Edmund Optics Scientific Outreach - edmund scientific optics

 2024-09-18 16:51:31

As you can see, I loaded those images into Photoshop, set guides to mark the centers of each light (easier than I expected to get within a few pixels of the center) and then used the ruler tool to measure the distance from center to center. Those distances are marked on the images above.

As far as Plossl eyepieces go - and eye relief, there is general formula - it is about 2/3 of EP focal length. You have to know what sort of eye relief you'll be comfortable with - say 8mm is ok for you, then you shortest FL plossl that you will be happy using is around 12mm (8/2 * 3).

How tocalculate focal lengthPhysics

It turns out that 25mm FL lens is equivalent to 50mm lens for 4/3 sensor and 30.86mm FL lens is equivalent to 50mm lens on APS-C sensor (and bunch of other equivalent FLs ).

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Many photographers misunderstand the concept of focal length and don’t realise that the focal length of a lens has little to do with physical dimensions. It’s more the way they think of focal length than any actual difference.

50mm on full frame will have same field of view as 25mm on 4/3 sensor or 30.86 on Canon APS-C sensor - that is why they are "equivalent".

After doing this, I went back and measured the focal length of the lens at each of these distances from the subject using the original method that I outlined above. My measured focal lengths were:

I'd like to know what telescope focal lengths are considered long or short. I'm guessing about 1200mm and above is long and 700mm and below short???

I've come across bunch of things that daytime photographers use that can be described as intuitive and helpful - but are misleading at best.

Surely it's the FL that decides if it's long or short ? I mean a 400mm FL with a 30mm Aperture would be an F13.3 (long) by this definition, but it's surely still 400mm - which isn't long surely ?

If that guide didn’t match up with the beginning of the ruler’s scale, then adjust your camera and re-shoot or, if you’re comfortable with the ruler tool in Photoshop, figure out how far away the guide is from the start of the scale, and add or subtract that amount. You can avoid using the ruler tool or making estimates by centering your AF point at the 10cm mark rather than the very end, and then subtracting 100mm (plus or minus adjustments) from your final measurement, but since that adds in another opportunity for errors to creep in to the process, I prefer starting at 0.

I’m still chewing on some of the details, but instead of using Wolfram Alpha, you can simply paste the formula: 24/(2 * tan(3.22 degrees)) into Google and it’ll give you the answer as well: 213.299798393

I’ve measured a several lenses this way, each lens multiple times, and the numbers have all come out as I’d expected (after a brief moment of confusion in one case). It’s not my formula, but the equation is simple enough, and certainly makes sense to me. That said, I’m certainly open to the possibility that there’s a mistake or that it’s missing something that would make it more accurate. If it is, and you can explain it to me, I’d be very grateful. Please let me know in the comments below!

Focal lengthof lens formula

To begin, mount your camera on a tripod near a wall or other vertical surface. Measure from the center of your camera’s sensor to the floor (you can use the vertical center of the lens mount). We’ll call this H. Then, tape something like a metric precision ruler (or metric measuring tape, depending on how wide the lens is) vertically to the wall, with the low end at the exact height of the center of your camera’s sensor from the floor (H). This is just to set up the square angle of a triangle. It’s also important to make sure that the camera is vertically aligned with the ruler.

I've come across bunch of things that daytime photographers use that can be described as intuitive and helpful - but are misleading at best.

In the fields below, enter your X measurement for adjacent side, and your Y measurement for opposite side, and press “calculate”. That will give you an the angle we need. We’ll call it A.

...and they mire themselves in that nightmare term, 'crop factor' in which chip size risks becoming an equivalent of focal length. (They also mire themselves in the notion that full frame cameras have a greater depth of field but this is only because, at present, full frame cameras tend to have larger pixels.)

I'd like to know what telescope focal lengths are considered long or short. I'm guessing about 1200mm and above is long and 700mm and below short???

Next, load the image into Photoshop. Create a horizontal guide at 50%. That line should match the beginning of the ruler’s scale in the image. From there, it’s simple: look at the top border of the image and read where the ruler leaves the frame. That measurement is the opposite side of the triangle, which we’ll call Y.

