Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-25-2012, 08:13 AM   #31
Veteran Member




Join Date: Mar 2010
Location: NewYork
Posts: 899
Results with front mount teleconverters can very widely. Some will eat huge amounts of light while others will eat little to none (in generally the larger the front glass, the less light loss as the lens is gathering more light). Of course the rear diameter can matter if it is small enough to vignette. The image quality on that canon probably doesn't look like it came from a 40$ converter because it didn't. Those were over 100$ (can not recall exactly how much but I'm thinking $140). There are other issues you can run into also. In general an afocal teleconverter can be closer or further from the main lens and work but its not completely true. If they are mounted too close to the main lens it can cause problems for some. I gather from experience (not completely sure) that video lenses have more issues with this but I imagine it matters what camera video or still, they were originally designed for. That canon was originally designed for still cameras so maybe that helps (it was for the early g series if I am not mistaken or maybe that was the tc-dc52c). Actually looking the tc-dc58? might be the ones for the g series (and have larger glass if I'm not mistaken so less chance of vignetting. The only one in that series I have is the tc-dc52 2.4x.

Due to all the compatibility issues and the fact that they are only guaranteed to work with a very limited number of cameras ( the ones they were designed for), the resale value is crap. That is why they are $40 new. I have seen plenty of various teleconverters go for 10-20% of their original price often enough on ebay. They can be pretty cheap to play with if you don't mind some gambling on what works. There are some with a reputation for working very well and or working with most cameras but that also makes them sell for a lot more used.

here are a couple of pics with and without a 1.4x front mount converter. Maybe my eyes are off but I do not see any light loss. These were shot manual so same aperture and shutter speed.
sony_14x_tc Photo Gallery by Richard Homeyer at pbase.com

07-25-2012, 08:46 AM   #32
Inactive Account




Join Date: Jan 2012
Photos: Gallery
Posts: 14
moon shots
f/10 - 1/200sec - Iso 800 - with tripod
55-300 + TC, at 300mm (ca 480mm) ... crop 62% to get size equivalent with 300mm crop 100%
edit: sharpening ...



several test without TC at 300mm, can't get better result than this when crop 100%...

Last edited by raimaster; 07-25-2012 at 08:58 AM.
07-25-2012, 09:00 AM   #33
Veteran Member




Join Date: Mar 2010
Location: NewYork
Posts: 899
front mount coverters often get a bad rap (maybe because there are quite a few cheap junk ones out there) but there are also some very high quality ones too.
08-01-2012, 02:10 PM   #34
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
The reason this converter gives a low effective f-number is that its rear element is small, about 26mm; with the DA 55-300 wide open this results in an effective f-number of about 300/26 ~ f:11.

Dave in Iowa

08-01-2012, 09:30 PM   #35
Inactive Account




Join Date: Jan 2012
Photos: Gallery
Posts: 14
Hi Dave,
yess, this TC has about 26mm real element, its same size as lens f/2.8 (Tamron 17-50 or pentax 50-135), even 55-300 (f/4.0-5.8) has only 18mm rear element. And Rear elements lens 50mm f/1.4 is about 30mm.
do you mean if i attached another TCs- Canon TC-DC58A (real elements 48mm) into 55-300 the effective f-number will increase to f/6.2, because of 300/48? how about tamron 17-50 and pentax 50-135? both has same size of rear and front elements but different FL?

I agree of lost light and about 1.5-2 stops slower but i am not sure if the effective f-number now becomes f/11?

Anyway here is another sample - DOF comparison ( tried to shot at same dimension/size, in exactly same position with tripod, unable to set the same aperture) but i dont know why the FL is 30mm different. Anyway if we look at DOF comparison, it is unlike f/5.8 vs f/11?


300mm f/5.8 - 1/640sec iso 800 around 5m to target.


170mm + 1.6x (270mm) f/5.6 - 1/640 - Iso 800


Crop 100% : 300mm f/5.8 - 1/640sec iso 800 around 5m to target.


Crop 100% : 170mm + 1.6x (270mm) f/5.6 - 1/640 - Iso 800

Last edited by raimaster; 08-01-2012 at 10:33 PM.
08-02-2012, 09:33 AM   #36
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
observations

QuoteOriginally posted by raimaster Quote
Hi Dave,
yess, this TC has about 26mm real element, its same size as lens f/2.8 (Tamron 17-50 or pentax 50-135), even 55-300 (f/4.0-5.8) has only 18mm rear element. And Rear elements lens 50mm f/1.4 is about 30mm.
do you mean if i attached another TCs- Canon TC-DC58A (real elements 48mm) into 55-300 the effective f-number will increase to f/6.2, because of 300/48? how about tamron 17-50 and pentax 50-135? both has same size of rear and front elements but different FL?

