Lens combo for 1.35x to 2.95x (extendable to 3.9x)

NikonUser · Post by **NikonUser** » Sun Mar 09, 2014 9:24 am

In a couple of recent discussions re. how to get decent images of less than 4x magnification on a DX-format sensor (23.6x15.8mm in this example); I received excellent helpful advice (alphabetical order) from Blame, ChrisR, jazzper,Planapo, ray_parkhurst.

http://www.photomacrography.net/forum/v ... hp?t=23019
http://www.photomacrography.net/forum/v ... hp?t=23027

Trials involved a fixed aperture (f/2.8) Apo Componon enlarging lens, 50mm and 80mm El-Nikkor enlarging lenses, a 105mm Micro Nikkor, a Raynox DCR-250.

Because my specimens vary in size it is most convenient for me to have a setup that allows for magnification changes by changing the length of extension via bellows. After several reiterations with single and stacked lenses, with likely more to come, the following currently serves my needs and it may prove useful to others.

D90 + 13mm extension tube (smaller would be better) + PB-6 bellows + 80mm El-Nikkor enlarging lens (not reversed) + Raynox DCR-250 clamped directly onto the 80mm.
At minimum bellows extension, FOV=19mm, magn.=1.24x, working distance from a 32mm lens shade=50mm.
At maximum bellows, FOV=6mm, magn,=3.9x.

Images of a bristly Tachinid, Winthemia datanae complex (ID: D.M. Wood)

Top:Full frame image; total extension, camera mount to El-Nikkor, 75mm; FOV=17mm, magn.=1.35x
I tried 2 f-stops on the 80mm, f/8 and f/11; IQ indistinguishable; f/11 for this image.

2nd image; head and thorax full frame; total extension 150mm; FOV=8mm, magn.=2.95x
f/stop on 80mm = f/8

3rd image; 100% crop from near center of 2nd image

4th image;100% crop from bottom right corner of 2nd image

NU14-03-09

jazzper · Post by **jazzper** » Sun Mar 09, 2014 9:54 am

It certainly looks like you've found an excellent combo here - well done!

Blame · Post by **Blame** » Sun Mar 09, 2014 9:55 am

But why?

The whole point of stacking is to use the front, objective, lens as close to its optical optimum as possible. This based on the assumption that optimum is with forward convergence close to it's focal length. With the 80mm as tube lens that would occur at about 80/125= 0.64x.

They should be the other way round. Particularly as it is the front, objective, lens that should be stopped down. That would give you an optimum magnification of 125/80 =1.56x which is actually achievable with your bellows.

Set the bellows so it gives you that and then replacing the 80mm with your 50mm will give you 2.5x while your 40mm (with paper stop added) will give you 3.125x.

ChrisR · Post by **ChrisR** » Sun Mar 09, 2014 11:14 am

Blame - It seems pretty reasonable: the enlarger lens is a bit over its focal length distance from the sensor (thick bellows), which is about where it's designed to work. Then the "Close-up" Raynox lens is doing exactly what it was designed to do. Were you thinking NU was using different lenses?

Seems to work pretty well too!

Blame · Post by **Blame** » Sun Mar 09, 2014 11:38 am

ChrisR

No. I have been trying to explain that shortish enlarger lenses work best close to infinity. The results may be good but they can be better. The 80mm should be front and reversed.

However good the raynox are as macro filters they will work even better reversed as tube lenses. They have the right properties for the job. A thin lens that will work well at small apertures. In front with aperture controlled by the tube lens they are being asked to handle aperture = (80mm lens aperture)/magnification. That is far too wide for a triplet to be sharp.