High resolution back and RZ67 glass

[Steve Hendrix]

Steve,

Nice to see your tests. I presume, it's a given that you adjusted the camera to subject distance in order to equalize the image scales between the 80mm and 110mm lenses?

Regarding lens sharpness performance, I am mostly interested in wide open, and 1 and 2 stops closed down. Beyond that, lenses pretty much all converge towards similar performance, unless you get a real dog. So when I want to test a lens, or compare lenses, I take shortish exposures of the starry sky. Point sources at infinity, scattered all over the image, are the best test of sharpness and aberrations (as well as infinity point accuracy). It's no test of bokeh though! Although it can be a great test of foreground bokeh, if you defocus deliberately.

I just wonder why more people don't do this. I know there's the "I take photos of people/landscapes, not stars" attitude, but that is completely missing the point. It's not (necessarily) about astrophotos as the end usage goal; it's about the starry sky as "Nature's free optics testing lab" :thumbup:

Ray

Correct on your presumption. I agree about lenses - generally - that you'll find the differences at wide open (and also, to a lesser degree, stopped down). But normally the difference in lens prices is largely attributable to the wide open performance. Most of the time - that's what you're paying for.

My question about your tests are - you say "shortish" exposures of a starry sky. Can you define shortish? Objects at that distance would - to me - seem prone to introduce all manner of intrusive phenomena, shifting environmental matter and reflections, etc. I've never been a sky shooter, so completely at the mercy of your experience.


Steve Hendrix

 

[ondebanks]

My question about your tests are - you say "shortish" exposures of a starry sky. Can you define shortish?

A good rule of thumb to freeze the trailing of stars due to Earth's rotation is to shoot at about 600 or 800 seconds divided by the focal length of the lens.
So, about 8-10 seconds for an 80mm lens, about 12-15 seconds for a 45mm or 50mm lens...

It's not a hard limit though...there's a little hint of trailing at those times but not enough to affect the tests; while pointing your camera north (if you live in the northern hemisphere) buys you more time as the circles described by the stars are smaller, so trailing is slower.

For longer lenses like 200-300mm, 600/F doesn't give a lot of time to capture much starlight. OTOH their greater diameters largely compensate for this. And with their narrower fields, you can be more selective of which stars you image i.e. you can point them at richer constellations to maximise the number of bright stars.

Another tip is to re-aim the camera a couple of times to ensure that you get at least one bright star near different edges/corners of the field, where optical problems will be most apparent.

Objects at that distance would - to me - seem prone to introduce all manner of intrusive phenomena, shifting environmental matter and reflections, etc. I've never been a sky shooter, so completely at the mercy of your experience.
Steve Hendrix

No need to worry about that. Up to really high focal lengths (telescopic, like 2000mm), and as long as you avoid hazy nights and low-altitude stars, a star will remain a clean point source regardless of atmospheric conditions like wind, light pollution or moonlight. But it's preferable to avoid light pollution or moonlight, to improve visibility of the outer "wings" of the stars' images.

Ray

 

[MaxKißler]

Here you go: Kaiser: Lupe 4x Plastik fr 6x6<br>Kaiser: Loupe 4x Plastic for 6x6 | Lupen und Diabetrachter / Loupes and slide viewers - DE
Its very cheap and distorts heavily, but does cover 36x48mm and is plenty bright and sharp in the center. Very light too!

Hey Marko,

thanks for your advice on the loupe. I decided to kill my original waist level finder and install that loupe as a WLF dedicated to digital capture. With this thing, my focusing accuracy improved significiantly. I think this is exactly what the RZ67 ProIID is missing: A waist level finder that is magnifying the whole crop and is therefore actually usable in combination with a focusing screen like the SA705...