Recently I wanted to photograph the moon framed by some trees and thought, oh, my husband’s 1970s-era Tamron zoom lens has a focal length range from 80mm to 210mm, and 210mm ought to be plenty long enough to get a really nice large image of the moon using my 35mm “full frame” sensor. I had in mind a composition where the moon occupied maybe a third of the short side of the frame.
Well, there’s assumptions and then there’s reality. Reality is that on a full-frame camera, when photographing the moon using a 210mm focal length lens, the moon doesn’t cover very much of the frame.
Many of you know far more about lens focal lengths vs sensor sizes than I do. But for people like me who never thought about it, here are some comparisons for “size of moon vs size of sensor for a 210mm lens”:
To get the composition I originally had in mind for a “trees and moon” picture, I’d have to use something like a 600mm lens on my full-frame camera.
Here’s a wonderfully informative discussion (including some DIY approaches to solving the problem) in which reality hit home for a person who wanted to fill the frame while photographing the moon using a 4x5 camera:
While on the topic of photographing the moon, here is “time and place” information for anyone planning to watch or photograph the January 31 “super blue moon eclipse”:
For this kind of work, I find Stellarium to be useful. It is a ‘planetarium’ program that displays the sky (stars, planets, sun, moon, deep sky objects) at any position on earth at any time. There is a feature that allows you to configure sensors, eyepieces, and telescopes (which can be lenses). You can then see the area covered by sensors (or film) with different lenses displayed as a frame projected on the sky. You can easily change lenses and sensors to pick the best combination for your target, and plan the time and place for your shot. Windows, linux, Mac.
There is also an obligatory xkcd for the rather silly “supermoon” stuff that’s been promoted by news outlets and the internet in recent years. The difference in average moon size and perigee moon size is about the difference between shooting it with a 100mm lens and a 105mm lens.
Here is the moon as framed at 540mm (400m with 1.4x extender) on my Canon APS-C camera:
On the actual picture (20Mpx) the moon has a diameter of 1150px.
The difficulty with longer lenses is that you are shooting a moving object… The moon moves across the sky at 15°/hour and is .5° wide, so moves by its own diameter every two minutes. If you frame it too close you have to invest in an astronomical mount. On a tripod you also have to use relatively short exposures (better that 1/20s with the apparatus above if you want the moon to move by less than half a pixel during the exposure).
That’s about the amount of “frame-filling” that I’d hoped for when I naively and hopefully attached our old Tamron zoom to my full-frame camera.
I like having a full-frame camera, but the more I use the full frame camera, the more I realize that the APS-C sensor has its own advantages. For example my 55mm Micro-Nikkor lens made a nice portrait lens on my old Canon 400D, but not so much on my Sony A7.
When you are a “nature photograpĥer”, the difference is on your shoulders. For the same framing, the FF lens has a focal length which is 1.6x longer… and is at least twice the weight, and 4 times the price. And these aren’t small and inexpensive lenses:) The smaller sensor also gives a bit more DoF in macrophotography.
Glad you like Stellarium. It’s extremely useful. You can even load a 360 degree panorama of the ground and horizon at your location if you’re ambitious. A fork of it is used to run professional planetaria.
When Stellarium is launched, there will be a box in the upper right of the window with four symbols in it. The far right is a wrench in a circle in a square. That’s the icon that will bring up a configuration window for eyepieces, lenses, sensors, and telescopes. Use the sensors tab to enter data for your camera sensor size and resolution, and the telescopes tab to configure your lenses. (The lens tab is for configuring doublers, triplers, and focal reducing lenses used with telescope eyepieces.) Diameter on the telescopes tab is the diameter of the lens aperture in mm, which I’m sure you know how to calculate from focal length and f-stop. To add new optics or sensors, click on the Add button to the lower left, below the list of lenses and sensors.
There is no need to compensate for focal lengths relative to full frame, as you’re giving Stellarium the raw data.
