Best lens for 4x5 (or even 8x10?) astrophotography?

[Dhuiting]

I’m looking to do some astrophotography with Provia 100 on 4x5 (or ideally 8x10, but I am not sure that there is a lens that is fast enough to get me down to a ~25 second exposure or so, the max before you start to see stars blur) and I’m wondering what would a good lens be that is very fast but still quite sharp for 4x5.


I guess if I had to, I could also use a roll of film back if that helps find faster lenses that are a little cheaper, and just shoot 6x7, 6x9, 6x12 Format, etc.


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[Nodda Duma]

Even in the heyday of film astrophotography, it was difficult to find a lens fast enough to give you exposures in that short of a time while taking a satisfying photo. You pretty much had to switch to a Schmidt Camera running at f/1.2 or so.

Your maximum exposure before stars begin to blur is a function of the field of view. For wider FOV you can expose longer. I don't recall the exact equation, though.

Alternatively, you can construct a simple barn door tracker (google it) which will allow you to take longer exposures without blurring.

[Pfsor]

I’m looking to do some astrophotography with Provia 100 on 4x5 (or ideally 8x10, but I am not sure that there is a lens that is fast enough to get me down to a ~25 second exposure or so, the max before you start to see stars blur) and I’m wondering what would a good lens be that is very fast but still quite sharp for 4x5.


I guess if I had to, I could also use a roll of film back if that helps find faster lenses that are a little cheaper, and just shoot 6x7, 6x9, 6x12 Format, etc.


You must find books or web sites about astrophotograhy, there is a plenty to read and it will answer your questions. Google wide angle astrophotography.

[Pere Casals]

i’m looking to do some astrophotography with provia 100 on 4x5

A choice may be Aero Ektar 178 f/2.5 , perhaps some Uran aerial russian lens may be cheaper, also you can push Provia a bit

[xkaes]

Alternatively, you can construct a simple barn door tracker (google it) which will allow you to take longer exposures without blurring.

One book on this is "The Handbook for Star Trackers" by Ballard. He shows how you can make a DIY, manual tracking device. I made one, and have one. It works. The shorter the exposure the better. The wider the lens the better. It's cheap to make, and the more precise you are, the better. If it's not good enough for you, you can get electric trackers for sure.

For what's it's worth, sitting next to the manual tracker and turning the dial 1/8 of an inch (or whatever) ever 30 seconds (or whatever) gets pretty old FAST. But it works. Bring a watch that beeps every 30 seconds (or whatever). I've made some ten minute exposures, but finally decided -- I LIKE THE STAR TRAILS! I set up a 65mm (or whatever) well after sunset -- out in the desert -- and shut in off well before sunrise.

[konakoa]

Dan, I'm not aware of a lens and film combination to record record stars in color and under thirty seconds in large format. I wish such a thing did exist. With only a few exceptions like the Aero Ektar above the vast majority of large format lenses are going to be f/5.6. I've done astrophotography with my 4x5 for many years. The minimum exposure time for stars with a lens at f/5.6 and ISO 100 Provia is about eight minutes in my experience (during a new moon, and don't forget manmade light pollution). I have to use a equatorial telescope for 4x5 astrophotography. See the attached photo below. It works very well, but it's not walk outside, set camera down and immediately make photo quick to use.

[Dhuiting]

Dan, I'm not aware of a lens and film combination to record record stars in color and under thirty seconds in large format. I wish such a thing did exist. With only a few exceptions like the Aero Ektar above the vast majority of large format lenses are going to be f/5.6. I've done astrophotography with my 4x5 for many years. The minimum exposure time for stars with a lens at f/5.6 and ISO 100 Provia is about eight minutes in my experience (during a new moon, and don't forget manmade light pollution). I have to use a equatorial telescope for 4x5 astrophotography. See the attached photo below. It works very well, but it's not walk outside, set camera down and immediately make photo quick to use.

Wow! So the camera actually moves with the stars?

