Lenilenapi
Carbonite Member
Re: Backfocus, resolution and Luis C's LEX chart test
(I made this a separate thread because I think a discussion of backfocus is warranted)
Luis,
I've spent alot of time staring at focus charts with these adapters and while they are never perfect I think your looks pretty bad. Not only are the corners too soft, but there seems to be exposure vignetting as well. However ,how much of this is the fault of the Letus would be impossible to judge. As has been mentioned -using a Nikon @ f.14 is getting its worst performance and the specific faults will be poor corner focusing, vignetting and you may also exaggerate any lens back focus issues.
Regarding whether you will see these corner issues on footage - of course you will if its the right shot.
I am surprised in general at how little careful attention is placed on exactly how people back-focus these adapters (and moreover how they set backfocus on the lens mounts themselves - but that's another issue.) Maybe I'm prickly about it because I spent years in a film rental department checking the backfocus of every lens that went out.
I posted the following on a previous thread but it seemed worth redoing it here.
On every adapter I've tried (the SGPro might be an exception but i haven't looked recently) there is a difference between best center and corner focus marks. (This makes perfect sense since its hard to get an achromat to focus flat on that close a target.) If you use focus assist to focus on the grain in the center, you will sacrifice your corners and vice versa. Stare carefully at the screen on a good monitor and you will see this and you may have a devil of a time finding the best compromise between center and corners. Frankly its a pain.
I did this with a Letus Extreme the other day. I thought I had the focus correct but we were testing lenses and noticed that that the corners were soft on exteriors of a fence. We finally had three people staring at a monitor for almost a half hour trying to get it precise, so we knew which footage readout was the best to guarantee focus. Generally I think it is best to go just barely (farther ) past the best center focus and you will be at an optimum compromise - that's if you don't have a monitor big enough or enough time to check the corners. But definitely don't focus on the near end of the center.
I always bring a focus card with me which is a blow up from Kinko's (3' or 4' wide) of a standard focus chart you can find on the Web. After roughing it in with the standard method of focusing on the ground glass while looking at an out of focus image ( or one with no 35mm lens ), I put up a 35mm lens and look at the chart. Then I go back and forth focusing the 35mm lens and double checking the backfocus on my HVX checking always for corners and center. During the day I double check the chart whenever I get a chance.
To me that's the real test because you are looking through a lens at an image. Personally i don't trust the ground glass method, because i'm not sure exactly where the image is on the glass. Likewise I wouldn't trust the "auto focus" on the ground glass technique either unless I double checked it a bunch of times against the chart method. A chart will tell you the truth for sure. (If someone has compared the "auto" method with careful chart focusing please chime in here.- might be different on different adapters & video cameras also)
My skeptism about how most people seem to set backfocus extends to my skeptism about claims made for which adapter is the sharpest, because I haven't heard of anyone testing that while being extremely careful about their initial backfocus set-up using a chart. Likewise the numerous claims that any adapter has a perfectly flat field. Frankly all you have to do is very slightly bump the focus ring when you are taping it down and the comparison is tainted.
Maybe if I can bear to take the time I'll try comparing my Brevis , SGPro and Letus Extreme for focus one of these days, but its kind of painful so i don't know if I'll really do it. My gut feeling is that all three are close in sharpness, but that's an untested hypothesis.
- Lenny Levy
(I made this a separate thread because I think a discussion of backfocus is warranted)
Luis,
I've spent alot of time staring at focus charts with these adapters and while they are never perfect I think your looks pretty bad. Not only are the corners too soft, but there seems to be exposure vignetting as well. However ,how much of this is the fault of the Letus would be impossible to judge. As has been mentioned -using a Nikon @ f.14 is getting its worst performance and the specific faults will be poor corner focusing, vignetting and you may also exaggerate any lens back focus issues.
Regarding whether you will see these corner issues on footage - of course you will if its the right shot.
I am surprised in general at how little careful attention is placed on exactly how people back-focus these adapters (and moreover how they set backfocus on the lens mounts themselves - but that's another issue.) Maybe I'm prickly about it because I spent years in a film rental department checking the backfocus of every lens that went out.
I posted the following on a previous thread but it seemed worth redoing it here.
On every adapter I've tried (the SGPro might be an exception but i haven't looked recently) there is a difference between best center and corner focus marks. (This makes perfect sense since its hard to get an achromat to focus flat on that close a target.) If you use focus assist to focus on the grain in the center, you will sacrifice your corners and vice versa. Stare carefully at the screen on a good monitor and you will see this and you may have a devil of a time finding the best compromise between center and corners. Frankly its a pain.
I did this with a Letus Extreme the other day. I thought I had the focus correct but we were testing lenses and noticed that that the corners were soft on exteriors of a fence. We finally had three people staring at a monitor for almost a half hour trying to get it precise, so we knew which footage readout was the best to guarantee focus. Generally I think it is best to go just barely (farther ) past the best center focus and you will be at an optimum compromise - that's if you don't have a monitor big enough or enough time to check the corners. But definitely don't focus on the near end of the center.
I always bring a focus card with me which is a blow up from Kinko's (3' or 4' wide) of a standard focus chart you can find on the Web. After roughing it in with the standard method of focusing on the ground glass while looking at an out of focus image ( or one with no 35mm lens ), I put up a 35mm lens and look at the chart. Then I go back and forth focusing the 35mm lens and double checking the backfocus on my HVX checking always for corners and center. During the day I double check the chart whenever I get a chance.
To me that's the real test because you are looking through a lens at an image. Personally i don't trust the ground glass method, because i'm not sure exactly where the image is on the glass. Likewise I wouldn't trust the "auto focus" on the ground glass technique either unless I double checked it a bunch of times against the chart method. A chart will tell you the truth for sure. (If someone has compared the "auto" method with careful chart focusing please chime in here.- might be different on different adapters & video cameras also)
My skeptism about how most people seem to set backfocus extends to my skeptism about claims made for which adapter is the sharpest, because I haven't heard of anyone testing that while being extremely careful about their initial backfocus set-up using a chart. Likewise the numerous claims that any adapter has a perfectly flat field. Frankly all you have to do is very slightly bump the focus ring when you are taping it down and the comparison is tainted.
Maybe if I can bear to take the time I'll try comparing my Brevis , SGPro and Letus Extreme for focus one of these days, but its kind of painful so i don't know if I'll really do it. My gut feeling is that all three are close in sharpness, but that's an untested hypothesis.
- Lenny Levy
