I took this image a couple of weeks ago:



I really like the image, but was disappointed that it was rather soft away from the centre. I had not noticed this problem before, mainly because most of my photography is at weddings, where edge sharpness isn't really an issue. After some research and discussion on the landscape forum, I have discovered the cause.

Firstly, some data:

Camera: 5D2
Lens: 24mm f/1.4 II (Hoya Pro1 Digital UV filter attached)
ISO 800
Shutter 1s
aperture 5.0

One potential cause was the filter, so I did some tests this morning, with and without the filter:

Filter On



Filter Off



Looking at the images it is clear that the filter is affecting the sharpness quite a lot. Even the centre is a bit softer, but as you look nearer the edges of the image the effect is very severe.

Conclusion: If sharpness is an important thing you are looking for in an image, don't use UV filters, especially on wide angle lenses.

Hi Stuart, first I congratulate you on actually testing the lens and filter to determine suitability. In general, wide angle primes and some zooms "are" sometimes less likely candidates for a protective filter when absolute sharpness is important. I believe that the reason is the rather strong angles that light has to enter the lens, the bulbous curved front element of the lens and the differential angles between those surfaces (i.e. the flat plane of the filter vs the curved front element.)

However, in testing my own lenses there is not a "direct" correlation, as in some lenses are inexplicably less affected.

It's important to test each lens and filter combination as a set to understand their particular interaction.

In many (many) years of professional photo photography, I decided that:

1) A protective front filter is normally a good thing, and many lenses are perfectly fine to leave the front filter on continuously.

2) Testing shows which lens/protective filter combinations are a problem, and the extent of the problem. There is no valid generalization which applies for all lens/filter combinations.

3) Knowing which lens/filter combinations cause problems, and knowing when they cause the greatest problems (through testing), allows knowing when it's best to remove the protective filter.

4) Avoiding protective front filters altogether exposes the front element of the lens to scratches and damage, and exposes the front filter threads to damage. A protective front filter helps to avoid or mitigate some of this damage. (Yes, a lens hood also provides a measure of physical protection and yes, I generally use both.)

5) In the summertime, airborne pollen, which can be very sticky, can also attach without your knowledge. The pollen can travel in "clouds" and cause problems you may only discover later.

6) At events, people can cough or sneeze onto the lens without warning, and leave a sticky mucous mess. (Yes, that really does happen.)

7) Protective filters are much, much easier to clean than the front element of a lens.

8) Repeatedly cleaning the front surface of the lens can cause micro-scratches, invisible to the naked eye, but which do cause dispersion softness in the images produced by the affected lens.

While I hold all of the above to be factual, it still presents the opportunity for individual choice, and situational options.

To the OP... Have you thought of trying out a better UV filter like a B+W?

Unless I need a filter for an effect, I've moved to the non-UV filter camp. The front element is very hard and with lens hood (which is always on), the chances of damage are small.

Originally Posted by adbsgicom Unless I need a filter for an effect, I've moved to the non-UV filter camp. The front element is very hard and with lens hood (which is always on), the chances of damage are small.

I would, except I mostly shoot motorsports. The chances of damage are much higher and have replaced a few UV filters due to flying objects among other things. lol

Here's a new image without the filter (I also used a slower shutter speed and less ISO):



Big improvement IMHO.

Should say that I used the water from the original shot as the reflections were much less impressive tonight. Should also mention that I used Photomatix on this one for the buildings/sky.

I've been shooting with and without filters for many years now, and more importantly I have been working full-time as a post-producer, editing tens of thousands of images per week for the past year.

These images look like field curvature. Canon wide angle lenses have a tendency to be pretty nasty with this; even when stopping down you may not get good sharpness at the edges of the frame, even if the center gets all the DOF that it ought to according to regular hyperfocal rules.

Specifically, it looks like the left hand side of your image is suffering from severe field curvature, and the right hand side at least a little bit.

This may be caused by the filter; although I've never had any experience with a UV filter being the cause. In my professional opinion, the UV filter isn't the cause.

Work on studying the hyperfocal characteristics of each wide angle lens you own. Oh, actually start by performing similar on/off filter tests with longer lenses. Then, get into the DOF characteristics of your 24 L... Often times, you have to focus at infinity and then stop down, if you want the edges to be in focus as well. This is especially common for the 24 L, and the 16-35 L mk2...

