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Thread: Ability to Focus with F/8 Lenses

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    PhotomanJohn's Avatar
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    Ability to Focus with F/8 Lenses

    I just purchased a AF-S Nikkor 70-200mm f/4 lens and am planning to buy a TC-20, 2X teleconverter. Nikon says the D7000 has autofocus issues with lenses with equivalent apertures smaller the f/5.6 due to how the autofocus system works. The system splits the image into two and judges the focus on how far apart the two images are on the focus sensor. When the effective aperture gets too small it interferes with the ability to create the two images.

    I have heard that others have not had an issue with a f/4 lens and a 2x teleconverter on the D7000. I wonder if it has to do with the focus point that is selected. It seems logical to me that the system would have more problems when a focus point on the side is selected as opposed to the center point. The image of the object of a side focus point might be more blocked by the small effective aperture than the center point which I would think would be affected very little.

    Does anyone out there really understand this subject? I plan to buy the teleconverter anyway and manual focus when necessary which won't be a problem in good light.

    John

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    Moderator Dave Humphries's Avatar
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    Re: Ability to Focus with F/8 Lenses

    Hi John,

    From what I have read;

    I would expect that in good light, with a 'sympathetic' subject edge to focus on, it will AF OK.

    I suspect your chances are higher if you do use the centre focus point.

    Hopefully someone that knows what they're talking about will correct me if I am wrong

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    rpcrowe's Avatar
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    Re: Ability to Focus with F/8 Lenses

    I don't know if this is true with Nikon gear but, the Canon Teleconverter will send a signal to the camera that the f/stop is smaller than f/5.6 and the AF wil not work.

    I have not tried this but, it is said that third party TC's such as Kenko do not report the f/stop and a lens can AF (albiet slowly) at f/8.

    Additionally, it is said that you can "tape off" the contacts on a Canon TC to "fool" the camera. I have not tried this either and don't know if it pertains to Nikon gear...

    OTOH... even some of the best Canon lenses do not achieve especially good IQ with 2x converters. The 70-200mm f/2.8L IS ii and Canon 2x TC iii appear to be an exception to the above statement. However, my EF 70-200mm f/4L IS and 300mm f/4L IS lenses work quite well with the Canon 1.4x TC. I would expect that your Nikon lens would work very well with the Nikon 1.5x TC; giving you a 105-300mm f/5.6 lens. That is a really useful focal range...

    Nikon folks are the people to ask. But, I just threw in my two-pennies worth of comment...

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    Melkus's Avatar
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    Re: Ability to Focus with F/8 Lenses

    Nikon makes 3 teleconverters, a 1.4x a 1.7x and a 2x. Adding a teleconverter increases the magnification, but also decreases the total light coming into the camera. Nikon cameras prior to the D4 and D800 require a maximum aperture of f5.6 or greater to autofocus properly. So, if you have a 300mm f2.8 lens, you can use any teleconverter and the camera will function properly. (f2.8 x 2 = f5.6) a f4 lens will work with the 1.4x teleconverter (f4 x 1.4 = f5.6), but but not the 1.7x or 2x, and so on. Do not use a teleconverter on wide zooms as the lens can damage the teleconverter. Although the teleconverters will function in manual focus mode on most lenses, they are primarily intended to be used with Nikon's long pro telephotos. While there are reports of teleconverters and autofocus working out beyond an effective maximum aperture of f5.6, proper performance should not be assumed. The teleconverter manual (available on line) basically implies that to use the 1.4x teleconverter the lens should be at least an f4, the 1.7x should be used with a lens faster than f3.3 and for the the 2x the lens should be faster than f2.8.

