Focus distances on FD lenses: Do they vary with the type of adapter

S

sankarz

Member
I was testing FD 50mm and 28mm on my GH1 with two adapters I bought on ebay (one for 20$ other for 15$) and found that the actual focus distances are slightly above the one marked on the lens.
For example when I set the focus distance at 50cm on 50mm lens at f1.8, the subject distance is 52cm .
And for 1m on the lens the subject distance is 105cm.
Also for the 28mm I found the focus distance is slightly above the one marked on the lens.

Just wondering if anybody found correct focus distances with other adapters on GH1 with FDs
 
It's because the only way they can guarantee Infinity focusing is by reducing the depth of the adapter from the technically perfect size, by as much as half a millimetre, but usually less, so that you can usually focus a touch beyond infinity, thereby guaranteeing that all lenses will work to infinity.

The problem with this approach is that when you want infinity you can't whack it right to the end stop and know it'll be spot on for infinity - it almost always won't with an adapter, but often you won't notice due to hyperfocal distances at that focal length.

The only solution I have found other than buying a precision adapter, which can be very expensive, or not available, is to adjust the lens - to rear focus adjust it to the adapter. This makes the lens forever tied in to that adapter, or else it will need re-adjusting to it's original state. I've done this on Contax Zeiss lenses and on Minolta Rokkor lenses, but not FD yet. The method is to remove the front ring and rubber focus band, remove 3 tiny screws so the helicoil is free, adjust to infinity and also a short measured distance so it's perfect, and then re-assemble with the screws. It's quite technical but fun and the only way to get perfect collimation on your lenses with an adapter. The precise process is slightly different between lens brands.

I hope that helps.
 
Or buy the adapter from ciecio7 on eBay. It is made in Poland to 0.01mm precision, and milled from a solid billet of aluminium. Very good and not overly expensive if you want reliable results without dicking around.
 
I find that even different lenses of the same makes and models don't even focus to the same marks on the same adapters. This is one of the best things about cine lenses is that the lens markings are calibrated to the lens, not to the design, therefor will always be 100% correct on every lens.

Best thing to do is just keep some tape around and mark your own markings on the lenses. You can also make notes and focus marks that are more intuitive.

I typically make color marks for focus points as I find that if I think about numbers while focusing I tend to react more slowly than just looking for a color.
 
The fastest and most accurate way to focus is...wait for it...look at the LCD or viewfinder of your GH1 while you focus. :)

You know that the GH1 provides a 5x or 10x manual focus assist if you need it? Left button, center button, then front dial if you want 10x.

I guarantee you, this is faster & easier than dorking around with the lens markings, and attempting to guess your subject distance.

J

p.s. FWIW, I use Canon FD-mount 50mm/1.4 and 100-300mm/5.6 on my GH1 with Fotodiox adapters ($30). FD macro coming soon.
 
Svart said:
I find that even different lenses of the same makes and models don't even focus to the same marks on the same adapters. This is one of the best things about cine lenses is that the lens markings are calibrated to the lens, not to the design, therefor will always be 100% correct on every lens.

Best thing to do is just keep some tape around and mark your own markings on the lenses. You can also make notes and focus marks that are more intuitive.

I typically make color marks for focus points as I find that if I think about numbers while focusing I tend to react more slowly than just looking for a color.
Click to expand...

Svart that's not strictly true about cine lenses - they sometimes need re-collimating, which adjusts all focus distances. I've watched the process being done to my own Zeiss lens at Arri (UK) and it makes me think that the markings are printed quickly in a normal factory fashion on cine lenses, based on a single measured prototype (rather than individually with a focus check per focus distance, per lens.) Certainly T stops differ from F stops in that they are measured per the prototype of the lens rather than merely theoretical, but again, they don't measure each and every lens, only the prototype. Anyway, if the collimation is out, as can happen on any lens, all of the distances are wrong.
 
"Focus is checked for each increment of
critical focus.
Focus scales are then engraved according
to the computer data entered for each
lens."

Well I never. So that's why they're such silly prices.

I also agree with you about the poor man's follow focus (white tape on the barrel, marked with stops), but I do think that getting infinity focus to land precisely on the end stop is a very worthwhile procedure.
 
Infinity focus is temperature dependent. A lens gets longer at high temperatures and shorter at low temperatures. This change in physical length of the lens changes the focus point. This effect is worst with long telephoto lenses. Many of those lenses focus past infinity (making the lens shorter) on purpose to allow for when the lens is hot in the sun. This is also why Canon and others paint their long telephoto lenses in the light gray/tan color. Shorter lenses don't suffer from this problem as badly. So, if you shoot indoors in a temperature controlled studio, then you can set up your lenses to have perfectly accurate focusing with infinity right at the focus stop. But, for long telephoto lenses and use in widely varying temperature conditions, it is still best to allow focus past infinity and do critical focusing.
 
NPrincen said:
Infinity focus is temperature dependent. A lens gets longer at high temperatures and shorter at low temperatures. This change in physical length of the lens changes the focus point. This effect is worst with long telephoto lenses. Many of those lenses focus past infinity (making the lens shorter) on purpose to allow for when the lens is hot in the sun. This is also why Canon and others paint their long telephoto lenses in the light gray/tan color. Shorter lenses don't suffer from this problem as badly. So, if you shoot indoors in a temperature controlled studio, then you can set up your lenses to have perfectly accurate focusing with infinity right at the focus stop. But, for long telephoto lenses and use in widely varying temperature conditions, it is still best to allow focus past infinity and do critical focusing.
Click to expand...

interesting piece of info... its some thing to consider when shooting in extreme conditions
 
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