Aperture number of Raynox DCR-150 and/or DCR-250

[zzffnn]

As title, please kindly advise me:
What is the aperture number of Raynox DCR-150 or DCR-250? And does aperture change when they are stacked?

Thank you very much!

[rjlittlefield]

You can divide the focal length by the glass diameter to get an F-number for either Raynox by itself. The resulting number will mean nothing when the Raynox is used in combination with any other lens that provides the limiting aperture.

Why are you asking the question? That is, what will you do with the answer?

--Rik

[zzffnn]

Thank you very much, Rik.

I am indeed trying to see which lens combo has clear image quality advantage, at 1x-2x on-sensor (m4/3) magnification:

1) a reversed 50mm F/1.4 lens on flocked extension tubes;
vs

2) Raynox 150 and/or 250 stacked on this Olympus zoom lens (40-150mm, F/4-5.6, used at 150mm and F/5.6): http://www.four-thirds.org/en/microft/telephoto.html

I am guessing 1) has much larger aperture, so much so that image quality would be determined largely by that advantage.

Combo 1)'s images look quite good to my eyes, even at 100%. Though I have not had chance to run extensive comparison, which I dread to do.

Edit: in 2) Raynox 150 provides 1:1, R250 provides 1.55:1, while R150+250 provides 2.39:1, per Johan's very helpful web page: http://extreme-macro.co.uk/raynox-adapter-techniques/

[rjlittlefield]

Well, there's always the "satisficing" approach, if #1 looks good then use it.

Or the purely experimental approach, test them all and see which one works best.

If you want some guidance from computation, then what matters is the F-number of the whole string of glass in each case. I think the simplest approach is to work in effective F-numbers at the sensor. With Raynox stacked in front of an ordinary lens, the effective F-number at the sensor is the same as if the Raynox were not present. That's because the cone angle of light hitting the sensor is determined by the rear lens and its aperture setting, and those don't change when you add the Raynox. The comparative number for the reversed 50 mm would be roughly (magnification+1)*F_nominal, where F_nominal is whatever you have set on the lens.

--Rik

[zzffnn]

With Raynox stacked in front of an ordinary lens, the effective F-number at the sensor is the same as if the Raynox were not present.

Your guidance is very help, thank you, Rik!

So to calculate effective F number of Raynox stacked over a telephoto lens, I also use
(magnification+1)*F_nominal, where F_nominal is whatever you have set on the lens,
correct?

That would mean that my 150mm F/5.6 lens with Raynox stacked on would be working at effective F number of 11.2 (5.6*[1+1]) at sensor. That is much smaller aperture than reversed extended 50mm F/1.4. I heard diffraction starts to kick in at around F/8 for m4/3 sensor.

[zzffnn]

Rik:

What focus stacking step size (in microns) should I use, at 1x effective F/3.6 on (m4/3) sensor and 2x effective F/5.4 on sensor?

Sorry for my greedy questions. I just though it would be easier to ask now, since I got your attention. Thank you very much!

[rjlittlefield]

With Raynox stacked in front of an ordinary lens, the effective F-number at the sensor is the same as if the Raynox were not present.

...

So to calculate effective F number of Raynox stacked over a telephoto lens, I also use
(magnification+1)*F_nominal, where F_nominal is whatever you have set on the lens,
correct?

No, I meant exactly what I said. If the Raynox were not present, then the effective F-number would be whatever you have set on the camera (assuming that the camera works in effective F-number). When you add the Raynox, the magnification increases, but the effective F-number does not change. If you have your 150 mm set on f/5.6, then the effective aperture is f/5.6 either with or without the Raynox.

The formula (magnification+1)*F_nominal is only appropriate for the reversed 50mm, stopped down using its own aperture.

What focus stacking step size (in microns) should I use, at 1x effective F/3.6 on (m4/3) sensor and 2x effective F/5.4 on sensor?

Use Table 2B at https://zerenesystems.com/cms/stacker/d ... romicrodof, the one for "Determine DOF (step size) from magnification and effective f-number".

1X at f/3.6 is midway between f/2.8 and f/4 (closer to f/4), so something like 25 microns should be OK. 2X effective f/5.4 would be around 20 microns.

As always, these numbers are rough approximations -- go smaller for safety, or test how far you can push the limits by going bigger.

--Rik

[zzffnn]

That clears a lot of my confusions. Thank you very much, Rik! You are very kind and helpful.