Black background

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

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Cornel
Posts: 26
Joined: Tue Feb 07, 2017 11:33 pm

Black background

Post by Cornel »

Hello everybody,

I'm looking for some advice regarding obtaining a black background for a high magnification photo. I know, usually you want to obtain some kind of color there and that works fine by placing something colored behind the subject but what if I want to make it black.
I'm mainly referring to 10x-40x magnifications and placing something black or leaving the background exposed has almost the same effect (some sort of gray). It probably has to do with my diy diffusion, I use a horizontal rig.
I was curious if someone could share a picture of a setup that can obtain a black background in camera, or some advice.

Thanks!

Beatsy
Posts: 1650
Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Post by Beatsy »

I'm not at my rig to take pics right now, but I spent some time tinkering with this. I got the blackest results from a piece of card with flocking stuck on it. This is positioned a few cm behind the subject and angled to left or right by 30 deg or so (my rig is horizontal too). The most important bit is shading the lighting so none falls on this background. This can simply be bits of card taped to the sides of your lights - allowing light to reach the subject or diffuser but casting a shadow on the backdrop. A similar approach works to stop light bouncing off the diffuser(s) and lightening the background, but I found the best approach was to also remove extraneous diffuser behind the subject (between subject and backdrop).

Hope that makes sense.

Edit: forgot to mention, when setting up I put a piece of white card behind the subject and adjust the lights and shading while looking at the live view. Adjust until you get the darkest background you can (it will likely still be light grey) then replace the temporary white backdrop with the flocked card when you're done.

Another (possibly better) idea: put a film canister (or similar) on it's side behind the subject with the open end pointing toward subject and lens. You'll be using the inside, bottom of the canister as your backdrop. This will reduce the faffing about with shading adjustments as the bottom of the canister should be fully shaded by it's own sides. I'd do this on my setup, but I don't have enough room behind the subject - a post for the adjustable specimen stage is in the way. A tinkering session for another day perhaps.

Cornel
Posts: 26
Joined: Tue Feb 07, 2017 11:33 pm

Post by Cornel »

Thank you for the reply.
It makes perfect sense, I thought of something similar but unfortunately for me it means I have to change everything, so I was hoping for some sort of hidden trick :D.
I use a single studio flash which lights from above the subject into a horizontal paper, tunnel like diffuser which has the lower half taped with reflectorisant duct tape. Inside this tunnel there's another white plastic diffuser that comes around the microscope objective. This is the best way I found to lit the subject uniformly with a single light source (that it's pretty big and can't be positioned very easy), it also means most of my subjects are held from the back and not from below, so it's hard to put something behind them.
Great idea with the film canister, I have some and will try it :D

What we need is this:
https://en.wikipedia.org/wiki/Vantablack

:lol:

rjlittlefield
Site Admin
Posts: 21126
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

One thing to watch out for...

At high magnification, you have to consider that the entrance cone of the lens can be unexpectedly wide. An objective with NA 0.28 (Mitutoyo 10X) accepts light from angles that include over 16 degrees off-axis (almost 33 degrees total width). Wider apertures have even larger angles, such as NA 0.42 almost 25 degrees off-axis (50 degrees total width) and NA 0.55 over 33 degrees off-axis (almost 67 degrees total width).

With such large angles, it's pretty simple to inadvertently include something bright that is "off to the side" of the subject and appears to be out of view even though it really isn't. Objects like that result in general brightening of the background, so the real cause can be difficult to figure out.

--Rik

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