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Archive 2013 · Fresnel Lens Questions
  
 
Bruce n Philly
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p.1 #1 · p.1 #1 · Fresnel Lens Questions


I am tinkering and building my own flash extender and am having trouble finding inexpensive lenses with short focal lengths. It appears that all these lenses you find on the 'net are 300mm sheets.

Comments on my following issues please:

1. Can someone with a Better Beamer measure its focal length please? Just take a shade off of a lamp and go to the other side of the room and focus the beam to a point (think frying ants) and measure the distance lens to wall. I suspect it is about 12" (300mm).

2. I have purchased two Fresnel Lenses so far... the Bausch & Lomb 2X acrylic sheet that I measured to have a 12" focal length, and another from a craft store that claimed to be 3X but has the exact same 12" focal length. I have found some sources on eBay/Amazon that claim to be 3X but then say focal length is 300mm Shouldn't the focal length change if power changes? By the way, I don't think B&L even makes their product as it is not even noted on their company website nor was there a data sheet there.

3. Are there any cheap 3X or 4X lenses out there (focal lengths shorter tham 12")? Edmunds Optics has many options with many different focal lengths but they don't mention the power - makes me think this power thing is a sales trick. Unfortunately... their prices are high... I don't need this quality for a simple flash extender.

Edmund Optics

Some background: I am seeking a shorter focal length lens than 12" (300mm). Why? I ran some experiments last night measuring the flash pattern and brightness through the B&L lens using the Better Beamer's structure of 6" and then at my lens's focal length of 12" from the flash. The differences were profound. I got way brighter and more focused beam holding the B&L lens at 12" from the flash. I also shot three different flash 24/50/105 settings also that had again, profound differences. I was about 75' camera to wall (house) for the test. When I had the beam narrow and focused at 12" flash to lens, and the flash set widest at 28, I got a killer bright throw that will work well with my 500mm f/f IS lens.

So....... 12" from flash to lens ends up being a pretty big extender......... If I can find one at 4X, or maybe more accurately to say a 6" focal length, I think I can achieve the same but having built a smaller unit.

By the way, I was getting at least 3-4 stop increase in flash power at 75'. A pretty big deal I think. I believe I got this power by putting the lens at it native focal length but had open sides. I plan to use highly reflective foam board to build a tube that should channel more light into the lens.

Any light to shed on this?

Peace
Bruce in Philly



Jul 18, 2013 at 03:48 AM
Roland W
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p.1 #2 · p.1 #2 · Fresnel Lens Questions


A big factor in the optimum focal length of Fresnel lens would be the beam angle of the flash you are using. A flash that has adjustable beam width, and is set at its longest setting, may indeed be a good match for the standard Better Beamer lens. I suggest you buy a replacement Better Beamer lens for $10 so you have it as reference, and can compare it to the other lenses you play with. The link at B&H is: http://www.bhphotovideo.com/c/product/683113-REG/Visual_Echoes_FX_FL_Fresnel_Lens.html

A reflective housing to try to capture side light and redirect it in to the Fresnel is not going to help much if any. For light to contribute to the beam, it needs to originate from near the focal point of the lens where the main flash tube and built in flash Fresnel are located. The side light rays will reflect back in toward the Fresnel, but will be coming from too much of an angle, and will just end up focused down range off of the main beam.

The over all length of the unit really does not matter much, so your search for a shorter focal length Fresnel may not have much benefit. The Better Beamer comes apart quickly for transport, so the longer length is not really an issue. A larger sized Fresnel might capture more of the beam angle of the flash, so that might help out with more power down range, but the specific flash and Fresnel size and Fresnel focal length will all enter in to finding an optimum.

The Better Beamer works really well as is, so improvement may not have a big payback. Another factor to consider is that if you put too much light on the bird, you may end up harming it temporally, which is not good. Some photographers will not even consider using beamed flash or any flash on birds because of possible harm. My thought is to keep on using beamed flash occasionally, but to cut the power down some depending on the range to the bird. I also try to stay up with factual information about possible harm, but tend to ignore rumors that are not backed up with true science.



Jul 18, 2013 at 06:15 AM





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