By my calculations the 12 foot width/height lighthouse at 1200 feet will
be 0.57 degrees wide or slightly wider than the nominally 1/2 degree
full moon. There may be reasons to use a shorter lens but not from the
standpoint of fixing the apparent diameter problem. At 1200 feet both
the lighthouse and moon will still appear to be about 1/2 degree
regardless of the focal length. If the focal length is shorter the
lighthouse will still obscure the moon except both will appear as
smaller objects.
Ken had it right that you will need to move further away. Moving
further away will have no apparent effect on the angular diameter of the
moon but will have an effect on the apparent size of the lighthouse. At
2400 feet it will be only about 1/2 the size of the moon.
If you'd like to do you own precise calculation for any distance it's:
2*ArcTan((W/2)/D) where W is the width of the light house and D is the
distance to the lighthouse.
If you use the Windows calculator in scientific mode you will not see
ArcTan listed as a function on the keys. To compute ArcTan click the
"Inv" (inverse) setting above the function keys and then click the
"tan" button. The expression ((W/2)/D) computes the tangent of the half
angle between camera and width of the lighthouse. Arctan converts the
tangent into its inverse which is the angle. Multiplying it by 2
converts the half angle into the full angle.
Chuck Norcutt
On 7/30/2013 4:12 PM, Ken Norton wrote:
>> This discussion is getting pretty close to something I plan to try and
>> do. There is a lighthouse off the coast of Morris Island near here. I have
>> an idea to photograph the top of the lighthouse with the rising full moon
>> directly behind it. Using TPE it is easy to locate the position and
>> time/date where I need to be to get the proper alignment. I plan to shoot
>> it with a 480mm/f4 refracting scope attached to my Olympus E-body. I know
>> from experience that the full moon will occupy about one third of the frame
>> with this setup. My question is: how large will the top of the lighthouse
>> appear at this magnification? The lighthouse is about 1600 feet from the
>> beach where I will be shooting and the top of the lighthouse is about 12
>> feet high and 12 feet wide.
>
> Ah, a geometry question. I'll leave the correct answer to our vast
> number of REAL engineers on this list. (I'm in name only).
>
> However, a couple of thoughts do come to mind:
>
> 1. 1600 feet is probably going to be a bit close as the moon will
> probably end up getting mostly obscured. The only way to be sure is to
> test this ahead of time with a shot of any moon and a shoot of the
> lighthouse. Merge in your favorite editor. You may find that you're
> better off with a slightly shorter lens (I use 300mm on 4/3 as my
> standard setup). You may also be a little close to the lighthouse and
> would be better served backup up a bit.
>
> 2. For a moonrise in the northern hemisphere, you want to start out
> high and to the right of where you'll end up. As the moon rises, you
> can keep shifting your position downward and slightly to your left and
> get more shots. A single-position, locked-down shot will give you all
> of about five seconds to get the shot and that's it. I almost
> exclusively use the monopod now as I can keep hoofing. If your beach
> has a slope, you'll start out in the position where the moon breaking
> the horizon will be immediate behind the lighthouse (get it as low on
> the horizon as possible) and as it rises, just walk down the beach and
> keep shooting. When photographing radio towers and other structures, I
> try to shoot along an east-west road so I can just walk down the
> shoulder of the road and keep shooting. As the moon rises, I get
> closer and closer to the tower. Sometimes, I end up quite a ways down
> the road.
>
> 3. For moonrises, you want to shoot BEFORE the full-moon. The day
> before seems to be about ideal for most locations, but depending on
> terrain, you can back it up another day. This way, you'll get sunlight
> on the lighthouse as well. Once the full-moon occurs, you'll end up
> with a silhouette shot.
>
> 4. Lock your focus. Seriously. I gaffer-tape my lens now. In the heat
> of the moment, you're timing has to be perfect, otherwise the moon
> will be slightly off position from where you want it to be. If you are
> hoofing it down the beach, the last thing you want to do is fight the
> focus. Frame and shoot. Frame and shoot.
>
> 5. Manual exposure. With the E-thingies, I personally find that I have
> to protect my highlights a lot--especially if the moon is warm
> colored. I generally keep the histogram peak at least a full stop away
> from clipping. I fix my aperture and shutter speed and then vary the
> ISO as necessary. To me, the aperture and shutter speed are the most
> important items to get right.
>
> 6. Invariably, there will be a distant cloud deck or haze layer that
> prevents the moon from being visible right at moonrise. Trust the
> numbers. Get a good understanding of elevation angles using your
> fingers. (BTW, for most humans, when you hold your hand out at arms
> length with the fingers just slightly loose, each finger is around 15
> minutes of moon/sun travel near the horizon at mid-latitudes--a great
> way to guess when a sunset will occur if you don't have your
> electronic toys at your disposal).
>
>
--
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