Some naive questions about helicoid focusing for large-format

[Jeroen]

Supposedly *the* best ground glass available http://www.stabitech.nl/Bosscreen.htm, made in The Netherlands.

[Cor]

Supposedly *the* best ground glass available http://www.stabitech.nl/Bosscreen.htm, made in The Netherlands.

Unfortunately maybe already 16 years out of buisness ("Last update Jan. 11 2004"), I have tried to contact them 3 years ago, never heard back. You wonder why their webside is still up.

Best,

Cor

[mani]

Yes, unfortunately every time I see a ground-glass recommendation, it seems to be for a manufacturer that's been out of business for a while.

There are a lot of new ground-glass sellers on eBay, but it's difficult to know whether any of them are any good? Other than that, someone had recommended Canham gg - but the shipping cost to Europe doubled the price, so I reluctantly had to rule it out.

The only other option I see in Europe for a reputable and known manufacturer is Linhof - so I'll probably choose one of those.
I've watched probably hundreds of LF camera and equipment reviews on youtube now, and read at least as many, but items like the ground glass or recommendations for loupes don't really ever get a mention.

[Oren Grad]

But if my initial assumption about how focussing works with a helicoid on large-format was actually correct, then the depth-of-field scales on each lens should obviously be correct for that lens.

To use a depth of field scale, table or calculator effectively, it helps to learn about how depth-of-field is calculated. There is no one set of depth of field numbers that is "correct" for a given focal length and format. The numbers on a given lens or lens mount are based on the manufacturer's assumption about what circle of confusion (CoC) is appropriate. But depending on how much you intend to enlarge your pictures and how close and how critical a viewer you are, your CoC may be smaller or larger (more or less stringent), and your effective depth of field at a given aperture smaller or larger than the markings would imply.

[mani]

To use a depth of field scale, table or calculator effectively, it helps to learn about how depth-of-field is calculated. There is no one set of depth of field numbers that is "correct" for a given focal length and format. The numbers on a given lens or lens mount are based on the manufacturer's assumption about what circle of confusion (CoC) is appropriate. But depending on how much you intend to enlarge your pictures and how close and how critical a viewer you are, your CoC may be smaller or larger (more or less stringent), and your effective depth of field at a given aperture smaller or larger than the markings would imply.

Hi Oren - I know about circle-of-confusion and how depth-of-field should be calculated on that basis. My point in the section you quoted was that for a larger format, and a lens made for that larger format, the scales marked on that lens would be appropriate for that format, and not for some other arbitrary format. I think I was pretty clear.

The reason I'm asking about how this should work, was because in the landscape images that I'd photographed the background was not slightly out-of-focus consistent with 'stringent' viewing (nose pressed up against a large print), they were completely blurred - consistent with something totally wrong.

[Richard Wasserman]

To use a depth of field scale, table or calculator effectively, it helps to learn about how depth-of-field is calculated. There is no one set of depth of field numbers that is "correct" for a given focal length and format. The numbers on a given lens or lens mount are based on the manufacturer's assumption about what circle of confusion (CoC) is appropriate. But depending on how much you intend to enlarge your pictures and how close and how critical a viewer you are, your CoC may be smaller or larger (more or less stringent), and your effective depth of field at a given aperture smaller or larger than the markings would imply.

I generally stop down 2 stops from the settings I use on the scale. So if I put the infinity mark at f/8 I set the lens at f/16, and assume my depth of field is what it would be at f/8. This works well for me and I don't have to think too much. I print up to 16x20—if I were making larger prints I don't know if my method would still be ok as I have not tested it.

[Oren Grad]

Hi Oren - I know about circle-of-confusion and how depth-of-field should be calculated on that basis. My point in the section you quoted was that for a larger format, and a lens made for that larger format, the scales marked on that lens would be appropriate for that format, and not for some other arbitrary format. I think I was pretty clear.

The reason I'm asking about how this should work, was because in the landscape images that I'd photographed the background was not slightly out-of-focus consistent with 'stringent' viewing (nose pressed up against a large print), they were completely blurred - consistent with something totally wrong.

Understood, the issue that prompted your post is whether your helical is miscalibrated. How to use the scale effectively in practice once the calibration is correct is a separate question, but it sounds like you know what to look for in figuring that out.

FWIW, I have a Wide 750 as well. Very handy for fast working in the field, if a wide view is what you want. Good luck sorting out the helical, and enjoy!

[Oren Grad]

I generally stop down 2 stops from the settings I use on the scale. So if I put the infinity mark at f/8 I set the lens at f/16, and assume my depth of field is what it would be at f/8. This works well for me and I don't have to think too much. I print up to 16x20—if I were making larger prints I don't know if my method would still be ok as I have not tested it.

Yes, I think many of us figure out through experience some seat-of-the-pants approach that's "good enough" and lets us work quickly in the field.

[mani]

Understood, the issue that prompted your post is whether your helical is miscalibrated. How to use the scale effectively in practice once the calibration is correct is a separate question, but it sounds like you know what to look for in figuring that out.

FWIW, I have a Wide 750 as well. Very handy for fast working in the field, if a wide view is what you want. Good luck sorting out the helical, and enjoy!

Thanks Oren. I really enjoy using the Cambo - it’s given me a totally different and meditative experience shooting, and even these ‘failed’ images are lovely, if I can reconcile myself to looking at them at web dimensions (and that’s where c-o-c would be ‘acceptable’).

[Emmanuel BIGLER]

Hello from Good ol' Europe!

Coming back to the original question of a de-calibrated helical mount, it is easy to estimate the required helical travel that will properly focus to infinity.

Assume that when the helical is (erroneously) set-up for "infinity", the image is actually sharp at "D" = 3.5 metres.
The amount of required helical travel "T", shortening the lens-to-film distance, to properly focus a infinity, for a focal length "f", is given by:
T = f²/D
(Newton's formulae)
For example if your lens is a 150 mm, you have to shorten your lens-to-film distance by (150x150)/3500 = 6.4 mm
If your lens is a 90 mm, (90x90)/3500 = 2.3 mm.

If you have an infinity stop, unfortunately you cannot shorten the lens to film distance "beyond infinity", except if the helical as a whole can be moved by a few mm shortening the lens to film distance as indicated.

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Regarding the supply of a simple ground glasses for a large format camera, there is somebody in France who is selling ground glasses at an affordable price; no web site, but he has communicated on the French LF forum and people seem to be satisfied.
He has an ad' on Le Bon Coin
https://www.leboncoin.fr/image_son/1597003390.htm/
See this discussion (in French ...) and as usual, "I am not affiliated, etc.."
http://www.galerie-photo.info/forumgp/read.php?6,181397