Chris_Malcolm
Lens hood crop factor equivalence
The above lens is an ancient second hand Minolta 80-200mm f2.8. It is fitted with two lens hoods for comparative purposes. It originally came when new with a small white lens hood roughly a bit shorter than it was wide. The hood was bayonet mounted, but the lens has only a 72mm filter thread, so part of the hood assembly must have been a screw-in bayonet mount.
These old lenses are now often without the original hood. Replacement hoods are hard to find. The usual favourite, seen here (the wider shorter hood), is the Korean-made Matin generic reversible hood. You're right: it does look a bit like a wide angle lens hood. A long lens really should have a longer hood. As it happens I had a much longer metal hood, but for 77mm. So I fitted it with a 72-77mm thread converter. Would it vignette at 80mm? Not in the slightest.
I tried some test shots of a shaded garden wall above which was white clouded sky with a mild exposure compensation of +1. I tried using no hood, the wide Matin hood, and the longer metal hood., all with both 80mm and 200mm zoom. With both focal lengths there was a similar slight improvement in contrast and wall detail detail with the Matin hood compared to no hood. There was a much bigger improvement in contrast and shadow detail between the Matin hood and long hood. So hood length definitely mattered.
I wondered how much room there was for an even narrower or longer hood. I looked through the viewfinder at the lens' 80mm widest while poking my finger in from a corner of the image across the end of the lens hood. Setting the aperture to f22 and using the aperture preview button allowed me to see a clear slightly blurred image of my finger. To my surprise there was roughly 5mm extra clearance at the image corners, i.e. 10mm in diameter. Fitting a 72mm threaded tube of the same length instead of this 77mm one would take up only half of that clearance. So clearly that narrower 72mm tube could also be a lot longer, maybe even twice as long! I don't think I've ever seen a lens hood that long.
But of course my camera (Sony A77) is a crop sensor DSLR, so hoods can be longer and deeper than using the same lens on a full frame camera. I'm also not impressed by the blackness of these lens hoods. I recall using a telescopic (i.e. zoomable) lens hood on a zoom lens, and forgetting to zoom it back when using the wide end of the lens. I could see the blurred edges of the hood in the image. It looked black except for the part which was sunlit, which looked a bright white, and was clearly flaring veiling haze out into the image.
So how much more improved shadow contrast and detail can I expect if I use the longest possible hood (for my APS-C sensor) and make it a lot blacker? Can IQ be improved over what a full frame camera would get from the lens?
(I have acquired a black flock lined 75mm long 72mm straight tube metal hood, which shows no signs of vignetting. See www.flickr.com/photos/chris_malcolm/38797159770
(to be continued)
Original: DSC09223FBX
Lens hood crop factor equivalence
The above lens is an ancient second hand Minolta 80-200mm f2.8. It is fitted with two lens hoods for comparative purposes. It originally came when new with a small white lens hood roughly a bit shorter than it was wide. The hood was bayonet mounted, but the lens has only a 72mm filter thread, so part of the hood assembly must have been a screw-in bayonet mount.
These old lenses are now often without the original hood. Replacement hoods are hard to find. The usual favourite, seen here (the wider shorter hood), is the Korean-made Matin generic reversible hood. You're right: it does look a bit like a wide angle lens hood. A long lens really should have a longer hood. As it happens I had a much longer metal hood, but for 77mm. So I fitted it with a 72-77mm thread converter. Would it vignette at 80mm? Not in the slightest.
I tried some test shots of a shaded garden wall above which was white clouded sky with a mild exposure compensation of +1. I tried using no hood, the wide Matin hood, and the longer metal hood., all with both 80mm and 200mm zoom. With both focal lengths there was a similar slight improvement in contrast and wall detail detail with the Matin hood compared to no hood. There was a much bigger improvement in contrast and shadow detail between the Matin hood and long hood. So hood length definitely mattered.
I wondered how much room there was for an even narrower or longer hood. I looked through the viewfinder at the lens' 80mm widest while poking my finger in from a corner of the image across the end of the lens hood. Setting the aperture to f22 and using the aperture preview button allowed me to see a clear slightly blurred image of my finger. To my surprise there was roughly 5mm extra clearance at the image corners, i.e. 10mm in diameter. Fitting a 72mm threaded tube of the same length instead of this 77mm one would take up only half of that clearance. So clearly that narrower 72mm tube could also be a lot longer, maybe even twice as long! I don't think I've ever seen a lens hood that long.
But of course my camera (Sony A77) is a crop sensor DSLR, so hoods can be longer and deeper than using the same lens on a full frame camera. I'm also not impressed by the blackness of these lens hoods. I recall using a telescopic (i.e. zoomable) lens hood on a zoom lens, and forgetting to zoom it back when using the wide end of the lens. I could see the blurred edges of the hood in the image. It looked black except for the part which was sunlit, which looked a bright white, and was clearly flaring veiling haze out into the image.
So how much more improved shadow contrast and detail can I expect if I use the longest possible hood (for my APS-C sensor) and make it a lot blacker? Can IQ be improved over what a full frame camera would get from the lens?
(I have acquired a black flock lined 75mm long 72mm straight tube metal hood, which shows no signs of vignetting. See www.flickr.com/photos/chris_malcolm/38797159770
(to be continued)
Original: DSC09223FBX