split focus

SmoothOperator wrote:

There are some nice split focus filters out there, but I'm wondering if other lenses like a pair of bifocals can be repurposed, or these graded optometrist test lenses, that step up a quarter of a diopter.

Does anyone make a circular splitfocus, where the center is closer, and the outside can be focused on the background. Somewhat like the minolta variable field lens.

I've never seen anything like that sold for photographic use. I do have a few strong aspherical lenses elements for playing around with where the strength varies across the radius. I've not yet managed to get anything looking remotely like an image with them!

Does the accessory lens need to have the same optical center, or can you basically wave a magnifying glass in frontof the lens.

You can just use a magnifying glass the image quality probably won't be quite as good. In experiments I've actually used Fresnel lenses too (IQ definitely takes a hit with these)

SmoothOperator wrote:

There are some nice split focus filters out there, but I'm wondering if other lenses like a pair of bifocals can be repurposed, or these graded optometrist test lenses, that step up a quarter of a diopter.

Does anyone make a circular splitfocus, where the center is closer, and the outside can be focused on the background. Somewhat like the minolta variable field lens.

Does the accessory lens need to have the same optical center, or can you basically wave a magnifying glass in frontof the lens.

The old Minolta 24mm f/2.8 VFC (Variable Field Curvature) lens is definitely the way to go. There's one under $400 on eBay now (and several more expensive).

The really cheap alternative is, if you have a sufficiently programmable camera, to capture 2 or more shots in a burst varying focus and then align and merge them in postprocessing. Quite a few cameras allow focus bracketing, but usually not with a very large change in focus distance....

glassmandavo wrote:

Spiratone sold an inexpensive adapter in the 70's, that was essentially a bifocal add-on for your lens. It had a semicircular element in the lower half of the housing that screwed into your lens' filter ring (or slipped over the outside, I can't remember exactly now). The effect was near focus in the foreground, infinity or distance focus in the upper half.

Check on Ebay.

I think those sort of split focus diopters were made by just about everyone. I think mine is a Pentax version, but I've also seen them in a Cokin catalogue, and models from quite a range of other manufacturers listed on the bay. The OPs post seems to be talking about something different however.