4x5 internal flare

dslater · Aug 31, 2007

Hi all,
A while I took some close-up pictures of a black raspberry bush with my Fujinon 300mm F/8.5 lens. I wasn't really in macro range, but I was close enough that my bellows were extended to about 450mm. I took several pics some color and some B&W. When I developed the film, all the pictures have sever flare in the center of the image. I'm quite sure it's not a light leak. I think what is happening is that the 300 lens has such a large image circle, that light is reflecting off the inside of my bellows and onto my film.
Any ideas on what I can do about this? Is there something I can use to safely darken the inside of my bellows? Another thought I had was that I could attach some kind of black shroud to the back of the lens as a baffle to block the edges of the image circle. Does this sound doable?

Dan

Steve Goldstein · Aug 31, 2007

Did you use a lens shade? With that much extension you could get away with quite a narrow one and prevent most of the non-image-forming light from getting inside in the first place.

steve

dslater · Aug 31, 2007

Steve Goldstein said:
Did you use a lens shade? With that much extension you could get away with quite a narrow one and prevent most of the non-image-forming light from getting inside in the first place.

steve

Hi Steve,
The problem is not non-image forming light - it's that the image circle is so large, that the image forming light is hitting the inside of my bellows and reflecting off the bellows onto the film.

Dan

David A. Goldfarb · Aug 31, 2007

That would be non-image forming light--light that is part of the excess image circle beyond what you want to hit the film that instead reflects off the bellows to produce bellows flare. The solution is to shade the lens properly, ideally with a compendium shade, and probably a mask if you're using a 300mm lens on 4x5" focused as closely as you describe.

That said, bellows flare usually shows up as additional edge density, rather than in the center, so it could be something else. I'd start by using a sufficient lens shade, and work from there.

dslater · Aug 31, 2007

David A. Goldfarb said:
That would be non-image forming light--light that is part of the excess image circle beyond what you want to hit the film that instead reflects off the bellows to produce bellows flare. The solution is to shade the lens properly, ideally with a compendium shade, and probably a mask if you're using a 300mm lens on 4x5" focused as closely as you describe.

That said, bellows flare usually shows up as additional edge density, rather than in the center, so it could be something else. I'd start by using a sufficient lens shade, and work from there.

I don't see how a lens shade is going to help - the sun was behind me and light was not falling on the lens elements as I was shading the camera with my body.

What I need is a way to reduce the size of the image circle.

BTW - I consider non-image forming light to be the light that reflects off internal elements of the lens and is not focussed. Just because part of the image circle is outside the film area doesn't mean it isn't forming an image - you're just not recording the image.

Nick Zentena · Aug 31, 2007

dslater said:
I don't see how a lens shade is going to help - the sun was behind me and light was not falling on the lens elements as I was shading the camera with my body.

What I need is a way to reduce the size of the image circle.
.

Have you ever put a filter on a lens and had it vignette? A shade is like that. At least a shade that's adjusted to the format and the lens.

Dan Fromm · Aug 31, 2007

dslater, glancing reflections off the inside of the bellows -- good heavens, is the thing shiny? -- don't form images.

I've had this problem with a mount Steve Grimes made for me to hang a 12"/4 TTH telephoto entirely in front of a Graphic board. This because the back of the lens is too fat to pass through the front standard. The mount screwed into the back of the lens, the board was held on the back of the mount by a retaining ring. Yes, stepped tube. As delivered, the mount had a nice shiny anodized interior. This yielded a bright spot in the center of the frame as you described. I sent the mount back to Steve and he cut threads, as for mounting a filter, on the inside. This solved the problem.

So there's a solution that isn't easy enough to implement. No, don't cut threads on the inside of the bellows or even put baffles inside.

Mount the lens on a baffled tube in front of the board. Or put a baffle on the back of the lens, to vignette most of the cone of rays the lens projects.

David A. Goldfarb · Aug 31, 2007

The compendium (bellows) shade is designed to vignette the excess image circle (which is generally referred to as non-image light, because it's light that isn't striking the film) and prevents this excess light from bouncing around inside the camera. You extend the shade until it is just outside the image area, and if the shade isn't long enough, you can put a mask on the front standard to vignette the image further. Some shades are designed with adjustable masks to make this easy.

How do you know when the shade is extended adequately? If you have a groundglass with cut corners, you can sight the shade through the corners, or if not, you can look through the front of the lens and see if you can see the corners of the groundglass with the shade extended and adjust the shade so that you can just see the corners of the glass.

This isn't just a large format issue. If you do close up photography with small formats, a compendium shade will also improve contrast by reducing the amount of non-image forming light reflecting inside the camera.

dslater · Aug 31, 2007

David A. Goldfarb said:
The compendium (bellows) shade is designed to vignette the excess image circle (which is generally referred to as non-image light, because it's light that isn't striking the film) and prevents this excess light from bouncing around inside the camera. You extend the shade until it is just outside the image area, and if the shade isn't long enough, you can put a mask on the front standard to vignette the image further. Some shades are designed with adjustable masks to make this easy.

How do you know when the shade is extended adequately? If you have a groundglass with cut corners, you can sight the shade through the corners, or if not, you can look through the front of the lens and see if you can see the corners of the groundglass with the shade extended and adjust the shade so that you can just see the corners of the glass.

This isn't just a large format issue. If you do close up photography with small formats, a compendium shade will also improve contrast by reducing the amount of non-image forming light reflecting inside the camera.

O.K. - now I see - I've never used a lens shade in this way before - looks like I'll have to find a compendium shade. In the meantime, I can work around this problem by using a shorter lens with a smaller film format.

4x5 internal flare

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4x5 internal flare

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