I guess I could always do the “star trails” type of photos where they spin around the North Star, etc.


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[LabRat]

A choice may be Aero Ektar 178 f/2.5 , perhaps some Uran aerial russian lens may be cheaper, also you can push Provia a bit

The AE used to be the "go-to" choice for a fast, cheap (during the 90's about $35) astro lens, but about all lenses need to be stopped down at least 2 stops for better correction from wide open (most noticeable is coma, where star images near the edges sometimes look like seagulls)...

A Biogon 75mm from an aerial camera performs well wide open and should cover most of an 8X10 (made for 9X9)...

But really, using film/LF for astro work is difficult as film will buckle/wave while facing upward on damp, cool nights and flop around in a holder unless a vac holder is used... 35mm sits more stable, but I recently went to a meeting of amateur astronomers and I mentioned I use film, and my guide had to find a polite way to tell me that film was dead in astrophotography, as he showed me a before and after image on his phone of a constellation shot uncorrected that looked just like a so-so night shot with a heavy veil of light pollution, then a shot with hundreds or thousands of frames stacked, and ran through free software that can detect very minor differences of light and pull an image from it... The after shot had full dark sky detail, with the nebula showing in many colors... This was shot with a webcam, laptop, and free software... He might be right...

Steve K

[Jac@stafford.net]

A Biogon 75mm from an aerial camera performs well wide open and should cover most of an 8X10 (made for 9X9)...

Your statement has me wondering! The only ~75mm for LF are the Biogon and the 3" of another make (http://www.digoliardi.net/super-wide-biogon-1/super-biogon-3-inch.jpg) or two. They cover 5x5", but 9x9" ? You devil. I'm going to have to cut another lens board to find out. :) Thanks for the new Winter project, Steve!

[Jim Galli]

Schneider Xenotar

[Jac@stafford.net]

Dan, I'm not aware of a lens and film combination to record record stars in color and under thirty seconds in large format.

Here is what we had to deal with the subject in the Sixties (http://www.digoliardi.net/skc/skc1.jpg). 8x10. Wide angle sky camera. Not so cool today.

[Jac@stafford.net]

Schneider Xenotar

Got one? Cheap?

[LabRat]

I think so, but what I was told (but not tested)... I have the "pick-of-the litter" 75mm f4.5 Biogon of the surplus lenses (in barrel, no iris) that was slated for astro work by me, but never got around to it... Really, I can't believe it, but supposed to be...

Easy enough to test the IC, and is supposed to be even illumination and correction across... Test first!!

Steve K

[Jac@stafford.net]

I think so, but what I was told (but not tested)... I have the "pick-of-the litter" 75mm f4.5 Biogon of the surplus lenses (in barrel, no iris) that was slated for astro work by me, but never got around to it... Really, I can't believe it, but supposed to be...

Easy enough to test the IC, and is supposed to be even illumination and correction across... Test first!!

Steve K

Later we could collaborate by private messaging. I have a few 75mm Biogons including Linhof, a military unit converted to a shutter, a 3" oddity also with shutter added, and at least one very strange one I cannot begin to understand; it's dirty and in remote storage. (All purchased in the days when military surplus meant cheap.)

Seriously, I'm looking at my universal iris-type lens holder for the Green Monster. Life is interesting again.

[Jim Galli]

Got one? Cheap?Yeah! But it suffered some collateral damage in a large explosion years ago:o

[Dan Fromm]

The AE used to be the "go-to" choice for a fast, cheap (during the 90's about $35) astro lens, but about all lenses need to be stopped down at least 2 stops for better correction from wide open (most noticeable is coma, where star images near the edges sometimes look like seagulls)...

A Biogon 75mm from an aerial camera performs well wide open and should cover most of an 8X10 (made for 9X9)...

4.5 x 4.5, Steve, and its an f/4.5 lens.