=Matt=

It looks like your latest image is showing the exif from water layer, not the new shot. What settings did you use for the latest shot? It does look sharper.

Originally Posted by kolibri It looks like your latest image is showing the exif from water layer, not the new shot. What settings did you use for the latest shot? It does look sharper.

The new image is a HDR created from 3 images (apart from the water). But the middle image has the following settings:

ISO 320
shutter 1.6
f/5.6
Same 24mm lens with no filter
Same camera
Same Tripod

The other 2 images in the HDR simply had longer and shorter shutter times, but were the same otherwise.

The lower ISO definitely helped a lot, and the new image has much less noise. But the extra sharpness is mainly due to the filter not being on.

Originally Posted by Matthew Saville I've been shooting with and without filters for many years now, and more importantly I have been working full-time as a post-producer, editing tens of thousands of images per week for the past year. These images look like field curvature. Canon wide angle lenses have a tendency to be pretty nasty with this; even when stopping down you may not get good sharpness at the edges of the frame, even if the center gets all the DOF that it ought to according to regular hyperfocal rules. Specifically, it looks like the left hand side of your image is suffering from severe field curvature, and the right hand side at least a little bit. This may be caused by the filter; although I've never had any experience with a UV filter being the cause. In my professional opinion, the UV filter isn't the cause. Work on studying the hyperfocal characteristics of each wide angle lens you own. Oh, actually start by performing similar on/off filter tests with longer lenses. Then, get into the DOF characteristics of your 24 L... Often times, you have to focus at infinity and then stop down, if you want the edges to be in focus as well. This is especially common for the 24 L, and the 16-35 L mk2... =Matt=

Hi Matt,

Thanks for your thoughts. I must admit, I had to google field curvature, and I'm still somewhat baffled

To me it's quite simple, filter on = soft image (especially away from the centre). Filter off = sharp image.

If you look at the images of the back wall of my house above. You will see that the image without the filter is sharper throughout, but much sharper towards the edges. I'm not saying for a moment that the 24mm 1.4 II is perfect, but that this particular lens filter (not a cheap one) on this particular lens has a major detrimental effect.

I would imagine that this effect is greater on wide angle lenses, but for me at least, it was convincing enough to remove the filters from all my lenses before shooting a wedding party last night.

Before doing the test I was pretty convinced that my lens needed a service to sort out the problem, so I was quite surprised it was so easy to fix.

I'm not suggesting that everybody gets rid of their filters right now, but for those that haven't run this type of test, maybe give it a try and see what you find?

I still saw no mention of what filter was in use. JWW's point still stands to me, as not all filters are created equally. Second, why are you using a UV filter in the first place? With film, that made sense, but I am not aware of digital sensors being sensitive to UV. If you want a protective filter, get one. That is not the purpose of a UV filter.

I use protective filters on most of my big glass. I prefer the Hoya HD for reasons that will be obvious to those familiar with them. But I don't have a UV filter on any glass I own if I am shooting digital. When I shoot film, it's a different story.

Originally Posted by perroneford I still saw no mention of what filter was in use. JWW's point still stands to me, as not all filters are created equally. Second, why are you using a UV filter in the first place? With film, that made sense, but I am not aware of digital sensors being sensitive to UV. If you want a protective filter, get one. That is not the purpose of a UV filter. I use protective filters on most of my big glass. I prefer the Hoya HD for reasons that will be obvious to those familiar with them. But I don't have a UV filter on any glass I own if I am shooting digital. When I shoot film, it's a different story.

If you look at my original post I did state that it was a Hoya Pro1 Digital Filter (checking now, it was actually a 'protector' filter, not a UV filter). It was in place purely to protect the lens, as recommended by the store. I think there are more expensive ones available, but it's hardly cheap.

Obviously I would have removed the filter if I had thought it could cause this sort of problem, but until now, I always thought any effect would be negligible.

Originally Posted by Stuart-M If you look at my original post I did state that it was a Hoya Pro1 Digital Filter (checking now, it was actually a 'protector' filter, not a UV filter). It was in place purely to protect the lens, as recommended by the store. I think there are more expensive ones available, but it's hardly cheap. Obviously I would have removed the filter if I had thought it could cause this sort of problem, but until now, I always thought any effect would be negligible.

My apologies sir, I missed it. While you do have a quality filter, I would offer that perhaps it's not flat for some reason.