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    PhotomanJohn's Avatar
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    Re: Ability to Focus with F/8 Lenses

    Thanks to all for the replies. Since I had read some users commenting that the they had success with the lens/TC combination on a D7000 I wasn't considering the fact that the camera might not even try to AF knowing what it was looking thru. As Richard said the safe choice would be the 1.4 which has no issues with this combination but doesn't give me a lot more reach. The 70-200mm f/4 is super sharp and is supposed to work well with all of the Nikon TCs which are about the same price. I may end up with a D7100 or more likely a D600 fairly soon which both AF with f/8 lenses. I don't mind manual focusing for now which I do often anyway. Decisions, decisions, decisions.

    John

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    Re: Ability to Focus with F/8 Lenses

    Quote Originally Posted by rpcrowe View Post
    I don't know if this is true with Nikon gear but, the Canon Teleconverter will send a signal to the camera that the f/stop is smaller than f/5.6 and the AF wil not work....
    I'm pretty sure this is the case with Nikkor lenses/telecoverters. I don't believe the camera will even attempt to focus if the lens/tele combo is greater than 5.6.

    With non-Nikon lenses it then becomes a crap shoot. I used to shoot a Sigma 100-300 f4 lens on a D300 body which has the same f5.6 limitation. With a Sigma 2x teleconverter the afforementioned "lockout" did not occur. In good light the camera would focus if there was adequate contrast in the target. In low light/poor contrast it would hunt a lot and sometimes fail to lock on at all.

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    rpcrowe's Avatar
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    Things I wish for besides A/F at f/8!

    BTW... I would hapily trade the print from camera capability and the specialized JPEG + RAW button for the ability to autofocus at f/8!

    I no longer pine for the ability to do five shot incremental AEB bursts (facilitating HDR imaging) since there are now ways to accomplish this using either the Magic Lantern or using a modified I-Phone to control the camera...

    I do wish however, that my 7D had an audio record capablity to record information about the images I shoot!

    I also wish that the 7D had a two memory card capability. I have only experienced CF Card failure once. Luckily it was not when I was shooting anything of great importance. However, a two card setup would be an insurance policy against missing future images...

    Finally, wish that I had ISO 50 and ISO 25 capability...

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    Re: Things I wish for besides A/F at f/8!

    Quote Originally Posted by rpcrowe View Post
    ...I do wish...

    I also wish...

    Finally, wish that...
    What, Santa would bring you a Nikon for Christmas?

    I just couldn't help it. Now go ahead, I deserve whatever's coming

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    Moderator GrumpyDiver's Avatar
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    Re: Ability to Focus with F/8 Lenses

    Quote Originally Posted by NorthernFocus View Post
    I don't believe the camera will even attempt to focus if the lens/tele combo is greater than 5.6.
    This is definitely not correct. My wife has the 150-500mm Sigma which has a maximum aperture of f/6.3 at 500mm and she has lots of shots on it at the focal length on her D90. I suspect that reliability of focus might go down, but the camera does not refuse to autofocus.

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    Re: Ability to Focus with F/8 Lenses

    Quote Originally Posted by GrumpyDiver View Post
    This is definitely not correct. My wife has the 150-500mm Sigma which has a maximum aperture of f/6.3 at 500mm and she has lots of shots on it at the focal length on her D90. I suspect that reliability of focus might go down, but the camera does not refuse to autofocus.
    Manfred, I indicated "with Nikon lenses/teleconverters". I also related the same experience with Sigma.

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    Moderator GrumpyDiver's Avatar
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    Re: Ability to Focus with F/8 Lenses

    Quote Originally Posted by NorthernFocus View Post
    Manfred, I indicated "with Nikon lenses/teleconverters". I also related the same experience with Sigma.
    You are right Dan; I should have read more carefully. Time for a coffee...

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    Re: Ability to Focus with F/8 Lenses

    I got a Kenko 1.4x TC to use on my D5000 with a Nikkor 70-300 f/4.5-5.6 lens. It will autofocus in good light, although it hunts more than without the TC. But the big downside to my mind is that the contrast of the images suffers dramatically and the sharpness is impaired noticeably. I could live with the loss of sharpness more than the loss of contrast. AIUI, 2x TCs are substantially worse in both regards than 1.4x TCs. If you have a chance to try before you buy it, you may want to take some test photos and play with contrast enhancement in post to decide whether you will actually find the results usable. I am keeping the TC in my bag, but am much more reluctant to use it in the field than I expected to be. FWIW.