I agree of lost light and about 1.5-2 stops slower but i am not sure if the effective f-number now becomes f/11?
I did an exposure test with and without the tc.- without tc the exposure at f5.8 was 1/200 sec and with the tc the exposure increased to 1/30 sec; an increase of 6.67. Since exposure is proportional to f-stop squared this is equivalent to an effective f-stop of 5.8SQRT(6.67) = f14.9

You appear to be using the zoom lens at 170mm; in this case I'd expect the effective f-stop to be about 170/26 = 6.5 consistent with your comments

I don't understand the exposure on the images you posted; they appear to be identical, 1/640sec at ISO 800, yet you talk about a 1.5-2 stop difference - where did this difference go?

Dave
08-02-2012, 10:15 AM   #37
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
observations

QuoteOriginally posted by raimaster Quote
Hi Dave,
yess, this TC has about 26mm real element, its same size as lens f/2.8 (Tamron 17-50 or pentax 50-135), even 55-300 (f/4.0-5.8) has only 18mm rear element. And Rear elements lens 50mm f/1.4 is about 30mm.

I agree of lost light and about 1.5-2 stops slower but i am not sure if the effective f-number now becomes f/11?
I did an exposure test with and without the tc.- without tc the exposure at f5.8 was 1/200 sec and with the tc the exposure increased to 1/50 sec; an increase of 4. Since exposure is proportional to f-stop squared this is equivalent to an effective f-stop of 5.8SQRT(4) = f11.6

You appear to be using the zoom lens at 170mm; in this case I'd expect the effective f-stop to be about 170/26 = 6.5 consistent with your comments

I don't understand the exposure on the images you posted; they appear to be identical, 1/640sec at ISO 800, yet you talk about a 1.5-2 stop difference - where did this difference go?

QuoteQuote:
...do you mean if i attached another TCs- Canon TC-DC58A (real elements 48mm) into 55-300 the effective f-number will increase to f/6.2, because of 300/48?
Yes. the f-stop will increase to about 300/48.

I just did a similar test using a too heavy Konica-Minolta ACT-100 1.5x TC which has a 43.3mm rear element on the extended da 55-300 at f5.8; it increased exposure time from 1/250s to 1/160s which is an effective f-stop increase to sqrt(250/160)*5.8 = 7.25. The f-stop estimated from the TC's rear element diameter is 300/43.3 = 6.9

You are doing a good job thinking about the implications of what you see! It is great these numbers and theories work!

Dave in Iowa

Last edited by newarts; 08-02-2012 at 10:22 AM.
08-02-2012, 09:21 PM - 3 Likes   #38
Inactive Account




Join Date: Jan 2012
Photos: Gallery
Posts: 14
Thanks for the respons, now its bit clear to me, thanks for the explanation..
anyway here are another sample why i guess the f stop - being increase about 1.5-2 stop. I tested with same aperture and speed but different Iso:
note: pic at 300mm without tc - cropped and resize to 900x596
pic with 300mm + TC only resize to 900 x 596 (slightly out of focus)
pic at 190mm + 1.6x TC (304mm) cropped and resize to 900x596

300mm No TC - f/11 - 1/200sec - Iso 100 (crop & resize)


300mm no TC - f/5.8 - 1/200sec - Iso 100 (crop & resize)


300mm with TC - f/5.8 - 1/200sec - Iso 400 (resize only)


190mm + 1.6x TC (304mm) - f/5.6 - 1/200sec - Iso 200 (crop & resize)


300mm + TC f/5.8 - 1/200 sec - iso 800 (resize only)



Pic 2,3,4 are almost similar exposure (pic 3 and 4 slightly brighter in my monitor)
Pic 2 vs 3 has two stop different Iso 100 vs Iso 400
Pic 2 vs 4 has one stop different iso 100 vs iso 200 but pic 4 has slightly larger aperture f/5.6 vs 5.8

i think the conclusion base on this test:
at the same FL 300mm or 190+TC (304mm) it has around 1 stop slower
at the longest FL 300mm vs 300+TC (480mm) it has around 2 stop slower


Rai - Bali


Last edited by raimaster; 08-02-2012 at 10:08 PM.
08-04-2012, 10:15 PM   #39
Senior Member




Join Date: Feb 2010
Photos: Gallery
Posts: 174
some more info


Any 1.4x TC with AF for HSM or SDM

Good discussion. I purchsed the Tamron 1.4 and it works great. One just has to be careful to get the correct model as discussed in the above posts.
08-05-2012, 04:03 AM   #40
Veteran Member
Frogfish's Avatar

Join Date: Dec 2009
Location: Shanghai
Posts: 4,490
Giving you a Like for the excellent work you have done in this thread Rai !
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
k-mount, lens, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Good Tele Converter for K-5 + DA* 60-250mm F4 Lens brosen Pentax K-5 8 03-11-2012 10:44 PM
Which tele-converter lens? Iroc Pentax SLR Lens Discussion 12 06-16-2011 12:41 AM
DSLR lens with SLR tele-converter kiafrika Pentax SLR Lens Discussion 13 02-15-2011 12:21 PM
Auto tele converter to manual lens? COULDBE2 Pentax SLR Lens Discussion 5 08-11-2010 03:28 PM
Tamron AF tele-converter with DA AF lens NRV Shooter Pentax SLR Lens Discussion 8 05-22-2010 12:39 PM



All times are GMT -7. The time now is 12:59 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top