Jac, I understand the desire to believe in impossible things, but f/4.5 Bertele Biogons cover only a little more than twice their focal lengths. Prayers to Saint Rita of Cascia won't stretch their coverage.

[LabRat]

4.5 x 4.5, Steve, and its an f/4.5 lens.

Jac, I understand the desire to believe in impossible things, but f/4.5 Bertele Biogons cover only a little more than twice their focal lengths. Prayers to Saint Rita of Cascia won't stretch their coverage.

Thanks for that, Dan... I never was sure about what I was told, but I did see the IC was much larger than a 4X5 when projected on the wall...

Still worth checking the IC on this sucker...

Steve K

[Pfsor]

I’m looking to do some astrophotography with Provia 100 on 4x5 (or ideally 8x10, but I am not sure that there is a lens that is fast enough to get me down to a ~25 second exposure or so, the max before you start to see stars blur) and I’m wondering what would a good lens be that is very fast but still quite sharp for 4x5.



Mind you, if you don't take a good book on Wide field film astrophotography you are in danger to be misled by this forum by those who don't know that in astrophotography - stars being a pinpoint light source - the actual absolute aperture diameter is more important than the f number of the lens.
Read - Wide field astrophotography by Robert Reeves (Willmann-Bell 2000) (he used to have a web-site with parts of his book on it). It will save you time and you will be able to recognise what answers are nonsense and what is you need to know - plus it will answer your questions...

[Jac@stafford.net]

4.5 x 4.5, Steve, and its an f/4.5 lens.

Jac, I understand the desire to believe in impossible things, but f/4.5 Bertele Biogons cover only a little more than twice their focal lengths. Prayers to Saint Rita of Cascia won't stretch their coverage.

I know, Dan. I was being polite and I will try the 3" Biogon on 8x10 to provide amusement. Lemmie have some fun, eh?

[Jac@stafford.net]

Yeah! But it suffered some collateral damage in a large explosion years ago:o

Oh, yeah, you science guys. Was it one of the lenses that used an explosive device for a super fast shutter? :) Anything to get the picture!

[Pfsor]

Mind you, if you don't take a good book on Wide field film astrophotography you are in danger to be misled by this forum by those who don't know that in astrophotography - stars being a pinpoint light source - the actual absolute aperture diameter is more important than the f number of the lens.
Read - Wide field astrophotography by Robert Reeves (Willmann-Bell 2000) (he used to have a web-site with parts of his book on it). It will save you time and you will be able to recognise what answers are nonsense and what is you need to know - plus it will answer your questions...

It seems you can even download it free?? http://jeansbet.com/book/1512792/wide-field-astrophotography-exposing-the-universe-starting-with-a-common-camera/

[Drew Wiley]

First of all you need a vacuum filmholder or nitpicking lenses is a waste of time. Plus a big expensive equatorial mount. Plus some high altitude in some remote dry area. There entire websites dedicated to amateur astrophography. But the best work I've seen used 4-element Apo Nikkor lenses. They're especially sharp and Apo f/11 to f/22.

[Drew Wiley]

Of course, you wanted a 25 sec exp, not 6 hours, so I didn't help you much. The most coveted system by widefield shooters without breaking into Fort Knox is the 300EDIF for the Pentax 67. This needs even better support than an 8X10. You can even get a vac plate for the back, but then have to respool 120 film w/o the paper backing since 220 film is scarce. These planet and comet hunter guys haul to stuff around in big trailers with "tripods" costing 50 to 75K. The ridgetop road of the White Mtns on the Calf/Nev border is a popular spot, esp up around the bristlecones at 11000 ft. I've seen vaguely portable reflecting scopes up there with 18" mirrors. Sorry to default to MF, but that is what lots of them use, either film or digi backs.

[Pfsor]

Your maximum exposure before stars begin to blur is a function of the field of view. For wider FOV you can expose longer. I don't recall the exact equation, though.


