Science Buddies: "Ask an Expert"

Hi, I am doing the photography project on the Fresnel Lens Focuser (https://www.sciencebuddies.org/science- ... 0&from=TSW) and, you guessed it, I need a professional interview by Monday. Sorry for the short notice!! I have emailed other people and posted on forums too. I only have 10 questions for someone to answer. My independent variables are the size of the Fresnel lens used to build the focuser and the depth of field. My dependent variables are uniformity of lighting, exposure, and depth of focus. I have a fully equipped photography studio in my house.

Here are the questions:

1. In what instances would you use a flash focuser?
2. If you have ever used a flash focuser, how far generally have you been able to successfully illuminate and stay in focus?
3. What kind of lens would be best for my project, zoom or prime?
4. Would it beneficial to test the different focusers with multiple lenses to get the most accurate results?
5. What is the best way to measure resolution/depth of field, or what type of chart would be best to use to measure depth of focus?
6. Are there any other factors besides exposure, depth of field, and uniformity of lighting that would be beneficial in measuring the quality of a photograph?
7. When testing uniformity of lighting, what type of a scene should I take a picture of? As in, should it be the resolution chart or a still life with various objects?
8. What kind of factors would you expect to interfere with the results of my experiment?
9. What kind of background do you recommend I use for the photograph, just plain black?
10. Last, do you expect my project to work?

Thanks!!!

-Jessica Schoenenberger, Glenelg High School

1. In what instances would you use a flash focuser?
I've used them to extend the useful distance of a flash when using long telephoto lenses and needed fill flash.
2. If you have ever used a flash focuser, how far generally have you been able to successfully illuminate and stay in focus?
I personally dislike the use of the word "focus" when it comes to flash extenders. The usual goal of flash illumination is to get a uniform light distribution on something within the image. In other words, you don't want the light to be focused and produce a high intensity illumination of something in the image. A more useful question is how far have you been able to extend the range of a flash. I've extended a Nikon SB-28DX at its 85mm zoom setting for use with a 600mm lens for fill flash use to about 20m.
3. What kind of lens would be best for my project, zoom or prime? It doesn't matter. What matters is the focal length of the lens has to be longer than the flash is designed for by whatever factor your flash extender lens provides. In my case 600/85 = 7 and the flash extender was a nominal 3x magnifier. If the ratio of the lens focal length to flash zoom is less than the flash extender magnification, your illumination won't fill the full image frame.
4. Would it beneficial to test the different focusers with multiple lenses to get the most accurate results? That would depend on your hypothesis, e.g. what you are attempting to test.
5. What is the best way to measure resolution/depth of field, or what type of chart would be best to use to measure depth of focus? How does this relate to a flash extender project? If you are testing flash exenders, the more useful things are how uniform the lighting pattern is. resolution and depth of field are lens and sensor (or film) properties and better be independent of illumination and flash properties.
6. Are there any other factors besides exposure, depth of field, and uniformity of lighting that would be beneficial in measuring the quality of a photograph? From an asthetic standpoint, yes, but this is a science project. If you are going to be able to evaluate uniformity of lighting, you are going to have to have an exposure that will allow you to see shadow detail. If you are mixing ambient and "fill" flash which is something photographers do for asthetic reasons, you won't be able to compare how well an extender behaves at various distances scientifically because you will be having to change the shutter speed to maintain the balance which will introduce an uncontrolled variable and skew results. If most of your illumination is coming from the flash, then the shutter speed is unimportant as long as the shutter is at or below the flash sync speed. This means that what you actually can have control over are aperture, flash power, subject distance, and focal length (assuming you have a zoom lens). You need to understand these factors better before you come up with a hypothesis and design your experiments.
7. When testing uniformity of lighting, what type of a scene should I take a picture of? As in, should it be the resolution chart or a still life with various objects? Flash manufacturers typically use a plain flat eggshell painted wall and a variety of room sizes. In small rooms, some of the flash output bounces off the floor, ceiling, and side walls to illuminate the subject wall. In larger rooms, the bounce effect light is attenuated by distance such that it does not play a significant factor in the illumination.
8. What kind of factors would you expect to interfere with the results of my experiment? see 6 and 7 above
9. What kind of background do you recommend I use for the photograph, just plain black? see 7 above
10. Last, do you expect my project to work? Without knowing your hypthesis, I have no way of knowing what you are trying to accomplish so I can't answer your question.