Findfocal lengthfrom image online

Once that’s done, center your middle focus-point on the end of the ruler. Since your AF point is most likely a rectangle, the end of the ruler should bisect it, dividing it in half (if your ruler’s scale doesn’t start right at the end of the ruler, then center the first markings on the scale in the center of the AF point instead).

I wanted a way to quantify this visual change, and Bob Atkins’ article on focus breathing gave me the idea for a way to do it. He used stars as subjects, but I live in Seattle, so that’s not a practical option (hello cloudy skies). So, instead, I used two small LED lights and did this:

My favorite of these is Wolfram Alpha, which is accurate and flexible. Click here to see Wolfram Alpha’s “Lens Angle of View” calculation page.

Surely it's the FL that decides if it's long or short ? I mean a 400mm FL with a 30mm Aperture would be an F13.3 (long) but this definition, but it's surely still 400mm - which isn't long surely ?

Astro and camera definitions of FL are indeed different. Something that often confuses those comming from a camera background.

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Lens equation

If you are talking about general purpose viewing - then I would say that you are pretty close to what would be considered short / long focal length. I think that you went a bit short on the long side. Maybe long is longer than say 1800mm rather than 1200mm.

Other than that - you are absolutely right, focal length is distance between objective and point where parallel rays bent by lens converge into single point.

I'd like to know what telescope focal lengths are considered long or short. I'm guessing about 1200mm and above is long and 700mm and below short???

In the default example, the calculation assumes that we’re measuring using a vertical angle (which we are) and a full frame 35mm sized sensor (36 x 24mm). If you’re using an APS-C camera or another size, enter the vertical size of the sensor instead of 24mm. So, in the computational inputs, enter the angle of view (A) that we found above in the computational input, and press “compute”.

I’d also love to hear your results from using other methodology, and your explanations of those methods. It would be great if I could find a testing method that’s easier than this and just as accurate or better.

Is it imaging? Then you have to look at "current" pixel sizes. Current pixel sizes are around 4µm. Just 10 or so years ago "regular" pixel size was around 7µm and another 10 years ago, more like 9µm was norm.

Visual: Over the past week, I’ve been looking at a couple of different ways to cross-check my measurements and make sure that what I’m getting makes sense. If you’ve seen my video comparison of the Canon and Tamron lenses and their breathing, you’ve seen a clip from Matt Granger’s video on the subject that shows the effect of breathing on his model while shooting video. It’s a simple, visual comparison but it shows what you’d expect: as the Tamron and Nikon lenses are focused towards the MFD, the subjects get smaller, but on the Canon, the subject gets larger.

This means that from our two different methods, one in which the camera is a stationary 26 feet from the target and one in which the camera is moved to as close as 5 feet from the target, the numbers support each other pretty nicely.

As far as I've gathered - photographers primarily think in terms of FOV on 35mm / full frame sensor rather than in terms of actual millimeters of focal length.

I am of the age where 35mm film was the norm for me. I still look at a scene and think of what 35mm lens I'd use, then mentally convert that by the camera's "factor'. It is no different from being brought up on feet and inches then trying to visualise something given in metres, I suppose.

Surely a 400mm FL camera lens has a FL of 400mm? It might be physically shorter in length because of the optical train, but then a telescope may be only 500mm long but if a barlow lens is placed in the optical train would have a focal length much longer.

In imaging the situation has been somewhat altered in recent years by the arrival of cameras with tiny pixels. When pixels were typically 7 to 10 microns or so, an imager would have wanted well over a metre of focal length for the smallest targets but this is no longer strictly necessary, so a metre has gone from medium to quite long in deep sky imaging terms.

As far as I've gathered - photographers primarily think in terms of FOV on 35mm / full frame sensor rather than in terms of actual millimeters of focal length.

There you go - another daytime photography pearl - 1" sensor - has nothing that is one inch long (neither sides nor diagonal - really no dimension that is 25.4mm).