As a rule of thumb, divide 1000 by the focal length (in mm) and you get the time in seconds that you have for the exposure before star trails start to appear. It also depends on how close the the North or South celestial pole your camera points - the closer you get to it the longer your exposure can be before you fall into the star trails trap. There you'll see that you don't need to have a big expensive equatorial mount for your camera to venture into astrophotography.

[Corran]

Mind you, if you don't take a good book on Wide field film astrophotography you are in danger to be misled by this forum by those who don't know that in astrophotography - stars being a pinpoint light source - the actual absolute aperture diameter is more important than the f number of the lens.

You still have never answered my question with regard to differences in focal length and angular movement as it relates to that. Should not a, say, 75mm lens have a different exposure than a 150mm lens due to differences in magnification from focal length? This would cause the star to "move" slower or faster, giving more exposure, no?

To answer the OP, the 75mm Biogon is a great choice for speed, especially as a wide angle (I am more partial to wide angle, startrail images). Jac, I can tell you that the Linhof 75mm Biogon and the oddball barrel-mount ones that some have remounted into Copal 1 shutters definitely only cover 4x5. I've hit the edge on them quite quickly! The remounted barrel mount one (can't remember the brand that was written on it, it's long gone) might've had a smidgen larger IC compared to the Linhof one. I don't use extensive movements with the Biogons for sure. I was just out shooting with mine today.\

[Pere Casals]

The AE used to be the "go-to" choice for a fast, cheap (during the 90's about $35) astro lens, but about all lenses need to be stopped down at least 2 stops for better correction from wide open (most noticeable is coma, where star images near the edges sometimes look like seagulls)...

A Biogon 75mm from an aerial camera performs well wide open and should cover most of an 8X10 (made for 9X9)...

But really, using film/LF for astro work is difficult as film will buckle/wave while facing upward on damp, cool nights and flop around in a holder unless a vac holder is used... 35mm sits more stable, but I recently went to a meeting of amateur astronomers and I mentioned I use film, and my guide had to find a polite way to tell me that film was dead in astrophotography, as he showed me a before and after image on his phone of a constellation shot uncorrected that looked just like a so-so night shot with a heavy veil of light pollution, then a shot with hundreds or thousands of frames stacked, and ran through free software that can detect very minor differences of light and pull an image from it... The after shot had full dark sky detail, with the nebula showing in many colors... This was shot with a webcam, laptop, and free software... He might be right...

Steve K

Yes I agree... stacking is an digital impressive tool for astro... Anyway the AE coma in the corners perhaps would not be seen much in a pictorial shot...

A 75mm for 8x10, beyond vigneting it would have incredible fall off specially well oppened... and a CF in the night could not be recommended...

[Pere Casals]

As a rule of thumb, divide 1000 by the focal length (in mm) and you get the time in seconds that you have for the exposure before star trails start to appear.

This is for DSLR formats, for 4x5 it would be dividing 4000 by the focal length (in mm)

[Pfsor]

This is for DSLR formats, for 4x5 it would be dividing 4000 by the focal length (in mm)

Has nothing to do with DSLR camera format. It is used for 35 mm cameras, for MF and LF too.

[Pfsor]

You still have never answered my question with regard to differences in focal length and angular movement as it relates to that. Should not a, say, 75mm lens have a different exposure than a 150mm lens due to differences in magnification from focal length? This would cause the star to "move" slower or faster, giving more exposure, no?
\
See the post n. 24. That should answer you. Don't know anything about your question you allude to.

[Corran]

You still don't seem to understand my premise / thought experiment. I believe the issue w/ regard to absolute aperture size is only relevant if one is shooting on a tracking mount. For long exposures and startrail images, not so much, due to differences in angular speed relative to the film. At least, that has been my experience.

[Pfsor]

To answer the OP, the 75mm Biogon is a great choice for speed, especially as a wide angle (I am more partial to wide angle, startrail images). \

There we go - as soon as the topic of lenses for astrophotography is discussed on this forum there is always somebody who doesn't know that the actual aperture diameter of the lens is more important in astrophotography than the f- number. In this case a 100mm/2.8 lens is better for pinpoint light from stars than 35mm/2.8 lens. It will gather more stars on the film.

