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wilsawn
07-19-2014, 12:10 PM
hi,

I'm using the bmpcc speedbooster and a nikon 50mm 1.8 -- when I stop down infinity focus is sharp, but when wide open it's soft. Is that to be expected or do I need to make adjustments? I tried adjusting the speedbooster via the screw on the back, but it didn't make any difference (not sure how much you need to turn it when adjusting.. I only tested micro adjustments...)

thanks,
-W

vicharris
07-19-2014, 04:10 PM
Yes. Little bit of research goes a long way as well :)

Steve Wake
07-19-2014, 06:28 PM
Instructions: http://www.metabones.com/article/of/infinity-adjustment-speed-booster-only

Just be aware that if you try a different lens and it will not reach infinity or overshoots infinity, you can't adjust the speed booster to be exactly on the mark for all lenses. Adjust it so that all reach infinity, meaning some will have to overshoot, i.e. reach infinity focus before the infinity mark.

bitcrusher
07-19-2014, 08:35 PM
hi,

I'm using the bmpcc speedbooster and a nikon 50mm 1.8 -- when I stop down infinity focus is sharp, but when wide open it's soft. Is that to be expected or do I need to make adjustments? I tried adjusting the speedbooster via the screw on the back, but it didn't make any difference (not sure how much you need to turn it when adjusting.. I only tested micro adjustments...)

thanks,
-W

Just a thought but it might be focus shift with that type of lens design.

"Focus shift can lead to blurry images and focus errors, when working with subjects at close distances and using fast aperture lenses. With the lens aperture fully open or “wide open”, incoming rays of light converge at different focal points"

Read more: http://photographylife.com/what-is-focus-shift#ixzz37y5soKoA

wilsawn
07-22-2014, 05:12 PM
Instructions: http://www.metabones.com/article/of/infinity-adjustment-speed-booster-only

Just be aware that if you try a different lens and it will not reach infinity or overshoots infinity, you can't adjust the speed booster to be exactly on the mark for all lenses. Adjust it so that all reach infinity, meaning some will have to overshoot, i.e. reach infinity focus before the infinity mark.

Thanks Steve. I'd been using that guide when attempting to adjust my speedbooster, however it doesn't state how much you should be turning it.

I assume the turning the element causes it to move in and out -- I would not want to move it so far out that it could interfere with the sensor. Does anyone know if there is any risk in that happening?

wilsawn
07-22-2014, 05:17 PM
Just a thought but it might be focus shift with that type of lens design.

"Focus shift can lead to blurry images and focus errors, when working with subjects at close distances and using fast aperture lenses. With the lens aperture fully open or “wide open”, incoming rays of light converge at different focal points"

Read more: http://photographylife.com/what-is-focus-shift#ixzz37y5soKoA

thanks bitcrusher

Steve Wake
07-22-2014, 08:45 PM
Thanks Steve. I'd been using that guide when attempting to adjust my speedbooster, however it doesn't state how much you should be turning it.

I assume the turning the element causes it to move in and out -- I would not want to move it so far out that it could interfere with the sensor. Does anyone know if there is any risk in that happening?

I just adjusted my Pocket SB on the BMPCC to match the witness marks on most of my lenses. Lenses had been focusing at infinity before the infinity mark, so I knew I needed to screw the optic "cell" clockwise to increase the flange focal distance (opposite of your problem). Using a chart, my 50/1.8 at a measured distance of 7 feet was in focus when the witness mark was about 5 feet. To my surprise I had to screw the cell in (clockwise) a little over 180 degrees to make the witness mark match. This also made the lens focus at infinity just a hair before the hard stop. I checked some other lenses and backed off the adjustment a few degrees to make sure lenses with hard stops would still reach infinity focus.

I also did a rough check of how much clearance was left between the Pocket booster element and the sensor cover glass. Roughly I'd say at least 0.5mm. So unless you totally overdo the adjustment there should be no danger of damage. (If brianc1959 is reading, I'm sure he knows how much wiggle room there is).

One more thing: I bought my Pocket SB used. While it looked new, it did have 2 previous owners, both of whom said they barely used it, and it was originally purchased from B&H, who will sell LNIB returned items. So I don't know if someone along the way tried to adjust the optic cell and that's why it needed a full half-turn. I do know the mount screws were loose when I got it.

Another tip, loosen the set screw just slightly-about 1/2 turn. It sets in a recess in the collar that hold the optic cell in place, and to avoid complications you want the screw to stay in that recess. Re-tighten when done, of course.

Hope this helps.

Note: My Sigma 18-35 is MUCH closer to being par focal now that the back focus is adjusted - still not perfect, but very close. I wish back focus on the plain MFT - F adapter was so easily adjustable.

wilsawn
07-22-2014, 09:57 PM
thanks again Steve.
does a counter clockwise turn make the element retreat inward or push out farther? still concerned about hitting the sensor.

to be clear, I can focus to infinity fine when stopped down, but wide open the image is definitely softer. can't tell if it's a focus thing or as bitcrusher suggested perhaps this is a characteristic of the lens I'm using.

Steve Wake
07-22-2014, 11:14 PM
thanks again Steve.
does a counter clockwise turn make the element retreat inward or push out farther? still concerned about hitting the sensor.

Push out, i.e. move closer to the sensor. If it focuses at infinity a hair before the hard stop, say stopped down one stop, you do not need to adjust it at all. Or you can check it at a measured distance (to the sensor, not the front of the lens).


to be clear, I can focus to infinity fine when stopped down, but wide open the image is definitely softer. can't tell if it's a focus thing or as bitcrusher suggested perhaps this is a characteristic of the lens I'm using.

I have that lens and yes, it is softer no matter what the focus distance when wide open, as practically all lenses are when WO. To me, the loss of contrast and the color shift WO are also bothersome - but those are pretty standard problems, too.

Your issue is not likely focus shift, since that can be corrected by manually refocusing if you change the iris (focus shift is mainly an issue with AF lenses that focus wide open, lock focus, then close the iris - and potentially shift focus - when the shutter is clicked). And that 50/1.8 doesn't have bad focus shift anyway. It (again, like most lenses) does lose some sharpness, contrast, and exhibit color shift when WO, though.

If it's real bad it could be a lens issue, but no way for me to tell without samples.