Hasselblad Flex Body and Arc Body focus screen differences

Hi folks,

I've been thinking about a Flex Body to use with my HB gear while shooting film and I've noticed that a lot of the bodies available don't include the groundglass adapter for the Flex Body (72405). I think that is because some (many?) people are shooting these with one of the digital backs and they can use live view instead, so I'm guessing the grounglass holders are getting misplaced a good bit. It seems that even if they don't have the GG, holder, they will sometimes include the focusing aid fresnel screens 10 and 20 degrees, which makes no sense to me at all...

I've seen the 72405 seperately pop up at time, and they are often quite expensive. I've also seen the Arc Body focus screen (41057) pop up and it is even more expensive, but that made me wonder what the differences are? They both appear to use the same offset slides (but the Arc Body has a different set of fresnels since the lenses aren't retrofocal; it appears that they included the same ones as the Flex Body when shooting the 75mm, but for the shorter lenses, they are diffferent). However, that shouldn't account for the differences in the product number, so there must be something else.

I'm trying to understand what the differences are between the two and whether there may be some value to getting the Arc Body GG holder instead of the Flex Body holder, if I have to choose one or the other as a seperate purchase from the camera body.

Is it possible the difference is that the Arc Body screens have an Acute Matte focus screen in them and the Flex Body focus screen is not one of the Actue Matte screens? That would make some sense, as I've seen that in the screen adapters for the SWC/M and the SWC903, where I think the biggest difference is the presence of the Acute Matte screen.

Anyone with firsthand experience with these groundglass adapters?


Thanks,

---Michael

Just a note... I have noticed that the 41057 includes a bubble level. I wonder if that might be the only real difference...

Hi Michael.

I have a FlexBody which I use from time-to-time just as you are considering, so perhaps I can offer a few thoughts.

I believe that the difference between the part numbers for the GG for each of the FlexBody and ArcBody is due to the design of the actual taking lenses being used for each camera, and design of the fresnel lens required to support focusing on these bodies.

With the FlexBody, all of the lenses have the same back focus as when they're used on a standard V series body, so that the fresnel lens/screen on the FlexBody GG adapter uses the same fresnel design pitch and orientation as do the standard focusing screens for those bodies.

Conversely, the ArcBody uses very different lenses with very short back focus (no need to fit a mirror between the lens and the film plane), and as the result, needs a different design of fresnel lens to capture the light rays when the GG is positioned so closely to the lens.

This is similar to my experience using the GG that's designed for use with the SWC/903, for which the fresnel seems quite different from that of the FlexBody: using the FlexBody GG on the SWC results in being able to usefully see only the centre portion of the image as the result of the very short back focus of the SWC's lens, whereas the GG that's designed for the SWC has much improved peripheral illumination on the SWC, but is not useful on the FlexBody.

I does seem very strange that people are selling correction frames without the actual GG adapter to use them. A full set of GG components should consist of the GG adapter for the FlexBody, 10 and 20 degree correction frames, a light blocking frame (which is a simple frame like each of the correction adapters, but without a fresnel, that's simply inserted to stop light from leaking in through the correction frame slot on the GG) and a 4x5 format frame insert that fits into the rear of the FlexBody for composition purposes.

In addition, using an appropriate viewfinder on the FlexBody is enormously helpful when working with tilts and shifts. In particular, the RMFx works beautifully, it's light & small and - most importantly - provides a correctly oriented (upright and laterally) image to view.

I hope that helps shed some light from one user's practical perspective.

There should be no difference in use, but getting a Flexbody NOT as a full kit is not a prudent approach.

I don't know if Flex slides fit into Arcbody GG adapter, that might be the only thing if slides for each are actually not framed exactly same.

I see no possible advantage of using Arcbody GG on a Flexbody,e even if it is fully compatible. I can't see using Arc slides on Flexbody either, different underlying correction, but it might work OK. Just not sure why mess with what was produced as a closed kit.

There are enough of fully equipped Flex kits available. But surely no money saving by getting a "cheap" Flexbody that is stripped of all or any critical components.

Note that some GG adapters have bubble level built in, others don't. That is because part of Flexbody production had a bubble level built into the body (on top of rise screw, if I remember correctly), others do not have that. If an offered kit is as original it will have one bubble in one of these two places.

Thanks for the info...

