Excellent,though like you said a bit technical.Originally Posted by ameerat42
A couple of weeks ago I would have had no idea what was being said. Now,I actually figured out what was being explained.
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Thanks useful information: 5
Excellent,though like you said a bit technical.
A couple of weeks ago I would have had no idea what was being said. Now,I actually figured out what was being explained.
CC always welcome.
Shane I have the impression you may have misunderstood how Nikon's AF system works.
The sensors aren't positioned for a specific aperture .. they are sensitive to a specific aperture value.
If you have ever used an old film camera that has a split prism feature in the focusing matte, you will basically understand how the PDAF AF system works.
If not, then in brief, the image that is formed on the sensor is split into two parts. As you view a specific subject and place the split prism area onto that subject the subject appears split into two parts .. disjointed!
As you focus closer to the correct plane, the subject becomes one .. ie. formed again. This is now in focus. PDAF sensors work the same way(CDAF works differently).
Again, if you've ever used a split prism, you know that at about f/5.6 the split prism is useless(on the whole) .. and at f/8 is totally useless. Looking through the viewfinder with an f/8 lens .. all you see is black in the split prism area. At f/5.6 you get a partial blackness .. in comes in and out dependent on the angle you're eye comes into the viewfinder.
PDAF works in the same way.
Nikon's f/5.6 sensitive focus points aren't positioned in a special way. That I can remember, they are all basically positioned in a central area of the focus point group. As Nikon have many and varied AF systems types for many of their camera, it's hard to describe it in a singular manner.
But (lets say) on a D750, the central vertical column of 15 AF spots are all f/5.6 sensitive. the rest may need f/2.8, and the central AF spot is sensitive to f/8 maximum values.
So you're comment that:
is incorrect.... the autofocus sensors are positioned in a way that corresponds with an aperture of ...
They are sensitive to f/5.6 only .. this sensitivity is probably dependent on both the hardware tech, as well as the software used to make sense of the hardware.
Also you said that:
No matter the sensitivity of the AF sensors, a larger aperture lens always allows more light into the system, which includes both vf, and AF sensors!
An f/4 lens allows less light through to the AF system than does an f/2.8 lens. Whether this translates to AF performance differences is is dependent on so many factors.
Light being the predominant one.
an example!
if you had two basically two identical lenses, other than the point that one was f/2.8 model and the other was an f/4 model .. otherwise all other design factors were identical.
We had this lens on a D750, which we know is good down to -3Ev ambient lighting for AF.
-3Ev lighting is basically a similar experience to the lighting from a full moon .. so lets say we're shooting portraits under moonlight only .. out in the sticks with only a full moon for the light.
In this situation, don't you reckon the wider aperture f/2.8 lens would AF more reliably(if at all!) than the f/4 lens would.
Nikon D800E, D300, D70s
{Nikon}; -> 50/1.2 : 500/8 : 105/2.8VR Micro : 180/2.8 ais : 105mm f/1.8 ais : 24mm/2 ais
{Sigma}; ->10-20/4-5.6 : 50/1.4 : 12-24/4.5-5.6II : 150-600mm|S
{Tamron}; -> 17-50/2.8 : 28-75/2.8 : 70-200/2.8 : 300/2.8 SP MF : 24-70/2.8VC
{Yongnuo}; -> YN35/2N : YN50/1.8N
Arthur, I think you are confusing the AF points that are visible in the viewfinder vs the actual sensors inside the AF module, which is what I'm referring to.
I highly recommend you read this thread on dpreview - http://www.dpreview.com/forums/thread/3713509. If you haven't already come across her, Marianne Oelund is a very well credentialled photographer as well as an expert on the design and operation of photography equipment and theory. She uses Nikon equipment.
Using the actual components of a Nikon D300 AF module set up on an optical test bench, she shows that as you stop down a lens from f/5.6 to f/8 to f/11 etc that the amount of light hitting the AF sensors decreases, and ceases altogether at f/11. However opening up to f/4 to f/2.8 does NOT increase the amount of light hitting the AF sensors. It seems counterintuitive, but it is a real physical characteristic of the AF module.
My own summarised understanding is this (although again I recommend reading the above link for a full correct description) - the sensors inside the AF module are located at the edge of the exit pupil when the main lens is set to f/5.6. I don't really know why f/5.6 was chosen, but I suspect it's a good compromise between getting enough light, having a large enough distance for the triangulation calculations, and giving the ability to work on almost every lens available.
BTW I realise that we have gone way off topic here. This subject interests me so if it's preferable we can start a new thread.
Cheers
Shane
Shane
Bluddy 'ell!
I started reading that thread, and didn't get out of it until about 2AM!
Anyhow .. I've read Marianne's threads on DPR previously, but don't frequent those boards due to the general nature of them.
Very illuminating information on the inner workings of the AF system, and so far she seems to have presented a very good argument for her case.
Two points I find unresolved as yet.
It's still a bit inconclusive that the amount of light at the AF module begins at an f/5.6 aperture equivalent, even tho much data has been posted to prove the idea.
The images captured by snellius appear to not only prove the point, but also to disprove it as well.
