Autofocus and aperture

[McLovin]

Hi,

In this great article about Understanding Camera Autofocus https://www.cambridgeincolour.com/tu...-autofocus.htm, there's a statement that says 'for these cameras autofocus is not possible for apertures smaller than f/8.0 and f/5.6'. The statement is made in the section titled Number & Type of Autofocus Points in which it's using the Canon 1D MkII and the Canon 20D as examples.

Does this actually mean autofocus will not work for smaller apertures than stated? If so, is this something unique to these two camera models or is there some principle that I don't understand about autofocus?

Thanks

[pnodrog]

The figure refer to the widest aperture (lowest f value) of the lens not the aperture you have set the camera to use during exposure. Most lenses will have a sufficient open aperture to auto focus unless they are being used in conjunction with a telephoto extender.

The limitation does not apply to manual focus assuming the scene is bright enough for the photographer to do the focussing via the viewfinder or alternatively use the distance markings on the lens.

[Mark von Kanel]

HI Mc Lovin. and welcome to CIC if you could add your real name and approximate location to your prifile that would be great, it allows us to give you good location related advice for suppliers and i dont have to call you "Mc Lovin" unless thats what you want to be called, if you do just put that in your real name field.

L.Paul has given you a good explanation but i found it a little confusing so ill put into words that even i understand.

I have a sigma 50/500 f5.6/f8 lens if my cameras auto focus minimum aperture requirement is 5.6 then it will work accurately when my lens is zoomed out and my largest aperture value f5.6 is available. if i zoom in to 500 my largest aperture available becomes F8 and the camera may not have accurate auto focus.

Its important to remember that these figures are guidelines and ive successfully used bodies on this lens in auto focus that shouldnt have worked, without problems, so some experimentation is needed.

[pnodrog]

Gee I am glad I said most....

Currently not having any lenses where the aperture goes higher when zooming than my camera can handle I over looked this possible problem. I am pleased Mark has mentioned it.

[Inkanyezi]

Hi,

In this great article about Understanding Camera Autofocus https://www.cambridgeincolour.com/tu...-autofocus.htm, there's a statement that says 'for these cameras autofocus is not possible for apertures smaller than f/8.0 and f/5.6'. The statement is made in the section titled Number & Type of Autofocus Points in which it's using the Canon 1D MkII and the Canon 20D as examples.

Does this actually mean autofocus will not work for smaller apertures than stated? If so, is this something unique to these two camera models or is there some principle that I don't understand about autofocus?

Thanks

It does apply to all cameras with autofocus systems using what is called "phase detection" AF, i.e. all single lens reflex cameras when used with the optical viewfinder, and a few other cameras as well. The term "phase detection" (PD) is misleading, as you might think of phases of waves. The PD system uses triangulation to estimate sharpness, and the triangulation base is what governs the smallest aperture that can be used for the AF system to work.

The AF sensor in a PDAF system, has two sets of microlenses in front of its sensels, and those two sets of microlenses are adjusted to point at an angle toward the lens, half of them pointed to one side, the other half to the other side. The triangulation base is the distance between the spots on the lens where the PDAF microlenses are aiming.

In order to work at all, those spots where the microlenses are aimed must both fall within the exit pupil of the lens; if not, they could not receive any light from the scene. The angle of the bundles of rays received from the lens to the AF sensor will govern the actual F-number with which it is possible to focus. As the shape of the exit pupil is somewhat distorted when seen from an angle, AF points farther from the centre may lose light from one side at a larger aperture than from the other side. Hence AF is lost at the outer AF points earlier than at the centre of the image.

In most PDAF cameras, AF works with all AF points at f/5,6 and may work down to f/8 in the centre. Therefore, it is sometimes possible to use PDAF with a 2x extender and an f/4 lens in the centre spot, even though its effective aperture is f/8.

There are also a couple of models that have extra sets of AF microlenses for a larger triangulation angle with large aperture lenses, for higher precision. Those are aimed at spots farther from the centre of the lens than the first set.

[IzzieK]

Hi to ???? Welcome aboard this forum. If you take the time to fill up your profile, we will know where you are from and what you want to be referred to...some people who call me Hey! do not usually get a response from me either if you know what mean...

