Can filters ruin the quality of an image?

Toddb,
We need Bob Gross to comment on this one. I, too, am in a quandary about increasing the shutter speed by the crop factor. My understanding of the 1.6 crop factor geometry is identicle to yours. But, the more I think of it, the increase may be correct. If you take two shots of the same subject from the same distance with the same focal length with the same shutter speed (equal to the 1/focal length) but with one camera with a full sized sensor and the other with a cam with the 1.6 crop factor and you then print a standard 4x6 (assuming the same amount of hand shake), what you have is an enlargement of the center of the image from the cam that had 1.6 factor. I've got to conclude that the hand shake would be more evident - its magnified. Right? If so, that may well support the shutter speed increase due to the crop factor - but is the increase necessarily "equal" to the crop factor? Not sure on that one either.

I think that is why I am confused. Would the shake be just as bad then if you took that same focal length on a 35mm and just cropped it to the 1.6 crop factor. So in a sense, that middle part of the 35mm film would not have a fast enough shutter speed according to the inverse rule. That's the part I can't seem to get out of my thoughts on this. The way I see it, and I may be wrong, that the image on say a 50mm lens is still showing 50mm perspective...just that part of that image is spilling outside of the sensor and the 10D is only capturing that center of the image. My logic tells me that if this where the case (and maybe what I just said is where my logic fails) that the inverse rule applies only to the lens's focal length and does not matter what the crop factor of the sensor in the camera would be.

The issue with minimum shutter speeds for handholding is based on the assumption that two pictures from cameras with different crop factors are both enlarged TO THE SAME SIZE. Since the smaller sensor (10D) must be enlarged more than the larger (say 1Ds) any blur caused by camera shake becomes more noticeable.

I have looked and cannot find an original explanation for the inverse focal length rule for 35mm photography, but I reason it is based on the same logic that gives us different Circles of Confusion for different degrees or enlargement and a 10x8 print is assumed. In fact, most of scrutinise our digital images at much greater enlargements: a 100% zoom on a 21" monitor equates to almost a 40x increase.

No question that using 1/40 with a 70-200 lens is too slow, although the inverse focal length rule isn't a rule, but a guideline and some poeple will be able to manage to get sharp pictures with slower speeds, while others will need a more conservative approach.

Regards,

So, say I know my 135mm lens just isn't enough reach...I know I'm going to crop it in post process....say my crop will then be something more like 5x instead of just 1.6x. So then if I'm using this guideline, I'll do the math of 135x5=675. So I better make sure my shutter speed is 1/675 or better to get safe results.



So I guess it's how much the original can be downsized to fit the output size which will make it more crisp looking because it's more condensed....compared to stretching the pixels out further to make the output size. I guess I can start to see it this way.

I'm so interested in this because I know allot of my out side shots tend to be a little soft...I know it has allot to do with how far away my subject is and I'm trying to figure out how to get the most out of the camera.

Wow, this has certainly caught fire. I'm begining to feel like I'm the one who's going off topic here by asking, how do I know if I have a bad CP? Its a COKIN 67mm made in France. I took some snaps yesterday at the same location of moving cars and I did get better results without the CP. But once again, this was just before sunset in a shady area.

Anyone know anything about the COKIN Filter? One guy here says he herad that they were not too good. Anyone else? This is my first L lens and I'd hate to bugger it up with a crappy filter.

On the point of the shutter speed, if I'm shooting at optimum speed, I get bugger all motion blur. Everything just freezes, especially the wheels. Because I'm shooting on public roads (a very quiet large roundabout) the cars can't really go much faster than 40 kp/h

I'm sorry, I didn't mean to drive this off topic.

I have a hoya CP and I like it quite a bit. I did however got it stuck on my 28-135IS....so I thought. I was trying to use rubber goves, anything to get it off. Turns out if I just turn the CP (the part that turns) to the right posistion, it unscrews very easily. Wierd eh?

Another note that I'm not possitive, but I think I remember reading that the CP makes the most difference when the sun is not at high noon....so like at a 45 degree angle. I have notices that turning the CP at high noon I can't see much difference in the sky, but say around 3 O'Clock or something, it's more noticable.

1. Make sure that your polarizer is a circular polarizer and not a linear polarizer.

2. Make sure that you know what to expect out of the exposure metering in your camera. For any given scene and it's normal metering, if you add a circular polarizer on, then the meter should shift by up to two stops. Depending on which exposure mode your camera is in, that could leave you in a bad place for shutter speed.

3. Circular polarizers get their maximum effect when the camera is pointed 90 degrees off from the sun direction. If I have a circular polarizer on at noon, and then I notice that I turn the ring and get no change in effect, then often I will remove the circular polarizer altogether so that I don't pay the two-stop penalty.

---Bob Gross---

Rebel, it would have helped a lot if you had told us from the beginning you were shooting moving cars, whereby you wanted to show car detail while still showing some motion. If this is the case, other than the f stops the filter costs you, the filter is secondary. Shooting moving cars handheld at low shutter speeds is a triple whammy.

Further, a CirPol needs some constants in the direction of light, as stated before, 90 degrees to the light source to be effective. On a cloudy or hazy day, the light is diffused in all directions making a CirPol practically useless, only taking away light that you need for higher shutter speeds and f stops..

Would you like to give us a more complete idea of what you are trying to do so that we can be more helpful?

Overcast conditions mean that there are no bright sun and dark shadows (and that might be good), but if you don't have enough light to get a proper exposure without dropping the shutter low, then why in hell aren't you using a good stiff tripod?

---Bob Gross---