TO FILTER OR NOT TO FILTER......THAT IS THE QUESTION.

I've just acquired a 150mm-500mm Sigma lens for wild life photography and, according to some arguments, using a UV [86mm] filter on it would spoil the image quality. The reasoning behind it is that you're sticking a flat piece of glass, of perhaps lesser optical quality, in front of a lens thereby possibly distorting the image by taking the "edge" off it, even though the filter is of a high standard. They also argue that a filter will also affect the light gathering ability affecting the apertures
The front lens of the Sigma is a big piece of glass with only a small rim protecting it and it could easily get scratched in general use. I have a lens hood that I can use with or without the filter, but the question is - will using a UV filter on such a large lens for long distance work degrade the image quality?? .

 

As an ex-member of No.III Squadron I forgive you, Tony, for having been a member of No.74 Squadron [didn't their Squadron colours include a Tiger?}

Yep, you're right, Tony, that was something I was thinking of while trying to learn how to use the lens to get the best shots.

You're right, Sheal, that was the argument put forward in the Pentax Forum where the two sides of thought were being discussed. I have to decide on the day whether to use a filter or not. I did some test shots in the garden of the bird feeders about 60 feet away and, to be honest, I failed to see any difference. But when it comes to scenic and really long shots there may be some aberrations to the quality of the image then.

 

Thanks Strongy, your opinion is greatly appreciated as your pics are always of such a high standard. What I'm concerned about is will a UV filter affect a large len's ability to pull detail sharpness when operating at a distance, if you get my meaning.

My knowledge of quilting is not a patch on yours, Mum

That's another thing I'm useless at, Dan, I get bored and start yawning at the mention of sport

 

That's along the lines of my thoughts, Dave. To me these long lenses are a technological marvel and when you see the optical standard of the actual glass I would be mortified if I scratched any lens through carelessness or accident , so I would have more peace of mind having the protection of a filter.
. I think I was more worried about losing quality of optical image when pushing the lens to the maximum where the resolution would tend towards being slightly soft anyway and having a filter glass in front might not help.

 

Thanks Steve, for your expert advice and offer, it really is much appreciated
I was intending to do just as you suggested.i.e: put the 500mm Sigma and K5 on a tripod take a few varied shots in reasonable light.
I have used the Spyderlenscal on the Bigmos and so far I haven't see anything to pick at. What started this line of thought was a discussion by the "pixel peeping people" on the Pentax Forum who are very knowledgeable but also "fanatical" in their schools of thought.
There are two opposing camps, those purists who wouldn't dream of putting a filter on their large lenses, and those who would for various reasons, such as protecting the big piece of glass on the front of their prized lens and/or for UV or polarising qualities. So having just got the Bigmos I asked the question on the forum "Should I use a filter or not?" and was surprised at the depth of division in opinion. Seeing how little rim protection there was I was a bit apprehensive [knowing how clumsy I can be] about even screwing on the metal lens hood frequently knowing how easy it is to scratch a lens. So that is why I asked the question on here. My personal opinion leans towards using the filter I have now just for protection reasons. I never keep a lens hood permanently on a lens, and the lenses used on a trip are always alr cleaned and wiped before putting them away.
I have in my initial tests, using the Bigmos, not seen any different Apertures or Shutter speeds being pulled up by the K5. But those shots were of targets only 60 feet away and my thoughts were what about when at the full 500mm length and long distance when image quality will already "suffer" with some softening?.
Obviously, size of aperture and ISO speed will also affect resolution and I've been constant with those on the initial tests. But I was wondering what other peoples thoughts were on why they used a filter [if they did}, and what their experiences were.
I also trying to do some tests with a 2X tele-converter on all of my lens, which results another GC member is also interested in. I know I will lose two stops and image quality using it so bright light will be needed and it will be interesting to see how good the images are on the 320mm lenses and the Bigmos. I've finished my contract phase in Manchester and I'm going to take a couple of months off before I start in Birmingham on the final phase so I should have time to do the tests properly.
If I can't come to a firm conclusion on the filter question, Steve, I will take up your kind offer and see if our thoughts coincide My computer is colour calibrated so I have a known scale there and images should be constant regarding that.