Re: [ATM] ronchi accuracy

["John Sherman" <atm@mail.johnspics.com>]

Hi Jeff,

>Isn't using a Ronchi program with the ruler mode very similar
> in accuracy to using pins or a couder mask? You don't get numbers, 

Yes. Well, I'm certainly not the expert, but I do believe you CAN get good numbers from a Ronchigram. Can you put your own Ronchigram into your favorite program and use their ruler? That's what I'D like to see.

There are some who think the Lateral Wire Test is pretty accurate. You take a wire and move it sideways across the mirror, recording numbers each time you get to a zone marker in your mask. Then enter the info into Sixtests using the caustic. 

But, isn't a Ronchigram a "backwards" LWT? In other words, you have a bunch of wires with predetermined spacings - you don't have to move them, all you do is measure the zones they fall in. Same info as the LWT, right? Only it's a lot easier to get those numbers, since they are coming off of a photo. Just enter the info into Sixtests the same as with the LWT. Look at the post by Bill Thomas from 11/11/5 for a very similar approach, and for the method of extracting the numbers from the Ronchigram.

Maybe someone else will comment on this. I do hope it works, because I have yet to see an easier method of getting good numbers. I did it on a 22" f/4.5 a couple of years ago. I like it!

Have fun,

John







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