Af min/max focal length /aperture explanation

scott199

scott199

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Hi

Don't really know how to explain this, but as I don't understand could anyone explain in basics please.

On a few threads, mainly the tamron 150-500, I have noticed a few comments about AF not working above a certain F point.

So is this basically that the AF range is F6@400mm (random figures)

So anything above this F range will stop AF working ? Or at least throw up some focus issues at that length

Also if the above is true does it depend on distance to the object your trying to focus on ?

I have many other question about this but I think one at a time.

Thanks
 
Many cameras auto focus systems are rated by the manufacturer to work up to a specific aperture. Most DSLRs will autofocus down to f/5.6, more modern one will autofocus down to f/8, though most have fewer sensors capable of this above f/5.6 - usually the centre ones (can be one or maybe up to 9 if I recall correctly). The thing here is that AF will work below these numbers, they just become more unreliable is all. You really notice it in poorer light, in good light most AF systems will work ok at almost any aperture.
 
To expand a bit, the camera focuses with lens wide open. There are different types of AF sensor that have different sensitivities, some need more light than others to get a reliable focus. For most DSLRs the sensitivity is about f/5.6 - if the minimum aperture is less than this (e.g. f/6.3) then the sensors don't get the minimum light required for a reliable focus. Some higher-end bodies are fitted with sensors that can cope down to f/8. This also comes into play if you start adding tele-converters, so a x2 tele-converter that costs you two stops of light turns your f/4 lens into an f/8 (as far as light reaching the sensor whilst wide-open is concerned) and you're into the realms of "does my camera have AF sensors that are reliable with this?"

I've put the emphasis on reliable focus, because you may get a good focus or you may not, but you're going to be less certain about getting it. With big variable aperture zooms you can find that at shorter lengths the aperture is within the AF sensitivity range, but as you zoom to longer focal lengths the minimum goes beyond the reliable AF range.

To check what sort of AF sensors you have, check the specifications for your body (DPReview will usually list this). You'll often find that the centre point is different to the others.
 
Thanks, so to try and get my head around this and make sure i have the basics correct (for simplicity i will just use random figures rather than exact)

For the benefit of ease, i will say my D5100, using sigma 70-200 F2.8 DG HSM, using single point AF setting.

in basic.

This is dependent on the light hitting the sensor, so if im shooting football with my sigma 70-200 @ 200mm and 2.8, lets say im getting a shutter speed of 1/800

am i right in saying, theirs plenty of light so AF should be fine ?

same scenario, but i step down to F8, 200mm @ 1/60 for example, but now i pushing the limits of light, hence the AF could/will struggle ?
 
scott199 said:
Thanks, so to try and get my head around this and make sure i have the basics correct (for simplicity i will just use random figures rather than exact)

For the benefit of ease, i will say my D5100, using sigma 70-200 F2.8 DG HSM, using single point AF setting.

in basic.

This is dependent on the light hitting the sensor, so if im shooting football with my sigma 70-200 @ 200mm and 2.8, lets say im getting a shutter speed of 1/800

am i right in saying, theirs plenty of light so AF should be fine ?

same scenario, but i step down to F8, 200mm @ 1/60 for example, but now i pushing the limits of light, hence the AF could/will struggle ?
Click to expand...
Almost correct. Because the lens is capable of opening up to f2.8 your af will be fine at f/8 because the camera will open the lens up to it's maximum aperture (f/2.8 in this case) to achieve focus and then just before taking the shot the camera will then close the aperture down again to f/8. If you were to swap lenses and put the new Tamron on, and then zoom in to 600mm the lens maximum aperture at this focal length is f/6.3 which is outside of the D5100's AF sensitivity, so in this case, as the AF system cannot open the aperture any wider than f/6.3 it will struggle particularly in poor light, in very good light it will, most likely, still work ok.
 
At f8 yes you would struggle and at f2.8 you would auto focus with no issues. The shutter speed makes no difference to auto focus as it focuses before the shutter fires.
Edit as Paul says if it's a f2.8 lens it will still auto focus and then shut down to f8 to take the image.
 
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The phase-detect AF systems of all DSLRs work by 'looking' through both sides of the lens and comparing the two images. When the aperture is physically smaller, ie at higher f/numbers, the two images are too similar to differentiate. With Canons, the AF system simply switches off at f/numbers higher than f/5.6 (all except 1D-series cameras and the 5D3). Nikon is similar.

Tamron and Sigma have several lenses with maximum apertures higher than f/5.6, ie f/6.3. They can still AF because they don't tell the camera's AF when the aperture has gone above f/5.6, so they will still try to focus. But that doesn't change the fact that the aperture is now too small for fast and accurate working. It's not that the amount of light is reduced, though that doesn't help.

To give the camera its best chance, use the centre AF point (it's more sensitive) and find an area on the subject with good light/dark contrast.
 
My old Nikon D70 could AF my old Sigma 170-500 at the long end, as could Dad's D50. Pretty sure the lens was f/6.3 at the long end. IIRC, it didn't hunt too badly under those circumstances either, although finding a camo'ed plane from above over the coast was ... a challenge! The D70 can also AF a 70-300 on a 1.5x Kenko teleconverter at the long end. Quality suffers though.
 
Ok I see, so the constant 2.8 will be fine.

Now does the distance to subject have any effect on this. (AF ability )
 
scott199 said:
Ok I see, so the constant 2.8 will be fine.

Now does the distance to subject have any effect on this. (AF ability )
Click to expand...

No. The key point is the physical diameter of the lens aperture (as seen by the lens) at max aperture - ie the f/number. At f/2.8, AF is often better as on some cameras as f/2.8 and lower activates additional AF points.

To summarise, phase-detect AF works best at low f/numbers, particularly f/2.8 and lower. It works less well at higher f/numbers, and above f/5.6 it will either not work at all, or may be slow and hesitant. Note that neither Canon nor Nikon has any lenses with a native max aperture above f/5.6. If the AF is having difficulties, it will have its best chance using the centre point, and positioned over an area of high light/dark contrast. Good light helps, but good contrast is the most important thing. For example, if you try to AF on say someone's white top, the AF may struggle, but put the AF point over a shirt button and it will lock on immediately.
 
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