I’ve been involved in an interesting exchange of ideas over the past couple of days with Tony Northrup about the effective focal length of Canon’s 70-200mm f/2.8L IS II lens when it is focused at distances closer than its infinity setting (ie, focus breathing). We’ve been coming up with wildly different numbers (mine were presented in a video last month), and we don’t have many outside sources to check ourselves against… perhaps because making the calculation is not a very straight forward process. So, if you’ve ever had the desire to see how your lenses really measure up or how much they breath, here’s how I do it, with a few of the tools that I use to make it faster.

Or has long and short got a weird astro definition as well as every other camera term I've learned in the last 40 years ?

It is the astrophotographers who have the proper understanding of focal length, field of view and resolution. (Says an astrophotographer! ) Focal length is written on the lens. Chip size determines field of view. The ratio of focal length and pixel size determines resolution measured in arcseconds per pixel. Deviate from these simple facts and you plunge into the mire! Daytime photographers do have an interest in perspective, however, since the naked eye creates a relationship in size between foreground and background objects which they may wish to preserve or to over-rule and here the vile term 'crop factor' might be spared from summary execution! Astrophotographers always work at infinity. (Infinitely difficult, infinitely expensive, infinitely exasperating...)

Focal lengthto magnification calculator

Surely the focal length is the (optical path) distance from objective to focal plane? By definition. Matters not if it's a camera or 'scope.

I then focused the lens to about 12 feet away (using the range indicator on the lens) but did not move the camera. The idea here is to get the same effect as Matt Granger’s racking the focus of his lens in his video, but at some distances that I could measure.

Surely the focal length is the (optical path) distance from objective to focal plane? By definition. Matters not if it's a camera or 'scope.

Say you want to observe planets with up to x200 - in that case, get telescope with 1200mm FL and barlow x2. F/6 is still very fine for plossl use. I have 12mm plossl as my shortest FL plossl and use it in my 8" f/6 dob - works very well and I have no issues with eye relief (but I don't wear glasses when observing - take that into account).

They’re not a perfect match, of course, but part of that reason is that there was a lot of room for slop in this second measurement; I had to set the focus to 7 feet by looking at the range indicator window on the lens rather than by actually focusing on something, for example. But they’re still pretty close… the worst difference is about 3.75%.

Other than that - you are absolutely right, focal length is distance between objective and point where parallel rays bent by lens converge into single point.

So, if you found an angle of 22 degrees, then the calculation will return 61.73mm as a focal length. Wolfram is an education site, so you can get a full, step-by-step solution if you want one, too.

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calculate the focal lengthof a lens of power -2.5 d

Astro and camera definitions of FL are indeed different. Something that often confuses those comming from a camera background.

How tocalculate focal lengthof parabola

[Update: 10/4/2022] I have updated this article to show a method that is more intuitively easy to understand, and which attempts to measure the angle of view of a lens rather than a focal length directly.

(above is of course "arbitrary" and you can say that slow scopes start at F/9 for example instead of F/8 - there is not hard line to be drawn).

In telescope terms, I don't feel that I'm hampered by being a long-time "daytime" photographer. Additionally, "daytime' is, itself, misleading because I often take night-time shots, some of my best shots are such. Perhaps "terrestrial" might be a better label?

Once you have found A, you have half of the vertical angle of view of lens, so to get the complete angle of view, just double it. From there, the easiest course it to look up the corresponding focal length based on that angle; there are numerous standardized tables of such things all over the internet, and also tools that will do the exact calculation.

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While part of answer does relate to actual focal length - as in what sort of plossl eyepiece would be usable on such telescope (and answer is all but shortest FL plossl eyepieces that are not generally usable due to short eye relief - there are much better alternatives in short FL range), I guess another part of question has to do with speed of scope - as plossl eyepieces are not best suited for fast scopes if one wants excellent edge correction.

Visually, F10 is the boundary between long and short for me. Anything longer falls into the LONG focal length catagory, while anything shorter falls into the rich, wide field catagory. It's largely academic from my visual point of view, as many modern short scopes are perfectly capable of delivering views at high power that equal their longer siblings. Conversely, I've used long focal length telescopes to observe exquisitely defined deep sky objects. We can if were not careful, get bogged down believing this does this and that does that, when in reality they all do everything very well despite having their own specialities.

