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Thread: TC - focus distance, repro size and DOF

  1. #1
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    Default TC - focus distance, repro size and DOF

    hi,

    don't know where to post. so just tot newbie would be a good place. i'm a newbie in area of tcon..

    I can't find info anywhere off the net.. so post here for experienced tcon-ers to fill me in.

    I'm thinking of getting a 2x or 1.4x tcon for my D70 with a 70-300APO. My question is.. how will the teleconvertor affect the closest focussing distance (currently at 95cm giving 1:2) image size and DOF.

    eg: a 2x tc at 300mm will give equivalent of 600mm, if i use f/5.6 i will lose 2 stop of light to become f/11, correct? so.. what is the DOF now? at f/5.6 or f/11?

    also if at 300mm i was able to shoot at 95cm.. using a 2x tcon to 600mm. will the focussing distance change? and.. what will the reproduction size on the size be?

    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Quote Originally Posted by yanyewkay
    hi,

    don't know where to post. so just tot newbie would be a good place. i'm a newbie in area of tcon..

    I can't find info anywhere off the net.. so post here for experienced tcon-ers to fill me in.

    I'm thinking of getting a 2x or 1.4x tcon for my D70 with a 70-300APO. My question is.. how will the teleconvertor affect the closest focussing distance (currently at 95cm giving 1:2) image size and DOF.

    eg: a 2x tc at 300mm will give equivalent of 600mm, if i use f/5.6 i will lose 2 stop of light to become f/11, correct? so.. what is the DOF now? at f/5.6 or f/11?

    also if at 300mm i was able to shoot at 95cm.. using a 2x tcon to 600mm. will the focussing distance change? and.. what will the reproduction size on the size be?

    Greetings,
    I won't be able to answer your questions directly but do allow me to share my thoughts about your intended purchase. Its my opinion only so take it FWIW.

    You need to consider things like optical quality degradation and focussing ability when putting on a TC. The Sigma 70-300 APO, the nice good budget lens it is, is not stellar optically (I had one). If you put on a 2x TC, I think the optical degradation will be significant. Generally for a 2x TC to give acceptable pics, the parent lens needs to be optically outstanding. Next, at 300mm x 2, you are at f11. Will your camera even be able to focus at those apertures? I don't know about the D70, but the Canon 20D will not. Even when I tried to fool the cam by taping the TC pins (with a different lens, also f5.6 at long end), the cam will not focus lock.

    IOW, there is no free lunch, try it before you buy. My 2x TC is sitting in the dry box, unused for the last 1 year because of the issues above.

    Cheers,

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    Default

    Quote Originally Posted by yanyewkay
    hi,

    don't know where to post. so just tot newbie would be a good place. i'm a newbie in area of tcon..

    I can't find info anywhere off the net.. so post here for experienced tcon-ers to fill me in.

    I'm thinking of getting a 2x or 1.4x tcon for my D70 with a 70-300APO. My question is.. how will the teleconvertor affect the closest focussing distance (currently at 95cm giving 1:2) image size and DOF.

    eg: a 2x tc at 300mm will give equivalent of 600mm, if i use f/5.6 i will lose 2 stop of light to become f/11, correct? so.. what is the DOF now? at f/5.6 or f/11?

    also if at 300mm i was able to shoot at 95cm.. using a 2x tcon to 600mm. will the focussing distance change? and.. what will the reproduction size on the size be?

    TC don't change minimum focussing distance.

    If you were shooting at 1:2 at the min focussing distance without a TC, the addition of a 2X TC means you now shoot at 1:1 at the same min focussing distance.

    The DOF will be that of the 'new' effective aperture, ie f5.6 on the lens aperture ring coupled with a 2X TC means you have a DOF of the new effective aperture, f11.

    Hopefully I'm correct as to the above....

  4. #4
    Senior Member glennyong's Avatar
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    yyk, ur 70-300 APO do not support tc. aperture above f8 will have problem focusing.

    and further more that lens is very not recommended for tc stacking.

    1) Image will degrade like crazy
    2) AF Super slow
    3) AF Hunting like mad
    3) Light hungry.

    i stacked 70-200 f/2.8 already slow liao.. and showing signs of image degradation... .. not recomended...

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    To clarify a bit, your Sigma APO 70-300mm WILL WORK with 2x and/or 1.4x TC on D70 (I have all the above).

    BUT....

    ...as mentioned by others,

    - image quality degraded, as this lens is not a VERY lens to start with, esp on the 2x TC, the (lack of) sharpness amplified. You might as well interpolate the original image without TC to a larger image size.

    - focusing difficulty / hunting, all because there isn't enough amount of light to allow the cam to focus due to the resulting smaller aperture. If there is (enough light), even stacking the 2x + 1.4x TCs will focus. Tried.

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    thanks everyone for the feedback. I've heard of image degradation but i din't know it's going to be that bad. guess TC is not going to come soon for me then. Though i could get a 'free lunch' like what dkw said. Gain some focal length with minimal loss.
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    wow.. thanks for your valuable feedback HeWolf.
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Senior Member glennyong's Avatar
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    do the calculation of the aperture size, anything above F/8, ur lens will start to go hunting...

    i am still in the process of doing a test on a f/11 aperture size which will eventually give me a lens going hunting all the time...

    safest bet. get a afs 300 f/2.8 and stack 2x converter u get a 600 f/5.6.. heh.. which is onli half a stop slower den the afs f/4 version ...

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    brudder.. thanks for the advise but i'm not that rich...
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Quote Originally Posted by yanyewkay
    brudder.. thanks for the advise but i'm not that rich...
    A fairly 'affordable' long tele-zoom is the Sigma 50-500. Under 2K, and a lot of good things said about it.

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