Center point is F8 but?
- Thread starter mikew_RIP
- Start date
I don't really get what you're asking.
The f/8 focus ability of the later Nikons is the ability to focus lenses to which a TC is connected. How many focal points depends on the specific cam. For the D7100 it was solely the center one but for other cam's it's up to 11 focal points (If I remember well).
Wider aperture lenses will use more points but you'd have to check how many which cam can use at what aperture.
I just checked and for the d7100 it seems only the center focal point is compatible with lenses slower than f/5.6 up to f/8.
Nikon | Imaging Products | Capturing power - Nikon D7100
The Tamron has an effective focus of f/5 to f/6.3 if my EXIF data is correct. I start to understand why you have more issues to focus the Tam than me.
To add: I read somewhere some "trick" is being used to have the Tam focus on certain cams but I'd have to think hard and dig deep to find that article again. It does focus on my D3300 but the results are poor. I'd have to check the max ability to focus of that cam.
The f/8 focus ability of the later Nikons is the ability to focus lenses to which a TC is connected. How many focal points depends on the specific cam. For the D7100 it was solely the center one but for other cam's it's up to 11 focal points (If I remember well).
Wider aperture lenses will use more points but you'd have to check how many which cam can use at what aperture.
I just checked and for the d7100 it seems only the center focal point is compatible with lenses slower than f/5.6 up to f/8.
Nikon | Imaging Products | Capturing power - Nikon D7100
The Tamron has an effective focus of f/5 to f/6.3 if my EXIF data is correct. I start to understand why you have more issues to focus the Tam than me.
To add: I read somewhere some "trick" is being used to have the Tam focus on certain cams but I'd have to think hard and dig deep to find that article again. It does focus on my D3300 but the results are poor. I'd have to check the max ability to focus of that cam.
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Now this seems interesting - "seems" because I have no idea what you guys are talking about.
I'd appreciate it if you care to explain or point me to the relevant authority.
Edit: Just busy reading the article in that link from J-see an will need to read it a couple of times to understand it.
I'd appreciate it if you care to explain or point me to the relevant authority.
Edit: Just busy reading the article in that link from J-see an will need to read it a couple of times to understand it.
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Every lens has an effective aperture at which auto-focus is done. For an f/2.8 prime, all AF is done at f/2.8. Other lenses like zooms often have a variable effective aperture; the Tam 150-600 is f/5 at the short end and f/6.3 at the long end. If you'd add a TC to it, you extend the lens and thus create a smaller aperture. For an f/4 with a TC2x attached that will be about two stops resulting in f/8. It is not just the light or contrast but also the width of the angle the light enters the lens that affects the performance of focus. By making a lens "longer", the ° of that angle shrinks (30° becomes 25° as an example). That's why those exotic primes have such massive front elements.
New cams can focus even with an effective aperture of f/8 while older cams can no longer focus under such conditions and start hunting.
The D7100 has only one focal point that still functions at an effective aperture between f/6 and f/8 which can make it hard to focus the Tamron 150-600. Especially at low light.
But there is some trick being used by Tamron since I have been shooting her with my D3300 at 600mm length and that cam should not be able to focus beyond an effective aperture of f/5.6. It clearly did although the results were poor.
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When you attach a lens to the cam, the aperture is always wide open. That is the effective aperture of the lens. The lens will only close down to the aperture you select the moment you push the trigger. After the shot, it opens up again.
The "selected" aperture plays no role during focus. Only the "effective" aperture.
All modern DSLRs auto focus at the maximum effective aperture i.e. with the lens wide open and only close down to the set aperture when you press the shutter. When you look through the viewfinder you are seeing the scene with the lens open. If you press the preview button you will see the scene at the set aperture.
So if you had the Tamron set at 600mm its maximum effective aperture would be f/6.3. If you have your aperture set on the camera to f/11 it would still focus first using f/6.3 then when you press the shutter it would close down to f/11 to take the shot.
Hope that helps.
OK thanks - that now makes sense to me.
I only ever use the center point as i use my lens at 600mm a lot,wasn't sure if the surrounding points where F5.6 with a programed shut off or where capable of F8 in perfect contrast/lighting situations,in the early days a AF system would work until it didn't get enough light,although a manufacture would claim maybe F4 it would often hunt around at smaller stops and actually focus in the right conditions.
If its a programmed shut off there is no point in me considering using multi point,think i need to wait for a bright day and select an off center point and try it at F6.3.
BackdoorArts
Senior Member
Here's what Mike is asking. Current Nikons are said to be able to center focus at f8. I interpret that to mean that when using the center point of your 51 pt focus system you should be able to focus with a 300mm f4 and a 2X converter, which is effectively f8 at minimum aperture. If, however, you move away from the center point, will the camera still be able to attain focus with the same f8 minimum aperture?
I do not know the answer, but I would speculate that you might have an issue beyond the dark red circle above, particularly in a low contrast situation.
I suppose the next logical part of my question would be with a F6.3 lens if you chose multi point could this actually work against you achieving focus,or would the other points just not try to do anything,i think i need to wait for decent light and do some tests.
BackdoorArts
Senior Member
I think in any situation decent light and contrast in the focusing area is going to be key to getting auto-focus to work properly, regardless of the camera specs and lens.
The crux of your question is really how quickly does the ability to lock focus degrade as you move away from center at near-threshold conditions? To truly know that you're going to need a controlled environment and a set of focus pattern charts, which will then give you a technical answer, but not necessarily a practical one. I shoot with my D7100 and Sigma 150-500mm quite a bit, and that's f6.3 at the long end. I rarely have an issue with it focusing anywhere in decent light, even with dull birds on a cloudy day. Occasional focus hunting, but that's usually when I vary distances greatly (i.e. going from a hawk flying over to a bird 20 feet away on the deck).
I recall reading something about slow lenses cheating by giving the cam a different aperture value than the effective one used to get around a programmed problem with focus but I'm not 100% sure it was the Big Tam and related to Nikon cams.
I know she works on my D3300 and I even used the 70-300mm with the TC2 attached. It only hunted at the long end which implies nothing gets "shut down".
It's possible the one focal point Nikon guarantees is simply because, when testing, it has to perform at f/8 during a specific range of circumstances and all focal points that don't meet that standard will be labeled as "not functioning" to eliminate future complaints of users.
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All this makes sense and I fully understand it. BUT. If looking through the view finder and having the tamzooka attached with a teleconverter. You would still see the whole scene as apposed to having the edges cut away such as it vignetting. So if the whole scene is still in view than why do the focus points become inactive around the edges?
Good question. I suppose that's the difference between our eyes which can instantly adjust to any light situation and the cam or AF which requires a certain amount of light.