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Reversed stacked lens DOF vs aperture calculation

 
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zzffnn



Joined: 22 May 2014
Posts: 1682
Location: Texas USA

PostPosted: Tue Mar 13, 2018 5:49 pm    Post subject: Reversed stacked lens DOF vs aperture calculation Reply with quote

Please kindly advise me on how to select aperture to obtain enough depth of field, to finish in-camera automatic focus bracketing in one round, without moving camera or subject (micro sea shells).

I am using a Canon FD 50mm F/1.4 lens, reversed on an Olympus m4/3 auto zoom lens 40-150mm F/4-5.6. Zoom set to 50mm for 1x and 100mm for 2x on sensor magnification. I am using Olympus' in-camera automatic focus bracketing function; basically, I set the rear auto lens to closest focus (0.9m), set step size (1-2), then the camera drives auto rear lens to infinity focus and stop focus bracketing there.

When I tried focus bracketing micro seashells, I set the front reversed Canon 50mm to F/1.4 and the rear lens to maximum aperture. In that case, one round of focus bracketing is not enough to image entire micro seashells, at either 1x or 2x (missing roughly around 1/3-1/2 of depth). I know I should not use front lens at max aperture, though I tried it that way.

To get my micro seashells in one focus bracket, should I stop down the front 50mm lens to F/2 or more? I don't currently have an easy way to move camera or subject precisely at 1x or 2x, so I would like to finish focus bracketing in one single round.

Is there a formula or close approximation that tells me, for x mm thick subject to be focus bracketed in one round, I should set front reversed lens to F stop Y? Or do I set it to say between F/2.0-2.8 and just try multiple times to see what works?

Another way to do it for me, is to use a 1x NA 0.03 (Nikon vintage achromat) microscope objective and a 2x NA 0.09 ((AO Spencer vintage achromat) microscope objectives, on a microscope stand, using microscope focus drive and afocal camera (without using automatic focus bracketing). But I heard 1x and 2x microscope objective usually don't image as well as camera lenses at the same magnification, due to smaller aperture and worse optical correction, correct?

In my shoes, which approach (stop downed reversed camera lens or microscope objective) would you take, to get good images, conveniently and quickly Twisted Evil Reasonably good images are good enough for me.

I know I can also put Raynox 150 and/or 250 over my Panasonic 100-300mm F/4-5.6 tele lens, but my gut feeling says that combo's aperture may not be large enough for good studio macro (even though it works well for single frame field macro using flash).

Thank you very much!
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zzffnn



Joined: 22 May 2014
Posts: 1682
Location: Texas USA

PostPosted: Fri Mar 16, 2018 11:31 am    Post subject: Reply with quote

Any comment is better than nothing, thank you gents. Please feel free to answer/comment as little/concise as you want, if that makes it easier.

Judging from view count, I probably asked an interesting question; but I probably asked too many questions? Thank you for your patience.
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mawyatt



Joined: 22 Aug 2013
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Location: Clearwater

PostPosted: Fri Mar 16, 2018 11:55 am    Post subject: Reply with quote

The Canon 35mm Bellows lens is very good down to slightly above 1X, and the Nikon 55 F2.8 micro-Nikko AIS is very good up to about 1X. Of course the Printing-Nikkor 105 F2.8 is excellent around 1X, but very expensive also!! See results to compare here.

http://coinimaging.com/lens_compare.html?

Best,
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Fri Mar 16, 2018 12:44 pm    Post subject: Reply with quote

You wrote:
Quote:
basically, I set the rear auto lens to closest focus (0.9m), set step size (1-2), then the camera drives auto rear lens to infinity focus and stop focus bracketing there.

If this is what you mean by "one round", then you're stuck. There's no way to get a larger focus range by changing apertures. Stopping down would extend the front and back limits by just a hair due to smaller blur circles, but the nominal focus range would not change.

Have you thought about using one of your microscopes just to provide the focus-stepping, while using macro optics to do the imaging? I'm thinking of a setup like http://www.photomacrography.net/forum/viewtopic.php?p=94797#94797 .

--Rik
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zzffnn



Joined: 22 May 2014
Posts: 1682
Location: Texas USA

PostPosted: Fri Mar 16, 2018 2:41 pm    Post subject: Reply with quote

Thank you very much, gents.

Rik:

Yes, 0.9m (closest focus) to infinity was what I meant by "one round".

And yes, I have thought about using a microscope focus drive, though that would require me to raise my camera much higher, on a stable stand. Currently, I can only use a tripod to achieve that; that tripod is not stable enough for 10x NA 0.25 on-sensor magnification, though may be OK for 2x.

I could get an Olympus 60mm macro (1:1) lens that can move focus from 0.19m to infinity, in the future. That should extend focus range quite a bit?

That 60mm macro lens maxes out at 1:1 though. To get 2x, what do you think would work better:

1) put Raynox 250 in front of the 60mm macro;
though isn't Raynox 250 is only a F/6 lens (would it stop down aperture too much to kill resolution)?

2) reverse a full frame fast 30mm lens in front of the 60mm macro.

Thank you again!
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