Distinguish flat vs curve field projection lens

[weasel_bar]

I project my slides. Bought a kodak ektagraphic with a Golden Navitar lens (see attached). How do I tell if it's a flat field or curved field lens? Other than the fstop and focal length, the only distinguishing thing on it is the number "53586. " 

Also, in general, if I'm buying another lens for my projectors, how do I know if they're flat or curved field? Do manufacturers usually specify? Is there a default to assume if not specified? 

Thanks for any help

[SCL]

Can't speak to yours, but Leitz produced a Colorplan 90mm/2.8 projection lens (for 35mm slides) in BOTH a flat field and curved field version and they were distinguished by different catalog numbers. I sold my curved field and purchased a flat field lens. On most of my slides at my projection distances there was only a slight difference. I would think a manufacturer's default would be curved field, as the flat field lenses were more difficult to manufacture and usually more expensive.

[jose_angel]

On projection lenses I`d think they should be all flat field designs, like process or copy ones; at least it is what makes sense to me. I think a projection 2" f2.8 lens must be a flat lens design!!

But there are other considerations; size, number of elements, glass quality, etc. 

I`d say what really count is how much sharp the image is at the whole projection surface. When working with enlarging lenses I have been really surprised on how big the field curvature is on some supposedly good ones that I need to stop down a lot to have a -really- sharp image both at the center and the corners. Also, a bad performing "flat field" lens is not a big deal, specially if you cannot get a bigger enough or contrasty image.

So at the end, I think the thing is to know which lens perform better, despite of being flat or not.

 

Edited May 10 by jose_angel

[AJG]

Kodak at one point marketed lenses specifically with a curved field to compensate for the inevitable curved film in cardboard mounts that most people used.  The Buhl lens you showed us would have been a substantial upgrade on the usual Kodak Ektanar lenses that came standard on Carousel projectors.  It was probably used in some kind of auditorium situation (since it is 50 mm, probably back projection which would require a short throw) where glass mounts would be common practice.  So it is probably intended to be a flat field lens.  My experience with Buhl lenses is that they  were usually good quality lenses and fairly expensive so you usually found them in institutional settings.

[glen_h]

I am not so sure how consistent it was, but Kodak used to make flat field lenses with focal length in inches, and curved field in mm.

So, 4 inch or 102mm were common Kodak projector lenses.

Shortest I have is 2.5 inch, as far as I know flat field.  Nice for small rooms.

The 2 inch, lens is for 2x2 mount 110 slides.