Stupid question about lenses, vignetting, groundglass illumination

[PaulSchneider]

Hi guys,

I'm new to LF and just got a 8x10 LF camera and a 240 Sironar-S lens.

Today, for the first time, I looked at the groundglass with the lens and saw that the image is well illuminated only in the center. At the edges it gets quite dark.

Everything was set up correclty, no movements such as rise or fill, I realize that I don't have too much room with the 240 ...

When shooting a sheet of 8x10 film, how come the image will be well lit overall whereas on the groundglass only the center is really getting much light?

Is there a difference between what one sees on a groundglass in terms of vignetting than what one will get on film?

I know this might be an easy answer, but I'm new to this kind of photography, i.e. working on a groundglass ...

Kind regards

Paul

[Louie Powell]

Paul -

Welcome to the crazy world of LF.

A 240mm lens is a wide angle in 8x10 - the diagonal of the 8x10 image frame is 12.8 inches, so the 'ideal' normal lens would be 325mm.

There is always light loss at the edges of the image frame - even in 35mm. Remember the inverse-square law - light loses intensity as the distance between image (source) and film increases. With LF, this is more of an issue, and it's not uncommon for there the image in the ground glass to be dimmer at the edges of the frame. That's something that we deal with by using a dark cloth, and simply by sucking it up and living with it.

A greater concern is when that loss of light translates into a loss of exposure on the film. Obviously, it happens in every case, but its not always noticeable, and good printers tend to use a bit of edge burn both to increase separation at the edges and also to compensate for this effect. (It happens in enlarging also.) But when you are using a wider angle lens in LF, that reduction is exposure is sometimes significant. Architectural photographers often use 'center filters' to compensate for this effect. A center filter is a graduated neutral density filter that is denser in the middle, and gradually fades to clear around the edges. Don't rush out to buy one until you conclude that it is really necessary - they are expensive.

The other consideration is the coverage of the lens. The image circle thrown by the lens at the typical lens-film plane distance must exceed the diagonal dimension of the frame in order to avoid vignetting. Your lens has a 372mm image circle which is only slightly greater than the diagonal of the film plane. That means that the vignetting that you get with the camera controls zeroed out may be negligible, you you won't have much margin for rise, fall, swing, tilt, etc. The point is that vignetting and loss of light at the edges are really two different things - loss of light is simply the inverse-square law at work, while vignetting is where the image circle is too small, and the front of the lens is actually shading one or more corners of the image frame.

It's hard to differentiate visually between true vignetting and simply the loss of light at the edges. One trick is to look into the lens with no film-holder in the camera - if you can see all four corners of the ground glass, then you won't have any true vignetting. Of course you still could have noticeable darkening at the edges.

[Leigh]

There are a couple of points to consider.

Rodenstock recommends your lens for use on 5x7 format maximum.

The Rodenstock literature shows an image circle of 372mm, as Louie said, but that's at f/22. When you use the lens wide open that diameter decreases dramatically. This probably explains the difference you're seeing between the ground glass image and the film image.

I can't find published specs for the Sironar-S wide open, but as an example the comparable Nikkor W 240/5.6 has an image circle of 336mm @ f/22 but only 278mm wide open. That's a big difference, considering the diagonal of your 8x10 film is 325mm, which defines the minimum image circle to fully cover the film.

The full-open aperture is available primarily as an aid to focusing. The recommended taking aperture for your lens is f/16 to f/32.

Center filters are not appropriate for this focal length. They're used in situations where the angle of view exceeds roughly 100°. Your Sironar-S has an angle of view of about 75° at taking aperture, less when wide open.

- Leigh

[engl]

When shooting a sheet of 8x10 film, how come the image will be well lit overall whereas on the groundglass only the center is really getting much light?

Is there a difference between what one sees on a groundglass in terms of vignetting than what one will get on film?



The ground glass, unlike the film, works by diffusing the incoming light from the lens. This diffusion is not equal in all directions, most of it ends up going along or near the axis of the incoming light. If you position your head so that you have the corner of the GG between you and the lens, you will notice that the corner is a lot brighter than when you had your head in front of the GG center.

If you were to put a matte white sheet of paper, which reflects in all directions, in the place of the GG and looked from inside the camera, the image would look just like it is captured by the film. The corners would still be darker than the center due to vignetting, but this depends on the image created by the lens and will be visible in the final image as well.

What camera do you have? A modern ground glass is brighter than an old one, and you can also add a fresnel lens to minimize the "dark corners" effect on the GG.

Edit: Apart from this, as the others have pointed out, there is also vignetting due to the lens. When you focus and compose you likely have the lens wide open resulting in heavy vignetting of the image as created by the lens. When you stop down to take the picture, vignetting is much reduced. The corners are not made brighter by stopping down, but they are made brighter relative to the center of the frame, resulting in an image with less vignetting.

