digital lens, image circle using roll film ?

[JW Dewdney]

6x8??? what is it - a fuji??
no, mamiya motor drive 6x8 back on an arca swiss Fline69... much better than the arca/horseman back !

a screw-in corrector plate is required on the rear of the lens... rodenstock sold them ?

Bob will certainly beg to differ on this and try to find an argument for getting you to purchase a corrector plate... but if you're shooting at f/8 and beyond... it really won't matter. It probably wouldn't matter at any aperture - when you consider the tiny differences in field curvature it would make. But the proof is really in the pudding, and you should test for yourself. Worst case scenario - and you really wanted to shoot flat-field type stuff wide open (most photographic subjects are NOT flat-field anyway) - then you could always make-do with a UV filter screwed onto the rear of the lens. Just make sure you focus with it in place, etc..

[Bob Salomon]

"then you could always make-do with a UV filter screwed onto the rear of the lens. Just make sure you focus with it in place, etc.."

No that would not work as the UV filter glass is much thicker then the corrector plate.

" but if you're shooting at f/8 and beyond... it really won't matter. It probably wouldn't matter at any aperture - when you consider the tiny differences in field curvature it would make"

What the corrector plate does is modify the point where the rays come to focus on the film.

Stopping down will change the depth of field. The corrector plate changes the depth of focus and the depth of focus does not change with the aperture..

[JW Dewdney]

See? Told you Bob would find an alternative argument. Anyway try it and see.

Depth of field, Depth of Focus? Depends on which end of the lens your looking through, it seems to me.

[Bob Salomon]

See? Told you Bob would find an alternative argument. Anyway try it and see.

Depth of field, Depth of Focus? Depends on which end of the lens your looking through, it seems to me.

What "alternative argument"?

If you don't know the difference between depth of focus and depth of field you better look it up.

While depth of field is easily adjusted depth of focus isn't. If the depth of focus is not properly adjusted your images just will njot be sharp.

[JW Dewdney]

Bob - alternative to MY argument.

Depth of Field assumes a certain tolerance of 'circles of confusion', varying subject distance, with a fixed distance to the film plane. Depth of Focus assumes a certain tolerance of 'circles of confusion', fixed subject distance (normally flat), with a variable distance to the film plane (meaning, that for a certain size C.O.C., there is SOME play in the nodal point-to-film distance.

I can't really think of a better living metaphor of 'two sides of the same coin'.

My point was that the resulting curvature in NOT using the corrector plate is somewhat moot, due to;

1. it being completely absorbed by 'depth of focus'.
2. the fact that all subjects in the real world don't exist on a flat plane (the only condition under which your argument would be truly meaningful).

Sorry - I'm not trying to erode your sales quotas... just trying to help a brother out.

[Oren Grad]

It probably wouldn't matter at any aperture - when you consider the tiny differences in field curvature it would make.

You're right about depth of field/depth of focus being two sides of the same coin, of course.

But do we know that what the corrector plate is correcting for is field curvature? The Linos literature doesn't have anything to say on this beyond the nonspecific claim that the characteristics of the CCD cover glass "were taken into the equation of the optical correction".

[JW Dewdney]

Oren - it was established in the thread below - that the effect of placing a plane of glass of a specific thickness behind a lens (pretty much any lens) that, the result would be (EFFECTIVELY) a spherical aberration, as well as a focus shift of ALMOST 1/3 glass thickness, depending totally on reproduction ratio/magnification. Thus, I think it is reasonable and quite defensible to extrapolate that the result of removing such a plate from an optical system designed for a flat-field WITH it, would be field curvature. i.e. - the system would no longer be PERFECTLY flat-field corrected. But still enough, IMO, for EXCEPTIONAL quality film photography under 'normal' (scenes with varying distance relationships) circumstances.

http://www.largeformatphotography.in...ad.php?t=24489

I think it was Helen's post (with the graph) that actually established this fact most convincingly.... though I was (less convincingly apparently) arguing this would happen, in different words.

[Oren Grad]

Ahhh, got it - thanks for the link, I'd missed that thread. Very interesting.

Bob's comment makes sense if one interprets it as referring to the focus shift.

Anybody have a clue as to the thickness of a typical CCD cover glass?

[JW Dewdney]

yes, except you wouldn't get ANY shift if you're using the lens without the plate in the first place. it's only between the state of 'with plate' and 'without plate'. On a view camera - you just focus on the GG... and that's all there is to it.

[Oren Grad]

WYSIWYG! But then if you're being ultra-critical using it in a digital application, you'd better be focusing off the acquired digital image, unless your focus analog has a corrector too.