Infrared Hot Spots


bforbes

Link Posted 09/06/2019 - 12:59
Is it normal for the same lens to display hot spots over differing f stops depending on the rating of the IR filter used?
Example IR720 filter no visible hot spots from f2.8 to f22. IR 850 filter hot spots from f11 to f22.
Barrie
Too Old To Die Young

http://www.pentaxphotogallery.com/artists/barrieforbes

petrochemist

Link Posted 12/06/2019 - 23:46
I've not noticed any effect from the filter used, but there are certainly a huge number of variables that affect hotspots:
I've had a lens cause them every time with one body, yet never at all with another.
Another lens only seems to give hotspots when focused close
Then we have aperture setting, Direction of lighting...

I wouldn't consider it surprising for a 850nm to be worse than a 720nm - it is after all working further from the wavelengths the lens is designed for!
Mike
.
Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.

.[size=11:].Flickr WPF Panoramio

Helpful

bforbes

Link Posted 13/06/2019 - 04:46
Thanks Mike. I've heard similar experiences on other forums. It seems to call into question the usefulness of the tables of good and bad IR lenses.
Barrie
Too Old To Die Young

http://www.pentaxphotogallery.com/artists/barrieforbes

petrochemist

Link Posted 13/06/2019 - 09:16
bforbes wrote:
Thanks Mike. I've heard similar experiences on other forums. It seems to call into question the usefulness of the tables of good and bad IR lenses.

They do need to be taken with more than a pinch of salt.
If contemplating a new lens for IR they may have some benefit, but I've never bothered with them. I've got quite enough lenses in all reality!
Mike
.
Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.

.[size=11:].Flickr WPF Panoramio

bforbes

Link Posted 13/06/2019 - 10:44
I'm looking to replace a lens that came highly recommended, but did not work out for me. This time I'm taking all the kit to try using the lens as I'm going to.
Barrie
Too Old To Die Young

http://www.pentaxphotogallery.com/artists/barrieforbes
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