You can still happily use plossls in say F/5 scope. I used plossls with ST102 with 500mm of FL. However, that is wide field achromat and I never really wanted to observe planets with that. Regardless - plossls performed quite well in that scope (and I was not really looking for edge of the field aberrations).

There you go - another daytime photography pearl - 1" sensor - has nothing that is one inch long (neither sides nor diagonal - really no dimension that is 25.4mm).

Agreed, however the "50mm-standard lens" is historical and relates (approximately) to the 35mm film frame diagonal (it's 43mm in reality). A 50mm lens "sees" roughly the same as the human eye would (although not in terms of FOV). So comparing that makes sense to a photographer brought up on film. The F/L is the only way to change the image size for a given sensor (or film format) diagonal, for perspective, FOV or whatever other reasons. Since 35mm film was the dominant film format for many decades, it made sense to use it as "the standard". Indeed, consider the the so-called full-frame digital camera, the frame size is an arbitrarily chosen one in that is the same as, er, 35mm film. It's not full-frame compared to 120 film, 110 film or any of the other (many) film formats used. Likewise, micro 4/3rds is "full-frame" equivalent to 35mm half-frame.

Now, focus the camera and take a photo. If your setup is not completely rigid, take several photos to ensure that the exact center of the image is over the end of the ruler scale.

It turns out that 25mm FL lens is equivalent to 50mm lens for 4/3 sensor and 30.86mm FL lens is equivalent to 50mm lens on APS-C sensor (and bunch of other equivalent FLs ).

50mm on full frame will have same field of view as 25mm on 4/3 sensor or 30.86 on Canon APS-C sensor - that is why they are "equivalent".

Some people have claimed that the Canon 70-200 f/2.8L and IS II do not breath at all. If that were the case, then both of these sets of measurements would have to be off by at least 20% at the 5 foot mark, but hardly at all at the 26 foot end!

Landscape photographers often know the entrance pupil as the “Nodal Point” of the lens, and use nodal rails on their camera when shooting panoramas. If you’re shooting a panorama and the pivot point of your camera is directly below the entrance pupil / nodal point, then you’ll avoid parallax problems. Although finding the nodal point is relatively simple, it’s beyond the scope of this article, but there are numerous good tutorials out there already.

Or has long and short got a weird astro definition as well as every other camera term I've learned in the last 40 years ?

What isfocal lengthof lens

Again, it's from historical comparison as far as I understand. Wikipedia says the 1" format is from vidicon tubes used in early television and the 1" is the tube size not sensor area. I agree the "comparisons" are all somewhat random though and may confuse as much as be useful. However, I think it's handy for a photographer (non astro) to have a yardstick in their mind to use when thinking of composition etc.

In 2016 I had an email discussion with the great Pierre Toscani who really clarified the matter of where to make the camera-side measurement for me. The entrance pupil of the lens is the point where the angle of view begins, and unless you have a lot of technical data about the lens formula of the lens in question and are adept at ray-tracing, it is best to find it via experimentation.

First, determine the location of your lens’s entrance pupil (described below), and then measure that distance to the wall (X in the diagram above) I like to use a precise laser measuring tape. My Bosch GLM 50 is accurate to about 1.5mm, but some others are even better. If you’re careful and have some help, a tape measure will also work, though with reduced accuracy.

I took an under-exposed photo, shot at f/22 (to cut down on bokeh/circle of confusion) with the camera sensor about 26 feet away from the lights, with the lens focused at that distance. The lens, incidentally, was the Canon 70-200 f/2.8L non-IS, since I had read that it behaves similarly to the IS II, but I hadn’t measured it for myself yet. It looked like this:

If you remember your geometry, this is pretty easy, but it’s even easier using an online calculator, like this one at EasyCalculation.com.

Even without the numbers, though, it’s plain to the eye that the lights are getting further apart; the distances involved are not especially subtle. This can be seen simply by looking through the lens and racking the focus. (Note that the CENTERS of the lights are getting further apart, not just the the edges. If I had simply opened the aperture to increase the size of the blur, we’d see results mimicking what you’d expect from zero focus breathing).

Astro and camera definitions of FL are indeed different. Something that often confuses those comming from a camera background.