Astro Landscapes: Super wide vs. standard focal lengths

BlackgumNate said:

Newb/dumb question relating to clear aperture; does stopping a lens down reduce the clear aperture as equally as if the lens' fully open aperture was where it's stopped down to? ie: does stopping a 35mm f/1.4 lens down to f/2 equate its light gathering capacity to a 35mm f/2 lens?

Thanks,

Nate

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BlackgumNate said:

Newb/dumb question relating to clear aperture; does stopping a lens down reduce the clear aperture as equally as if the lens' fully open aperture was where it's stopped down to? ie: does stopping a 35mm f/1.4 lens down to f/2 equate its light gathering capacity to a 35mm f/2 lens?

Thanks,

Nate

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DerekSurfs said:

rnclark said:

Light gathering by aperture area is more than just stars. It applies to all subjects. See:

http://www.clarkvision.com/articles...-and-lenses-for-nightscape-astro-photography/

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Roger,

Thanks for the reference to your articles. They provide a lot info.

While I'm sure you are correct regarding general photography, Its fair to say it also matters less for most shooting under normal lighting conditions. It wasn't until I started researching night astro photography that it really mattered at all. That said, I'm sure there is probably a case for someone shooting fruit in a bowl in their studio with artificial less. But still, not as major an issue as dark skies with stars millions of miles away.

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DerekSurfs said:

BlackgumNate said:

Newb/dumb question relating to clear aperture; does stopping a lens down reduce the clear aperture as equally as if the lens' fully open aperture was where it's stopped down to? ie: does stopping a 35mm f/1.4 lens down to f/2 equate its light gathering capacity to a 35mm f/2 lens?

Thanks,

Nate

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Hi Nate,

Nice to see other newbies asking questions on here as well. Based on the formulas used by lonelyspeck and Roger's website, it would seem that it does reduce the clear aperture. And if that is so, it does beg the question, 'do I really need a 1.4 aperture lens' if I am really going to be shooting at f2 or f2.8? I know there are other benefits to stopping like such as removing aberrations and improving sharpness. And that is why some do it for their astro work. But still, its a valid question.

Case in point, I was shooting the night sky this week next to a guy who had just purchased all of this high end gear for night photography. He even had a gigapan which automatically takes the panos for him. So, he was taking 30+ shot panos with apparent ease using an f1.8 55mm prime lens. However, toward the end of the night I asked what aperture he was shooting at and he said 2.8! It was then that I couldn't help but think, isn't this the same as shooting with 24-70mm 2.8 lens at 55mm? Now of course there are other lens differences depending on what their individual characteristics are. But what if a zoom such as the Tamron 15-30mm f2.8, Canon 24-70mm L f/2.8 or some other zoom was used which is just a sharp at 2.8 with well behaved coma, CA, etc..? Is it basically the same as shooting astro with that high end prime?

That is also the same general photography rationale for using high quality zooms over primes such as the new Sigma 18-35mm f/1.8 or Canon 24-70mm f/2.4 as long as one is ok with shooting at those apertures for their base.

Derek

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rnclark said:

BlackgumNate said:

Newb/dumb question relating to clear aperture; does stopping a lens down reduce the clear aperture as equally as if the lens' fully open aperture was where it's stopped down to? ie: does stopping a 35mm f/1.4 lens down to f/2 equate its light gathering capacity to a 35mm f/2 lens?

Thanks,

Nate

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Yes. Clear aperture = focal length / F ratio, so 35 f/2 = 35/2 = 17.5 mm diameter.

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Digitaliz said:

What about the Transmittans? I would like to include that into the equation for lenses.

Lets for example say two F1.4 lenses. Tests show they have T1.5 and T1.6 in Transmittans. Wouldnt it be logic to say that the T1.5 will gather more photons to the sensor?

Also the vinjetting still confuses med and T values. Two lenses can have similar T-values and F-values. But one of the can have like 2eV more vinjetting. I would also in this case say that the less vinjetted lens will gather more total photons to the lens.

The Sanyang has Same F and T valus, but the Tamron has a vinjetting of -1,2 eV and the Samyang has -2,6 eV.
Having had both lenses, I can tell that there are significant differences in the image quality.

/Stefan

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DerekSurfs said:

Digitaliz said:

What about the Transmittans? I would like to include that into the equation for lenses.

Lets for example say two F1.4 lenses. Tests show they have T1.5 and T1.6 in Transmittans. Wouldnt it be logic to say that the T1.5 will gather more photons to the sensor?