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    rpcrowe's Avatar
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    Re: Things I wish for besides A/F at f/8!

    Quote Originally Posted by NorthernFocus View Post
    What, Santa would bring you a Nikon for Christmas?

    I just couldn't help it. Now go ahead, I deserve whatever's coming
    Or a Canon 1D Series. I strongly suspect that the reason Canon has not put my dream parameters into the 7D is that it might seriously eat into the sales of the 1D (series) cameras, which have all of my wants. Additonally, most customers seem to flock to video, gargantuan ISO and articulating LCD's as selling points.

    If I shot with a single camera, I would have seriously considered a 1D model, But a pair of 1D (series) cameras would be quite heavy and way out of my ball park financially. Speaking of being out of my range, both financially and weight wise, the new Canon 200-400mm f/4L 1.4x TC lens is certainly in that grouping. It would solve the problem of a long lens not focusing at f/8. But I could never justify the price of this lens unless I were shooting for some pretty good pay. That is, I couldn't afford it until Canon decides to accept one of my kidneys as a trade in through the Canon Loyalty Program!

    Actually, I am quite happy with the 7D, it is the nicest camera that I have ever used (and than covers a lot of cameras). I am so happy with it that I purchased a second 7D. This is the first time since I have shot with DSLR cameras that I have had the luxury of shooting with a pair of identical cameras.

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    Re: Things I wish for besides A/F at f/8!

    If you get the right tool for the job things often get on better and in view of the frequent threads about TCs and their problems I cannot help feeling that the long zoom bridge camera using contrast detection instead of phase detection will work better ...it has drawbacks of course, just different to the DSLR-TC route, and if the x50 zoom [ 1000mm reach ] has given you a tight framing and the camera was held steady to help OIS work the results should be good enough for most purposes after editing.
    But since the OP intends to buy a TC anyway ... well
    I know my CD snaps into focus at f/10 if there is any contrast in the focusing target area. I believe some DSLRs come with CD these days.

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    PhotomanJohn's Avatar
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    Re: Things I wish for besides A/F at f/8!

    Just a quick update. I finally decided that if I was going to get a teleconverter I would make it worth my while and buy the TC-20E III which is Nikon's newer version 2X even if I had to manually focus often. I am pleased to say that the combination of the 70-200mm f/4 and the teleconverter on my D7000 snaps into autofocus even indoors in average lighting. Manual focus with the setup is pretty straight forward as well.

    Now I need to get a better handle on the camera movement issue to get fully sharp shots. The VRIII system in the lens which works great without the teleconverter is not up to the job with the teleconverter installed (no surprise there). I am going to order a RRS lens collar tomorrow which I hope will help. The balance point of the setup is at the teleconverter so reversing the collar so the foot points back toward the camera should put the ball head pretty much at the balance point. Then hopefully good technique will do the rest.

    Any words of wisdom would be appreciated.

    John

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    Re: Things I wish for besides A/F at f/8!

    Only the centre 9 focus points on the Nikon D7k are of the cross type with a sensitivity of -1 to +19 EV. Any of the other outer points are nowhere near as sensitive and the specific sensitivity is not given, but I would say that you would have far less reliability of AF when using any of these focus points with a TC mounted.

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    PhotomanJohn's Avatar
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    Re: Things I wish for besides A/F at f/8!

    Graham - Thanks for your comments and it was my understanding that only the center focus points would work with an f/8 equivalent lens. I am very pleased with the results with the lens / teleconverter when mounted to a rigid tripod. I have read many comments from others who did not have good luck which I attribute to inadequate support systems.

    John

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