Astrophotography with 4x5

Lee L · Apr 10, 2007

When astronomers talk about sky glow, they're talking about man-made artificial light or the moon lighting up earth's atmosphere, local stuff, not what's out beyond our atmosphere, and definitely not the glow from our galaxy, nebulae, etc. This atmospheric sky glow overwhelms and limits what you can see of space.

As for aperture, a telescope's limiting magnitude for viewing pinpoint sources like stars is calculated with the formula: 7.5 + 5 x log(aperture in cm) For photography, about 2 magnitudes are added because the film can accumulate light over time, unlike the eye. The quality and condition of the glass, mirror coating, light loss due to large numbers of glass elements, etc, come into play, but not focal length.

If you don't want to believe this or research it, that's fine by me. My wife has students that "don't believe in graphs". You could test it for yourself by setting two different lenses to the same physical aperture in mm and shooting the same film under identical conditions. The wider field lens will see more stars across the sky that appear more crowded, but if you look at the same section of sky seen in the longer f.l. lens, you won't see any more stars, even though you used a "faster" f-stop on the shorter lens.

Lee

Sparky · Apr 10, 2007

well thanks for the further explanation lee... the problem is, well... now my BRAIN hurts... it just doesn't make sense to me! I'll have to research this further! Thanks for the patience.

Lee L · Apr 10, 2007

Sparky said:
well thanks for the further explanation lee... the problem is, well... now my BRAIN hurts... it just doesn't make sense to me! I'll have to research this further! Thanks for the patience.

I'll freely admit that this behavior seems somewhat counterintuitive. Think about it this way: Why build all those large diameter telescopes on mountaintops? You get up above the thicker atmosphere, away from light pollution, and you build big because square area gives you "signal gain". More square meters = more incident photons, all of which are coming in parallel to each other because of the distances involved - the point source. You get the same number of photons striking each square meter regardless of focal length, so you increase your capture area to increase gain, regardless of focal length. So it's absolute aperture that really counts, not f-ratios.

Longer focal length will give you more magnification, but with nearly all stars, that's a futile approach for anything but spreading the stars apart in the field you're viewing. Only a handful are close enough to measure radius optically, with even the most powerful methods and instruments available.

The term "limiting magnitude" is applied in several different ways, but check out the term with google where it applies to telescope apertures for a variety of explanations, some of which may work better for you than others.

Lee

George · Apr 10, 2007

Sparky,
nothing to worry too much about. The universe is full of mystery. Thanks LeeL for the explanations - I hate writing long explanations myself.

konakoa · Apr 10, 2007

A little disclaimer here: I don't claim to be a astronomer. I'm a photographer who happens to enjoy capturing images of the night sky. My abilities in astronomy only go so far as to finding a few constellations and identifying a few of the brighter stars.

George, Lee, what you've brought up is perfectly valid and true. If theories and formulas help describe why something works, it certainly helps myself and others to better understand the process. Large apertures mean fainter stars can be recorded regardless of the f/stop. I get it now. (But couldn't you both have just said that from the beginning?) Personally I'm just not that complicated. I simply use what camera equipment I have and do the best I can with it. I don't have the Keck telescopes at my disposal. Nor, I doubt would they let anyone hook a film camera up to one of them.

I'm certain this is flawed, but here's a comparison of Orion on two of my cameras. On the left is Orion taken with my 35mm camera. On the right is Orion taken with my 4x5. These were taken months apart, scanned on different kinds of scanners (desktop 35, flatbed 4x5) different resolutions, etc. I tried to make them similar for this comparison (levels, color balance) yet this is the best I can do. I can say this with certainty: the 4x5 records far more stars than I can get with my 35.

BTW Lee, you used the exact same example out of Covington's book in your posting yesterday. For shame.

DrPablo · Apr 10, 2007

Danny, what film was that? E100G for both of them? How long did you expose for?

Sparky · Apr 10, 2007

Danny Spence said:
but here's a comparison of Orion on two of my cameras. On the left is Orion taken with my 35mm camera. On the right is Orion taken with my 4x5.

