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Fast, big aperture lenses


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#1 tkr

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Posted 07 March 2006 - 04:45 AM

I think it's generally accepted that a faster/big aperture lens would have better AF performance than a slow lens.

I've being comparing the AF on the 60mm F2.8 micro Nikkor to that on the 18-55 F3.5-5.6 Nikkor on the D50. I've had the zoom at 55mm and tried shooting in a very dimly let room on some very low contrast subjects.

Using central spot focus the AF on the zoom lens is just as fast to lock on to its target - and makes a lot less fuss and noise about doing it.

The 60mm F2.8 does of course give a brighter viewfinder but I'm wondering if this particular Nikkor is a bit of a slow AF performer. I also wonder if lens design is moving on at such a pace that even a cheap lens like the newish 18-55 Nikkor is actually very good.

Is it a myth that fast lenses have better AF?

Any views?

#2 james

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Posted 07 March 2006 - 06:01 AM

Hi,

No, it's not a myth. You tested an AF-S lens verses an old screw-drive lens so of course the older lens is slower.

All lenses focus with the aperture open to the max. An f5.6 lens just won't let as much light through to the AF sensors.

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#3 mattdiver

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Posted 07 March 2006 - 06:05 AM

Hi tkr,

You can't really compare the 60mm and the 18-55mm head-to-head in a focusing competition, as the latter is a AF-S lens, which is a newer generation as compared to the 60mm.

On the other hand, it is correct to say that for lenses of similar build (ie. comparing AF lens to AF lens, AF-S to AF-S, etc.), a lens with a faster lens focuses faster. This is because lenses focus at their widest possible aperture, which makes it easier to detect contrast in low light situation.

Cheers,
Mat

#4 TheQ

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Posted 08 March 2006 - 12:13 PM

In the Canon lineup, many of the camera bodies have high precision focus sensors that are only enabled when using lenses with large aperatures (f/2.8 and f/4 or larger).

I'm not certain if that is also the case on Nikon bodies.

#5 herbko

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Posted 08 March 2006 - 12:19 PM

The camera autofocus at the widest aperture, so a faster lens that lets in more light will help the sensor especially in low light settings. However, that's not the only factor. The exsistance/performance of the motor in the lens makes a big differences also. I've used both the Sigma 105mm F/2.8 and the Canon 100mm F/2.8 and the Canon is much, much better at autofocus.
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#6 fdog

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Posted 09 March 2006 - 08:29 PM

<snip>
Is it a myth that fast lenses have better AF?

Any views?

<{POST_SNAPBACK}>

No, it's not a myth. It has enough of an impact that ALL my lenses cost twice as much as the cheaper, slower lenses.

You don't really notice the difference until you start shooting in lower light levels.

All the best, James

#7 Glasseye Snapper

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Posted 12 March 2006 - 08:55 AM

No, it's not a myth. It has enough of an impact that ALL my lenses cost twice as much as the cheaper, slower lenses.

You don't really notice the difference until you start shooting in lower light levels.

All the best, James

<{POST_SNAPBACK}>



One situation where low light levels are more common is the use of Magic Filters. I still haven't found any information on how much they attenuate light (1, 2, 3, more stops) but from the stories it seems they like clear sunny environs with above-average ISO settings.

For the magicians out there that have used the filters what has been your experience with respect to both AF and getting a correct exposure. In particular I'm interesting shooting with the filter for fish portraits whereas all lenses listed on the Magic Filter web site are in the widish angel range (10 to 40mm).

Alex has sent me two beautiful fish portraits shot with filter (the last two on his "magic gallery") so it can be done. Wonder what lenses people have been using for this purpose. I'm thinking going a bit extreme with the Canon 50mm f1.4 but if the EF-S 60mm f2.8 macro will do just fine then it will be a more versatile lense.

Apart from the lense question I would be interested in knowing if people think light loss by the filter has a bigger impact on AF or on getting a decent exposure with reasonable shutter speed.

Bart
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#8 CeeDave

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Posted 12 March 2006 - 04:30 PM

Bart,

Many of the lenses that've been used with the Magic Filter are 2.8 (e.g., Nikon 16FE and 10.5FE, which I shoot), so I don't think you need to go faster. In my experience, if there's enough light for an ambient light shot, then focus is no problem (even with the filter). I've been using wide lenses, which are commonly quicker to focus than macros -- but then they weren't EF-S-equivalent (just AF, not AF-S).

Hope this helps.

Chris
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#9 Glasseye Snapper

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Posted 12 March 2006 - 06:48 PM

Thanks Chris,

Alex didn't seem overly concerned about AF either, more about getting a good exposure setting (shooting shutter-priority and letting aperture go where it needs to go). Sounds like you're running into exposure issues (long) before hitting AF limits. Longer focal lengths will need bit shorter shutter times so that may add to the challenge.

I'd still be interested to hear if anyone has shot at a white wall with and without filter using identical settings and look at the average reading in the red, green and blue channels to see how much filtering is going on. I'm going to do some experimenting myself soon. Have booked the holiday and ordered the filters. Now I just have to buy the accessories such as camera, lens, and housing :blink:

Bart
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