hood size for hasselblad planar 80 (bay50)?

[icono]

Hi all.

I seem to have a slight problem - i wanted to buy a shade hood form hasselblad planar 80 mm chrome (bay50), this one - http://cgi.ebay.com/Hood-Sun-Shade-for-Hasselblad-80mm-f-2-8-C-T-Lens_W0QQitemZ390087419447QQcmdZViewItemQQptZLens_Accessories?hash=item5ad305a637#ht_6055wt_1166.
But accidentally i bought the wrong one - for 100-250mm... yeah. So now i'm thinking of "trimming" it a bit, as it's made of plastic. The problem is that i don't know the right sizes. What i need to know is the lengh of hood's side:

Does anyone know?))

BTW, what if using hood for 100-2500mm for 80mm lenses? what's the result? Vignetting?

 

[Q.G.]

Sure!
37 mm, from one end to the other. The hood itself, i.e. without the mounting ring) is 32 mm.

You could try the longer hood and find out. But yes, it will vignet, more or less visible depending on focussing distance and aperture.

 

[BenZucker]

From the lip of the hood to the base of the mount ring it is 49mm. I am using a bay60 hood because i have a CF 80......I'm not sure if this is different for the bay 50....

 

[Q.G.]

It is different indeed, because the bayonet 50 is less wide. So the bayonet 50 hoods have to be shorter.

 

[icono]

ok, thanks. i decided to "cut" it eventually)) though it seems to be not that easy..

 

[lxdude]

Remember both height and width factor in.

I cut down a petal type hood once to fit the angle of view of a wider lens. I set the lens at its smallest aperture, then stopped it down and looked through the finder. The edges of the hood were visible. I estimated how much to take off and took off a little less. By knowing how much coverage your finder gives, you can figure out pretty well how much additional to take off after you see no more vignetting. It's not much. I checked it again and removed some more until I was satisfied that it was not vignetting. A 6 inch metal ruler came in handy to estimate by eye how much to take off.
Be conservative. When close, it's easiest to remove material with sandpaper, by affixing it to a flat surface and moving the hood back and forth on it. That also allows small corrections as needed. It's a good idea to cover the outside of the hood with tape so it can't get scratched accidentally during the process. Remove the burrs along the edge with medium fine sandpaper wrapped around a flat piece of wood or metal. I think wood is better. Fine sandpaper finishes it off and makes it look good.

Check with test shots, or just shoot and allow for a little possible vignetting. Take off a little more if needed.

 

[icono]

ok, thanks for very detailed comment.
i tried your "method" and i didn't see hood's edges in the finder at all. does that mean i do not need cutting it?))
i tried all apertures with different focus legnths.. but still no sign of vignetting.. don't really understand

 

[lxdude]

Interesting. I don't know what amount of the picture area on film is not visible on the focusing screen. 80mm and 100mm are not far apart, so I guess if some of the image does not show on the screen the vignetting could just be not visible.
A quick test to see would be to hold something at the edge of the hood and then move it toward the center and note how far you have to move it before you see it. I don't think it will be much.

The best way to know for sure will be to take a test shot with the hood on the lens and one without it. Then compare the images to see how much vignetting you have. It might be obvious or it might be subtle. If it's not obvious, look for any difference in brightness on the outside of the image, which will indicate vignetting. If there's not much then trim only a little bit. It might take a couple of times to get it right.

Something else: sometimes non-manufacturer items are not really precise. So that could mean that it doesn't vignette where the actual Hasselblad item would.

 

[Q.G.]

The finder, with diffusing focussing screen, is not the right place to check for vignetting.
Take the back off the camera, set the shutter to B, and press the release and keep it that way.
Then you can look through the back. The thing to do is put your eye in the extreme corners of the film gate, looking from there at the aperture to see if the hood becomes visible or not.

Try it with the lens wide open, and set to infinity.

But why do it the hard way? Just use the dimensions of the original hood.


I found it very easy to cut the chinese copies. Rather soft plastic.
I used a hobby knife, and scored the line along a steel ruler. After the first go, the cut was deep enough not to need the ruler anymore. Just a few cuts and the knife was through.
Then rotate the hood 90 degrees and tackle the next side.

I don't think it necessary to be too precise. An extra 0.1 mm removed too much doesn't make a difference. An extra 1 mm too much will, but only a very small one.

The original Hasselblad hoods were a lot harder, and took quite a lot longer to cut to size.

 

[lxdude]

The finder, with diffusing focussing screen, is not the right place to check for vignetting.
Take the back off the camera, set the shutter to B, and press the release and keep it that way.
Then you can look through the back. The thing to do is put your eye in the extreme corners of the film gate, looking from there at the aperture to see if the hood becomes visible or not.

Try it with the lens wide open, and set to infinity.

I always thought that wasn't reliable, due to the image of the hood going through the lens optics. Maybe I'm wrong.

But why do it the hard way? Just use the dimensions of the original hood.

Which is the way to do it if the width is the same as the original. If the OP knows it is, that's the way to do it.

 

[Q.G.]

I always thought that wasn't reliable, due to the image of the hood going through the lens optics. Maybe I'm wrong.

You are.

You really are.
That (the image going through the lens) is exactly the reason why it is the only reliable way of checking for vignetting.
Think about it: vignetting is bad, because it leaves its permanent mark on film. But how does that end up on film, if not "through the lens optics"?

 

[lxdude]

You are.

You really are.
That (the image going through the lens) is exactly the reason why it is the only reliable way of checking for vignetting.
Think about it: vignetting is bad, because it leaves its permanent mark on film. But how does that end up on film, if not "through the lens optics"?

The difference being that it's an aerial image. The image on the retina is at a different distance than the film plane. Viewing through the back of the lens does not produce the same image to the eye as it does with a diffuser at the film plane. Optics can displace an image of an object from where it actually is.

When you look through the back of a lens, do you see the same image as what ends up on the film? Nope.

 

[Q.G.]

Indeed.
And what that means is that you do not see a vignetted image, but the thing that is causing the vignetting.

It really is the only reliable way (short of exposing some film, processing that film, and inspecting the result).
That's why, for instance, the corners of viewing screens on LF view cameras are clipped. So you can see through the resulting holes towards the aperture of the lens. To see if you get vignetting.