Chris_Malcolm PRO 2:43am, 17 March 2012
500mm with red dot sight (side)

These long lenses are hard to aim, especially if the thing you're aiming at is against a featureless background, and mosr especially if the thing is moving fast and unpredictably, like a bird in flight.

This kind of sighting aid allows you to use both eyes open, one looking through the sight. You see a red dot against the place where the centre of the lens is pointing (less parallax error). Makes it extremely easy and fast to aim the lens at anything.

See the description of the photograph for more details
boxcustom 5 years ago
do not aim this near armed citizens. or police.
Overly Precise Tim Posted 5 years ago. Edited by Overly Precise Tim (member) 5 years ago
Are you sure you know what a red dot sight is?

Edit: the previous post was removed. It seemed to discuss a red laser sight rather than a red dot sight.
^M^ 5 years ago
Know what???

.... that's a great idea ... rapid acquisition of target, makes complete sense. Those 500's are a pain to get zero'd in, and I can see where a red-dot would assist.

I think any opposition on this would come from someone who is either inexperienced with using a 500mm or a Red Dot.

opinioneditor98 5 years ago
Can you adjust for parallax using the windage and distance adjustments on the gunsight?
Chris_Malcolm PRO 5 years ago
For any particular distance, yes. But the error will then increase the further you move from that distance. I prefer the idea of keeping the axes of gunsight and lens aligned and making the adjustment by aiming a constant few inches low.

That few inches of parallax error makes no difference at all if what you're doing is getting the target in the field of view of the lens, and then you shift to viewfinder or LCD to compose and focus. But it does make a difference when following a bird in flight and there's no time to do that, so you're completely relying on the red dot to give you sufficient accuracy to place the target on the central focus sensor.
Analyst 1 5 years ago
I picked up a hot shoe mounted laser sight from an astronomy website with a similar idea in mind. Not as elegant in use as your model, I would think (more parallax), but it was nice & cheap.
dannyt. Posted 5 years ago. Edited by dannyt. (member) 5 years ago
couldn't you mount it on the hotshoe? Doh analyst beat me to it sorry...
Chris_Malcolm PRO Posted 5 years ago. Edited by Chris_Malcolm (member) 5 years ago
I tried mounting it on the hot shoe. The problem is that the hot shoe isn't a completely stable mount. There's a bit of wiggle in it. You have to zero the sight every time you put it on, and if you knock it while using it you have to zero it again. Whereas my tube mount stays zeroed.

The amount of wiggle doesn't matter if all you want is to find the object in your viewfinder or LCD once you hit it with the red dot. But if you want the red dot to be accurate enough to locate the object on the central autofocus sensor without having to move your eye to viewfinder or LCD then that wiggle is an important source of error and frustration. You need that accuracy for tracking birds in flight with a 500mm lens.
Chris_Malcolm PRO 5 years ago
Note that the 500mm reflex can only use the central focus sensor in AF. So to get the AF to focus on the bird you must get the bird accurately centred in the image and keep it there for long enough for the AF to grab it. Whereas a lens which can use the other AF sensors too allows the camera to do much smarter focus acquisition and tracking.

I've discovered that although this slit tube mount wiggles much less than a hot shoe mount, there is still a small residual wiggle which developed over time which threw off this central AF sensor aiming accuracy. Adding rubber bands at each end of the tube to improve the strength of its grip on the lens body seems to lock the whole thing down quite securely.

A problem with birds in rapid flight which is changing their distance rapidly seems to be the focus-shoot lag time. The camera will only shoot when focus has locked. With stationary objects that's nearly always a perfect focus. But quite often with fast moving birds it's not.

It's possible that my camera's focus tracking (an A550) isn't quite smart enough to handle this problem on a lens with such a narrow DoF. For example I'll sometimes get a wing tip in focus and the rest of the bird out of focus.

I may be expecting too much of such a difficult focusing situation.
Especially since so far I've had to do most of my experiments in dim cloudy conditions where the noise at the ISO necessary to keep shutter speed above 1/400th at f8 is degrading detail enough that perfect focus can't be assessed anyway. That may also not be a fast enough shutter speed when I'm swinging the camera around rapidly to follow a bird.

With stationary or slow moving objects such as small distant craft in wide expanses of featureless water this device works very well indeed. But there is time enough there to steady the camera on a support and correct the focus aim carefully with viewfinder or LCD.
dannyt. 5 years ago
Man that must be a difficult shot...a bird flying and shooting it with a 500mm lens at f8...

Not sure about Sony but Canon cameras have a predictive auto focus mode but that is still hit and miss. Perhaps some real careful planning and a laser trigger? :*)
Chris_Malcolm PRO 5 years ago
Sony have predictive focus but I think it requires the use of all the AF sensors, and the f8 500mm can only use the single central focus sensor. Note that most cameras can't autofocus an f8 lens at all.

But if the red dot sight is carefully aligned with the central AF point it's not difficult to catch birds in flight with this lens. Without the sight and lots of gulls wheeling round I could only catch a shot of maybe one in ten or twenty gulls. But with the sight I can usually get a few to several shots of nearly every bird I go for.

The problem is simply that the depth of field is so shallow and the bird usually changing distance so fast that it's luck whether the AF manages to nail the focus exactly. Even if the bird is flying across the view at the same distance the preferred focus is on the bird's head. But the autofocus is just as likely to settle on the near wing tip or the far wing tip.
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