Fousing On A Rainbow

Hi

I tried to take a picture of a rainbow today but my camera would not focus in AF I had to go on manual . Is ther any reason for this ?
Paul

Your camera manual described situations in which the AF would not work. This is one, there are no sharp edges. Focus on something at infinity and recompose.

Paul Beckner said:

Hi

I tried to take a picture of a rainbow today but my camera would not focus in AF I had to go on manual . Is ther any reason for this ?
Paul

Click to expand...

Sometimes AF works well. This image was taken with AF and the AF point used was on top of the middle right part of the half circle

Hi olyfler
Very nice pcture I thought it might have something to do with the color spectrum
Paul

I have taken a number of rainbow shoots on Maui. over the years. As a general rule, I use the widest portion of the rainbow as a focus point. If I cannot get a focus lock, I point it at a portion that seems darker in color to me. When focus comes in, I press the shutter half way down and lock it. Then, I recompose and take the picture. Other times I simply point the camera out at infinity and take the shot. That's what I did in the shot, below. I mounted the camera on a tripod in order to get a level horizon. Then, I took a couple of overlapping shots. Later, I stitched the two shots together to arrive at this image.





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truview

Paul Beckner said:

Hi olyfler
Very nice pcture I thought it might have something to do with the color spectrum

Click to expand...

Hi Paul,

Thanks for the comment on my image. In fact, I think focus is more about contrast than colour. I think the problem with rainbow is that the contrast is often not very high, but if you try several parts of it some may have good enough contrast to lock focus. The other thing which may disturb focus is that many times, like even here, there is also a heavy rain. Also, I prefer using single point which I move around to give me the best composition. I do as little focus and recompose as possible.

How far away is the rainbow? 400m, 800m, 1200mm?

At those distances and with lens wide angle, focus is not critical. Guess the distance, set focus manually and shoot. Basic photographic principles, learn them or be forever in the "dark".

cary

Most lenses these days don't have a reliable infinity stop. So picking something quite far away to AF on is probably the best.

MalbikEndar said:

Most lenses these days don't have a reliable infinity stop. So picking something quite far away to AF on is probably the best.

Click to expand...

Right. Here is where older lenses had some advantage, though I think modern lenses are still better, if they have focus distance markings then the markings are in fact quite reliable. Also, with a WA lens, set to f/5.6 or f/8 it isn't very critical. If you for example set the 16-85 between the infinity marker and 1.5m then I am pretty sure everything will be in focus. Anyway, if AF on a distant object works than it is also OK but if you focus on something too far away it may not work. I don't know how the DOf is if you focus too far away, beyond infinity. My rainbows are normally not very far away, maybe 2-300, possibly maximum 500 meters away, so focusing on something at around 100m from the camera may be a better idea and is also easier to get a focus lock on a point which is nearer than infinity.