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Forum: Photo Critique
01-23-2019, 11:33 AM
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If I am thinking correct the moon moves a bit slower across the sky than the stars do by about 4% or so but for most things that doesn't matter much. A 20 second exposure for stars with 300mm on a 2x teleconverter would show a lot of trailing. Generally when shooting stars people follow the rule of 500 (full frame) or rule of 300 (APS-C) to avoid having visible star trails. To get point stars going to the rule of 400(full frame) or 200(APS-C) will produce much better results. Since the moon's movement across the sky isn't that different from the stars these work well enough if one isn't using an equatorial to track for a long span of time. Basically these rules are (Number)/(focal length)=Max shutter length.
Using the rule of 300 that 600mm (300mm plus the 2x converter) would mean that the maximum exposure one could use to get a fairly sharp image would be .5 seconds and going to the rule of 200 would mean an exposure of up to .3 seconds would produce a nice sharp image. I got fairly good results using a 300mm f/4 lens stopped down to f/5, at .5s and ISO 1250 (I think that is what I was using)
If you have a pile of images of it in totality I would almost suggest stacking them in a tool like Autostakkert, RegiStax, or even photoshop and then using the wavelet feature in RegiStax to bring out the detail. You may be pleased with the results although if you haven't worked with wavelets before there is a learning curve to understand them. I am having to do that with my shots as I got to shoot through high level clouds and with the one I finished last night was fairly pleased with what I managed to pull out.
So at 2 seconds there is some motion blur from the moon moving. The 2x converter on a long zoom lens isn't helping much either. If the lens was set at f/8 then the 2x converter means you were really shooting at f/16. Add in that the zoom at the long end may not have super sharp to begin with and the converter only magnified that unsharpness.
As far as other pictures looking like they are slightly backfocused on the moon that is more an artifact of how the moon is illuminated. The depth of field (assuming you focused properly) at that distance will easily engulf the entire moon and everything farther out. You get the appearence of sharpness near the poles and edges because the illumination isn't as flat there as it is as you go towards the middle of the moon. The shadows from the craters create that contrast and depth, which is why moon pics near when the moon is half full look great as there is so much texture.
Frost on the lens barrel isn't an issue if that is only where it is and I frequently get that when I have been out. On the front element it is a problem but a hood will cure most of that and if that isn't enough you can always rubberband some of those air activated hand warmers around the extended hood which will cure it.
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