Exposing for the sun (transit of Venus)

[polyglot]

In case you hadn't heard yet, there's a transit of Venus next Wednesday with perfect viewing from the Australia/New Zealand kind of longitudes. There was one 8 years ago but the next isn't for more than a century.

The proper way to take a photo of it is with something called Baader AstroSolar Film, a very dense (3.8D or 5D, i.e. 12 or 16 stops) attenuating foil with little to no flare. However it's sold-out everywhere now so I was hoping to fake something up with an R72 (about 8-12 stops depending on red-sensitivity of the film). Glass filters like that have too much flare to be able to render good sun-surface detail but it should allow me to get an image of Venus; if you have the real thing you should be able to get all the texture, sunspots and coronal detail, etc, if you have a long-enough lens.

Can anyone tell me what EV the sun is in a direct exposure? I think it's going to be up near 30 or so, but having a number would be a good start as I'd like to do some test-exposures on Pan-F tomorrow...

I gather (from looking at a few sunspot photos) that the Baader film gives about f/8 1/500 ISO100 depending on conditions, but I don't know if that's the 3.8D or the 5D. If it's the 5D (16.6 stops), that puts the sun at about EV32.

Has any one here successfully photographed the sun and have exposure guidance?

Second problem: my longest lens is now only 240mm, so I think I get a 2.2mm image. Not going to be much detail in there!

 

[Lionel1972]

Wow, your technical knowledge of the art of photography never ceases to impress me. I wanna be like you when I grow up! Great project! Unfortunately I can't help you but it made me think of maybe another way to do it: how about building a long focal large format pinhole camera mounted on a motorised synchronised tripod to follow the sun's apparent trajectory?

 

[Bill Burk]

My eclipse photo in the gallery was taken indirectly using a 12" lens - focused onto a piece of black construction paper... then I took a macro shot of the projection on the black paper. That reduced the effective brightness about 10 stops. If clouds come in, there will be a great drop in brightness. That made many successful eclipse shots possible.

 

[Worker 11811]

As Lionel and Bill suggest, why not make a camera obscura out of a shoe box or similar materials then photograph the projected image?

That way you avert the risk of permanent damage to your vision or your equipment. Further, I imagine this would allow you to take pictures in a lot more comfortable setting rather than squinting and peeking at the bright sun.

If you have some type of synchronized mount like a telescope tripod you could mount it thus but, if you have something like that, why not just use the telescope to project your image onto a screen of some type then photograph the result.
All it would take is some gaff tape, some cardboard and a few pieces of coat hanger wire and you could build a projection screen for your telescope that would move right along with it. Just focus on the screen and snap away.

If you don't have a telescope or some sort of synchronized mount, you'll still have a minute or so to take your photo before you have to move your camera obscura again.

 

[kurtdriver]

It seems that Baader safety film is still available.

Dead Link Removed

http://www.firstlightoptics.com/solar-filters/baader-astrosolar-safety-film-nd-50.html

 

[andrew.roos]

A common value for light meter calibration sets the luminance for 0 EV at 100 ISO to 0.125 cd/m^2. According to Wikipedia, the luminance of the sun at midday is 1.6 x 10^9 cd/m^2. This equates to 33.6 EV at ISO 100.

 

[polyglot]

A common value for light meter calibration sets the luminance for 0 EV at 100 ISO to 0.125 cd/m^2. According to Wikipedia, the luminance of the sun at midday is 1.6 x 10^9 cd/m^2. This equates to 33.6 EV at ISO 100.

Excellent; thanks. I reckon I'm within no more than 3 stops of it being achievable with just the one filter, so maybe if I slap a green on top of the R72 it'll get me there. Completely clouded in today so the experiments are postponed...

With respect to photographing a projected image, I'll certainly try that too. The most magnification I can get is 1:1 though and that only on digital, which means I'm still going to get something pretty small. And there will be quality losses from the diffusing material too if it's particularly rough. I think I'm going to try a bit of unglazed ceramic tile as it's the finest non-flammable thing I can think of. Clay-coated inkjet paper might be good too.

 

[couldabin]

Where does ~34 EV @ EI 100 put the sun's disk on a gray scale? Middle? Closer to white?

 

[polyglot]

EV is just a radiant flux density (brightness) and where it will be placed tonally depends on your exposure. For reference, say something is EV15, it will be middle gray if exposed at f/16 t=1/ISO. I was planning on exposing the disc at about +3, i.e. setting the camera+filter as if the meter reading had been EV31, which would put the sun at Zone VIII (close to but not quite paper-white) on a straight print.

Anyway, my shoot-through-IR-filter experiments failed, mostly because I had someone else perform the tests while I was off at work during the day when the sun was out. Apparently there might have been a cloud involved

Transit has been going for about 75 minutes so far and we're still under 90% cloud cover. I have my hopes up and the university just down the road has announced that they've publicly-accessible scopes setup this morning, so I'll go there once it clears a little. I'll probably just end up with a digital macro photo of a projected image.

 

[Poisson Du Jour]

No sign of the Sun or any sunshine here in Geelong. And that doesn't look like changing any time soon...

 

[Newt_on_Swings]

I totally missed it, forgot to look out as I was in darkroom at 6:40 for sunset. Didnt get out of work til now. Oh well maybe I'll live to see it again with the advances in medical tech. On the upside the students made excellent still life prints while we were in there.

 

[polyglot]

You ain't missed it yet, there's still about 45 minutes left (assuming you're not on the dark side). We finally got some breaks in the clouds in Adelaide (about 50% coverage now), and I got my (digital photo) of a projection out of a telescope, complete with a bunch of hairs in the picture because someone's eyepiece ain't real clean...

 

[couldabin]

OK, as usual I'm confused. I thought EV was bound to f-stop/shutter combinations. The exposure that results in will depend on the light energy and the film speed. So my question was (before the transit) what density one would expect for the sun's disk with ISO 100 film and EV 33 (or 11 stops less than 1/1000 and f-64)? And, FWIW, do you know if the welder's glass rating is on an EV scale? That is, #12 reduces brightness by 2^12 times?

 

[polyglot]

EV is "exposure value", which means a quantity of light coming from a source in the scene. You can think of it as being source brightness, or you can think of it as the camera settings to capture that particular source brightness as a midtone on your film. It's also the value returned by a reflected-light meter.

So EV15 is 4096 cd/m^2 or it's ISO100 f/16 1/100s. Likewise, the sun (according to andrew.roos) is 1.6e9 cd/m^2 = EV33.6, which would be ISO100 f/16 1/39,000,000 (a 25.6 nanosecond exposure) to hit Zone V.

I think you're right about the numbering of welding glasses being stops but I don't know. They also have some odd spectral effects (kind of green, lots of UV-blocking) so it's not a simple ND and therefore you can't state a simple filter-factor without knowing the spectrum.