Focus is on the wrong area with Speedlite..why?

Started Mar 29, 2013 | Discussions thread
John Deerfield Veteran Member • Posts: 3,075
Re: Focus is on the wrong area with Speedlite..why?

leecamera wrote:

John Deerfield wrote:

And if you are using a UV filter... don't.

I'm intrigued John...  Why is that?  A couple of my lenses have UV protection filters and I've not noticed any issues.


First, I am in complete agreement with everything Thom Hogan writes in his article about filters. In other words, the use of any filter is going to degrade image quality. The catch is whether or not you can perceive any loss of quality and/or if the benefits outweigh the negatives. There are situations where a UV filter may be advisable to protect the front element of the lens. Shooting on a beach where sand and water may be kicked up is one example. But for most shooting, the lens hood is as much protection as you need.

In terms of image degradation, again, it isn't going to be perceivable on every image. And the better the filter the less chances for visible issues. When light strikes the surface on anything, three things can happen: the light is reflected (what we see); the light is absorbed which turns to heat (the dashboard of your car!); or the light is transmitted. With most surfaces, it is some combination of the three. With a filter, we want to transmit as much of the light as possible, but physics is going to prevent 100% transmission. When the light is transmitted, it usually isn't at the same angle upon which it entered. On big reason for using a lens is to focus the light. There are exponentially more ways for the light to be out of focus than in focus. The use of any filter simply means that the light is behaving differently prior to hitting the front element of the lens.

All of that said, if you feel more comfortable using a filter than not, great! It's a personal choice. However, if you run into an issue with a soft image, or ghosting, or back-fcousing or what-have-you, the filter will always be suspect. If I gave two photographers the same exact camera and lens combo only one lens had a filter on it and then had them go shoot a wedding, I would be able to tell you which lens had the filter on it as I went through and edited each wedding. It wouldn't be a  glaring difference, but one set of images will have more flare & more focus issues. I have a friend who shoots AAA baseball (usually from the third base line). He had an issue with maybe one out of ten images not being tack sharp. He sent his lens into Nikon twice and twice Nikon sent it back saying the lens was fine. When I asked if he was using a filter, he was. I asked him not to use it for one game. When he did, every image was what he expected. Now he still uses a filter because someone sliding into third base might kick something up into the lens!

In the end, you just have to accept that the use of any filter degrades image quality. It's just a matter of whether it's perceived or not. And I suppose that you have to accept that a cameras auto-focus system isn't infallible, it can get it wrong. Putting another element, no matter how good that element is, isn't going to help the camera's AF system!

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