What was your exposure, Danny? I'm curious!

konakoa · Apr 11, 2007

Yes, both shots were on E100G. The 35mm exposure was two minutes at f/2.8. For the 4x5, it was fifteen minutes at f/5.6.

I've also used Kodak Elite Chrome 100 for astrophotography with identical results. I've heard the Elite Chrome 100 is the same emulsion as E100G -- I don't know if that's true, but I don't think it's a much of a stretch. I can't tell the two apart when they're side by side on my light box. YMMV.

Elite Chrome 100 and E100G record the sky (or just my local light pollution) with some blue. I like the look myself. However, most of the photos by other astrophotographers I've seen heavily favor intense reds. If you're after good reds, I've shot and found that Kodak Elite Chrome 200 is marvelous. Reds just pop with this film. Unfortunately, it's only available in 35mm.

Dan Fromm · Apr 11, 2007

Danny, EB was introduced decades before E100G and seems to be much the same as EPP. E100G is a T-grain film and much finer-grained.

Historical facts aside, EKCo may well be selling E100G labeled Elite Chrome nowadays. But in beginning, relatively, was EB ...

George · Apr 11, 2007

Danny Spence said:
Yes, both shots were on E100G. The 35mm exposure was two minutes at f/2.8. For the 4x5, it was fifteen minutes at f/5.6.

...

Danny, your pictures confirm the theory - the 85mm on 2.8 (not 1.4 as you have written on the picture) has the actual opening just 30.3mm. The 300mm on 5.6 has 53.5mm opening - much bigger, much more open for the beautiful stars you admire. What is more, you gave the 300mm lens much longer time to eat the stars...

Sparky · Apr 11, 2007

I'm still trying to wrap my head around this 'absolute aperture' phenomenon! Do you think it has to do with the fact that the rays from the celestial light are coming in parallel?? - and are therefore 'captured' once past the iris?

snaggs · Apr 11, 2007

Danny, can you give us a crop of the orion section so we can compare the details there?

Cheers,

Daniel.

PS. Anyone tried E100VS? I've got some expired E100VS I thought I'd experiment with.

Danny Spence said:
A little disclaimer here: I don't claim to be a astronomer. I'm a photographer who happens to enjoy capturing images of the night sky. My abilities in astronomy only go so far as to finding a few constellations and identifying a few of the brighter stars.

George, Lee, what you've brought up is perfectly valid and true. If theories and formulas help describe why something works, it certainly helps myself and others to better understand the process. Large apertures mean fainter stars can be recorded regardless of the f/stop. I get it now. (But couldn't you both have just said that from the beginning?) Personally I'm just not that complicated. I simply use what camera equipment I have and do the best I can with it. I don't have the Keck telescopes at my disposal. Nor, I doubt would they let anyone hook a film camera up to one of them.

I'm certain this is flawed, but here's a comparison of Orion on two of my cameras. On the left is Orion taken with my 35mm camera. On the right is Orion taken with my 4x5. These were taken months apart, scanned on different kinds of scanners (desktop 35, flatbed 4x5) different resolutions, etc. I tried to make them similar for this comparison (levels, color balance) yet this is the best I can do. I can say this with certainty: the 4x5 records far more stars than I can get with my 35.

BTW Lee, you used the exact same example out of Covington's book in your posting yesterday. For shame.

konakoa · Apr 11, 2007

George, it's a old habit for me to label my lenses by the maximum aperture. It makes identifying them easier for me. I stopped my 85 f/1.4 down to f/2.8 for the exposure I posted. Most of my 35mm lenses suffer some diffraction -- stars literally appear as UFOs near the frame edges -- when shot wide open. Depending on the lens I stop down to f/2.8, f/4, or f/5.6 - this improves the edges a good deal - for my 35mm astrophotos.

Did you calculate for a 85 f/2.8 lens or for a f/1.4? I can measure the physical size of the front element of these lenses if you need it. And yes, the 35/4x5 exposure times are dissimilar - I start to record skyglow (light pollution) with my 35 long before it shows up on the 4x5. I dunno why.

