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 p.1 #3 · Step ring + cardboard DIY lens hood | |
First, here's some general background info:
http://www.vanwalree.com/optics/lenshood.html
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Rectangular lens hood dimensions ###:
note: This part came from some Zeiss-lover site. My apologies as I didn't save the url, so can't give proper credit:
Contax designed their lens hoods to be used with only one filter at any one time. As these hoods are cylindrical they only work at maximum effectiveness at the corners of the rectangular film plane with other areas being less effective. Yashica used to provide rectangular lens hoods for their 'ML' and earlier lens range. These were attached to the lens body by a neat clamp. These were very effective and were beautifully made.
Problem - How does one get a simple SINGLE rectangular hood shade effective for 21, 28, 35, 45 and 90mm lenses without spending a fortune on a bellows hood which would anyway screw up the auto-focusing?
Contax/Kyocera have already done all the computing for you by (hopefully) optimally fixing the lengths of the three hoods for the three lenses, 28, 45 & 90mm (see below for 35mm lens). All one has to do is buy a Contax hood cap (GK54) or better and a lot cheaper generic plastic 57mm cap and cut a rectangular hole in it.
The ratios are- 2 : 3 : 3.6 (H : W : Diagonal)
So for a 55mm diagonal, the height of the hole is 30.5mm and the width is 45.8mm.
Cut a rectangle (H = 30.5mm W = 45.8mm) of stiff paper and using this as a template, mark out your plastic hood (you are practising first aren't you?). Start cutting with your knife taking all possible precautions against removing digits, flaps of skin etc. Remember to cut slightly too small a hole and then gradually enlarge up to your marks if you are in doubt. Chamfer the outer edges of the hole very slightly.
Mark the top centre with a dab of red marker ink so that you can always correctly align it with the black mark just in front of the aperture ring. Paint the outside to a colour of your choice and check that the inside surface of the cap is painted black before use.
Slip your new hood cap onto the front of the hood, align and voila even more protection for your lens from stray light. It works with all five prime lenses (not including the Hologon) using optimally one filter or filter ring in place and the correct Contax lens hood or just one filter in the case of the 21mm lens.
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MAKING YOUR OWN RECTANGULAR HOOD: (my personal ideas)
NOTE: This requires that you know the optimum minimum width & depth for your lens, which can theoretically be taken from the stock round hood. A little research should turn up these numbers. The round hood will theoretically be correct for the far left/right, but way too open for the top/bottom.
for lenses with unknown hood needs, use an existing rectangular, short, and shim some cardboard until you find the vignetting points at all apertures.
The following is for the Rokkor 58 / 1.2 It has 55mm threads, and the factory round hood is 73mm X 24mm deep.
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Needed:
A good disc-belt-sander, maybe with a jig to hold the cap 90-degrees vertical.
Get a 55mm-77mm step-up ring.
Get a generic plastic hood, 77mm.
Tightly attach the step-ring to the hood. Possibly even epoxy or super-glue them together.
Thread the step ring/hood combo onto your lens until snug. Mark the point on the ring & front of hood that ends up on top.
Sand-down the front of the hood until the length of this combine ring/hood is 24mm.
Put the hood against the disc-sander, front-down to maintain a 90-degree cut, and sand its sides & top/bottom until the front is effectively 73mm X 48.66mm.
Epoxy some stiff plastic over the open areas, so that the front actually becomes a rectangle.
Sand the glue-lines smooth, then spray the whole thing with black plastic-paint.
For pizazz, add thin black felt on the inside, almost to the front edge, like Canon.
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The ratios are- 2 : 3 : 3.6 (H : W : Diagonal) And we are interested in the width vs the diagonal, hence 3 : 3.6 with the "3" being our desired final rectangle width, and the "3.6" being the required diameter of the round good we start with. -That gives a required, starting round minimum diameter of:
87.6mm
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from: http://www.vanwalree.com/optics/lenshood.html
Testing A Hood
A simple, practical approach to determine whether a certain hood (or filters, or a combination) causes vignetting on a certain lens consists of a series of test exposures. The subject should be an evenly illuminated object at a large distance. Vignetting is less of a problem at close range than it is at infinity, so when infinity poses no problems a nearby subject is also safe. A brick wall on an overcast day will do fine.
Four exposures are required, two at the lens full aperture (with and without hood) and two at the smallest aperture (again with and without hood). Slide film is preferred because of its short dynamic range, which will reveal vignetting more convincingly than print film. If the slides taken with the extension(s) show no additional corner darkening in comparison with the slides taken without, you are completely safe.
If there is a slightly increased, gradual corner darkening at full aperture, you probably won't notice the presence of the hood with other subjects than a brick wall or a blue sky and you are also safe. Black corners however are generally considered gruesome and the extension (or combination of extensions) is just not suited for the lens.
Edited on Aug 28, 2009 at 06:54 PM · View previous versions
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