[UncleVanya]

The lens sold for roughly 2k.

[kaseki]

When contemplating long range, high magnification images, the photographer has to keep in mind that atmospheric turbulence is always present, affecting both sub-image sharpness and distant structure distortion. Atmospheric turbulence is the motion of various size cells of air of differing temperature -- and hence index of refraction -- along the image path. One is living in a bubble of a bazillion weak lenses of various sizes and powers, all moving randomly but generally with both the wind and thermal updrafts. Longer exposure times will have blur due to averaging the turbulence; shorter will freeze the atmospheric distortion. For modest wind conditions, short means significantly shorter than 1/100 seconds; long means significantly longer than 1/100 seconds. The root mean square of the deviations to any distant point will depend on the varying structure constant of the air along the path. Higher altitude shots across deep valleys will have the least turbulence. One should plan on seeing 20 to 50 micro-radians, rms, for shots along kilometer sized paths when the image path is only a couple of meters above the average terrain.

The camera sensor quantization of the image, given by pixel size divided by the lens focal length, can be compared to the turbulence deviation to see which will dominate. For example, a 10 micron pixel with a 1000 mm lens will have a far field image size of 10 micro-radians.

[TDvN57]

I have already the 67 400mm f4 EDIF, 67 1.4x and 2x teleconverters and the 645 1.4x and 600mm f5.6 lens. So have been able to make some of those comparisons, Theuns.

For me, the 2x converter (while "ok" in quality) doesn't make sense to use. I find using the 1.4x and then up-rezzing the resultant file is actually superior. It seems that the 1.4x converter is the sweet spot and once you get to 2x, it loses more than it gains.

So, in practice, that means for me that the 400mm f4 with 1.4x converter (which gives 560mm) is a good option. Once I want more than that, the 600 (with and without the 1.4x) comes into play. There isn't much difference in quality between the 400mm with 67 1.4x converter and the unconverted 645 600mm lens. But the 645 with1.4x converter is better than the 400mm with the 67 2x converter. BUT because I know the 67 1.4x is better than the 645 1.4x converter, I wish I could use it with the 645 600mm lens!

If I got the 800mm lens, I wouldn't use it often. But I do find a surprising number of situations in which the 645 600mm with 1.4x just isn't enough. So it would be a nice one to have. It will have to wait though, as I cannot afford any major purchases for the foreseeable future!

Thanks Ed, very helpful info here.

---------- Post added 2020-11-11 at 08:11 AM ----------

When contemplating long range, high magnification images, the photographer has to keep in mind that atmospheric turbulence is always present, affecting both sub-image sharpness and distant structure distortion. Atmospheric turbulence is the motion of various size cells of air of differing temperature -- and hence index of refraction -- along the image path. One is living in a bubble of a bazillion weak lenses of various sizes and powers, all moving randomly but generally with both the wind and thermal updrafts. Longer exposure times will have blur due to averaging the turbulence; shorter will freeze the atmospheric distortion. For modest wind conditions, short means significantly shorter than 1/100 seconds; long means significantly longer than 1/100 seconds. The root mean square of the deviations to any distant point will depend on the varying structure constant of the air along the path. Higher altitude shots across deep valleys will have the least turbulence. One should plan on seeing 20 to 50 micro-radians, rms, for shots along kilometer sized paths when the image path is only a couple of meters above the average terrain.

The camera sensor quantization of the image, given by pixel size divided by the lens focal length, can be compared to the turbulence deviation to see which will dominate. For example, a 10 micron pixel with a 1000 mm lens will have a far field image size of 10 micro-radians.

Thanks Kaseki, I never thought of a lens in this perspective. Would you venture an opinion on the maximum practical focal length for the 645z sensor, versus say the 100mpx sensor?

[kaseki]

Thanks Kaseki, I never thought of a lens in this perspective. Would you venture an opinion on the maximum practical focal length for the 645z sensor, versus say the 100mpx sensor?

This can't be answered because the turbulence effect grows with distance (to some power I've forgotten; perhaps minus 5/3). So if you want a long lens to image a frog on a lily pad a dozen feet away, or an eagle 100m away, the turbulence will be much less than for a landscape 3 km away.

If the 100 Mpix sensor is the same overall size as the 645Z's 51 Mpix sensor, then the pixels are root 2 smaller in each dimension than those of the 645Z, so 'limiting' turbulence will occur at a modestly shorter range, or at the same range with a modestly shorter focal length.

The take home is not to avoid buying monster glass, but to recognize that there will be limitations in addition to lens aberrations and the diffraction limit when shooting through the atmosphere. Also worthy of recognition is that the larger the objective, the more aperture averaging that takes place and the lower the observable turbulence. So aim for the 4m f/4 Schmidt telescope you've always wanted. There should be room for an entire 645Z body at the focus.

