Need help focusing using only DLSR

[emagine]

Hi everyone. This is my first post on the here. I've been trying some astrophotography using only a Nikon D80 (18-135mm DX) for the past few months. All the time I've been shooting at 18mm, but recently (after fixing some problem with my barn-door tracker) I have been trying to get some shots at 135mm. Using manual focus involves great trial and error and is really time-consuming. I just need to know how reliable it actually is, to use auto-focus to focus on a distant objects (streetlights etc.) and then pointing the camera towards the stars. So far it has seemed much more reliable than manual focus through the view-finder. So is focusing like this good enough? Or can I do better using other much simpler methods with the same equipment? I really really want to get a few decent shots of M42 & M31...

[steppenwolf]

This is all a matter of what is infinity - and your distant street light might not be quite it.

Have you considered making a Bahtinov Mask for your particular 135mm lens? I have made one for mine and it works a treat - aim at the brightest star you can find and manually adjust focus until you get the right pattern (this will make sense in a minute!) then remove the mask and point at the object you want to image.

What am I talking about? Have a look at my website here

[Texy]

Also take a look at this thread :

http://stargazerslounge.com/beginners-help-advice/97162-bahtinov-mask-barlow-lens.html

It seems that the mask does not need to have so many veins - especially so considering the size of your lens objective.

Texy

[emagine]

No...I haven't considered making a Bathinov mask yet. No matter how many times I try to make something myself, it almost never ever works the way it should and that leads to great disappointment. Samething happened with my scotch-mount, but glad that is works now. I would love to know if there is another simple method, otherwise I might've to give the Bathinov mask a try.

[steppenwolf]

I would love to know if there is another simple method, otherwise I might've to give the Bathinov mask a try.

I have tried all sorts of methods but for me, this is the most reliable and, amazingly, the easiest to use. Making a mask is about an hour of your time using materials that you most probably already have in your home. I have made 4 different masks for my various lenses and telescopes and they all work just fine. Full construction details were published in the Sky at Night Magazine in their August 2009 issue.

[emagine]

Thanks Steppenwolf. I have had a look at the Bahtinov mask and it does seem to be a really simple concept. There is a generator for the mask on this site

astrojargon - Bahtinov Focusing Mask Generator: Version 0.4

So the values I have to enter here for my lens will be 135mm & 24.1mm respectively for focal length and aperture....right (135mm, f/5.6)? Sorry to sound stupid but I really do want to get this right the first time, especially since I have to wait atleast 2 weeks or so for the sky to clear up. If you don't mind can you also explain how you made your mask, what tools and materials did you use, how do I stick it onto my lens? I'll be grateful. Thanks.

[Texy]

The front of the lens is what, 67mm? So aren't the figures 135 and 67 respectively?

Texy

[emagine]

Oh...I haven't measured the front of the lens yet.. I assumed it would the aperture of the lens, and not the front size of the lens. I will start with these figures. Now all I need to know is what materials to use, and how to do it properly without making any mistakes.

[emagine]

I am thinking of cutting the patterns onto a plastic sheet using a razor blade. Does it have to be really really precise? Or is there any room for slight errors while cutting?

[themos]

"really really precise" is not a quantity. Just do one as carefully as you can and try it.

[themos]

Oh, and make sure you don't "stick" it on the glass surface of the lens. Build a cardboard tube that can fit around the lens and stick the mask on the cardboard.

[glowjet]

If you have a look at the threads on the topic" Bahtinov mask and Barlow lens" it has come to light that a simple Y will produce the same effect and give a brighter image, very easy to make and experiment with, it has already been tried out in a very basic fashion and has been proved to work just the same.

John.

[emagine]

Thanks to everyone here for all your help. Now I am going to go and give it a try! cheers!

[Psychobilly]

Based on Pete's testing as reported in this thread... the Y essentially behaves in the same way as a hartman in which case just make three holes in a replacement lens cap and away to go...

Peter...

[Texy]

Well thats 3 of us who have pointed to the same thread, LOL.

Texy

[Naz]

Sorry to jump in and give contradictory advise, as emagine is only using the 18~135mm lens, why go to all the bother of making a mask ?

