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05-28-2013, 06:21 PM   #1
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Front mount teleconverter light loss?

I'm just curious if a front mount teleconverter could have no light loss on a smaller sensor camera but have light loss on a larger sensor camera?

I have a couple of lenses that I had thought didn't have any light loss but trying them recently they do. One is a pheonix 2x that has 62mm rear mount (rear glass is even smaller) and 72mm front glass (glass is actually 72mm). The other is a sony 1.4x (rear glass is about 52mm and front glass is about 70mm though the front of the lens itself is about 80mm). The 2x is loosing a lot more light. The shutter speed about doubles with the telconverter attached. The 1.4x seems to lose a lot less but some. In one instance it changed from 125/sec to 100/second (one step).

Is there any way to get a rough calculation of the light loss?

Also these are being used on 300mm lenses (one is f5 and a couple are f5.6). They could wind up being used on a cheap 500mm f8 too. Would focal length change anything?

Lastly I just bought another front mount telconverter (because I like wasting my money doing things the stupid way, maybe I just like experimenting). I got it for $35 shipped on ebay (haven't got it yet though). Its a 1.6x century optics (schnider) and yes its a real one. According to the specs on adoramas site the front of the lens is 105mm and based on the pics the glass should be about that big. It claims no light loss but is focal length or sensor size going to effect that?


In case anyone is wondering what I am using it for, I had been thinking of getting the kids a cheap telescope. I figured (with a little help from some here) that I might wind up better with lenses on a camera and they can take pictures (quality doesn't have to be high). Even if I could afford some nice, fast big glass which I can not, there is too high a risk it might get used once or twice and then sit on the shelf collecting dust. I have a cheapo t mount tou 5 star 500mm f8 that has been doing exactly that. I was thinking of something a little faster. I have an old vivitar 100-300mm 5.0 constant so the 1.6x should turn it into a 160-480mm 5.0 constant. I also have a couple/few x-300mm af lenses with 5.6 on the long end (sigma, pentax f etc). Likewise I could use the 1.6x on the 500mm to get 800mm f8. I also have a couple of 2x rear mount teleconverters to play with.


I have also considered modding a tv lens (not c mount, I know about the flange focal distance) or an enlarger lens or something? I guess I like to experiment.


Any thoughts on any of it but in particular, how to calculate light loss and or how light loss works on front mount converters (effected by focal length or sensor size)?

05-28-2013, 06:36 PM   #2
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The light loss is 2 stops for 2x converters and 1 stop for 1.4x converters.

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05-28-2013, 06:51 PM   #3
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QuoteOriginally posted by Adam Quote
The light loss is 2 stops for 2x converters and 1 stop for 1.4x converters.
Isn't that what it is for rear mount teleconverters that go between the lens and the camera? I was referring to front mount teleconverters that screw into the front filter threads of a lens. I was under the impression that if they could gather enough light they would not lose light? I was wondering how focal length and sensor size might come into play if at all though?

This is the lens that I have on the way from ebay.

Century Optics 1.6x Tele Converter, Panasonic AG-DVX100 0VS16TCDVX

If its anything like the similar century optics wide angle I got (the bigger one anyway), it is just huge and weighs about as much as a brick. The 105mm front dimension leeds me to believe it may be that big.
05-28-2013, 07:03 PM   #4
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QuoteOriginally posted by ripit Quote
Isn't that what it is for rear mount teleconverters that go between the lens and the camera? I was referring to front mount teleconverters that screw into the front filter threads of a lens. I was under the impression that if they could gather enough light they would not lose light? I was wondering how focal length and sensor size might come into play if at all though?

This is the lens that I have on the way from ebay.

Century Optics 1.6x Tele Converter, Panasonic AG-DVX100 0VS16TCDVX

If its anything like the similar century optics wide angle I got (the bigger one anyway), it is just huge and weighs about as much as a brick. The 105mm front dimension leeds me to believe it may be that big.
Oh, like magnifier attached to the front of the camera? Those will degrade your image quality noticeably, but you're right, the light loss won't be as significant. I believe the new max. aperture value can be calculated using the (ratio of the magnification over the new front element diameter) times the original focal length, though I'm not 100% sure. If you then compare that to the original max. aperture, you'll be able to roughly calculate the number of stops of light loss. The calculation is only an estimate because there might be some additional light loss because of the extra glass element(s) in the converter.

In any case, if the new front element size is big enough, it can make up for most of the light loss associated with the magnification.


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05-28-2013, 07:26 PM   #5
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Thanks a lot for the clarification. I'm aware of the degradation but it might not be as bad as you would think on this lens (though perhaps still probably noticeably in a side by side comparison). I have dozens of front mount wide angle and telephoto lenses from before I had a dslr and a few I have gotten since. This one is top of the line. In case you didn't click the adorama link they are selling it for $650 despite being a bayonet mount for and old outdated semi professional dv tape camcorder (semi professional as in a few thousand dollars). Thats why it went for $35 shipped at auction (low demand for such a specialty fit lens). I have already converted a couple of these (wide angle though) from bayonet to threaded and It wan't particularly hard. While degradation is unavoidable with such a lens, its not terrible. If it can compete with a walmart $50 telescope for the kids to play with and do a little star gazing it will serve its purpose (kids are 6 and 7 so they don't exactly need high end stuff). Of course I might play with it to, just a little.

Do you by chance know the formula (I recall reading it but can not find it) to calculate how big the front element must be for a particular focal length at a particular f stop?

Edit: Opps, missed that you posted it. I'll see if I can figure it out from that.
05-28-2013, 07:37 PM   #6
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Seems like it might just work. 300x1.6/100 = 480/100 = f4.8 Thats a 300mm lens with a 1.6x telconverter and the lens happens to be f5 constant (100-300mm f5).

500 x 1.6/100 = 800/100 = f8 Thats a 500mm f8 with the 1.6x telconverter

The adorama specs are 105mm front diameter (not sure if that is outside, threads or glass) so I just used 100.
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