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I have a Canon Rebel XTi and some decent lenses. I am considering getting a 500-1000mm f/8 telephoto zoom lens.

For photographing birds that are 25-100m away, should I use an automatic focus or a manual focus lens?

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    Which lens are you considering, exactly? What's its max aperture range? – Hueco Jan 28 at 17:11
  • max aperture is 8 – AdrienDaBoss Jan 28 at 17:41
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    Do we talk about Walimex and co. lens? Because they are only manual focus. – Romeo Ninov Jan 28 at 20:24
  • If you are buying a lens for bird photography and have to ask the question avoid those cheap 500-1000 mm lenses. Save up for some 150-600mm from tamron or sigma. Remember to check used prices, with these autofocus will work and you can choose to focus manually or not depending on situation. – lijat Jan 29 at 19:22
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With a Canon Rebel XTi and a 500mm - 1000mm f/8 lens, the question isn't really, Should I use Autofocus or Manual Focus but more so Will Autofocus Work At All?

The Rebel series is a consumer grade series - it was never designed for tracking birds in flight, for example.

You've got 9 focus points and, for the life of me, I can't remember or find in the manual what aperture is required to run them. This question has a wealth of knowledge on the subject though: What apertures are required to enable autofocus, including cross-type or high-precision focusing, on Canon DSLR cameras?

To put it bluntly, with an f/8 lens, I'm not sure if any AF point except the center will work for you at all. Perhaps on a very bright day, but I wouldn't expect much from it during golden hours.

At best, you'll have some limited AF functionality or center point only functionality. At worst, you won't have AF at all and will have to manually focus.


If your subject isn't moving, I'd try using AF first and then go with manual. The problem with manually focusing with a modern day SLR/DSLR is that they don't expect you to - there are no focusing aids built into the camera (like a split screen/microprism). Some newer cameras offer Live View, which may help you out - it won't do well for birds in flight but stationary ones may work out.

For birds in flight, the same holds true - try using the center AF point and AI Servo to track...but realize that you are asking a lot from your camera here...keepers may be few and far between.

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Two things that make the question moot:

  • Your EOS Rebel XTi/400D is only rated to autofocus with lenses f/5.6 or wider (lower f-number). There are third party lenses with f/6.3 maximum apertures at the longest focal lengths that still autofocus on Canon f/5.6 limited cameras. Many have theorized that either the third party lens "tricks" the camera into thinking it as f/5.6 lens currently set to f/6.3 or that in this context f/5.6 really means "anything wider than f/8." (f/8 is the next standard "full stop" past f/5.6)
  • Every 500-1000mm f/8 lens I've seen is really a 500mm f/8 lens with a converter attachment that makes it a 1000mm f/16 lens. They're also all manual focus. And not very good optically...

For more on the whole "aperture limited" thing with regard to autofocus, you can read this answer to Comparing Canon EF 100-400mm f/4.5-5.6L IS II USM and Canon 70-300mm 1:4-5.6 IS II USM with Kenko 1.4x MC4 DGX

There is a difference between being rated to focus at f/5.6 as most of the older Canon cameras are and being firmware limited to f/5.6 like some of the newer Canon cameras are. Older Canon cameras rated to focus with f/5.6 or wider lenses will sometimes focus at f/8 depending on conditions. But that ability will be fairly limited to very bright light and will be excruciatingly slow, especially with cropped APS-C cameras. I once tried it with a Canon EOS 7D + Kenko C-AF 2X Teleplus Pro 300 DGX + EF 24-105mm f/4 L IS. The only thing it would AF on, and then only some of the time, was a bare light bulb using the center AF point.

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For birds at rest, you can probably get away with manual focus. For birds in motion/flight, you would have to be very good with your manual focus to be successful. Autofocus would have a much better chance in that case, and preferably one of the tracking autofocus modes if your camera has it (e.g. Canon's AI Servo mode or the equivalent for whatever manufacturer is appropriate).

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  • I do not know the Canon Rebel xTi in particular, but based on the age and class of this camera, I find it highly unlikely that it has an autofocus module capable of tracking birds in flight. That is a difficult task even for current high-end cameras. – jarnbjo Jan 28 at 18:12
  • @jarnbjo The specs list the xTi as having a "Predictive AI Servo" AF mode. But, as you mention, it's probably a lot more primitive than a considerably more recent body. My 7DII does a pretty decent job, as long as the flight isn't too fast or erratic... – twalberg Jan 28 at 18:17
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You are going to use manual focus, because on these cameras the AF is not functional past f/6.3 (which is why the big 150-600m zooms from Tamron/Sigma are f/6.3 at the long end). Still true on my 70D which is several generations younger and one market band higher.

With a 1000mm lens your depth-of-field is quite shallow. On this online computer:

Subject distance  50 m

Depth of field 
Near limit      49.6 m
Far limit       50.4 m
Total           0.74 m

In front of subject     0.37 m (50%)
Behind subject          0.38 m (50%)

Hyperfocal distance     6579.9 m
Circle of confusion     0.019 mm

This means that you can't set your lens to some focus distance and shoot without focusing.. you are not too likely to catch sparrows in flight. You can at best get sitting birds or gliding seagulls.

Don't get too hooked on focal length. Before shooting a bird you have to see it, so the longer your lens, the better your eyesight.

You lens is likely not stabilized. So at 1000mm on a crop-sensor camera, you have to shoot at 1/1500s. Compound that with the f/8 aperture and a fairly old sensor that has a low ISO sensivity, you will need a lot of light, even an overcast sky will be a problem.

As I said before, a $20 a ghillie suit will get you closer to the birds than a $2000 lens.

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  • "A $20 ghillie suit..." and fieldcraft. ;-) – scottbb Jan 29 at 4:01
  • @scottb the $2000 lens also requires fieldcraft :) – xenoid Jan 29 at 8:01

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