Fast Lens
Mar 18, 2024 17:36:43 #
scoundrel
Loc: Wytheville VA
agillot wrote:
Maybe we should call it a brighter lens ???
Some photographers do.
Mar 18, 2024 18:57:46 #
Mar 18, 2024 19:42:57 #
scoundrel
Loc: Wytheville VA
BebuLamar wrote:
So when they say a lens is "Fast" they don't mean that it would auto focus fast but rather the lens has large aperture and transmit more light. However, a lens with larger aperture can help in auto focus speed but it's not the only factor. How fast the lens can focus also depends on whether its built in AF motor is fast or it has to rely on the camera AF motor, how heavy the focusing mechanism is etc...
All true enough. The fast/slow descriptor for lenses long predates the widespread use of autofocus technology. A lens' "speed" was quantified by its wide-open f-number. The terminology also predated widespread use of zoom lenses, including zoom lenses with a focal-length-dependent maximum, i.e., wide-open, f-number.This is still the primary meaning of the fast/slow descriptor, though the lens' speed of focus lock seems to be creeping in as a secondary meaning.
Mar 19, 2024 08:02:51 #
petrochemist
Loc: UK
I think the Delta 77 projection lens mounted in my 5x4 monorail here might class as a fast lens. I've not yet determined the focal length (it's probably under 100mm from the FOV). It's bright enough that the image on the ground glass is clearly visible without a dark cloth
delta 77 - deluxe & GG by Mike Kanssen, on Flickr
delta 77 - deluxe & GG by Mike Kanssen, on FlickrMar 19, 2024 09:05:41 #
petrochemist wrote:
I think the Delta 77 projection lens mounted in my... (show quote)
The F stop is the tell...not the focal length.
Mar 19, 2024 10:02:35 #
petrochemist
Loc: UK
Canisdirus wrote:
The F stop is the tell...not the focal length.
Yes but the focal length will allow me to calculate the f stop. It won't be far from f1 which most people consider fast :)
Mar 19, 2024 12:56:15 #
petrochemist wrote:
Yes but the focal length will allow me to calculate the f stop. It won't be far from f1 which most people consider fast :)
Well then...what's your F stop.
Mar 19, 2024 13:17:24 #
petrochemist
Loc: UK
Canisdirus wrote:
Well then...what's your F stop.
I may remember to let you know once I've been able to calculate it.
FOV suggests a focal length around 90mm which together with the front element diameter of ~100mm would make it around f /0.9 - I find that very hard to believe in a lens I got for less than $10.
More careful measurement is definitely needed, but I'm 99% sure it's faster than f1.2
Unlike photographic lenses this info is not printed on the lens, or even apparently available to google with the model number - Though google can at least find several references for the Delta 77, showing the correct manufacturer & indicating it was often used in rear project TVs.
The 150mm/1.3 lens adapted to 5x4 doesn't even have a manufacturer shown, but I've had that mounted for longer so have got round to determining it's actual focal length & entrance pupil diameter.
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