Daniel, I'll pm you with a few. I need to rescan 'em for that.

konakoa · Apr 11, 2007

Jonathan, I'm likening George's "nominal aperture" to a bucket out in the rain. The bigger the bucket, the more it collects. The f/stop of the lens is just a spigot letting the water out.

BTW Dan Fromm - nice to see some history on the film. I've been shooting it since the old Elite II. I thought I was one of only five people who actually liked and used the stuff.

dslater · Apr 12, 2007

Sparky said:
I'm still trying to wrap my head around this 'absolute aperture' phenomenon! Do you think it has to do with the fact that the rays from the celestial light are coming in parallel?? - and are therefore 'captured' once past the iris?

Hi Sparky,
I think the easiest way to think about this is to consider 2 lenses that have the same absolute aperture but different focal lengths. For example, say you have a 50mm f/2 lens and a 100mm f/4 lens. For both lenses, the absolute aperture is 25mm i.e. 50/2 or 100/4. When you take a picture of an extended object, the 100mm lens will need 4 times as much exposure as the 50mm lens because the light gathered by that 25mm aperture is spread out over 4 times the area than the 50mm lens.
Imaging stars is different. Stars are point light sources and a lens can't actually make an image of a star. What happens is that the star's light is spread out into a small disk called the airy disk. The smearing out is caused by diffraction and the size of the disk is determined only by the absolute aperture of the lens. So for a star, the 50mm lens and the 100mm lens will create the same sized disk and will need the same exposure.

Hope this helps.

Sparky · Apr 12, 2007

well, having a physics degree and everything - I DO get the idea of airy disks and Rayleigh's criterion and all that stuff...

but this is very elucidating! Very. The 50 and 100mm examples you provide DO hold true - but ONLY given a constant distance to film (dependent on retrofocus!). Diffraction is dependent on TWO things. Aperture size, and distance to film. This is why Chris Perez' lens tests suggest that longer focal length lenses get worse and worse resolution specs as they get longer and longer - and why medium format and 35mm cameras get superior results. I'm absolutely sure of it. I wonder (NOT TO RE-OPEN A CAN OF WORMS HERE) if the rule of thumb previously mentioned was meant only for fixed nodal point to film plane distance...? That would make sense considering using SLRs was the norm for such work...

Helen B · Apr 12, 2007

Try looking at it another way. As you say, for a given wavelength the diameter of the Airy Disk is a function of the f-number, not the absolute aperture. Therefore a 100 mm lens at f/2 and a 50 mm lens at f/2 both produce the same diameter of Airy Disk. The area of the entrance pupil of the 100 mm lens is four times that of the 50 mm lens (one has a diameter of 50 mm and the other has a diameter of 25 mm). Therefore the 100 mm f/2 lens will collect four times as much light from a star into the same image area as the 50 mm f/2 lens.

How does that sound?

Best,
Helen

Sparky · Apr 12, 2007

Helen - What is GREAT about you is your clear writing style - so thanks for that! At any rate - I agree with what you say - but I suspect you've overlooked something (??) - you state flat-out that the airy disk diameter is a function of the f-number. But is it possibly that this is different for retrofocus than other lenses? THAT'S the question that I'M interested in. The relationship that you take as a given depends really on the distance travelled from the iris TO the film plane - that's what sets the airy disk size. Agree with me on that? SO - the problem HERE seems to be that, for retrofocus lenses, since the focal length is only an EFFECTIVE one, does it not stand to reason that the airy disk size would vary not as we expect?

If I'm just chasing my tail here, maybe you could point out the logical flaw in my argument...?

thanks.

Helen B said:
Try looking at it another way. As you say, for a given wavelength the diameter of the Airy Disk is a function of the f-number, not the absolute aperture. Therefore a 100 mm lens at f/2 and a 50 mm lens at f/2 both produce the same diameter of Airy Disk. The area of the entrance pupil of the 100 mm lens is four times that of the 50 mm lens (one has a diameter of 50 mm and the other has a diameter of 25 mm). Therefore the 100 mm f/2 lens will collect four times as much light from a star into the same image area as the 50 mm f/2 lens.