Also the vinjetting still confuses med and T values. Two lenses can have similar T-values and F-values. But one of the can have like 2eV more vinjetting. I would also in this case say that the less vinjetted lens will gather more total photons to the lens.

The Sanyang has Same F and T valus, but the Tamron has a vinjetting of -1,2 eV and the Samyang has -2,6 eV.
Having had both lenses, I can tell that there are significant differences in the image quality.

/Stefan

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Stefan,

That's an interesting point. Aperture seem to be held up as the only contributing factor in light gathering ability which I've wondered about myself. If transmittance is also an important contributor, then how great is it? For example, could a lens with minimal vignetting shot at F2.0 gather as much light as say a lens with poor vignette performance shot at F1.8? Or is it minor in comparison? In addition, I know some of these lenses end up getting stopped down anyway due to poorer performance wide open.

BTW, which lens focal lengths are you talking about? 24mm, 35mm, or..? Just curious.

Thanks,

--
Derek

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DerekSurfs said:

Digitaliz said:

What about the Transmittans? I would like to include that into the equation for lenses.

Lets for example say two F1.4 lenses. Tests show they have T1.5 and T1.6 in Transmittans. Wouldnt it be logic to say that the T1.5 will gather more photons to the sensor?

Also the vinjetting still confuses med and T values. Two lenses can have similar T-values and F-values. But one of the can have like 2eV more vinjetting. I would also in this case say that the less vinjetted lens will gather more total photons to the lens.

The Sanyang has Same F and T valus, but the Tamron has a vinjetting of -1,2 eV and the Samyang has -2,6 eV.
Having had both lenses, I can tell that there are significant differences in the image quality.

/Stefan

Click to expand...

Stefan,

That's an interesting point. Aperture seem to be held up as the only contributing factor in light gathering ability which I've wondered about myself. If transmittance is also an important contributor, then how great is it? For example, could a lens with minimal vignetting shot at F2.0 gather as much light as say a lens with poor vignette performance shot at F1.8? Or is it minor in comparison? In addition, I know some of these lenses end up getting stopped down anyway due to poorer performance wide open.

BTW, which lens focal lengths are you talking about? 24mm, 35mm, or..? Just curious.

Thanks,

--
Derek

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Trollmannx said:

DerekSurfs said:

Digitaliz said:

What about the Transmittans? I would like to include that into the equation for lenses.

Lets for example say two F1.4 lenses. Tests show they have T1.5 and T1.6 in Transmittans. Wouldnt it be logic to say that the T1.5 will gather more photons to the sensor?

Also the vinjetting still confuses med and T values. Two lenses can have similar T-values and F-values. But one of the can have like 2eV more vinjetting. I would also in this case say that the less vinjetted lens will gather more total photons to the lens.

The Sanyang has Same F and T valus, but the Tamron has a vinjetting of -1,2 eV and the Samyang has -2,6 eV.
Having had both lenses, I can tell that there are significant differences in the image quality.

/Stefan

Click to expand...

Stefan,

That's an interesting point. Aperture seem to be held up as the only contributing factor in light gathering ability which I've wondered about myself. If transmittance is also an important contributor, then how great is it? For example, could a lens with minimal vignetting shot at F2.0 gather as much light as say a lens with poor vignette performance shot at F1.8? Or is it minor in comparison? In addition, I know some of these lenses end up getting stopped down anyway due to poorer performance wide open.

BTW, which lens focal lengths are you talking about? 24mm, 35mm, or..? Just curious.

Thanks,

--
Derek

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Modern lenses have splendid coatings and there is very little light loss from reflections, absorbtion and scattering. So f/numbers and T/numbers are pretty close.

Vignetting is the really bad one, especially when it comes to wide angle lenses. A fast and wide f/1.4 or f/1.8 lens will vignette a stop or two (talking FF, less vignetting if using a smaller image sensor). So the extra f/stop will apply to the center of the image, not towards the vignetted corners.

Use even my Zeiss lenses at f/4 to get a really really sharp image across the entire field of view, and to get more even illumination all over the field. The light loss is recovered by using a star tracker and longer exposures - which is well worth the extra effort.

To record fainter stars I simply use longer focal lenghts (greater physical aperture). There is always a balance between aperture and abberrations, and with my lenses f/4 is the sweet spot. Loosing a bit light to get better image quality from corner to corner is the price I pay using this strategy. There is no ideal solution working wonders for all.

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