I agree that there is no point in making things more complicated, but the Arc Body version 41057 can clearly be used on the Flex Body because I see old comments of people using it. For that matter, the older SWC/M and SWC905 groundglass products (41025 and 41050) can also be used, but they will have problems with the fiewfinder going dark as soon as some tilt is applied. Actually, I thinnk the 41025 may not have an Acute Mattte screen (just a regular groundglass) so it may actually do OK when tilts are applied. But without the tilt adjustment fresnel, all of them will get pretty dark in the side that the tilt is applied. I have a 41050 here for the SWC/M but if I go with a Flex Body, I won't use that on that camera.

The question is whether there are any compromises (or if there is a benefit) to using the Arc Body one on the Flex Body one.

David mentions that the Arc Body uses non-retrofocal lenses whereas the Flex Body is designed for the HB lenses that all are retrofocal in design and that swapping the SWC groundglass for the Flex Body causes the fresnel in those GGs to be converging the light in the wrong spot, causing poor corner illumination. So the same question then applies for the Arc Body, would the GG designed for the SWC GG work effectively on the Arc Body when using the shorter lenses? I somewhat suspect it could, because the 35 and the 45 are both close to the 38 biogon focal length, and while the optical designs are somewhat different between these lenses, they probably have similarly diverging light coming out of them to reach the film plane.

Because of this, I suspect that the fresnel in the Arc Body is not going to work well on the Flex Body and it would be best to be going with the Flex Body version.

One of the reasons I'm trying to suss this out is because there isn't a single kit that I've seen in recent searches that actually has all of the parts... They are all missing one of the film plane masks or more (most don't have any of the film plane masks at all, and almost all of them are missing the 6x6 frame intended to be used when the 4x5 or 6x4.5 masks aren't being used. So as I search, if I find one that has some of these various parts but is incomplete in other respects, I need to understand how big of a deal it is with respect to the missing piece.

Last question... does the Flex Body GG adapter 72405 have a removable/replaceable focus screen like the normal Hasselblad bodies? In the photos I've seen they appear to have a metal frame around the GG that looks a lot like a normal focus screen, so I'm wondering if you can swap in a gridded one or whatever... since they are dealing with the same main lens distance as the normal cameras, I can't imagine why they'd be different such that they aren't compatible.

Michael Mutmansky said:

Last question... does the Flex Body GG adapter 72405 have a removable/replaceable focus screen like the normal Hasselblad bodies? In the photos I've seen they appear to have a metal frame around the GG that looks a lot like a normal focus screen, so I'm wondering if you can swap in a gridded one or whatever... since they are dealing with the same main lens distance as the normal cameras, I can't imagine why they'd be different such that they aren't compatible.

Click to expand...

I have not opened mine to look, especially mine is the latest version, but read it somewhere, screen is NOT swappable with Hass camera screens. I think it's in the Hasselblad Compendium. I've n intention to let screws come off to check though.

Thanks...

One note, looking back at old listings on Ebay, it seems about 1/3 of the Flex Body cameras that include a screen are sold with the Arc Body screen adapter and not the Flex Body screen adapter... That makes me suspect that HB started including that one with the Flex Body after the Arc Body became available.

Michael - you've got a message from me.

I just went on eBay and it does look like there are a number of them with mix and match parts. I also see a few that do have the correct parts, the non user removable grid focus screen in the rear frame.

I have a couple of Flexbodies that are truly fantastic to use and when mounted on the back of a 500 series camera, the rear frames are great for verifying focus on those bodies. It’s worth holding out for one with all the correct parts, especially for how much they are going for these days.

I have used the Ground Glass holder of the Flexbody on my SWC/M and 500C/M without any problems.
I don't know about the Arcbody, but I wouldn't be surprised if there ware a lot of similarities in the dimensions as a lot of the 'standard' Hasselblad accessories do fit on the body, even the Ground Glass holder...

There were two versions of the Flexbody's Ground Glass holder: one with and one without the round Spirit Level.
The first version of the Flexbody had a round Spirit Level built in the knob for adjusting the vertical shift, then it was omitted when the new frame was introduced (for the Arcbody).
As I saw it in my documentation, the frame without the Spirit Level was the original, and when the Arcbody was introduced it had a Spirit Level in the holder, and became the standard for both bodies.
That's why I think these can be swapped effortless (and fit the SWC/M and 500C/M).

BTW, selling an incomplete Flexbody kit is somewhat a crime as it is so beautifully well designed whole!

Philippe-Georges said:

I have used the Ground Glass holder of the Flexbody on my SWC/M and 500C/M without any problems.
I don't know about the Arcbody, but I wouldn't be surprised if there ware a lot of similarities in the dimensions as a lot of the 'standard' Hasselblad accessories do fit on the body, even the Ground Glass holder...

