Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
rnclark Senior Member • Posts: 3,755
Re: If you want the best, a dedicated cooled CCD cameras are far better than all the DSLRs and ILCs

kiwi2 wrote:

landscaper1 wrote:

O.K., kiwi, leaving aside the question of one's manners, Roger has posted a number of discrete statements as facts. Are you seriously suggesting ALL of those statements are "nonsense," or just some of them? If the latter, it would be helpful if you were to be more specific instead of issuing a blanket ridicule.

He has just used a lot of words here to basically say that he zoomed in on the moon with longer focal lengths (figure 4 of his link) and recorded more light. (because the moon was larger over the frame) As the focal length increased from 28mm to 70mm to 200mm, the f/ratio was kept constant at f/4 by increasing the aperture obviously. So he proclaims larger apertures recorded more light.

Glad you see this. But I didn't proclaim anything. The math showed that.

This is hardly groundbreaking news as it is pretty well established practice to fill your intended subject in the frame as much as possible. This is why people like astronomers and bird shooters chase after as much focal length as they can get their hands on. The more you can fill the frame with your intended subject, the more resolution you'll have.

Now you've lost it. It has nothing to do with focal length. Do you think focal length creates light? No, only aperture area can collect the light.

As you'll see though the surface brightness of the moon was the same in all three shots. That's dictated by the f/ratio and shutter speed and film/sensor sensitivity. The focal length governed how much it filled the frame. If the same aperture of the 28mm shot was used with the 200mm focal length, then it would have been f/28.

The light collected has nothing to do with the focal length. Try the same experiment with stars. The stars will be brighter in the larger aperture lens.

As I said yesterday... "Bigger telescopes (or camera lenses) give you longer focal lengths and more magnification. It's f-ratio tells you how bright things will look (at that focal length) and what kind of exposure times you'll need" That's exactly what you see here with Roger's examples. Nothing more, nothing less.

Again you are confusing focal length and light density with total light from the subject.

But when you do have subjects that already fill the frame to exactly where you want them, Roger's argument falls apart.

Not in the least. In fact your argument falls apart all the time regarding total brightness.

You just declare something falls apart. It is an indication that you do not understand Etendue and are misapplying it. If you believe otherwise show us the math.

He admits that the f-ratio of different focal lengths and apertures is a constant if "the scene is exactly the same intensity everywhere, like a blank, uniformly lit wall!

Yes, tat is correct regarding light density in the focal plane and total light in the frame. It works because the SUBJECT is being changed--the size of the wall.

Other scenes that would foot that bill (of his blank uniformly lit wall example) could be landscapes uniformly lit by the sun or even the dpreview test scene uniformly lit by a studio light. Or even the photos of my back lawn I posted yesterday. Or about 95% of photos posted here at dpreview on a daily basis for that matter.

Again you change the subject to make you case. That is a flawed example and only works with a uniform scene. Figure 2a and 2b here:

shows an example where this idea does not work. Yet Etendue describes it perfectly.

Rodger conveniently leaves out mentioning the inverse-square law as to why the f-ratio is what is used to calculate exposure rather than working out the aperture of the lens.

I showed you how Etendue includes the effects of the inverse square law.

If anybody here has some old slide film and a lightbox, try this demonstration for themselves. In a dark room, place one slide on the lightbox and then cover the rest of the lightbox with some rags or cardboard or something so the only light is shining through the slide. Now with the lights off in the room, take an old hand held light metre and hold it close to the slide. Note the reading. Now slowly move the light metre away and note how quickly it drops.

So, that is the inverse square law, and Etendue describes exactly how the light meter will respond.

This is exactly the same thing that is going on with the image circle that the lens is projecting to the sensor. The longer focal length the lens has, the more the light drops before it reaches the sensor. But having a larger aperture keeps the brightness up sufficiently at the longer focal lengths. This is precisely what the f-ratio is and why it is used rather than calculating aperture. Aperture is only relative to the focal length. The f-ratio.

Once again you are confusing light density with light from the subject. Your example fails with stars. Etendue describes the results exactly/

That's what makes this statement wrong....

rnclark wrote:
_"DPreview changes focal length while keeping f-ratio constant. That means lens aperture area changes between cameras, thus the light delivered to the sensor is changing, and that means the amount of photon shot noise is different due to the lens, not the sensor.

Again you misunderstand and mix up light density in the focal plane and total light from the subject.

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