Live view & exposure adjustments

TCampbell · Dec 5, 2014

As many of you know, I don't use Nikon cameras so I'm not familiar with this particular aspect of them.

The other night a school photography club came out to use our observatory to shoot lunar images and we had a bit of difficulty with focus.

When a DSLR is attached to a telescope, and the telescope becomes the lens. The telescope focus is normally handled manually and you typically use the "live view" image on the rear lcd screen (and often magnify that to 10x, etc. so you can see fine detail while you work on focus.) There are other focusing methods -- e.g. we also use Bhatinov focusing masks or computer assisted focusing such as "full width half max" method -- but the FWHM method requires that the focusing software on the computer can support the camera and very few astro-imaging programs support Nikon cameras.

So this brings me to the problem and, ultimately the question.

When the school photography club attached their camera to the scope (they were using a Nikon D5200 but had many other Nikon camera models) the "live view" on the rear LCD screen was completely blown out when trying to focus on the moon. It just a large white blob. We were trying to back off the exposure in live-view mode but couldn't find a way to do this.

On a Sony or Canon DSLR, if you're using Manual mode and you adjust the exposure settings, the image on the live-view screen changes to show roughly what your exposure will be (an exposure preview). This allows you to back-off the exposure (or drive it up if necessary) to see the necessary detail to achieve focus.

We could not find a way to get the Nikon to do this. We tried with a D7100 and had the same issue. We had to "guess" our way to better focus and in the end, the focus wasn't as good as it could have been.

I'm guessing there's some menu option for this and most DSLR users probably don't spend a lot of time in live-view mode so this may just be something the camera owners didn't know how to adjust.

But I think they'd like to come back to the observatory to do more imaging and I'm hoping I can help them with their camera settings in the future.

Any tips?

tirediron · Dec 5, 2014

You've got me curious Tim; I know less about astro-photography than I do about particle-matter physics, but the first thing that strikes me is, "How do you compensate for the lack of a diaphragm?" Is there some way of using Waterhouse stops or something similar?

480sparky · Dec 5, 2014

I'm always using a laptop to drive both my scope and my camera when out under dark clear skies.

Tethering the camera allows me to fire a shot to see what to focus/exposure is like.

TCampbell · Dec 5, 2014

The larger scope is a Celstron C14. This is an f/11 scope (Schmidt Cassegrain Telescope design). This means you treat the camera like you've dialed in an f/11 aperture and you can only use shutter speed or ISO adjustments to change the exposure. (well... assuming you have not used a focal reducer or "barlow" (think tele-extender) because these would effectively change the focal ratio.

I was hoping to run the ISO down to 100 and set the shutter speed to 1/100th (this is the "Loony 11" exposure which does a nice exposure for the moon.). BTW... Those same settings did create the correct exposure in the image... Just not on the live-view screen. The live view was blown out (likely trying to auto compensate for the black sky)

The is also an 80mm f/5.4 apochromatic refractor in the observatory.

We have a bahtinov focusing mask for the large scope. This looks like a slotted cover but the slots run in different directions. When you look at a single bright star, the mask throws diffraction spikes in your image which resembles an X shape with a verticle line running through the middle... Except it won't be in the middle if the star is not focused. You adjust focus until all the spikes converge in the middle and that confirms you have accurate focus.

You can see the mask and an animation showing how they work here: Bahtinov Mask (Just one of many vendors. Astro-Jargon.net has a focus mask "generator" -- you enter the dimensions of the scope and it prints a template for you... transfer it to a card stock and cut out the slots.)

Once the focus is adjusted so the spikes converge, you can remove the mask (don't forget to remove the mask... don't ask me how I know (*whistles innocently*) then point the telescope back at the object you want to I age (without touching focus) and start imaging. At the distances in space.... when anything is in sharp focus, everything is in sharp focus.

My challenge is that I don't have a Bahtinov mask for the smaller scope so I typically just use "live view" at around 10x magnification to get everything nice and sharp.

That turned out to be a challenge. It seemed as if the camera was trying to auto set the exposure on live view -- independent of the manually dialed in exposure settings we selected.

I'm hopeful there is a setting that lets do that. The school club owned a Nikon D5200 and the teacher who runs the club owns a D7100. I'd like to help them learn to get the image with their own gear. While we have cameras that will certainly work... it's just not the same -- I'd like to help them get the image with their own cameras. I plan to pick up a focusing mask for the smaller scope.

tirediron · Dec 5, 2014

runnah · Dec 5, 2014

As far as I know, some Nikons don't have live view exposure simulation.

I would suggest a tethered shot.

runnah · Dec 5, 2014

More digging. It appears as if the d800 is the only model with exposure sim. The newer models might have it as well but the 5200 and 7100 certainly don't.

It's too bad because it's sure a useful feature.

TCampbell · Dec 6, 2014

Thanks for the help on this. Now I just need to get one of you guys with a lot of pull at Nikon to tell them to add this to the next firmware update.

I've ordered a Bahtinov focusing mask for the smaller scope so we can use that to achieve focus and not worry about the live-view.

qleak · Dec 6, 2014

According to this thread, there is some limited exposure simulation if you switch a D7100 to live view video.

I've checked with my D7100 and it appears to at least darken the frame.

To switch to live view video flip the switch on the live view button. Note the shutter still takes pictures (not video) when in video mode.

I don't know about the other nikons, but i'd suspect it's worth a try