[dje]

By way of further explanation - with a DSLR the lens sits there wide open until you press the shutter. Then AF is set (whilst the lens is still wide open), then the required aperture is set and then the shutter operated. This is why it is the maximum aperture that matters. From then on, it's as Urban describes.

Dave

[ajohnw]

Minimum light levels are also sometimes specified as well as slowest aperture figures. It's not uncommon for a camera to have one AF point, the central one that will work down to F8 and it may also be more light sensitive.

If some one is choosing a camera and wants to look at the figures for various cameras the Dpreview reviews are probably the best ones to look at. The easiest way to find them is to google dpreview make and model. I'm assuming that as you are reading you may be thinking about buying.

John
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[ajohnw]

If the OP wants a simple view of how it works the wiki shows how the range finding aspect works

http://en.wikipedia.org/wiki/Autofocus#Phase_detection

It may be possible to demonstrate the effect after a fashion using a camera lens. Place something in front of it with 2 holes in it. These will form 2 images that will only coincide when the set up is focused. This will also result in maximum light intensity. It's a technique that is sometimes used to accurately focus telescopes. The bigger they are the easier it is. Smaller holes have more apparent depth of field making judging alignment more difficult.

I always wonder why the term phase is used. It generally signifies strange interference effects when say in simple not all that accurate terms an image is split into 2 and one passed through a wave plate that delays it slightly and then the 2 images are recombined. The delay light gets is dependent on the refractive index of the medium it's passing through Bet people will be a lot happier now they know that.

John
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[MrB]

Thank you Urban, I think I understand your explanation. I have always wondered why they quote a minimum aperture limit on PD AF systems when, assuming my lens's fully open aperture is below that limit, my camera will focus accurately almost in darkness (even without its focus assist lamp).

Cheers.
Philip

[Inkanyezi]

The easiest way to estimate the idea behind "PDAF" is, if you have a lens that has a wider aperture than a finger, that you hold a finger vertically in front of the lens and look through it focusing manually back and forth. You will then see two images superimposed, that move sideways and coincide when in focus.

[Mike Buckley]

Am I remembering correctly that this issue also comes into play when using a teleconverter? Though I have never used one, I seem to remember that a teleconverter reduces the effective maximum aperture of a lens. If I'm right about that, Mark's f/5.6-f/8 zoom lens would effectively be rendered at least an f/8-f/11 zoom, depending on the strength of the teleconverter.

Did I get that right?

[Shadowman]

Am I remembering correctly that this issue also comes into play when using a teleconverter? Though I have never used one, I seem to remember that a teleconverter reduces the effective maximum aperture of a lens. If I'm right about that, Mark's f/5.6-f/8 zoom lens would effectively be rendered at least an f/8-f/11 zoom, depending on the strength of the teleconverter.

Did I get that right?

You are correct.

[Inkanyezi]

It is particularly when a teleconverter is used this property is important, as only then would you experience the situation where AF will not work at all. Otherwise, as one would expect, autofocus works with all autofocus lenses specified for the camera.

Only when you alter the effective largest aperture, will you encounter a situation where it is not sufficiently large.

So also when using "bokeh" filters, actually masks with cut out patterns, you might run into AF problems.

[McLovin]

Thank you everyone for the responses. I'm very sorry I didn't respond sooner but I want you to know that I really do appreciate all of you taking the time to help me. I'll be better about responding in a timely manner.

Mark and Izzie, I've updated my profile with my vitals. Thanks for pointing that out.


So I think I understand the statement I asked about. It sounds like if a camera's AF system will not work for apertures smaller than f/5.6 then it won't work if you use a lens with a maximum aperture of f/8. I think my lack of experience with a wide variety of lenses contributed to my confusion. It didn't occur to me that there may be lenses with apertures that wouldn't open more than this.

I asked my question because I think it really helps to know how and why things the way they do - it helps me at least. I recently upgraded to a Canon 6D. After still having some focusing issues in a couple of situations I started to research how AF really works and of course quickly got lost in a world of contrast and phase AF.

So the AF specs on my camera are: contrast detect (sensor), phase detect, multi area, etc. which leads me to another question - does either contrast or phase AF take precedence over the other or do they work together somehow?

Thanks again,
Jim

[Inkanyezi]

When you use the viewfinder, the camera uses PDAF, as no other means is available.

Only in Live View can the camera use contrast detection for autofocusing.

