Except it has no bearing for startrails, which was my point. Thank you for sharing.
You're very much mistaken because the real aperture size has a lot to do with star trails - remember, star trails are just moved pinpoint sources of light registered on the film. And pinpoint sources are registered on the film in the direct dependence on the real aperture size. You seem to be particularly inapt when it comes to learn about optical laws. Why is that?
You can lead a horse to water...
Can't we all just get along?
Stone, do you have anything actually relevant to add? Or just more useless banter with thinly-veiled insults? As for getting along, the hostility is from Pfsor. It is unfortunate he can not have a civil discussion.
From the very beginning my question to him has been for more explanation. The simple fact here is you can't just boil down the entire image to just the physical aperture size. For one, the focal length is integral to the composition that the photographer chooses. Therefore I am never going to go out to make a photo with a 90mm lens and then say "oh, well I'll use this 300mm f/4.5 instead because the hole is bigger!" because that lens would be so much different and I couldn't include the landscape foreground as I intend. I assume, perhaps wrongly, that most are shooting startrails with a foreground interest. If not, the FL still determines how many stars are in the frame. And as I've already said, if one is shooting on a tracking mount, it would be much more important. This entire thread was predicated on startrails from a stationary camera though.
Secondly, and more importantly, is the movement of the stars on the film as Pfsor has just mentioned. Consider a 90mm lens and a 180mm lens. Both are set at f/8. The physical aperture size of the 90mm lens is about 11mm, while the 180mm lens is at about 22mm. So the 180mm lens gets more light. Okay - however, the stars are moving across the film at a relatively faster rate due to the longer focal length. What I am asking here is, doesn't this then negate the larger amount of light collected due to the larger aperture size? Surely there must be a difference. This is why the commonly cited "Rule of 600" is used - to determine how long an exposure one can shoot (on 35mm or a DSLR generally) before the star starts to trail. The longer the lens, the shorter you can shoot before a trail starts to show up (and this is also dependent on where you aim the camera relative to the poles). Anyway, this change in speed of the star vs. the focal length explains why you can use shorter lenses on smaller formats and still get plenty of startrails. It seems to me this movement is the source of the confusion and why you can't boil down the entire exposure into one simple concept.
I have still yet to see any kind of formula or something like that in any of the links provided to help calculate this. I am asking these questions and posing these discussions to help further both my understanding and to provide a broader base of information and experience to the forum. I find it really unfortunate that Pfsor chose to instead make this some sort of battle instead of a discussion to further understanding.
There's tech info and there's artistic choice. The ideals of each don't always line up.
My statement above was supposed to be helpful, meaning, you can be given all the info, but you can't be forced to use it, you can and will pick and choose what info you want to utilize.
I think Pfsor is just giving the info. Like use a longer lens with wider physical aperture doesn't mean you have to use it, it's just useful to know and understand it, and then you can choose to use it or not.
You're both kinda just being argumentative and ignoring each other's perspective. It takes two to tango.
The thing I want to understand is why my CF on my 150 SS XL isn't affective wide open, I'm not sure I understood why. If I want to use that with some 8x10 Velvia50 or 8x10 Provia100f for a wide view, why can't I shoot at f/5.6 or f/8 to reduce coma, and it won't truly work effectively? Why?
Thanks Bob, I mean, why? As in the science behind it.
Also, how far out does it actually affect? Like, the 150 covers 11x14, but if you use an 8x10 or 4x5 will it matter more or less?
But more importantly, "it's how they work" doesn't answer WHY. Because I physically see a reduction in the visible light when I add the filter, even wide open, so why does it need to be stopped down? What changes when stopped down that it won't work wide open, is there a "TOO SMALL" aperture on the other end that also doesn't have an affect?
The fall off begins about ⅓rd out from the center and increases to the edge.
Read page 26 of the following, it is the digital lens CF works the same way and for the same reason as the analog lens CF except the way that they are made by Rodenstock are different since digital is more demanding then film.
http://www.rodenstock-photo.com/Arch...grafie%20e.pdf
Unfortunately, in the past, Rodenstock did have a brochure specifically for center filters but no longer is on their web site.
You might check to see if Dan Fromm has a copy on his site, if he does it would be for the earlier version and not the current manufacturing system.
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