Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It only takes a minute to sign up.
Sign up to join this community
I'm looking to get a Sigma 150-500mm f/5-6.3 for my Nikon D3300. I've been trying to wrap my head around the crop factor and how that works and how it affects auto focus.
Spec sheet for my D3300 say it can't autofocus past 5.6 but I just tried setting the aperture higher than that on a different lens and tried to autofocus and it worked but maybe I'm just being super daft.
Will I be able to use autofocus in good lighting at the high end of the zoom?
Your Nikon D3300 (and pretty much every other film SLR & DSLR) meters and autofocuses with the aperture of whatever lens is mounted on it wide open at maximum aperture. It doesn't stop the lens down to the selected aperture setting until the instant before the shutter opens to expose the image. What aperture setting is selected is immaterial in terms of AF performance - it's the lens' maximum aperture that counts.
Here is the way the crop factor affects autofocus in terms of the lens' aperture and whether the lens is a DX or FX lens: it doesn't.
A DX lens at f/6.3 is the same as an FX lens at f/6.3 when both are used on an APS-C camera, just as a 300mm DX lens and a 300mm FX lens are the same focal length and provide the same field of view when both are mounted on an APS-C camera.
The narrower baseline allowed by the smaller mirror of an APS-C camera will mean some reduction in the maximum theoretical AF performance when compared to a FF camera with the same generation of AF technology. This is because the width of the baseline from one side of the system to the other can affect the speed and accuracy of a PDAF system. But that is true no matter what lens you have mounted on any APS-C camera. In the case of Nikon, they seem to concentrate more on making their AF systems work with as many lenses as possible at the expense of giving up the superior performance for which they might be capable using very wide aperture lenses.
The only AF issues associated with this lens and Nikon DSLRs that I've been able to find widespread reports of are only associated with the very early examples of the lens when it was first introduced around 2010 and then later issues with some subsequent models of Nikon cameras (particularly the D5300) that required a firmware upgrade to make the lens compatible with the newer camera. The first issue was resolved and copies of the lens made since then have not had that issue. The second issue can potentially occur anytime a new camera model is introduced. It's just part of the risk one takes when buying a reverse engineered third party lens. Sigma has usually been very good about updating firmware for current and recent lenses when incompatibilities with newer camera models are discovered. But eventually discontinued lenses stop getting updates for the latest cameras. Since your D3300 was introduced while the lens in question was still in production that shouldn't be much of an issue for you.
So as far as the f/6.3 maximum aperture at focal lengths of 313mm and above, there don't seem to be any widespread reports that Nikon cameras won't AF with the lens. Of course any camera should AF better (faster, more accurate, more consistent from shot-to-shot) with a wider aperture lens than a narrower aperture lens all else being equal. It is also true that any camera/lens combination will AF better in brighter light than in dimmer light and that high contrast targets are easier to AF than low contrast targets. One shouldn't expect an f/5.6 lens to perform as well with regard to AF as an f/2.8 lens will, even when both are stopped down to the same aperture when the picture is actually taken. As mentioned above, they all meter and AF with the aperture at its maximum setting. Often when manufacturers say their AF system is good to f/5.6 what they really mean is anything wider than f/8.
There are, however, some important considerations one should weigh when entertaining the possibility of using the Sigma 150-500mm f/5-6.3 DG OS HSM APO:
As much as with any other type of lenses, when one is talking about long focal length telephoto lenses it really is true that you get what you pay for. If you can afford the older Sigma 150-500mm f/5-6.3 but can't quite swing the cost of the newer Sigma or Tamron 150-600mm f/5-6.3 offerings then perhaps that is a good option for you. But if you can save a bit longer and get either the Sigma 150-600mm f/5-6.3 Contemporary or the Tamron SP 150-600mm F/5-6.3 Di VC USD instead you'll probably be much happier with the results you can get with either one of those lenses.
You can find some very good reviews and comparisons of all of these lenses at The-Digital-Picture. At the end of the review for the Sigma 150-600mm Contemporary that lens is compared to both the Sigma 150-500mm and the Tamron 150-600mm as well as a couple of other lenses.
Personal note: I've got a friend that had a Sigma 150-500mm f/5.6-6.3 DG OS HSM in the Canon EOS mount back in around 2012. It wasn't as sharp past 350mm as he wanted and he sold it after only having it about 6 months or so as he could just crop images from a quality 70-300mm and get results that were just as good.
Software in the Sigma lens tricks the camera into thinking it is at f/5.6 when it is really f/6.3
When using a 2x teleconverter with a slow lens, some people will tape the contacts to "fool" the camera into AF when it should be able to.
More info:
However, some third party lenses (e.g. Tamron and Sigma zooms with a max aperture of f/6.3 at the long end) effectively trick the AF system into thinking there's an f/5.6 lens attached. Likewise, although not condoned by Canon, it is possible to use tape to block some of the contacts on a Canon 1.4x extender used with an f/5.6 lens, resulting in the camera attempting to autofocus with an f/8 lens on bodies which are limited to f/5.6. Sometimes, it even works...
