What telescope focal lengths are considered long or short?

[bendiddley]

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???

[ollypenrice]

2 minutes ago, bendiddley said:

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???

That's about it but the terms are very vague. 

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.

Olly

[vlaiv]

4 minutes ago, bendiddley said:

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???

Depends in which context you define long and short.

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.

Maybe you think visual, but what sort of visual? Planetary? Wide field? General purpose?

Again - here you need to match that to usual eyepiece focal lengths and wanted magnification range.

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.

[bendiddley]

12 minutes ago, vlaiv said:

Depends in which context you define long and short.

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.

Maybe you think visual, but what sort of visual? Planetary? Wide field? General purpose?

Again - here you need to match that to usual eyepiece focal lengths and wanted magnification range.

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.

Thanks, sorry I should have been more specific. It's in the context of general visual use, using plossl eyepieces.

[vlaiv]

Just now, bendiddley said:

Thanks, sorry I should have been more specific. It's in the context of general visual use, using plossl eyepieces.

In that case, maybe you are more interested in speed rather than focal length.

below F/6 - fast scopes

F/6-F/8 - medium speed scopes

above F/8 - slow scopes

(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).

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).

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).

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).

[mikeDnight]

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.

Edited April 27, 2021 by mikeDnight

[powerlord]

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 ?

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 ?

stu

Edited April 27, 2021 by powerlord

[vlaiv]

1 minute ago, powerlord said:

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 ?

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 ?

stu

Well, question was asked in context of plossl eyepieces and there seems to be related discussion here:

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.

In that sense - both actual FL and F/ratio are part of answer.

[johninderby]

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

Edited April 27, 2021 by johninderby

[vlaiv]

6 minutes ago, johninderby said:

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

How so?

[johninderby]

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. 

[jetstream]

My 2499 mm fl f4.1 dob has a long fl but also a fast f ratio...

Edited April 27, 2021 by jetstream

[Seelive]

1 hour ago, johninderby said:

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

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.

[wulfrun]

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.

[vlaiv]

56 minutes ago, wulfrun said:

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

One such thing of the top of my head is "50mm equivalent lens" for either APS-C or 4/3 sensor.

What could be equivalent of focal length? And surely different numbers can't all be equivalent to certain focal length?

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 ).

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.

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".

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.

[Seelive]

Good job that plates and 120 roll film are virtually a thing of the past then 😉

[wulfrun]

1 hour ago, vlaiv said:

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

One such thing of the top of my head is "50mm equivalent lens" for either APS-C or 4/3 sensor.

What could be equivalent of focal length? And surely different numbers can't all be equivalent to certain focal length?

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 ).

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.

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".

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.

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.

[vlaiv]

31 minutes ago, wulfrun said:

Likewise, micro 4/3rds is "full-frame" equivalent to 35mm half-frame.

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).

[wulfrun]

31 minutes ago, vlaiv said:

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).

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.

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.

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?

"Long" and "short" F/L may be rather different in the astro world but they are just pigeon-holes.

Edited April 27, 2021 by wulfrun
typo

[ollypenrice]

6 hours ago, johninderby said:

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. 

...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.)

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...)

Olly

[vlaiv]

11 minutes ago, ollypenrice said:

(Infinitely difficult, infinitely expensive, infinitely exasperating...)

Don't forget - infinite exposure length required to achieve wanted SNR