Set the D80 onto manual shutter, aperture and focus, set the focus on the lens to infinity, set aperture wide open, select shutter speed to desired time, press shutter and hey presto

No need to look through the view finder, as the depth of field on of the lens will take care of the focus. That is how I use my D70, and had to do it in the dark days of fully manual cameras and silver salts spread on to a plastic surface

Archie

[glowjet]

The Hartman mask with two or three circles produces a double or triple circular diffraction pattern merging into a single pattern at the focal plain, where Babinet`s Principle states that the diffraction pattern produced by a single slit is the same as that of a bar of the same width, the diffraction pattern produced always lies perpendicular to the bar and is broken into strips ( called orders ) the brightest strip at the mid point is the zero order and those either side are 1st, 2nd orders and so on.

hope this makes things a little clearer.

John.

[themos]

Set the D80 onto manual shutter, aperture and focus, set the focus on the lens to infinity, set aperture wide open, select shutter speed to desired time, press shutter and hey presto

That would work with most subjects but not stars. You get too much bloat if you're not bang on, especially at the wide open apertures we have to use.

[Naz]

That would work with most subjects but not stars. You get too much bloat if you're not bang on, especially at the wide open apertures we have to use.

As the distance to the stars are at infinity, surely a lens focus at infinity, will be bang on. If you are getting bloated stars, then the lens is not focused on infinity.

At wide apertures the depth of field is shallow, but that will be more evident at close distances, as you focus further away, the apparent depth of field will increase, so focused at infinity, everything from several tens of meters away to infinity will be in focus. Regardless of the aperture used, there is always only one point of focus, the smaller the aperture, the apparent depth of field, where things appear in focus, will increase.

Online Depth of Field Calculator

Archie

[themos]

If you want to assume that the infinity mark on the lens is correct and that you are able to align the marks with enough precision, go ahead.

[emagine]

Sorry to jump in and give contradictory advise, as emagine is only using the 18~135mm lens, why go to all the bother of making a mask ?

Set the D80 onto manual shutter, aperture and focus, set the focus on the lens to infinity, set aperture wide open, select shutter speed to desired time, press shutter and hey presto

Archie

I would've tried doing that if only I had any idea how to set the focus to infinity. I don't see infinity marking on the focus ring or the lens. Is there a manual way for doing it with the D80 and the lens I'm using?

[themos]

If you trust Nikon precision, then infinity should be on one end of travel of the focus mechanism.

[MishMich]

My Sony 85-300mm focuses at infinity, but my Canon 50mm has to be taken about 2-3mm back from infinity. Infinity on the lens may not necessarily equate to infinity for the lens. I have come across this discussion on photographic forums - and the advice is not to trust that the end stop marked infinity is infinity, check it. Take it back a millimeter and see if the image is sharper, take it back again, until you find the sharpest point. The only way you can really test this is using some form of mask - which gets you there in a couple of minutes; unless you want to spend half an hout thinking you've got there, and find an hour's worth of frames have consistently bloated stars (or worse, doughnuts). Autofocus won't help, because the camera cannot focus on nothing.

I was wondering about masks for the 35mm SLR lenses I have for my DSLR - this has been very helpful, because I can create masks just like the ones I have for my telescopes.

If you have a lens that just does infinity without a problem, then think yourself lucky (or wise for buying a good quality lens). Stock lenses don't always, and (as with my 50mm prime Canon) neither do some bought lenses.

The links here are really useful. Thanks to people who have contrinuted sensible suggestions. This should be the subject of a sticky, in my opinion.

M.

[emagine]

I do trust Nikon...but perhaps not on this one. Mine doesn't do infinity whether I take it to this end, or that end. So technically, I'm better off focusing manually through the view-finder(looking at a bright star, or mars, or some distant streetlights), taking a few test photographs, viewing them and trying again if not satisfied. Results are a bit variable though, can't trust everything through the view-finder. I will most likely stick with the Bahtinov mask, since it seems to be quick, reliable & easy. I'm really patient when I'm in my backgarden, but want to get everything done as quickly as possible when outdoors before the clouds show up again. And I certainly don't want to grab any unwanted attention at all when I'm outdoors, even though I think its a lot safer here in Eastbourne. Once again thanks to everyone for their informative posts, learned quite a few things from here.

[themos]

My Canon has Live View which means I can hook it up to a laptop, bring up a display of what the sensor actually sees, magnify that and adjust the focus until a star is the thinnest possible. Now that someone has hacked a separate program to capture that Live View stream it should be possible to run a routine on the incoming frames that measures the width of any star-like features and displays it in nice big bold letters on the laptop. Doesn't your Nikon have a similar feature to Live View?