BIF with a red dot sight on the 500mm reflex

Started Jun 8, 2012 | Discussions thread
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Chris Malcolm
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BIF with a red dot sight on the 500mm reflex
Jun 8, 2012

This lens is allegedly most unsuitable for birding for many reasons, at least some of which I think are mythical. To demonstrate what it can do I've used large images here which you can click through twice to see the maximum resolution. I've processed them the way I do most of my shots with this lens -- up contrast and colour a bit, downsize to 67% of original, sharpen, then crop away any excess. With good light, a tripod, and remote release I can sometimes get shots which lose detail when downsized from the original. But not when whirling it around handheld after flying birds!

On a crop sensor DSLR (such as my A550) a 500mm lens has an angle of view of about 3 degrees. So it's rather difficult to aim it at what you want to see. Some hunting around using visual navigation clues is usually required, just as with a powerful pair of binoculars. Which makes it good for photographing relatively stationary objects, such as this standing heron.

But almost impossible for catching birds in flight!

Some kind of sighting aid is required. The kind of red dot gun sight often mentioned here seemed particularly good. A zero magnification version of that kind of sight lets you aim with both eyes open, freely surveying the entire field of view. You see a red dot superimposed on the view. You don't have to get your eye into some special aiming spot to see it, you can see it from anywhere you can see through the sight, which simply looks like a plain see-through tube. No eye focus problems, no shifting of the dot as you move your eye relative to the sight.

If you set the sight up exactly parallel to the lens there will be a constant parallax error of the distance between lens axis and sight axis. A few inches, and at its smallest if you keep the sight close to lens. Since you don't have to put your eye up to the sight this means you can mount it on the lens barrel for least parallax, plus that gives a much bigger and more stable mount than the hot shoe.

A quick and dirty experimental bodge which worked surprisingly well. The cardboard tube grips the lens barrel gently but firmly. The expansion slot also gives room for the focus hold button on the lens barrel. The sight comes equipped with a special long wedge gun sight mount gripper. I simply glued that to the tube with some strong stiff rubbery glue, positioning it purely by eye to look straight. Then I aligned it with the lens by using the vertical and horizontal adjustment screws provided for zeroing the sight. I discovered that it was accurate enough that it could be set up so that placing the red dot above the target displaced up by the small constant parallax shift placed the target smack on the central AF sensor.

You're right by the way -- the end of the lens spins when focusing, so I posed the photo badly. The tube would stop it spinning, so it should have been mounted slightly further back. There's nothing critical about that.

The central spot is the only AF sensor that operates with this lens. Attaching the lens to camera switches AF mode to central spot automatically. The camera's default AF mode is to focus when the shutter is pressed, and not to take the shot until it gets a focus lock. So all I have to is to locate the red dot above the bird and keep it there long enough for the camera to focus.

I keep reading comments about how dreadfully slow this lens is to focus. It is a lot bigger than a 50mm lens. There's a lot of glass and metal to shift when focusing, and it has to travel a lot between infinity and closest. That long travel might take a second. But when it's already close in position to being focused the focusing is instant.

So you can speed up initial focus acquisition by first focusing on something very roughly in the same distance range as the birds, focus preloading if you like. Then if you have hold-down-repeat shooting enabled and you track the bird with shutter down it will simply shoot every time it locks focus. I use the slowest repeat to avoid filling the buffer with strings of similar shots.

You can see the "doughnut" weird bokeh of the lens in the out of focus tree leaves behind the bird in that shot.

The very shallow DoF means that only in flat shots like this next one will you get most of the bird in good enough focus. Side on shots are a gamble because you'll often get shots focused on one or other wing tip. Aiming for the head or tail helps if you can manage it.

This was my first trial of this device in good bright light. Once I raised shutter speed to around 1/1,000th sec I was impressed with how many reasonably sharp shots I got. Higher might have been even better. I'm a novice at BIF and action shots in general. But I'm now an enthusiastic learner having discovered how easy it is to aim and track this long lens using a red dot sight. Far faster and easier than using a zoom to catch the target and then zoom in on it.

So my next step in this project will be to paint the sight and its tube a more professional looking black or camo and go after my local herons in flight. They fly up and down a small river with largely wooded and often sloping surrounds. So there will be a lot of background and foreground foliage clutter to throw off the autofocus, not to mention trip up the photographer!

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Chris Malcolm

 Chris Malcolm's gear list:Chris Malcolm's gear list
Sony Alpha DSLR-A550 Sony SLT-A77 Sony 50mm F1.4 Sony DT 18-250mm F3.5-6.3 Sony 500mm F8 Reflex +7 more
Sony SLT-A55
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