I don't know the particulars of the 6D in this respect, but there are various ways in different cameras to use one or the other way of focusing, when available. You'll have to consult your user handbook. There may also be resources on the web explaining it.

There are a few cameras that can use PDAF also in Live View mode, by quickly flipping down the mirror for AF, and there are also cameras that have PDAF sensels on the image sensor itself, to enable PDAF also in Live View. In DPReview it looks as the 6D uses neither of these approaches, hence you should expect only PDAF when the SLR viewfinder is used, and only contrast detection in Live View.
http://www.dpreview.com/reviews/canon-eos-6d/7

[William W]

Am I remembering correctly that this issue also comes into play when using a teleconverter? .. .

Additionally -
With Canon EOS, there are also other caveats when using an EF Extender.

A most common example is the use of an x1.4 EF Extender with all three (if I recall correctly) of the EF 70 to 200/2.8L Lenses: this moves the usable Maximum Aperture to F/4 for the lens/extender combination, but the added caveat is that - for many of the EOS Series Cameras, only the CENTRE point AF will be operative, whereas when those cameras are used with a native F/4 Lens, all the AF points are operative.

With Canon EOS, when using third party extenders there are also other caveats. One that I have encountered is the use of the Kenko x1.4 and the EF 85 F/1.8.

I am not sure of all the additional nuances with Nikon DSLR and Nikon Extenders: but I know that there are some.

On the other hand, there are some zoom lenses that have a varying maximum aperture F/x ~ F/6.3 (Sigma and Tamron) and it is widely reported that these lenses WILL allow AF at the F/6.3 Lens Aperture, when combined with EOS cameras which have an F/5.6 AF limitation.

When using an Extender in combination with a lens which places the resultant maximum aperture smaller than the AF Limitation of the camera, one can tape over contacts on the extender which will exorcise the communication of data to the camera, about the Maximum Available Aperture of the lens/extender combination. In this case of taping the contacts, the AF will (on many occasions) still function adequately - sometimes, however AF will not work adequately, but that also depends upon the ambient light level and also the CONTRAST level.

A common example where 'contact taping-over' is employed and seems to work quite well, is when the x1.4EF Extender is used with the EF 100-400 F/4.5~F5.6L IS USM

These are the pins to tape on the EF Extender :


Whilst we are on the topic of the 100 to 400L with an x1.4 EF – this folder of four images might be of interest to folk who are considering that combination. Though these particular images are from a long series of images where the shoot was a test comparing hand holding ability (both with and without IS) vs. the value of using a tripod: the last two images give an indication of the acceptable quality that the x1.4 + 100 to 400 can provide.


WW

[William W]

. . . I have a sigma 50/500 f5.6/f8 lens if my cameras auto focus minimum aperture requirement is 5.6 then it will work accurately when my lens is zoomed out and my largest aperture value f5.6 is available. if i zoom in to 500 my largest aperture available becomes F8 and the camera may not have accurate auto focus. . .

You could try taping the contacts on the lens, as per above.

WW

[PhotomanJohn]

I am not sure of all the additional nuances with Nikon DSLR and Nikon Extenders: but I know that there are some.

I have limited experience in this area but it appears to me that Nikon doesn't automatically disable focusing at some aperture value. This is only based on how my Nikon D7000 performs with a Nikkor 70-200mm f/4 lens and a 2X teleconverter. The camera is rated to AF with f/5.6 or larger aperture lenses. The combination of the f/4 lens and the converter provides an effective max aperture of f/8. At least with the center focus point (the most sensitive one and the only one I typically use) the combination focuses flawlessly even in low room light. Again, this is a sample of one but leads me to believe that taping the lens pins on a Nikon may not have the same effect that you mention with Canon cameras.

I would like to hear if anyone has some more insight on this subject.

John

[William W]

. . . this is a sample of one but leads me to believe that taping the lens pins on a Nikon may not have the same effect that you mention with Canon cameras. . .

I don't know either. Does Mark von Kanel have a Nikon Camera?

What I was referring to when I mentioned that Nikon Cameras / Extenders / Lens have ‘nuances’ (i.e. ‘exceptions to the expected rule’) was that for Nikon it seems MORE dependent upon the particular camera and lens combination than there being a general rule.

Your observation regarding your D7000 is an example.

My reportage on this topic is second-hand information that I have gathered: I have not used a Nikon Kit for about a couple of decades.

WW