To toggle the currently chosen sensor/lens combination frame display on the sky, just click on the empty rectangle in the group of symbols to the upper right of the full window. That will also bring down a window where you can change among sensor and lens combinations with left and right double arrows. Lenses show as Telescopes again in this window.
If you want to find an object in the sky, just Ctrl-F and type in what you want to see. (⌘-F on Mac.)
What about taking other advantage of the large sensor. During the moon transition through your sensor you would be able to shoot several images, which you could scale up, stack and align to get higher resolution. I was never successfully with this technique and the moon, but I did not try very hard, so that doesn’t mean that it doesn’t work .
Could be harder than you think due to the change in parallactic angle. This angle changes around 10°/hour at its slowest (around the 45th parallel). If you want to ignore it (to avoid rotating the various frames), you would need under a half-pixel of change, so on my set up (1150px diameter) this is atan(1/2300), roughy 0.024 degrees, that translates into shooting all your frames in less that 10 seconds (even less so when it goes faster, typically in summer with the moon low on the horizon).
It’s relatively easy to find the angle of view of a given focal length lens. As Ofnuts stated, the moon is very close to 0.5 degrees in diameter. Given lens angle of view and the moon’s angular diameter, it’s pretty easy to calculate the fraction of a frame the moon will occupy. BTW, the sun’s angular diameter is also very close to 0.5 degrees. This is also approximately the angular size of your little finger’s nail held at arm’s length.
IIRC, the diagonal angle of view of a 200mm lens on 35mm full frame is about 12 degrees. Therefore 24 lunar diameters will fit across the diagonal of the frame using a 200mm lens.
Thanks! That seemed easy enough to do, well, except I have a couple of questions
Is “pixel width and height in microns” the same as “pixel pitch”? I think “pixel pitch” means the distance from the center of one pixel to the center of an adjacent pixel? Apparently the pixel pitch on my camera is 5.9 microns. I couldn’t find a “pixel size” for the camera, and surely there is a bit of space - however tiny it might be - around any given pixel?
Hmm, well, I didn’t know , but maybe I do now. By “f-stop”, I’m guessing that refers to the widest aperture the lens has? Surely it doesn’t refer to the f-stop that was used to take the actual photograph?
So on my old Tamron zoom lens the “wide open” aperture is f3.8. Unless it’s 4.0 - here’s a description of the actual lens:
This page gives the relationship between focal length, f-stop, and lens diameter:
So I’m guessing the lens diameter is either 210/3.8=55.3mm, or else it’s 210/4.0=52.5mm. Which after typing everything into Stellarium makes a frame that looks about right, with the moon filling about the same portion of the frame as actually happens when I take a picture of the moon at 210mm zoom.
Thank you, thank you! for explaining how to set up a new “camera+telescope” in Stellarium!
First pixel size/pitch: As far as I’m aware, manufacturers don’t publish information that includes the gap size, so we’re forced to assume that pixel pitch is the size of the actual pixel. That also gives the correct correlation between pixel count and sensor size, so that works in a practical sense for Stellarium.
I am familiar with telescope use as well as cameras, and since telescopes are fixed apertures, lenses are treated the same in Stellarium. I always use the largest lens aperture for Stellarium, so the diameter of the aperture in mm is focal length divided by the f-number of the widest aperture. This makes no real difference in the way Stellarium works. It may help to know that telescopes are often referred to by aperture diameter and f-number. If I say I have a 150mm f:5 telescope, it would mean that the aperture (meaning the lens or mirror) is 150mm in diameter with a focal length 5 times that aperture. Astronomers would just mentally calculate a focal length of 750mm for that 'scope. In astronomy, aperture determines light gathering power and resolution, which are the critical features of most telescopes, so that’s the primary characteristic used to describe a telescope. (Eyepiece focal length determines viewing magnification and eyepiece optical design determines apparent field of view.)
Zooms often have maximum apertures that vary with focal length. Your Tamron is one of those. It’s f:3.8 at 80mm and f:4 at 210mm. Since you’re likely using it at 210mm focal length for moon photos, the f-number would be f:4, so your calculated 52.5mm would be the correct diameter.
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