Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Jan 19, 2010, 1:01 AM   #1
Member
 
Join Date: Dec 2009
Location: Los Angeles, California, USA
Posts: 87
Default Filter mounted teleconverters?

Are these teleconverters that screw into filter tings rings any good?
I want to put it on my 300mm telephoto without loosing light, but how much image quality do I loose?


NothingRare is offline   Reply With Quote
Sponsored Links
Old Jan 19, 2010, 1:55 AM   #2
Super Moderator
 
peripatetic's Avatar
 
Join Date: Nov 2004
Posts: 3,599
Default

Quote:
Are these teleconverters that screw into filter tings rings any good?
No.

Quote:
I want to put it on my 300mm telephoto without loosing light, but how much image quality do I loose?
You lose all of it. Seriously these things are a complete waste of money and will turn the images from your SLR into far worse quality than you would get with a P&S.

If you want a proper teleconverter you should get the ones that go between your camera and lens. They typically have a multiplication factor of 1.4x or 2x.
__________________
My gallery
My X100 blog
peripatetic is offline   Reply With Quote
Old Jan 19, 2010, 2:27 AM   #3
Member
 
Join Date: Dec 2009
Location: Los Angeles, California, USA
Posts: 87
Default

Oh, haha. Can't expect much from $15 lol

Last edited by NothingRare; Jan 19, 2010 at 2:48 AM.
NothingRare is offline   Reply With Quote
Old Jan 19, 2010, 4:42 AM   #4
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

Quote:
Originally Posted by NothingRare View Post
I want to put it on my 300mm telephoto without loosing light, ...
(emphasis mine - TCav)

Can't be done.

A 2X teleconverter doubles the focal length and halves the angle of view. That means the lens is passing only 1/4 the light it would without the "Teleconverter" (1/2 the horizontal angle of view and 1/2 the vertical angle of view equals 1/4 the scene, and therefore 1/4 the light.) That's a loss of 2 f-stops of light.

Since the camera dosn't know about the filter-thread-mounted-teleconverter, it doesn't adjust the actual f-stop, but you'll still lose the light, just like you would with a camera mounted teleconverter.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jan 19, 2010, 5:45 AM   #5
Junior Member
 
Join Date: Apr 2006
Posts: 20
Default Some are ok...

Some screw-on teleconverters are good. They are not inexpensive.

The best I have is a Nikon TC-E15ED (1.5X) Also the Nikon TC-E17ED (1.7X is highly prized and also highly priced!) The TC-E15ED is much less expensive but requires surgery.

See C.K.Shene's excellent comparisons at:
http://www.cs.mtu.edu/~shene/DigiCam/
(look under FZ30).

These TC's do not necessarily decrease the effective f-stop. Instead, the new f-stop will be the (original focal length)x(magnification)/(TC's-front-lens-diameter). There's a minor amount of light loss because of the extra glass elements, maybe 1/3 stop.

In most circumstances, putting such a TC on a 3.5 lens won't change the f-stop much.

Here's a test comparing sharpness of a good pentax prime lens without and with two teleconverters, the Nikon TC-E15ED and an Olympus B-300 (same as Olympus TC-17.)


Dave
newarts is offline   Reply With Quote
Old Jan 19, 2010, 5:59 AM   #6
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Also note that if your lens design allows it to rotate when focusing, the extra weight from a filter mounted TC can put more strain on the focus motor and lens mechanism and potentially damage it.
JimC is offline   Reply With Quote
Old Jan 19, 2010, 6:08 AM   #7
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

F-number is a function of focal length. When you change the focal length, you change the f-number. With a filter mounted teleconverter, the camera doesn't know you've changed the focal length, and therefore the aperture, so the EXIF will contain just the data the lens (minus the teleconverter) told it.

A 1.5X teleconverter (ANY 1.5X teleconverter) will reduce the aperture by about 1.2 f-stops, a 1.7X teleconverter (ANY 1.7X teleconverter) will reduce the aperture by about 1.5 f-stops, and a 2X teleconverter (ANY 2X teleconverter) will reduce the aperture by 2 f-stops.

Nothing can change that.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jan 19, 2010, 6:19 AM   #8
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

I'm not sure the same rules apply to a front mounted TC. IOW, technically, it may not be increasing the focal length, just changing what the lens receives by magnifying it. You'd have to try it to see. For example, if you need to use a slower shutter speed for the same lighting, aperture and ISO speed in order to get the same exposure. The TC manufacturers usually advertise that their front mounted TCs do not cause loss of light.
JimC is offline   Reply With Quote
Old Jan 19, 2010, 9:13 AM   #9
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

If you use a smaller angle of view, then you are reducing the amount of light.

If you frame an image of something, and then change the angle of view so that the frame contains only half of it, then you're only getting half as much light.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jan 19, 2010, 9:52 AM   #10
Senior Member
 
VTphotog's Avatar
 
Join Date: Mar 2005
Location: Extreme Northeastern Vermont, USA
Posts: 4,214
Default

If you increase the objective lens diameter, you are encreasing the amount of light entering the camera, effectively reducing the f/number. If this factor is the same as that of the t/c magnification, the effective f/stop does not change.

brian
VTphotog is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 8:45 PM.