There were two versions of the Flexbody's Ground Glass holder: one with and one without the round Spirit Level.
The first version of the Flexbody had a round Spirit Level built in the knob for adjusting the vertical shift, then it was omitted when the new frame was introduced (for the Arcbody).
As I saw it in my documentation, the frame without the Spirit Level was the original, and when the Arcbody was introduced it had a Spirit Level in the holder, and became the standard for both bodies.
That's why I think these can be swapped effortless (and fit the SWC/M and 500C/M).

BTW, selling an incomplete Flexbody kit is somewhat a crime as it is so beautifully well designed whole!

Click to expand...

This is the point... the two different GG frames (with and without the spirit level) were effectively designed for different optical conditions (possibly) and therefore may not "really" be interchangable (even though they will actually be mechanically compatable). Basically, if the one was made for the FlexBody, the focal length of the fresnel could be approximately 100mm but the one made for the ArcBody COULD (not certain, but COULD) have a focal length much shorter, like in the 30-50mm range because the 35mm and 45mm lenses will have the rear elements much closer to the film plane.

Similarly, the ground glass screen for the SWC/M cameras will likely also have a much shorter focal length to the fresnel lens because of the same reason and in fact, I wouldn't be surprised if it is the same fresnel used in the ArcBody groundglass.

I will be able to test this out a bit in a week or so when I have the FlexBody that I recently purchased. I'll have the groundglass frame for it (the one made for the FlexBody) and I also have one for the SWC/M body, so it'll be easy to compare them to know for sure. I can't test the ArcBody groundglass frame because I don't have one, but, I'd expect it to match the SWC/M because the primary lens for the system was the 45mm Grandagon.

Michael Mutmansky said:

This is the point... the two different GG frames (with and without the spirit level) were effectively designed for different optical conditions (possibly) and therefore may not "really" be interchangable (even though they will actually be mechanically compatable). Basically, if the one was made for the FlexBody, the focal length of the fresnel could be approximately 100mm but the one made for the ArcBody COULD (not certain, but COULD) have a focal length much shorter, like in the 30-50mm range because the 35mm and 45mm lenses will have the rear elements much closer to the film plane.

Similarly, the ground glass screen for the SWC/M cameras will likely also have a much shorter focal length to the fresnel lens because of the same reason and in fact, I wouldn't be surprised if it is the same fresnel used in the ArcBody groundglass.

I will be able to test this out a bit in a week or so when I have the FlexBody that I recently purchased. I'll have the groundglass frame for it (the one made for the FlexBody) and I also have one for the SWC/M body, so it'll be easy to compare them to know for sure. I can't test the ArcBody groundglass frame because I don't have one, but, I'd expect it to match the SWC/M because the primary lens for the system was the 45mm Grandagon.

Click to expand...

There are a few elements in your description I don't understand, perhaps due to my restricted knowledge of optical science.

You speak of "...the focal length of the fresnel...", but as I understood it, and correct me if I am wrong, a Fresnel 'lens' is a purely catadioptric form of a lens, combining refraction and reflection.
So, to my humble insights, a Fresnel lens has no real "focal lengt" but, perhaps, rather a somewhat restricted refraction capability.
But, again plagued by my poor knowledge of optics, the Fresnel lens, as used in the Flexbody, is designed to work with lenses ranging from 50mm till 250mm, which is a rather 'wide' range.
But, this Fresnel works flawlessly on a SWC/M too, which, as you know, is fitted with a 38mm symmetrically made wide angle.
So may I consider that this accessoire can handle very wide angles, just as the Rodenstock 35mm which is the shortest of the tree (the longest being a 75mm which is close to the 80mm Planar), on the Arcbody.

You speak of "...the rear elements much closer to the film plane...", but that is exactly what happens with the Biogon's symmetric construction, hence the specially designed body.

I hope, Michael, that you will understand my confusion, and will enlighten (sic) me a bit more...

Phillippe,

You can think of a fresnel lens like a single-element lens (a singlet), but all the thickness of the cell has been eliminated by making the lens flat and eliminating the majority of the cell material but keeping the surface relationships of the cell constant… here’s a web page that talks about it:



The key to this is that the fresnels do have a focal length and it would need to be optimized for the distance from the ground glass to the lens and also taking into consideration the viewing distance from the ground glass to the viewer or optic in the eyepiece of the prism.

There will be a range of acceptable performance to these, but certainly very long compared to very short will cause considerable differences in corner performance, which is where the fresnel was intended to improve.

I now have the FlexBody in hand with the 72405 focus screen and I also have one of the SWC/M focus screens here (41050). These are clearly different focal lengths as the 41050 has much, much darker corners than the 72405 when mounted on the FlaeBody and with an 80mm lens. I haven't tried all of the lenses, but since most of them are retrofocal, they will all have similar geometries from the rear element to the groundglass, (except the longer focal length lenses) and will likely function similarly.

