Optics question

[gayle]

I photograph birds as a casual hobby. Recently I upgraded from a camera with a 16x zoom to one with a 30x zoom. It is amazing because the camera viewfinder (when it decides to focus) acts like a binocular and through it I can see details of a bird's face from many yards away.

 

One thing I have discovered is that, when I use the 30x zoom on flying hummingbirds, the wings are often visible in the pictures. I used to see photographs like this and I thought it must take some kind of special camera to freeze a hummingbird's wings, which are an invisible blur to the naked eye. But what is amazing is that when I look through the viewfinder, at 30x, the wings are visible! If I was 30 times closer to the actual hummingbird, its wings would still be a blur to my eye. So the 30x is doing more than just bringing me closer, it is making it possible for me to see things that could not normally be seen by the human eye at all. How is it possible that the hummingbird's wings become visible with the 30x zoom? Does the act of zooming change the behavior of light in some way?

 

A related question, or maybe observation. Sometimes the same autofocus camera obstinately refuses to focus. And there's no manual focus option, only persistence. But I have discovered something very interesting in certain extremely out-of-focus bird shots that are taken on sunny days. I know that birds see ultraviolet and some of their plumage, especially features meant to draw the attention of other birds, such as sex features and features that distinguish closely related species, are ultraviolet (appearing to us white) or ultraviolet combined with the other color we see (especially red). I noticed in a series of extremely out-of-focus shots of a male downy woodpecker that the red spot on the back of his head radiated a huge "aura" that stood far out from his head. The white markings also glowed with an aura that stood out far from the body of the bird. This effect was not from the general blurring of the image. It happened only around the areas of the bird that would reflect ultraviolet. The camera is obviously interacting with the invisible ultraviolet light somehow, but how and why? And why would the areas of ultraviolet create this "aura"?

[Klaynos]

Ok, so you're probably not seeing an optical effect but a result of the camera hardware. The viewfinder is probably a PDF screen rather than an optical viewfinder from an SLR... you can see the wings as stationary as the exposure is much shorter, this is probably due to higher sensitivity sensor on the camera or a larger aperture on the lens. We'd need more details of the old camera and new to say much more.

[Endy0816]

Stroboscopic effect can cause something similar.

[gayle]

Stroboscopic effect makes sense ... maybe the camera is actually transmitting a series of views to the viewfinder, say at 24 frames per second (like a movie) -- whereas (just looked it up) a hummingbird's wings beat at around 50 beats per second. So in that case, the camera is only showing half of the views of the hummingbird at any given time, in effect slowing the hummingbird's wings down by half. That would make sense, if that is what is actually happening.

 

I want to clarify that the wings in the pictures are still more or less blurry (unlike some photos I see where you see the wings crystal clear) but just the fact that I could see and photograph the wings of a hummingbird at all was amazing to me. So could we say that a digital camera viewfinder is actually showing you a series of "preshots," maybe so fast (like 24 frames per second) that you don't know you are actually watching a series of stills, just like in a movie? And then, by pressing the shutter, you are selecting one of those preshots and making it a photograph?

 

 

. you can see the wings as stationary as the exposure is much shorter, this is probably due to higher sensitivity sensor on the camera or a larger aperture on the lens. We'd need more details of the old camera and new to say much more.

 

Is this saying the same thing as the above? And that a higher sensitivity sensor and/or larger aperture could do that?

 

The camera is a Nikon Coolpix (don't have the model number handy). The old camera I don't think has any significance to the question. But what kind of details would help, and how would I find them out?

On reflection, the strobing idea makes even more sense, because of what I have figured out about how autofocus works on these digital cameras. The camera sends out a tiny laser beam and uses its reflection to judge the distance of an object. I figured this out because under some conditions the tiny laser beam can be seen, especially if shooting through glass windows that have some reflectivity. This means that every time you move the camera, and the autofocus is trying to adjust by sending out tiny laser beams, each time the information from the laser beam changes there is a new potential picture in the camera. So it makes sense (to me) that what you are seeing through the viewfinder is actually a rapid series of stills, and that this creates a strobe effect that is not noticeable unless you are filming something like a hummingbird's wings in flight.

[Strange]

Interesting question. The strobe thing was the first thing I thought of.

 

 

 

But what kind of details would help, and how would I find them out?

 

I think there are a couple of things that might explain more about this. One is the speed at which the pixels are read out of the sensor (they have to be read serially). This could account for some of the remaining blur - the position of the wing will be slightly different when the fist pixel is read from when the last is read. (It may be that modern sensors store all the pixels as a single "snapshot" before they are read out. But this isn't a technology I know much about.)

 

The other is the one you say: the rate at which the viewfinder is refreshed. This might be 24 fps simply because that is simpler if the camera can also shoot video.

 

I have never seen either of these figures in camera specs so you might need to write to Nikon (customer support) to find out - send them a cool picture to grab their interest!

Edited July 8, 2016 by Strange