[PaulSchneider]

Wow, thank you Louie and Leigh,

that explains it all, I mean I forgot that there's a big difference between wide open and stopped down. I actually wouldn't shoot at 5.6 anyway ... but to get a family portrait in an interior setting means it won't be easy at f16 ...

brave new world!

[PaulSchneider]

engl: I have a 8x10 chamonix ... so at what aperture do you think will one not notice the light falloff too much? I wonder whether f8 is already ok ...

[Leigh]

... but to get a family portrait in an interior setting means it won't be easy at f16 ...
That's why they invented studio strobes.

LF cameras don't have hotshoes. :rolleyes:

- Leigh

[Mark Sawyer]

As Engl noted, the bright center of the ground glass is just a hot spot. The shorter the focal length, the more you get that hot spot. If you move your head around under the dark cloth, you'll notice the hot spot moves a bit.

Vignetting is a different phenomenon than fall-off. In fall-off, the corners are darker than the center because they are farther from the lens, but unless you go really wide on 8x10 (150mm or shorter), it's not an issue. Vignetting is when the lens simply does not deliver light to the corners, usually because the mechanics of the lens blocks it.

The 240mm Sironar-S is a modern plasmat with a lot of coverage. It won't vignette on 8x10 at any f/stop, even when focused at infinity, where the coverage is smallest, and it will even have a bit of coverage left over for movements. I use a 215mm plasmat on 8x10 without any issues. BTW, your lens is also convertible to a longer focal length if you remove the front element, but the f/stop scale will change.

[Jack Dahlgren]

Wow, thank you Louie and Leigh,

that explains it all, I mean I forgot that there's a big difference between wide open and stopped down. I actually wouldn't shoot at 5.6 anyway ... but to get a family portrait in an interior setting means it won't be easy at f16 ...

brave new world!

An 8x10 family portrait indoors will require lighting/flash unless they are in a very bright space or are working in a non-traditional mode. The traditional expectation is that everyone is pretty much in focus and not blurred. You may need stopping down more than f/16 if the group is large. Remember that this lens is also wide angle and you know what happens to people at the edges of wide angle images. They appear stretched...

[PaulSchneider]

yes but strobes change the look totally ... i prefer the natural light look of the struth portraits for example, take this image, what aperture do you think this was made at? It is a 240mm lens with portra 400 inside an office. I do not have enough experience see from the dof what f stop it is ...

http://www.flickr.com/photos/thomashobbs/5280705782/

[PaulSchneider]

Mark, so event at f5.6 this lens won't vignette?

[Mark Sawyer]

Nope, it won't. You're safe! (Unless you start stacking filters or a narrow lens hood on the front.)

[Mark Sawyer]

BTW, your ground glass may have the corners clipped. That's so you can peek through and see that area is still getting light from the lens, which becomes an issue when you start cranking in movements, especially swing and tilt on the front standard.

[Jack Dahlgren]

yes but strobes change the look totally ... i prefer the natural light look of the struth portraits for example, take this image, what aperture do you think this was made at? It is a 240mm lens with portra 400 inside an office. I do not have enough experience see from the dof what f stop it is ...

http://www.flickr.com/photos/thomashobbs/5280705782/

Take a close look at the plane of focus. Only her face and a tiny bit on the wall is in focus. This is OK for a photo of a single person, but not for a group of people unless you manage to get all their faces in the same plane. It also appears to be a fairly well lit working space - note the shadows under the work surface which are cast by bright lights above and behind the photographer.

A normal home would not be lit this brightly. You could easily light a studio like this though. But the main thing is that the DOF in this portrait is very small. If you had multiple people in this shot, at least some of them would be out of focus.

[engl]

yes but strobes change the look totally ... i prefer the natural light look of the struth portraits for example, take this image, what aperture do you think this was made at? It is a 240mm lens with portra 400 inside an office. I do not have enough experience see from the dof what f stop it is ...

http://www.flickr.com/photos/thomashobbs/5280705782/

I do not know which F stop was used, tilt was used, to have the wall upper wall, woman and foreground somewhat sharp. The OOF parts are not very blurred (as wide open 8x10 usually is) so I'm guessing a fair bit stopped down, F11-F16?

Anyway, the exposure time might have been very long for this shot, at least compared to how portraits are generally shot on smaller formats. With a DSLR you might consider 1/30 slow, but there are people shooting LF portraits with 30 second exposures :)

[Bob Salomon]

You apparently do not have a fresnel on your camera. Add one and the brightness will even out. Make sure it is a fresnel for cameras though. The 240 S fully covers 810. So does the 210s. But none were designed for optimal results wide open.