As I mentioned, I expected this because the 38mm Biogon is very, very different than these other lenses in design.

Since the lenses for the Arc Body is probably much more like the biogon than the normal V series lenses in this aspect, I can't imagine how they are making the fresnel work well for both of these cameras (the ArcBody and the FlexBody) using the same fresnel focal length... I wonder if they made a compromise on this and effectively made them both not really ideal but functional for both when they developed the 41057 ground glass, which was introduced with the ArcBody.

I will try to measure the fresnel focal length tomorrow if the sun is out and report back on both of these.

Mentioning the "darker corners", the widest Rodenstock lenses (for the Arcbody) were sold with a Center Filter added.
But yes, I know that this Center Filter wasn't in the first place meant to have a clear sight when focussing, but above all to produce an equally lightened image negative.
But if I compare it with the Ground Glass/Fresnel for my Silvestri SLV, which was originally designed for the Schneider Super-Angulon 47mm, there the corners are dark, knowing that this system was typically meant to be fitted with an array of extreme-to-very wide angled lenses to be used mainly for architecture (which it does perfectly).
But that camera comes with a very flexibel and good viewfinder (to say the least) which allows a straight, good and clear view 'in' the corners, while pointing the loupe along the light's ray path (1).

And I still wonder what to think of other camera systems, offering a Fresnel lens too and meant to be used with a very wide range of lenses having different angles. I am thinking of Linhof who sells a (good-) Fresnel lens as a separate accessory (2) to be attached 'over' the Ground Glass on the viewer's side.

Is there a Fresnel lens having a 'standard' focus lengt, fitting a wide array of recording lenses? And if so, why wouldn't Hasselblad have used it?

BTW, I think, and please do correct me if I am wrong, that the difference between these two Hasselblad Ground Glass attachments comes from their age of design, business model and progressive insights...


(1)

(2)


PS: anyway, these remarks are coming from a common user, only having some basic knowledge simply acquired by practice in the field and eager to learn as much as possible.

I just measured the apparent focal length of the SWC/M groundglass (41050) fresnel at about 35mm and the apparent FlexBody (72405) at about 55mm, so they are clearly very different in design, with the one being nearly twice the FL of the other. These values could easily be 40mm and 60mm because I was estimating the plane of the fresnel...

-----

Philippe, in a simple fresnel, the focal length will be a single value the sme way a simple spherical lens will have a single focal length. It is possible to have the focal length be varying on the frenel in a similar way that aspheric lens elements are done, or it would even be possible to create some kind of hybrid that literaly has two focal lengths, where each of the radial rings in the fresnel alternates to a differnt focal length.

However, I doubt that is done because I'm not sure it will be terribly beneficial. Maybe someone who knows better could speak to that.

I do know that there is a range of "beneficial" light modification that can occur even if the focal length doesn't exactly match the lens you are pairing it with, so for a lot of large format cameras, the focal length of the fresnel was probably selected to provide the best (most broadly beneficial) focal length when considering the range of lenses that you may put on the camera (based on bellows, I guess?). So if you can handle from a 65mm to 300mm on the camera, selecting something like a 100mm will probably benefit the wide angle lenses and still also benefit the longer focal lengths some (or more likely, not penalize the longer focal lengths too much, since they are not going to be to terribly bad without the fresnel).

However, this will be a balance of benefits and drawbacks and some situations, you may have worse performance to allow better performance in other situations. Wide angle lenses are the worst for this problem of needing corner falloff mitigated, so I'm guessing they favored those lenses when doing the design of the fresnel (when choosing the focal length).

I did mention that it would be possible to have two focal lengths in a fresnel (or more, actually), and it is possible that some designers did that so that no single focal length benefitted at the expense of other focal lengths. For example make one that is a 75mm and 175mm fresnel so that half the light on a wide angle lens would be better focused to bring up the corners and half the light on a longer lens would do the same and when the one is optimized, the other is just defocusing the light but it is never fully a positive or a negative so it broadens the usefulness of the fresnel. I'd be interested in seeing if that was actually done for LF cameras since the wide range of focal lengths would make it hard to find a good single one to work best for all applications, or if that ends up being horrible to work with.

Thank you, Michael, for the very extended explanation!

An insight how the tool you are regularly using works, brings some peace of mind and helps for what really counts: picture taking!
To my personal opinion, technicalities in photography are only something you must carry at the very bottom of your backpack, just in case you might need it, and yet not negligible...