[TDvN57]

So aim for the 4m f/4 Schmidt telescope you've always wanted. There should be room for an entire 645Z body at the focus.

I like your taste in long glass. Oops goes my budget.... again.

[RICHARD L.]

Straight 645 FA 200 mm f/4 lens.


645 FA 200 mm f/4 lens + 1,4X teleconverter.

Not bad but far from perfect. Plus you have to find critical focus manually, though you can use the Av mode to get autoexposure.

[w2ck]

Hi,

Yes. Atmospherics. Definitely a big issue. Here are center crops from two shots. They are taken in the exact same spot on two different Saturdays. Two weeks apart. The first is with the P645D plus A* 300/4 plus 2x TC on a tripod. The second is with a Nikon Df plus a Sigma 150-600mm super zoom with OIS and on a monopod. So. Not anything by way of equality or equivalence or what not. All I am showing in this long shot is the long way from where I get the boat and the Cape Lookout lighthouse. It's over three miles across.

So, no one bother to think this is some sort of a comparison between cameras. All this is showing is what the air and water and wind and temperatures are doing across the span. Atmospherics. The first day, with the 645D rig, was a lot warmer for both the air and the water than the second day with the Df. And, man, does that ever show up.

The risk here is someone thinking that the lenses had a lot to do with it all. They didn't. I have shot with long lenses long enough to have seen this before. I used a Nikkor 500/4 for-seemingly-ever and this sort of 'different day' effect is pretty common. In fact, it was that thinking that led me to shoot with the Df the second time out.

That trip was for walking the beach looking for shells. That is, with my wife. The first trip was to shoot the lighthouse and I had gone alone and done just that. Anyway, the Df is carried with a Nikkor 28-105 macro zoom and the Sigma 150-600 super zoom and monopod as my 'small and light take-with' kit. This was a case of recognizing that the atmospherics would be less on that second day, so I shot the view again from the same spot.

Maybe one other day in the future, I will go back here with both cameras and do a true comparison set of shots. But, for now, in this thread, here is what one can consider when it comes to long lenses and atmospherics.

These are crops out of the center of the shots.



P645D plus A* 300/4 plus 2x TC plus Tripod plus a Warmer Day



Nikon Df plus Sigma 150-600 at 600 plus monopod on a cooler day.

Oh. And that island is much closer to us than where the lighthouse is. It just gets in the way of us seeing the rest of the water. All of which is contributing to the atmospherics.

Here are the full shots, where things don't look quite as bad as in the crops.


P645


Nikon Df

Stan

---

Last edited by w2ck; 11-15-2020 at 08:28 AM. Reason: Added the full shots

[kaseki]

... Yes. Atmospherics. Definitely a big issue. Here are center crops from two shots. ...

Nice demo. For best seeing across water, the atmospheric structure constant (denoted C_sub-n_squared, which is not constant with almost anything) needs to be minimized. This might occur (with light for photography) just after dawn under conditions where the water temperature and the air temperature are nearly equal.

I couldn't find in a quick search a plot of Cn^2 with time of day, no longer having my vast literature collection on the subject, but astronomical seeing, part 1: the nature of turbulence seems like a good write-up for the interested.

[672]

800mm with dedicated 1.4 TC , go for it, it doesn't get better..........

[672]

u need an 800........

[w2ck]

Hi,

Yowza! That works.

I once had a Nikkor 1200/11 and really never used it for being too long. I also had the matching 800/8 and that one did get used once in a while. Ironically enough, I was thinking of the 645 800 as being what I really wanted to have when I was shooting the Cape Lookout lighthouse across the water with the 645D that day.

Edit: Sitting here thinking about it, I can see adding a 645 600mm to my kit. I might be wanting to take more of those 'lighthouse in the distance' photos. I have already done three of those using the A* 300/4 and the 2x TC. And, I have lots more lighthouses to go shoot on the Atlantic coast.....

Stan

[barondla]

Hi,

Yowza! That works.

I once had a Nikkor 1200/11 and really never used it for being too long. I also had the matching 800/8 and that one did get used once in a while. Ironically enough, I was thinking of the 645 800 as being what I really wanted to have when I was shooting the Cape Lookout lighthouse across the water with the 645D that day.

Edit: Sitting here thinking about it, I can see adding a 645 600mm to my kit. I might be wanting to take more of those 'lighthouse in the distance' photos. I have already done three of those using the A* 300/4 and the 2x TC. And, I have lots more lighthouses to go shoot on the Atlantic coast.....

Stan

Can't have "too big" a lens!

Thanks,
barondla