How to work out Aperture, Focal length and Focal Ratio

Hey there

Thanks for taking the time to answer my question.

If I could just clarify in that case, when it comes to camera lens' such as my one, the 120-400mm, when the lens is fully extended (zoomed in) the focal length would be at 400mm, regardless of how any other settings change (and then not zoomed in at all would be 120mm).

In the formula you typed: [2*arctangent ((D/2)/F) ], how would I work out the value of D? is that just L x W?

Thanks again

Hi Sorry, I think I worked it out.

D is the Square root of L sqaured + W squared.

Metastro said:

Hi Sorry, I think I worked it out.

D is the Square root of L sqaured + W squared.

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That's right. F is the value you set on your zoom ring, assuming the lens manufacturer calibrated it honestly.

Thank you very much for your help

Sittatunga said:

Metastro said:

Hi

I have access to an Advanced VX tracking mount for the next 10 or so months and I also have a Sony A7X and Sigma 120-400mm Telephoto lens.

What I would like to do is try and shoot some deep space objects. I know I will need an autoguider set up and I am not expecting any amazing results just yet, but before anything I want to be able to tackle some of the maths behind this.

I would like to be able to calculate using this FOV calculator,

http://www.12dstring.me.uk/fovcalc.php

approximately what particular space objects will look like, the problem is, I don't know how to work out the Aperture, Focal Length and Focal Ratio.

The pixel, image and sensor sizes I am not as concerned with just yet because they relate to the sensor in my Sony A7S, which will always be the same as far as I know.

But as for the Aperture (in mm), the focal length and focal ratio, I am unsure. I know that focal ratio aka aperture in camera lens' refer to the iris opening size in regards to light, but I don't know how this changes here.

It's not that I'm asking anyone to give me the answers because I do want to learn how to work these things out myself for the future if I acquire different gear, but can anyone tell me the formula behind working these figures out so that I can do it myself? Using the Sigma lens as my reference.

Thank you

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Aperture in mm (or inches, feet or whatever) is [focal length in mm (or inches, feet or whatever)] divided by the f/number; For example, the aperture of a 50mm lens at f/2 is (50/2) or 25mm. If you are used to describing that lens as a 50mm 1:2 lens, the aperture is still (50*1/2) = 25mm, just spelled differently.
The angle of view of a lens of focal length F depends on the sensor size, say L x W x D (Diagonal). Lens manufacturers usually specify the diagonal angle of view as the diagonal governs the size of the sharp image circle required and it sounds more impressive. The angle of the long side of the frame is given by the expression [2*arctangent ((L/2)/F) ]. (Arctangent is also spelled arctan or tan-1 with the -1 as a superscript. Your calculator might call it inv tan). The angle of the short side of the frame is given by the expression [2*arctangent ((W/2)/F) ] and the angle of the diagonal is given by the expression [2*arctangent ((D/2)/F) ]. You can have fun with the sums or just use that website to do it all for you

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Knowing these calculations is certainly a good idea, but just so you'll know, the simpler alternative is to use Stellarium (free from stellarium.org) and the FOV preview tool that it provides at the upper right corner of its display. It allows you to input values for any telescope or lens and the camera sensor, and will display the FOV of whatever part of the sky you select just by clicking on the rectangular "image sensor frame" icon. This won't teach you the math, but using the program will familiarize you with the celestial sphere, also something you'll want to know. Stellarium can also be used to control a go-to telescope mount.

Sittatunga said:

An autoguider is used to control a motorised equatorial mount to photograph deep sky objects over long time periods - to use your camera leness you would be better starting with something like a Skywatcher Star Adventurer or an iOptron Sky Tracker, both of which are affordable, portable and fit on a good camera tripod.

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I have to question this advice, except for the part about foregoing use of autoguiding at the beginning.

The AVX is a real equatorial tracking mount. If you have the use of one, I'd say definitely use it. As long as you have a way to attach your camera and lens, it should provide better tracking than lighter-weight tracker options. Celestron mounts also use a semi-automated polar alignment routine that produces pretty good alignment with relative ease. For that reason, you will probably be able to get acceptably round stars even at 400mm, as long as you keep your exposures of reasonable duration - 90 seconds give or take, as my very rough guess. From reports I've seen for the camera trackers, most users don't seem to recommend using them beyond about 250-300mm.

I'm also presuming you can live without the portability of a camera tracker. I use an older LXD-75 mount (basically of the same class as the AVX) as my own "portable" mount with a small "jump-starter" as its power source. As long as I'm setting up somewhere within a hundred feet or so of my car, it seems only minimally more trouble than a tracker would be (though I admit I've never used one).

Lyle's right, IMO. The AVX (I've got one, it's been perfectly adequate for my needs) is not really any harder to set up than most simple tracking mounts. it has the potential to be more accurately polar aligned, and since it's so much sturdier, I get a higher keep rate on my images than with my iOptron Skytracker Pro. I actually wouldn't put anything as heavy as that sigma tele-zoom on my skytracker; I personally feel like my D600+180/2.8 is pushing it. Note - the mount tracks fine, it's just that it's more difficult to get a really good polar alignment than with the AVX, and it's more vibration sensitive.

Edit to add: you're going to need an intervalometer. I personally use a JJC WT-868, works great. The other thing to bear in mind is the Sony "star-eater" - it invokes a noise reduction algorithm when you use it in bulb mode that will remove faint stars from your images. Assuming you're not running the very latest firmware, you can apparently use 30s exposures OK, but nothing longer. If you search this forum you'll see several notes on it. Not trying to discourage you, just letting you know that there are some challenges specific to your body; if you look for posts by "sharkmelley" you'll see what can be done with one with enough patience and practice.