[Pfsor]

You still don't seem to understand my premise / thought experiment. I believe the issue w/ regard to absolute aperture size is only relevant if one is shooting on a tracking mount. For long exposures and startrail images, not so much, due to differences in angular speed relative to the film. At least, that has been my experience.

You must rewrite astrophotography books with your experience. But it will be a pretty futile endavour.

[Corran]

Pfsor, I see you still can't comprehend that the focal length of the lens is integral to the photograph, when including foreground and shooting a composition. Exposure-wise, it doesn't matter whatsoever that a 300mm f/5.6 is a larger absolute aperture diameter, because you can't shoot a wide-angle photo with a 300mm lens (on 4x5).

I don't believe there are any 75mm lenses faster than f/4.5, so that is indeed the fastest you can get, with the most exposure, at that focal length. Yes, you could say the 90mm f/4.5 lenses available are a little larger in aperture size, and close to the same field of view, but I doubt they would be as good of a lens as the Biogon at full aperture.

I specifically mentioned startrail images, not tracked images with no foreground. That's not my bag.

[Corran]

You must rewrite astrophotography books with your experience. But it will be a pretty futile endavour.

Please see post #25 and enlighten us how angular speed differences as it relates to focal length on a fixed camera affects exposure, as I would really like to know.

[Pere Casals]

Has nothing to do with DSLR camera format. It is used for 35 mm cameras, for MF and LF too.

Even for DSLR cameras a different factor is recommended depending on if the camera is DX or FX, the time you may expose with DX it is dividing the FX time by 1.5. Another for way for DX it's not using the actual focal but the equivalent FX focal for the calculation.

[Pfsor]

Pfsor, I see you still can't comprehend that the focal length of the lens is integral to the photograph, when including foreground and shooting a composition. Exposure-wise, it doesn't matter whatsoever that a 300mm f/5.6 is a larger absolute aperture diameter, because you can't shoot a wide-angle photo with a 300mm lens (on 4x5).
...
I specifically mentioned startrail images, not tracked images with no foreground. That's not my bag.

The absolute aperture diameter is decisive for exposure from pinpoint light sources. That exposure is then what it is regardless of the foreground on your picture. The physics of the light doesn't change according to the foreground on your picture.

[Drew Wiley]

Just visit a dedicated Astro photo site and ask questions there. ... And drool over their gear.

[Pfsor]

Even for DSLR cameras a different factor is recommended depending on if the camera is DX or FX, the time you may expose with DX it is dividing the FX time by 1.5. Another for way for DX it's not using the actual focal but the equivalent FX focal for the calculation.

In case you haven't noticed - we are speaking film photography as to the OP post...

[Corran]

Is this a language issue? Please let me know how am I going to get a wide-angle image and see the foreground up to the north star with a 300mm lens on 4x5?

[Pfsor]

Please see post #25 and enlighten us how angular speed differences as it relates to focal length on a fixed camera affects exposure, as I would really like to know.

You too would benefit from the good book Wide field astrophotography by Robert Reeves. Don't hesitate - https://www.amazon.com/Wide-Field-Astrophotography-Exposing-Universe-Starting/dp/0943396646

[Pere Casals]

In case you haven't noticed - we are speaking film photography as to the OP post...

Sensors and film behave the same with star trails, different times for FX and DX is because different format size, if it was film it would be exactly the same.

[Pfsor]

Is this a language issue? Please let me know how am I going to get a wide-angle image and see the foreground up to the north star with a 300mm lens on 4x5?

Would you like to try a picture of a spruce top with Polaris star over it? You're free to go but don't ask me to guide you, please.

