Lenses and Infinity Focus

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Brandon Graham

Lenses and Infinity Focus

PostFri Feb 15, 2013 2:18 am

I wanted to throw this out there for the BMC engineers and people much smarter than me to consider when thinking about why certain lenses on this camera have problems with infinity focus.

I have been doing some work lately trying to calculate the hyperfocal distance and acceptable range of sharpness for my lenes on the BMCC EF.

Since we are using lenses that are formatted for full frame and thus designed around a different circle of confusion we must take this into account as it will change the behavior of the lens in terms of its depth of field, hyperfocal distance and acceptable range of sharpness.

We also must consider that this could exacerbate problems with manufacturing differences between lenses. Especially since we are talking about distances of maybe a millimeter or so.

I would surmise that those lens that have hard stops and no extra "play" in their focus mechanisms will have problems on this camera due to its different sized photo-sites when compared to standard full frame (which lens was originally designed for).

However, the BMCC has what I would call a significantly smaller photosite size compared to 35mm and thus could alter a len's capabilities in terms of hyperfocal distance (infinity) and range of sharpness.

I have whipped a a quick spreadsheet that allows you to add lenses to it and calculate hyperfocal distance and range of sharpness for any lenses taking into account the circle of confusion of the BMCC sensor. Using this you should be able to figure out if a lens has enough play in it to focus to infinity.

In short, the markings on your standard DSLR/SLR 35mm lens are set for the standard 35mm circle of confusion which directly impacts hyperfocal and range of acceptable sharpness. If a lens is very "strict" about sticking to these marking and doesn't provide enough range to account of the significant change on this camera it simply wont be able to reach hyperfocal (infinity) focus.

I have experienced first hand the significant effect this camera has in terms of where infinity actually is attained on my 24mm T1.4. It also helps knowing what the distance marks on the lens actually mean when taking into account the change this camera presents.

Here is a link to the spreadsheet. Feel free to download it and play around by adding lenses. I have also included an extra sheet with my assumptions. Currently only "feet" calculations works as I have not verified the meters calculation portion. I also did this in about an hour so be warned if the numbers of wrong, but they matched up to what I was getting on my Rokinon 24mm T1.4. Also feel free to provide any feedback on the spreadsheet or corrections in the formulas.

http://secondmovementfilms.com/DOF_HYPER.xlsx

TL;DR --> You need to use lenses with more play (room to go beyond the infinity mark) in their mechanisms to account for changes in len's parameters when shooting with this camera. Or find a way to shim that particular lens to account of its loss of focal range but be aware that you may be trading back focus in your search for infinity.
Last edited by Brandon Graham on Fri Feb 15, 2013 2:27 am, edited 1 time in total.

Brandon Graham

Re: Lenses and Infinity Focus

PostFri Feb 15, 2013 2:26 am

One last thing to keep in mind. The hyperfocal distance of a lens changes with aperture.

Meaning that the distance in which infinity is achieved at an f/2 is different than say f/4.

This also means that if you do not know what f-stop your lens is at you cant accurately calculate or use the distance marks on the lens to figure out where infinity is.
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John Brawley

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Re: Lenses and Infinity Focus

PostFri Feb 15, 2013 8:23 am

By my reckoning, the hyperfocal distance of a the Tokina @ 11mm at 2.8 on the BMCC using CoC of 0.00059" would be 9'5". I calculated this using pCam.

In other words, to hold infinity, that's the distance the lens needs to get to.

The CoC is the same as one would use for 1.78 extracted form a m4/3 sensor. Super 16 for example is normally 0.00050"

CoC and focus is, as always, highly subjective. What's sharp to some isn't sharp to others. Some use different CoC's for the same image size. Some focus pullers for example will use different CoC's for digital and film. Film seems to have more wriggle room than a lot of digital cameras, even though they have same imaging area.

I don't think the 135 format lenses would be the issue here, other than you're using the smaller, more central part of the lens.

jb
John Brawley ACS
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