Viewfinder illumination and astrophotography

This winter I plan to do some astrophotography with my Nikons and I'm

wondering if I should do something to prevent the viewfinder

illumination from entering (somehow) the film. I have F5 and F90X. I

plan to do exposures up to 15 min. Has anyone found this to be a

problem and what is the remedy?

The mirror is in the way. There is no posibility of fogging through the finder. In any event, the N90s has a built in cover for the finder (the small little lever on the left of it). I don't have an F5 so I don't know if it has it too. BTW, that was put there for when you are metering without looking through the finder, not to prevent film fogging.

The mirror is semitransparent; the camera has a secondary mirror under the main mirror which is used for something - exposure or/and AF. Maybe the secondary mirror blocks the semitransparent part of the primary mirror during the exposure. Anyway, there is a light leak in the F90X in front of the focusing screen that allows light from the viewfinder to enter the mirror space even during an exposure. Thus there is likely to be a small leak from the viewfinder display to film which may show up with very long exposures. On the F90X, the viewfinder display is illuminated during exposures, which seems stupid.

 

On the F5, the viewfinder display light is turned off automatically for the duration of the exposure, thus there won't be contamination. Surprise, surprise, yet another feature of the F5 which has received little attention ... I'm very glad that I bought it.

I remember a post from a long time ago about being able to remove the batteries from the F90 during a long exposure. That would certainly kill the illumination. <p>

I will have to ask though if you have actually observed this light leak problem yourself or is this just some rumor you have heard. It just doesn't sound right for this to be happening. I could see it being possible if the camera had wear or deterioration around the light seals but for this to be overlooked by Nikon I just don't know. Lots of people do long exposures with this body, this is the first time I have heard of this problem.

Ilkka:

 

Most amateur astrophotograohers in my circle use simpler MECHANICAL cameras such as an F or a Pentax or a Zenith, Olympus OM1 etc. A Zenith or an old Pentax Screw mount is really as good as a fancy AF F90s(I have one and do not use it for astrophotos thru my Televue Ranger), and it will not consume batteries and will work in the cold.

 

The actual box that attaches to the scope and which holds the film is the least important part of the system. Unless you are taking a shot of a full moon with a 1000 mm + lens the auto exposure is of no use, so there is no need for that, nor is the autofocus.

 

Cheers

Hal, I have not yet done really long exposures with the F90X (I need to get a data back to be able to do it) but I read on some astrophotography sites that the viewfinder illumination leaking to the film can be a problem (though this body was not specifically mentioned). The light leak between viewfinder and mirror space during the exposure is not due to a broken camera, it is clearly a design thing that the sealing is not quite complete. You can see a part of the focusing screen exposed by holding the shutter open with bulb (point the viewfinder at a window) and looking at the front of the focusing screen. The leak is very slight and it's not something that would affect any normal photographic situation. The light would have to be reflected from the back elements of the lens (a realistic possibility with an 85 mm f/1.8) to film. I don't know whether to expect this to be a real problem. Some people expose film for hours with telescopes ! The battery is clearly another consideration with these exposure times.

 

I am not planning yet to buy a telescope; I figured that I'd first get a motorized mount and using my normal telephotos with that (up to 300 mm). The price of a mechanical camera is very small compared to most decent telescopes though.

 

Perhaps I spend too much time measuring femtowatt light levels at work and not enough outdoors?! :-)

You may want to consider a Nikon EM or FG to attach to your lenses. All that you need for astrophotography work is the "B" setting. The N90s and F5 have no advantages over the least expensive Nikon bodies except for the interchangeable viewfinder screens. When working in the dark, there are definite advantages to simple cameras with no illuminated anything. Also, a good basic lens for widefield astrophotography is the Nikon 50mm f/1.4 used at either f/2 or f/2.8. Some examples can be seen at <A HREF="http://www.krages.com/leonid.htm">Leonid Meteor Showers, November 17, 2001</A>.