[Corran]

You too would benefit from the good book Wide field astrophotography by Robert Reeves. Don't hesitate - https://www.amazon.com/Wide-Field-Astrophotography-Exposing-Universe-Starting/dp/0943396646

As helpful as ever - as in, not at all. If you don't know the answer, that's fine.


spruce top

Ground, up to north star, is obviously what I meant. And please don't respond with a latitude that would afford such a view, you know what I mean.

[Pfsor]

Sensors and film behave the same with star trails, different times for FX and DX is because different format size, if it was film it would be exactly the same.

Because film gathers light from a pinpoint source differently according to its real estate behind the lens? Never noticed that during my star photography.

[Greg]

http://www.astropix.com/html/i_astrop/film_dig.html

Great comparison of film and Digital

[Pfsor]

Ground, up to north star, is obviously what I meant. And please don't respond with a latitude that would afford such a view, you know what I mean.

Change the spruce top for a mighty boulder on the ground or a mountain top a little bit far away and go ahead with your night time photography. Pursue your inspiration. Just don't ask me to guide you, please.

[Corran]

You are very adept at dodging the question, congratulations. Clearly you have no idea what the answer is. Thanks.

[Pere Casals]

Because film gathers light from a pinpoint source differently according to its real estate behind the lens? Never noticed that during my star photography.

No...

Using your formula for LF leads to absurd, 360mm with 8x10 with is a normal lens, 1000/360 it is 2.8s , so you say that the recommended exposure time for night sky pictorial shot with 8x10 and the normal lens is 2.8S

Is it that what you state ?

[Pfsor]

I’m looking to do some astrophotography with Provia 100 on 4x5 (or ideally 8x10, but I am not sure that there is a lens that is fast enough to get me down to a ~25 second exposure or so, the max before you start to see stars blur) and I’m wondering what would a good lens be that is very fast but still quite sharp for 4x5.
...


There you see what I meant by gaining more from reading the good book by Robert Reeves rather than asking on this forum... By the way, the book received a very good reception from Sky&Telescope in its time. You can read it from the cover to the cover in one go, so interesting and easy to understand it is. It's a pity Robert Reeves doesn't have his web site anymore or at least I couldn't find it for you.

[Pfsor]

No...

Using your formula for LF leads to absurd, 360mm with 8x10 with is a normal lens, 1000/360 it is 2.8s , so you say that the recommended exposure time for night sky with 8x10 and the normal lens is 2.8S

Is it that what you state ?

What the formula states is that dividing 1000 by the lens focal length in mm you get the time in seconds that you can use before star trailing appears. Not so much difficult to understand, I'm sure.
And by the way, 360mm is 360mm focal length regardless of the fact if it is a normal lens for some format or a long lens for the other. It still gathers light in the same way and exposes the film in the same way. Not much difficult to understand either.

[Greg]

This thread just brought back a long ago forgotten memory.... Memory is vague but here goes: When I was a student at RIT, my mentor was Nile Root. He ran the BioMedical Photography program at the time. He was also an excellent and very talented astrophotographer. I vaguely remember him telling me that he tried to do some astrophotography by piggybacking his 4x5 onto his professional quality telescope. He used his 150mm f/2.8 Xenotar. I believe he told me that after many times of trying to use his setup, he gave up because of the too many problems he had encountered. Only one I remember was that the 4x5 film buckled during the exposures, and he couldn't justify purchasing a vacuum back for his camera.

[Pfsor]

Only one I remember was that the 4x5 film buckled during the exposures, and he couldn't justify purchasing a vacuum back for his camera.

That's a good point. I also think that it is much more rewarding to pursue astrophotography with a MF rather than trying the larger film real estate, at least at the beginning.

[Nodda Duma]

The aperture determines the dimmest stars the imaging system will record above the background. The f/# and film speed determines how long your exposure will be before you can see them (and your longest exposure before the background fogs). The focal length sets the field of view. The optic / telescope design determines how big a piece of film you can stick behind the glass.