How does that sound?

Best,
Helen

George · Apr 12, 2007

Danny Spence said:
...
Did you calculate for a 85 f/2.8 lens or for a f/1.4? I can measure the physical size of the front element of these lenses if you need it. And yes, the 35/4x5 exposure times are dissimilar - I start to record skyglow (light pollution) with my 35 long before it shows up on the 4x5. I dunno why.

Daniel, I'll pm you with a few. I need to rescan 'em for that.

Of course I calculated the actual aperture for the f/2.8 (85:2.8= 30.3) And of course you started to record the sky glow quicker with the f/2.8 - 'cause it is a bigger (nominally) aperture than the f/5.6 and as the theory goes, the non point light (= sky glow) obeys the law of the nominal aperture. All in all, your pictures confirm the theory on both sides - to your greater confusion, I'm afraid

Helen B · Apr 12, 2007

Sparky said:
...but I suspect you've overlooked something (??) - you state flat-out that the airy disk diameter is a function of the f-number. But is it possibly that this is different for retrofocus than other lenses? THAT'S the question that I'M interested in. The relationship that you take as a given depends really on the distance travelled from the iris TO the film plane - that's what sets the airy disk size. Agree with me on that? SO - the problem HERE seems to be that, for retrofocus lenses, since the focal length is only an EFFECTIVE one, does it not stand to reason that the airy disk size would vary not as we expect?

If I'm just chasing my tail here, maybe you could point out the logical flaw in my argument...?

thanks.

Jonathan,

Thanks for the kind comment. I have a bit of a struggle with words, and have to make an effort.

Back to retrofocus lenses. I didn't forget about them, but I was trying to. Of course you are correct: the diameter of the Airy Disk depends on the absolute aperture (I'm going to call it the iris diameter from here on) and the distance from the aperture to the image plane. Perhaps the more rigorous statement is that the angular magnitude of the Airy Disk depends only on the iris diameter (all for a given wavelength, of course). Here's a Zeiss 35 mm f/3.5 retrofocus lens for a Contax 645:

Dead Link Removed

The iris - which should be visible as the gap between the faint grey lines to the left of the exit pupil - is much further from the image plane than the rear nodal plane is. At about 22 mm diameter is also larger than the entrance pupil - which is 10 mm (35/3.5 mm, actually 9.9 mm according to Zeiss) in diameter.

So, the angular diffraction created by the iris is about 10/22 of that which would be created by an iris of 10 mm diameter. However, as you so rightly point out, the iris is not 35 mm from the image plane, it is further away. There are also some bits of glass between the iris and the film plane, and they affect the angle of the rays passing through them. The rear elements change the apparent location and apparent diameter of the iris.

The apparent location of the iris - the optical path length to the image plane - is in the plane of the exit pupil. Easy? But wait. Should we use the exit pupil diameter and location to determine the diameter of the Airy Disk, or should we use the iris diameter and the exit pupil location?

If we use the exit pupil diameter (27.7 mm) and location (97.7 mm from the image plane at infinity), then the ratio of distance to diameter comes out at 3.5 - the f-number of the lens. No surprise. But is that the correct method?

Just as this is getting interesting, I'm going to have to sign off and continue later...

dslater · Apr 12, 2007

Sparky said:
well, having a physics degree and everything - I DO get the idea of airy disks and Rayleigh's criterion and all that stuff...

but this is very elucidating! Very. The 50 and 100mm examples you provide DO hold true - but ONLY given a constant distance to film (dependent on retrofocus!). Diffraction is dependent on TWO things. Aperture size, and distance to film. This is why Chris Perez' lens tests suggest that longer focal length lenses get worse and worse resolution specs as they get longer and longer - and why medium format and 35mm cameras get superior results. I'm absolutely sure of it. I wonder (NOT TO RE-OPEN A CAN OF WORMS HERE) if the rule of thumb previously mentioned was meant only for fixed nodal point to film plane distance...? That would make sense considering using SLRs was the norm for such work...

