I'll be shooting a project where part of it is about taking some stills from a mirror reflection.
I worked out that by using three mirrors I can get the right position to get the shot I want.
I was wondering if anyone knows about the physics :
- would it be any loss of stops when photographing from the last reflected image (third mirror)
- definition of the last image (from the third mirror),
- difference between using flash light or constant light,
- anything else I should consider?
Thank you all in advance,
DOP - Sao Paulo, Brazil
Diego Pascoalino wrote :
>>"- would it be any loss of stops when photographing from the last reflected image (third mirror) - definition >>of the last image (from the third mirror)"
In answer to those two points you absolutely have to use front surface mirrors. If two out of the three mirrors are simply to find the correct viewpoint and aren't meant to be part of the scene you're shooting you will want them to have as little effect on the optical quality of the shot as possible.
3 rear surface plate glass mirrors will give you horrible light loss (at best 90% of 90% of 90% - if that makes sense) and all sorts of double (and then quadruple and then... even more) reflections as you have a 'true' reflection off the mirror surface and a 'ghost' reflection from the top surface of the glass.
Make sure you use front-silvered mirrors so you don't get double reflections.
Most standard mirrors have the reflective coating on the back side of the glass so it is normal to have double reflections - one from the reflective coating and a faint one from the front of the glass. ?This might not be noticeable in wide shots but in close-ups it will be a problem. ?Make sure you use mirrors specially made for our purposes - and even then you might have multiple reflection problems. ?
>>- would it be any loss of stops when photographing from the last reflected image (third mirror) - difference >>between using flash light or constant light
I agree with the use of front surface mirrors. As for exposure loss, just use a spot meter and a good 18% gray card. I would use continuous light sources. A flash might look okay until you see the final image and you might get odd hot spots or flares that you didn't catch while shooting.
If you can, shoot some tests.
It’s best if you can use the exact mirrors you will use on the day, or similar.
Remember that mirror backed glass, adds a lot of green as the light effectively passes through the glass twice to reflect the image. Shining a light off all three mirrors and reading this against the same light, direct, with your colour meter usually takes care of this.
You may be able to combat some of the double reflections with polarising, and as they are stills you have plenty of post production tools to clean up and problems.
You might consider using acrylic (Plexigas/Perspex) front-surface mirrors. They're lightweight, easy to mount, and fairly unbreakable.