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Old October 8th, 2009, 08:37 AM   #1
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A thought on diffraction limits.

OK, here’s one of those “I woke up in the middle of the night with a random thought” things. I admit I’ve not given it much critical thought. I thought I’d just throw it out there and let the community pick it apart. It’s probably widely known, but I’ve not seen it discussed around here.

Here’s the problem: When using lenses designed for still cameras on camcorders with small sensor size diffraction limits the use of smaller apertures. This can necessitate the use of faster than normal shutter speeds and/or ND filters, both of which can have drawbacks in some situations.

Another possible solution: How about placing one or more close-up rings between the lens and lens adaptor? This should reduce the amount of light striking the sensor by one or more stops (depending on the number of rings and their length) while allowing the lens to work at larger non-diffraction limited apertures. It should also reduce the minimum focus distance while preventing the lens from focusing at infinity. With the long telephoto lenses we like to use these may at times be benefits. It would require rebalancing and possibly adjusting the lens support.

What do you think? What considerations have I neglected? Have any of you tried this? If not, does anyone have kit to hand to give it a go and report back? Thanks.
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Old October 8th, 2009, 10:51 AM   #2
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The amount of light reduction depends on not just the size of the rings but also the focal length of the lens.
For short focal length lenses even a small extension will limit your focus to just a few inches, so pretty useless. For telephotos you need massive amounts of extension to have any effect.
What's wrong with just using NDs? Put two ND8 filters in a matte box and you'll lose 6 stops, which even in outdoor sunny conditions should allow you to close the lens to under the diffraction limit.
Steve
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Old October 8th, 2009, 01:07 PM   #3
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Thanks Steve. I knew someone would know about this. I arrived at this train of thought while trying to solve another problem. I have a telephoto with a long minimum focus I’d like to reduce. In remembering the last time I used rings on a telephoto for stills (some years ago) I recall that 27mm on a 200mm reduced the minimum focus by about half while still giving a good working range. Certainly more than just a few inches. Am I misremembering? (Happens with age!)
“For telephotos you need massive amounts of extension to have any effect.” Are you just talking about light reduction or also focal effects (which admittedly wasn’t the “focus” of the post)?

“What's wrong with just using Nds?” Absolutely nothing. I use them all the time. I know some around here won’t use them because they refuse to color grade and the filters give a slight shift. Not a problem for me. (Personally, I dislike fast shutter speeds for most purposes.) Good filters can also be pricy in large sizes- a problem for those of us on tight budgets. Rings are inexpensive and I was wondering about them being a less costly solution. Apparently not.

I’d appreciate any further thoughts on using rings to reduce minimum focus. Is this also forlorn hope?
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Old October 8th, 2009, 05:46 PM   #4
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Quote:
Originally Posted by Mike Sims View Post
I’d appreciate any further thoughts on using rings to reduce minimum focus. Is this also forlorn hope?
There is a simple formula relating the amount of extension “x” to the focusing distance “d” and the focal length “f” and that is x = f squared divided by d. If for example (to simplify the arithmetic) you approximate the focal length of the EX3 lens to 5 - 80 mm and add an extension of 1 mm the focusing distance at the wide angle becomes 25 mm whereas at the telephoto setting it is only reduced to 64 m. (Incidentally this is a useful formula to use if you are making a lens mount and you do not have accurate measurements to work to).

With regards to your diffraction question, diffraction is caused by the edge of the iris. The effect becomes more noticeable at small apertures as there is less light entering in comparison to the length of the iris edge. Increasing the iris diameter lets in more light and so fogs out the diffraction effects. At an iris diameter of 5 mm or less the diffraction effect is important. So for a 5 mm focal length lens diffraction becomes a problem at f 1 or smaller whereas for the 80mm focal length lens can be stopped down to f 16.
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