a lens before?

Will partially throwing out the backfocus (by having additional glass between half of the projected image) muck up the diopter's focussing?

Thanks to Alexander Sutton Hough's recent discovery of an easy modification to allow 49mm filters on the rear of Sigma's Cine primes, I'm amassing a small collection of fun little filters. Would love to be able to add some split-diopters (as I use them too infrequently to justify a normal 4x5.65" purchase) but at 49mm it becomes possible to carry a set at all times. 

I just need to figure out whether they'd actually be usable.

Would love to hear any thoughts!

Cheers,

Mark

 

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Hi Mark,

interesting proposition!

I haven’t tried it, but I could theorise what might happen. Normally a diopter (or any single element) behind the lens will throw out the back-focus, as you say. Depending on whether it’s a positive or negative power, it will either shorten or lengthen the back-focus, effectively shifting the point of focus closer, or further away. So a split diopter behind the lens I imagine would work a bit like one in front of the lens, creating two seperate focus points. The altered half would also display the aberrations which get introduced - mainly spherical aberration and astigmatism. So far so good, but the other effect will be a small focal length shift, which manifests as a change in magnification. So I would think the altered half won’t line up properly with the other half of the image. 

All just theory though, so I’d be curious to see the results if you go ahead and source something!

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13 hours ago, Dom Jaeger said:

So far so good, but the other effect will be a small focal length shift, which manifests as a change in magnification. So I would think the altered half won’t line up properly with the other half of the image. 

This happens to an extent with split diopters in front of the lens too, correct?

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25 minutes ago, Dan Finlayson said:

This happens to an extent with split diopters in front of the lens too, correct?

Yes that’s true! 

For some reason I was imagining the magnification to be more with a diopter behind the lens, but actually now that I think about it, it must be the same mathematical addition of diopter powers either way.  A 50mm lens (diopter power of 1000/50 = 20) with a +1 diopter added (in front or behind) becomes diopter 21 or the equivalent of about 48mm, which is not much of a shift. A 200mm lens with a +1 diopter effectively becomes a 167mm lens, which is a much greater change. 

So I guess it’s the same issue matching the different magnifications with a split diopter in front of the lens as behind, only behind the lens the altered half will be much more abberated. 

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