Lens Identification - Guru Alert

[Dan Fromm]

John, I've also seen aerial camera lenses with the exact (to 0.1 mm) focal length marked. And process lenses with QC slips giving the exact focal length (again to 0.1 mm) or with the exact focal length marked on the lens. But I've never seen a lens for either application in a helical. Limited experience on my part, no doubt.

Peter Gowland must have had a good reason for putting a helical on his aerial cameras. I suppose we could ask him why he did it.

Cheers,

Dan

[Steve Hamley]

One more problem, I have 2 8x10 B&L tessar 1c lens, they are 1/2 that length. I don't think thats a 4.5 tessar.

Yes, it is a f:4.5 Tessar. It's marked as f:4.5 Tessar (but not 1c) but 1c Tessars certainly existed in this focal length - 12" - 300mm ranges. One just sold on eBay a day or two ago.

Cheers,

Steve

[orto]

Folks,

'm in a bit of a puzzle here trying to identify a lens. <snip>


Now why get so interested in an old uncoated Tessar? Well, other than curiosity, when compared to a 30cm f:4.5 coated Heliar, it has a far better transition to out-of-focus and shallower DOF than the Heliar, which would seem to violate the laws of optics given the difference in FL of 4mm or so. That would also possibly indicate the B&L was optimized for a different use than a standard view camera taking lens. It is quite sharp too, but just seems to have a smaller DOF which gives a nice 3D look.

So, any idea what this thing is?

Thanks,

Steve

Here you get to the bokeh highly subjective description and a non technical nonsense on DOF. Sorry, voice of reason: 300 mm is 300 mm is 300 mm... There are no cameras especially constructed for "nice 3D look" of some lens - there you are just dreaming. Other than that, if you like the lens, use it

[Steve Hamley]

Goldfart,

I know physics, I have a degree in it. I know it shouldn't have less DOF than another lens of the same FL, but I'm not so stupid as to believe dogma over my eyes. The difference is obvious on the GG.

The original post was about what it was, not who can explain its optical performance.

Steve

[Ole]

The Heliar has residual uncorrected spherical aberration by design, which gives it an illusion of greater DoF than (most) other lenses.

I thought that was well known and easily understood?

[Steve Hamley]

Not that much. Pictures attached, but do be aware that the GGs are different (brightness) as both lenses will not at this time fit on the same camera. The point of focus is the rock in both pics.

Cheers,

Steve

[orto]

Goldfart,

I know physics, I have a degree in it. I know it shouldn't have less DOF than another lens of the same FL, but I'm not so stupid as to believe dogma over my eyes. The difference is obvious on the GG.

The original post was about what it was, not who can explain its optical performance.

Steve

Relax Steve - if you know about the constant DOF then why to dream about some celestial exceptions from the rule? You yourself started to speak about the "optical performance" - I just told you that it is not technical to speak so. Now if you don't believe the "dogma" and you believe "your eyes" - isn't it a time to write a new optics' book? "The optics as seen - not believed" or something like that... eh, relax and keep to your degree

[orto]

Not that much. Pictures attached, but do be aware that the GGs are different (brightness) as both lenses will not at this time fit on the same camera. The point of focus is the rock in both pics.

Cheers,

Steve

To see the unseen? Steve, where is "the rock" in the picture??

[Dan Fromm]

Steve, with all due respect I can't tell anything from the thumbnails you posted. GGs are fuzzy, small digitized images are fuzzy, ...

I don't know why, but this all brings to mind the assertion that pinholes have infinite depth of field. I never understood that; if nothing is in sharp focus, how can the zone of acceptable sharpness have depth?

[Steve Hamley]

Apologies due gentlemen, too much caffeine this morning, but geez I love it.

I tried my best to get thumbs that were useful, but that's the limitations of jpegs of postable size and digital cameras. The rock is the small brownish object in the center of the sheet-metal "roof" of the bird feeder.

Anyway, that some lenses of the same FL have a different DOF or transition to out of focus (sorry Dan, I know you have problems with unscientific evaluations, and so do I, but I'm open to suggestions) was known by my local commercial pro who's been in the business for a long time. He was not surprised by the statement. I guess when I get it shootable I could take a couple of color transparencies and mail them around. Maybe the optimization or cell spacing might cause this effect?

Cheers,

Steve