It's been 15 years, but:

http://www1.iwvisp.com/opticman/astroimages/M42_031004.jpg

This was Provia 400F. It was a glorious film for astrophotography.

[Drew Wiley]

That's lovely.

[LabRat]

http://www.astropix.com/html/i_astrop/film_dig.html

Great comparison of film and Digital

Cool, but even more impressive is stacking technology... You are taking thousands of short exposures, and the software is looking at every frame (for several parameters), choosing the best for that parameter, and stacking these on top of each other (which cancels out the random noise from each frame)... Then the software has a microdensitometer function that reads very minute differences in brightness over the light pollution layer, and finds the spots that have VERY slight differences in brightness over the background and scales them... Then other processing that might last as long as 20-40 minutes, but the (extremely sharp) result looks like something from a coffee table book on the universe... Just a webcam on a modest scope, even on fairly crappy skies!!! Try to find examples of these...

The examples in the link are single exposures, but very different from stacked images...

Keep looking up!!!

Steve K

[Nodda Duma]

That's lovely.

It was taken near you, back when I lived in Inyokern.

[Dhuiting]

The aperture determines the dimmest stars the imaging system will record above the background. The f/# and film speed determines how long your exposure will be before you can see them (and your longest exposure before the background fogs). The focal length sets the field of view. The optic / telescope design determines how big a piece of film you can stick behind the glass.

It's been 15 years, but:

http://www1.iwvisp.com/opticman/astroimages/M42_031004.jpg

This was Provia 400F. It was a glorious film for astrophotography.

This is amazing!

I decided that I would like to try a shot with star trails, so I’m no longer worried about the exposure time, but the film buckling seems like an issue.

I’ve had other photographers mention “film” pop as something you can help by pulling the dark slide early and letting the film acclimate first, then doing your long exposure. But apparently that’s not enough.

If it helps, I’ll be in the Bisti wilderness desert in NM, about 6,200 ft above sea level (someone mentioned finding a dry, high altitude place. At least I’ll have the latter.)

I could try it with my roll film back on my 4x5, perhaps that film will buckle less? Only problem is my widest lens is a 90mm.

(I was really wanting to try my 120mm Nikkor SW f8 lens on my 8x10, but maybe that’s a lost cause with the film buckling issue.)

Thanks anyway for all the responses.


Sent from my iPhone using Tapatalk

[Pere Casals]

What the formula states is that dividing 1000 by the lens focal length in mm you get the time in seconds that you can use before star trailing appears. Not so much difficult to understand, I'm sure.
And by the way, 360mm is 360mm focal length regardless of the fact if it is a normal lens for some format or a long lens for the other. It still gathers light in the same way and exposes the film in the same way. Not much difficult to understand either.

Professor, you are completely wrong with that, this is from the book: "And all formats have the same recommended time to hide trails if using the normal lenses".

Look, 35mm film and 8x10 sheet film, each camera with normal lens, a Nikon F5 with a 50mm and the 8x10 with a 360mm lens, have same max exposure time for stars trails in a pictorial shot.

Shot both cameras 40s, then make the two prints to 30", you'll see the same.

Then make with each camera a 3min exposure and enlarge again the prints: you'll see exactly the same trails.

The trails are recorded in a 5 seconds exposure in a normal lens, when recommended value for normal is 40s. Astrophotographers don't use that formula because they use tracking gear. The 1000/F formula is for 35mm film only, for pictorial shots.

... and recommending 2.8s for 8x10/360mm has no sense.



The aperture determines the dimmest stars the imaging system will record above the background. The f/# and film speed determines how long your exposure will be before you can see them (and your longest exposure before the background fogs). The focal length sets the field of view. The optic / telescope design determines how big a piece of film you can stick behind the glass.

It's been 15 years, but:

http://www1.iwvisp.com/opticman/astroimages/M42_031004.jpg

This was Provia 400F. It was a glorious film for astrophotography.

Amazing shot, beautiful !!!!