[PaulSchneider]

@bob: thank you for the comment. would i replace the groundglass with a new fresnel groundglass or would I put it in front of the groundglass?

any fresnel lens brand comes to mind, that would fit the chamonix?

@engl: now i start to realize why it is so tricky to get good pictures ... 8x10 is so expensive, no room for dslresque trial and error ...

@jack: yes, you're right, i'm not sure this is a good idea to try to photograph a family in an interior room ... but how on earth does one get such a dof without flash in an interior room:

http://blog.richardtugwell.com/index.cfm/2010/6/21/Thomas-Struth--Kunsthaus-Zrich

...

[engl]

@jack: yes, you're right, i'm not sure this is a good idea to try to photograph a family in an interior room ... but how on earth does one get such a dof without flash in an interior room:

http://blog.richardtugwell.com/index.cfm/2010/6/21/Thomas-Struth--Kunsthaus-Zrich

...

Those shots are done with flash. Notice in the first picture, far right and far left, behind the the people, the light level is much lower and color temperature is different.

Also, I'm not familiar with this artist, do you know if he used 8x10 for all his work?

[PaulSchneider]

@engl: http://www.vimeo.com/7098979

Thomas Struth is part of the Düsseldorf School of Photography together with Andreas Gursky, Candida Höfer and Thomas Ruff. He uses a Phil. Compact II in 8x10 and a Technikardan in 4x5.

His prints are valued in the hundreds of thousands ... crazy market!

[Bob Salomon]

You would put it behind the GG so it is closest to your eye. With the smooth side towards your eye and the grooves facing the gg. If you put it in front you may create a focus shift since it might displace the gg frosted side which is what the focused image is formed on.

@bob: thank you for the comment. would i replace the groundglass with a new fresnel groundglass or would I put it in front of the groundglass?

any fresnel lens brand comes to mind, that would fit the chamonix?

@engl: now i start to realize why it is so tricky to get good pictures ... 8x10 is so expensive, no room for dslresque trial and error ...

@jack: yes, you're right, i'm not sure this is a good idea to try to photograph a family in an interior room ... but how on earth does one get such a dof without flash in an interior room:

http://blog.richardtugwell.com/index.cfm/2010/6/21/Thomas-Struth--Kunsthaus-Zrich

...

[Leigh]

yes but strobes change the look totally ...
It doesn't have to. It's all in how you use the lights. It can look every bit as natural as sunlight through a window.

- Leigh

[Leigh]

BTW, your ground glass may have the corners clipped. That's so you can peek through and see that area is still getting light from the lens...
Actually the corner bevels are just to permit free airflow as you adjust the bellows.

- Leigh

[Gem Singer]

Leigh,

Allowing the air to enter or escape the bellows, when it is expanded and contracted, is only one function of the clipped corners of the ground glass.

The most handy function is allowing the photographer to look into the camera through a clipped corner opening and check to see if the lens is stopped down sufficiently to cover the format, thus preventing vignetting.

When looking at the lens opening wide opened, the aperture appears to be football shaped.

As the aperture is closed down the football shape becomes round (the shape of the lens's iris) indicating that the aperture has been closed down to a point where vignetting no longer occurs.

[Noah A]

The Apo-Sironar S 240mm was my favorite lens when I was shooting 8x10. It should serve your needs very well.

I didn't hesitate to shoot it at fairly wide apertures, maybe even f/8-11, for interior portraits. Often with 400NC that would give me usable shutter speeds. I don't have any of that work online, but maybe I'll re-scan some of it and post it.

I was often doing portraits in the range of 1/8th of a second to 1 second.

I was doing single portraits though, not families. That would be tougher.

I know it's expensive, but you will just need to do some experimentation. Perhaps you could get some cheap B&W film, or even use paper negs, for some tests...

I've actually never used a camera without a fresnel, but I imagine one would help. Still, the darkened corners on the groundglass don't necessarily correspond to what you'll get on film. When shooting my 80mm lens on my 4x5, the corners are very, very dark on the GG. And while there is falloff with that lens, it's not nearly as bad as the GG would have one believe, even with a fresnel.

You also may need to move your head around...if you look at the corners of the frame from an angle you can sometimes get a brighter view. And the clipped corners are very helpful for checking coverage.

There's a difference between natural light falloff and mechanical vignetting. So it's definitely smart to check for the latter by looking through the clipped corners of the GG to see if the aperture is round. The 240 doesn't allow for a ton of movements, but I found it offered enough coverage for most uses.

[DaveTheWalker]

Wow, this thread explains a lot! I was just doing a search to try to find out whether the pronounced bright spot was just down to my setup or more universal. Thanks to all for the explanations.

Time to sort out a decent dark cloth...