(I'm not a DPR member and won't join in that forum every again(unless they seriously consider some level of moderation, so I can't post these questions directly to snellius or Marianne)
The images snellius shows from underneath the AF module definitely indicate that light levels are reduced starting at about f/5.6.
But, the images taken from the front of the camera/lens, don't correlate as well to support the arguments made.
Looking at the images coming from the front, at f/1.4 it's clear that the entire AF module is visible .. most notably the centre cross pattern portion.
At f/2.0, the periphery AF points are still visible, but at f/2.8 they are obscured .. even tho we can clearly see the centre cross pattern AF pattern.
While it appears that Marianne is probably completely correct, the images presented seem to leave a small level of doubt about the actual workings, as opposed to the implied workings.
The second point not touched on is that as the AF modules most important component is a imaging sensor(just like the sensor that captures the image) software allowances haven't been made. This would be either improbable, or impossible to determine and Nikon wouldn't be willing to reveal such info either.
That is, what level of gain does the AF module sensor receive at differing aperture values .. and different lenses mounted.
Considering all this electronic communication between camera and lens, I seriously doubt that Nikon haven't used some software tweaking in the AF system to cater for various lens types!
Anyhow, as you said, the last few replies are going a bit OT(although still pertinent to the OP's original point).
It is interesting and on topic to note that, if true the AF module can only see apertures of f/5.6 or so. I can't imagine Canon's and other brands using completely different PDAF module methods. While they may have differences .. the basic processes will be similar/same.
So that f/2.8 lens may not focus as quickly as it may otherwise do!
I'm happy to continue in a new thread if the interest to do so exists.
Arthur
I have started a new thread - http://www.ausphotography.net.au/for...-and-operation
Agree on the dpreview forums - I do frequent there for technical information but you have to filter out a lot noise, and there are only a handful of regular posters that I really bother to read at all.
Cheers
To the OP...
We got off track with a discussion about the effect of aperture on autofocus. However there are some undeniable reasons why 70-200 lenses tend to be recommended for sports in general:
- image quality tends to be at or near the best for each respective manufacturer
- having a fixed aperture of f/2.8 allows for good shutter speeds even in relatively low light.
- they do focus quickly if for no other reason than they tend to have powerful focus motors
- they have a reasonable zoom range of nearly 3:1
- usually they are solidly built with good sealing.
Having said all that you need to determine which lens suits your own circumstance best, which it seems like you are doing!
Threadstarter
I completely agree with the "general sport" recommendation, and the reasons for it. What had me concerned was the recommendation of that lens in the specific example, including all my own parameters. It seemed (and still does!) to be equivalent to being recommended a Ferrari when the question included the statement "I live at the end of a 10km rough dirt road". Sure the a Ferrari might be a great car, but is it really suitable for driving on dirt roads?
After much research and thought, I suspect that the majority of recommendations for the 70-200mm in the situation in question are gut reactions attributable to the "general sport" suitability of this lens and do not take the details of the situation in to consideration. In fact if I was considering using two bodies, I would certainly be using a 70-200mm on one of them. But for a single lens solution on a 7D, for the average "kids photo album" photographer, I'm still not convinced that a 24-105mm or 18-135mm (or something in that range of focal lengths) isn't a better choice. When the new season starts, I'll have my chance to find out!!
Threadstarter
I agree, this lens was on my shortlist and on paper I thought it looked ideal. Its only possible downside is that it's an APS-C lens, ruling out upgrading to a full frame body - though, of course, such an upgrade would probably make the lens "too short" for the job anyway! But it looked exactly right, in fact as an f/2.8 image stabiliser lens it certainly appeared that Sigma had designed it specifically as a 70-200mm "L" equivalent for APS-C.
Sadly it appears to be discontinued. Whilst the Sigmaphoto.com.au website states "Sigma mount discontinued" and lists Canon under "available mounts", I was unable to find a dealer who had one and most online sellers either don't list it or have it flagged as discontinued. Second hand there is/was one on Gumtree, but I found it odd that it was "purchased earlier this year", and "used mainly in the studio", yet was also stated as "just been serviced". Other than damage or fault, why would a lens less than a year old require service?
But if I ever find one on a shop it will be hard to leave it there!
Contact Sigma direct. Ask them which stockist might have one. They will check their inventory and give you the contact details of the stockists who last had some shipped. You might get lucky! I have done this. Meant a call to a shop in Melbourne, ordered over the phone, got it the next day. Otherwise it was a wait of 6 weeks or more, if I ordered one from my local store.
"It is one thing to make a picture of what a person looks like, it is another thing to make a portrait of who they are" - Paul Caponigro
Constructive Critique of my photographs is always appreciated
Nikon, etc!
RICK
My Photography
Personally I wouldn't worry too much about whether it's been serviced recently, but it depends on what was done. It could actually be a positive - if the service was done by an authorised Sigma repairer and the seller has the receipts from the service then you could have a bit more confidence that the lens is in good condition. Chances are it was just a basic service to check if everything was OK.
My local camera shop has a used version on the shelf for $795, but it's Nikon mount!!
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