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Old 07-08-2015, 07:16 AM
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troypiggo (Troy)
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camera lens external apertures - sanity check please

Wanting to make some external apertures for a couple of my lenses to get rid of diffraction spikes when stopped down a little. I just need to check my logic in terms of sizing the diameters to get the f-ratio I want.

Take the 135mm f/2L. I want to the extl aperture to stop it down to f/2.8. I know that the internal blade aperture of the should be about 135/2=67.5mm. If you set the lens to f/2.8, the blades should be about 135/2.8=48.2mm.

But where I'm uncertain is when the aperture is external - not at 135mm from the sensor. I'm thinking I should use the distance from sensor to front of lens where the extl aperture would be?

Say it's 156mm from sensor to front of lens. To get f/2.8, by this logic, the aperture should be 55.7mm as opposed to the 48.2mm.

Any thoughts/comments?
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Old 07-08-2015, 08:56 AM
cfranks (Charles)
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I've been doing this for my 6x7 lenses and they work OK but I don't know if my sizes are anywhere correct. Taking (assuming?) that 1 stop halves the amount of light through, I made the aperture disks reduce the open area of the front lens to whatever I needed. My exposures are nearly always 900 secs or greater so I wasn't too concerned with accurate sized holes. HTH
Charles
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Old 07-08-2015, 10:43 AM
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Thanks mate. I'm not trying to be too anal about it, but am trying to strike a balance between stopping down for sharpness, less vignetting, CA etc, vs keeping it wide/fast enough for shorter exposures as it will be tracking unguided.
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Old 07-08-2015, 10:49 AM
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multiweb (Marc)
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Bought one of these years ago. Came handy a few times.
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Old 07-08-2015, 11:25 AM
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Ooh. That looks like the ticket. I have, and intend to use, a 72-58mm step down ring. A set would be ideal.
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Old 07-08-2015, 12:01 PM
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I am using one of theese:
http://www.dx.com/p/wlxy-wlf41-11-wo...Q#.VcQe5vkYNqw

Perfect for cutting plastic sheet (for folders.. ).

BTW, F-number is simply a ratio between focal length of the lens and the diameter of the incoming beam of ligth (usually diameter of the front lens).
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Old 08-08-2015, 08:56 AM
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Phil Hart
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To clarify what Bojan said, the focal length of the lens is fixed and the size of the aperture should be calculated based on that not the distance from the sensor to where the aperture mask is.

Putting aperture masks on lenses gets tricky though, especially at shorter focal lengths. You should be ok with a mask on a 135mm lens. Just check that your vignetting is not getting worse.

Phil
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Old 08-08-2015, 11:08 AM
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Gotcha. Thanks guys.
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