Hey Sparky how about that - I have a physics degree as well. I was under the impression that you were confused as to why the brightness of stars in telescopes was only dependent on aperture, not f-ratio. Perhaps I was mistaken. In any case I was wrong about the Airy disk, the linear size of the Airy disk does indeed depend on the f-ratio. I found this formula:

D = 2.43932 x λ x F

where D is the diameter of the disk in mm
λ is the wavelength of light in mm
and F is the focal ratio

This is for a simple lens. However, I don't think that a retro-focus or telephoto design changes this. I think you can think of these designs as a combination of a simple lens that produces an airy disk and another lens that acts like an eyepiece to reduce or enlarge the image. I believe the end result is that the Airy disk size depends on the aperture and effective focal length of the lens - not the distance to the film plane.

Consider this, if it were the distance to the focal plane that mattered and not the ELF, then astronomers would be able to improve the resolution of a telescope by making it some kind of telephoto design.

Since we're both physicists lets stop talking about aperture and absolute aperture and say what we mean - namely aperture (the diameter of your lens/mirror) and f-ratio (the focal length of the lens / aperture) - seems much less confusing to me that way.

As to why stellar images only depend on the aperture, I think the real explanation is that even though the 100mm lens does indeed have a larger Airy disk than the 50mm lens, the disk is still much smaller that a grain of film, or a digital sensor, or a rod or cone in you eye, so even though the disk is larger, all the energy is still captured by one detector unless your f-ratio is very high.

walter23 · Apr 12, 2007

One possibility is to use tungsten balanced film (64T) to get a deep blue colour to the sky. I've seen other people do this but I've never used it myself.

Sparky · Apr 12, 2007

I think I'm going to need some more coffee before I can respond to, much less absorb, any of this! ack. (just woke up not so long ago!)

Helen B · Apr 12, 2007

I wrote: "Perhaps the more rigorous statement is that the angular magnitude of the Airy Disk depends only on the iris diameter (all for a given wavelength, of course)."

Re-reading that shows that it doesn't quite tally with the rest of what I wrote. It should have been clearer, and just referred to the initial angle of spread.

Anyway, I hope that the dependence on f-number and wavelength is clear enough. Few texts go into the treatment of anything other than a simple lens. The f-number is, of course, calculated from the diameter of the entrance pupil, and not the iris diameter. That is what should be used in the Airy Disk formula.

Only a small part of the story.

Best,
Helen

konakoa · Apr 12, 2007

George said:
Of course I calculated the actual aperture for the f/2.8 (85:2.8= 30.3) And of course you started to record the sky glow quicker with the f/2.8 - 'cause it is a bigger (nominally) aperture than the f/5.6 and as the theory goes, the non point light (= sky glow) obeys the law of the nominal aperture. All in all, your pictures confirm the theory on both sides - to your greater confusion, I'm afraid

George, I by no means want to be contrary but now I've just got to get a steady bead on this booger. Let's see If I've got this straight. I was under the impression that the larger the front element of the lens physically is, the more light from distant stars that can be gathered and condensed. The aperture controls WITHIN the lens (f/2, f/5.6, f/11, etc) in this case only affect non-point-source light -- skyglow, DSOs, nebulas -- anything that isn't a star.

In his book, Michael Covington says "When you're photographing stars, what matters is not the f-ratio but the diameter of the lens."

Got it. But Covington says that's for stars alone. I want to record the nebulae up there too.

Again Covington says, "Remember that most of the really interesting deep-sky objects are extended...Hence f-ratio is usually more important than diameter."

So, let me try to put this in terms of two commonly available 4x5 lenses. If I had a Nikkor 300mm f/9 and a Schneider 300mm f/5.6: given enough exposure time for the two lenses, they could both record the same nebulae; although it will be complete at dramatically different durations of time. However, the f/9, having a small front element, would only allow so many stars to record on the film. The f/5.6, having a much larger front element, will be able to gather the light of fainter stars and record a greater number than the f/9 optic. Nebulas like Orion would appear identical, but the number of stars around it will be different.

Have I got this right now?

Absolute apertures confuse absolutely.

Astrophotography with 4x5

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