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Focussing a DSLR

cyborg421

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cyborg421

cyborg421

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Hi All,

I have a nikon D40 and would like to image some deep sky objects. I understand about the B setting and remote control. I was wondering about the best way to ensure the camera is in focus on an obect. Is it trial and error or is there a better way? There is no "instant view" on the camera.

Thanks

Simon

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Squagnut

Squagnut

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I do it by peering through the viewfinder and adjusting the focus by small increments until stars or any other detail become sharp. There is certainly an element of trial-and-error, but practice gets results.

I use a Nikon D3000, which doesn't have a live view (i.e. you don't get to preview photos on the rear display screen), and this is actually useful, since the display is far too bright to use if I want to maintain night vision - I put red film over the rear display and use it to adjust settings, but that's all.

Edit: The mask suggestion is very probably the best way, but I've not yet tried that!

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Tinker1947

Tinker1947

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I have a Bahtinov mask and while it does work if the thing you are imagine is not very bright its not a lot of good, my cure, take a picture then view it on the LCD then refocus, take it past the point where it looks a good image then back again once you get to know the focus point it only requires a few attempts to get it right.

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rocketandroll

rocketandroll

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Definitely get a Bahtinov mask...

I started out focusing mine by just taking repeated short exposures of the subject at high ISO and tweaking the focus by viewing the images zoomed in on the camera screen between shots... though this can take 15-20 minutes and isn't dead accurate.

With the bahtinov mask I use the same technique, but do the focusing whilst I'm still on the final alignment star (usually a very bright one) so I get a good difraction pattern... again, I zoom in on each shot after taking it but with a bright star and a mask you can do it with 10 second exposures... only takes a couple of minutes max and you're done.

The image you get with the mask on is basically the star with a cross drifraction spike on it like an 'X' then a secondary horizontal line (two more spikes) across the star... these two spikes move up and down in relation to the 'cross' as you move focus... the aim is to get the line dead center of the cross... it's dead easy and works absolutely perfectly every time... I could never image without one now, I've even had a mask made for my camera lens :-)

Ben

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Sp@ce_d

Sp@ce_d

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Yup a bahtinov with Bahtinov Grabber is what you want.. Bahtinov Grabber goes 64 bit (and 32 bit update) Niels Noordhoek’s weblog

I've even had a mask made for my camera lens :-)

Ben

Ben, where did you get the mask made for your lens? I asked Keith Morris back in April when I got one for my ED80 but he said he didn't do them for camera lenses.

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rocketandroll

rocketandroll

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Yup a bahtinov with Bahtinov Grabber is what you want.. Bahtinov Grabber goes 64 bit (and 32 bit update) Niels Noordhoek’s weblog

Ben, where did you get the mask made for your lens? I asked Keith Morris back in April when I got one for my ED80 but he said he didn't do them for camera lenses.

Hmmm... dunno who the guy is who does them but ordered it through FLO.

Gave them the spec of the lens and the target focal ratio and got it back a week later! Haven't tried it yet but it looks right :-)

Ben

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Rebus

Rebus

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You don't need to buy anything. Just focus the camera to infinity. You can do this by putting the camera in AF then focusing on the moon. Switch back to Manual to adjust exposure time accordingly.

If the moon is visible, yes that will probably work ok for distant stars etc., but I tend to use live-view at 10x zoom on my Canon 7D, put the lens on manual focus then adjust focus until the object is sharp. I then tape the focus ring with a piece of electrical tape to stop me knocking it!

Using a smaller aperture will help slightly with any error, but depending on what you are shooting this is not always posible.

The problem is, the 'infinity' mark is quite variable on a camera lens. If you simply turn the focus ring fully round it actually goes past infinity (and beyond! :)) and will result in an out of focus image.

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Squagnut

Squagnut

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Hmm. It depends on the level of detail you want to capture. Keep exposures as short as you can - Dobsonians are alt-az mounts, so a long exposure through a dSLR would result in a twisted image. You'd also need to be sure you have perfect tracking and no vibration.

You could try and make/get a wedge to set under the dob. This is discussed here, but it doesn't look like an ideal solution - doing dSLR imaging on a dob is always going to involve compromises.

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Stagman

Stagman

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If the moon is visible, yes that will probably work ok for distant stars etc., but I tend to use live-view at 10x zoom on my Canon 7D, put the lens on manual focus then adjust focus until the object is sharp. I then tape the focus ring with a piece of electrical tape to stop me knocking it!

Using a smaller aperture will help slightly with any error, but depending on what you are shooting this is not always posible.

The problem is, the 'infinity' mark is quite variable on a camera lens. If you simply turn the focus ring fully round it actually goes past infinity (and beyond! :)) and will result in an out of focus image.

How or why does a smaller aperture help with focus?

Sent from my X10i using Tapatalk

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Stagman

Stagman

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By increasing the "Depth of Field"...

This can be thought of as the region either side of the focus point that still produces an acceptably sharp image...

Peter...

^^^ what Peter just said, the smaller the aperture (f/8, f/11, f/16 etc.) the greater the depth of field. Large apertures (f/2, f.2.8, f/4 etc.) have quite a shallow depth of field.

Online Depth of Field Calculator

But does this work at infinity?

Sent from my X10i using Tapatalk

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Rebus

Rebus

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But does this work at infinity?

Sent from my X10i using Tapatalk

I doubt this calculator is designed for such huge numbers (I gave the link for an example of depth of field in general)

What distance is 'infinity' though ?? :)

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