Lens design

I figured I wanted to try a m50 f1.7 on the k-5 for some astro shots.
Turned out it was not the best idea. The lens picks up the slightest light as massive flaring.

This got me thinking.
Using the original lens hood will not be very effective, so it would be better if I designed a new one instead.

I was not able to find the original lens design information for the lens, so maybe O'Connell's work could be sufficant for it.

Anyone able to correct me on this base data, is something wrong?




Most likely I will do a step-down before the actual lens hood will come.
How will it be when thinking about flare to lets say: 67->49mm -> (CPL) -> further step down -> hood.

Back to the drawing board.

The data from O'Connell can not be used for this, it is simply not correct.

Any suggestions on how to do this?

For starters I want to find the maximum step-down before vignetting.
What can I use to achieve this?
A piece of paper, made black by coal on both sides, with a circular hole of different sizes does not seem to do the trick. (For some reason it is almost like I can see through the paper...)
What aperture and focal length would be most ideal to spot the vignetting creeping in?

The second part will be a tulip shaped clip-on hood I will get molded.
But for this I need really good data. Original lens design prefered.

Maybe there are other people who might have found other ways to simplify this process

I think I'd go back to the very start with this and look at the lens. M lenses don't flare as a rule, so just maybe someone prior to yourself has been messing with it.

As for lens hoods, you can get copies of the proper Pentax hood on eBay for a fraction of the cost. Or a second hand Pentax one for slightly more.

Although the concept of always using a hood is sound, the lens on its own should not flare so badly as to be a serious problem.

Use black card from a craft shop to make a prototype. You may find that a standard food tin will be about the right size and shape, so make your prototype that shape and size (about 74mm diameter by 105mm high) The lens focussed at infinity and set to the minimum aperture will show up any vignetting. The sky in daytime is a good enough target for this.

You might find that the end diameter is a bit big, so you can make a circle to fit on the front.

Once this is proved to work, you are best off with a can that has a pressed on lid at both ends. Remove one end in the normal manner, and fit to it a step up ring or combination of step up rings, where the smallest male thread screws into your lens, and the biggest female thread is just big enough to accept the open end of the can.

Glue it all together. Once it's set and the rings are holding it in shape, remove the other end with the can opener. Remove all burrs and sharp bits as you go along. Paint it with blackboard paint after degreasing thoroughly. You could also use, I suppose, matt black car paint. A coat of spray primer first will make the spray paint more durable. Mask the threads that go into your lens before you start painting.

If you want to put a circle to go in the front, make it to fit the hole, with a row of tags all the way round to fold over at 90 degrees to give you something to glue to on the inside of the can. Use card or plastic card. I wouldn't bother with a petal ended or rectangular ended hood, I don't think it will make that much difference and you have to make sure that it lines up with the edges of the frame.

Having said all this, I did the above, but with one of those cheap 500mm mirror lenses that you see everywhere under various names. It looked great, was solid and did cut out some flare from the very exposed front element. However, I think I was getting flare from light reflected off the sensor, bouncing off the rear (uncoated?) element and back to the sensor again. Your 50mm lens should be better coated, so good luck.

Umm, why are you putting a CPL on the lens for Astro shots?

I only mentioned the m50 as an example. I will do the same for different 135mm primes aswell, and those I might use for different applications.
Imagine street photography, and instead of people trying to dodge me, as they see me point a big arse lens in their face, they are struck by curiosity. Simply because I stick a small old 28mm prime, with step down ring, and a cute tiny 30mm tulip on the end


@riley, Here is an example of my issue with the m50.


Original hoods are based loosely on the 135mm standard, and I say loosely because often they are not even ideal for that purpose

@Spotmetre, You will have to give me some time to understand that post

There seems to be general light pollution with this shot. It has that yellow tinge from the streetlights, reflected back from muck in the atmosphere. I think that the yellow smears on the bottom and up the left hand side are clouds. About all you can do is adjust your exposure until the sky becomes black, and avoid clouds. Or find somewhere with less light pollution.

The two curved lines are probably flare, perhaps from a couple of streetlights a little way off and about 45 degrees apart from the camera position. It might be easier to stand in the way of the light from the streetlights, or hold a black card to shade the lens.

What seems to be a very small amount of light can cause flare; compared to the light from the stars it will be very bright.

Depending on the length of exposure I wouldn't call that flare, it looks more like speading light pollution. Because the exposure is presumably quite long, the small variation in brightness in those areas may have been exagerated until it becomes an overwhelming smear.

The crossed lines look like they might be flare from a point light source. I would check that the lens is scrupulously clean. After all, smears on our glasses can so easily cause flare and camera lenses are no different in this respect.

I just got handed to me the solution by my 4 year old niece!

It was basically to roll some cardboard together to form a big enough circle.
Actually many circles in circles, so I only needed one copy.

Screw the entire thing on a lens like it was a lens hood, then take a test shot with 25mm opening, if I spoted vignetting, remove the inner most circle and I now had a 27mm opening etc. Continue until I spot the vignetting disappear at f-max and infinity.

I got some big suprises and I am curious if this sounds just too far fetched.
Pentax K200 f2.5 have a hood thread of 77mm, the front element is about 76mm, and vignetting stopped around 32mm

Considering this, would a 85 f1.2 end up with 33mm, and a 55 f1.2 with 28mm?

Only the wide ones showed much of high numbers. Like 21mm, 24mm and 28mm lenses that went up toward 37mm.


Now to the question, where to find suitable step-down rings?
(I am way to lazy to actually mold them as well.)

@Spotmetre, I have found a better spot now. That area had two street lights, might explain the lighting. It was also in the middle of the city!

@John, The lens was mint, and with 8sec exposure (no tracker..). I was however not actually trying to shade the lens from incoming light though.