focusing with p.series grey grad filter

[ianbullivant]

Hi again guys.
Just hoped you'd be able to help me with advise on whether I focus with filter attached or before I attach the filter when using cokin p.series grey grad and N.D filters as my first efforts were a bit ify to say the least.
I use a pentax k100d and cokin p attachment and filters. With my camera manual focus is a little tricky for me.
Cheers Big.E

[El_Sid]

I've found in the past that one filter is normally OK but that stacked filters can be an issue. I put this down to the increased number of reflecting surfaces reducing contrast enough to upset the AF slightly. I've also had similar issues when photographing into the sun.

I suspect it's a downside to slot in filters as, unlike screw-on types, light can in behind the filter and reflect back into the camera. I found it a particular problem with the Cokin IR filter where the resultant long exposures caused all sorts of flares and spots to appear on the image - a problem that has largely disapeared now I use a screw-on type...

One possibility with the focus issue is to use a Depth-of-Field calculator to create a table of hyperfocal distances for your lens(es) and use that to manually pre-set your focusing. Otherwise you may have to use the AF to focus then turn it off and slot in the filter(s) which is a bit of a fiddle but I presume we are talking tripods and landscape work here...

[ianbullivant]

Cheers for the advise. I will go out in the next few days and give this a go. (Yes we are talking tripods).It does'nt matterhow fiddly if I get sharper results. If this does'nt work do you think it may be mirror slap iv'e read about on the forum.
Thanks again Big.E

[PeteRob]

Cheers for the advise. I will go out in the next few days and give this a go. (Yes we are talking tripods).It does'nt matterhow fiddly if I get sharper results. If this does'nt work do you think it may be mirror slap iv'e read about on the forum.
Thanks again Big.E

Depends on the shutter speed- if speed is slow or fast then makes no odds - only if camera shakes over period of whole exposure so [others will know better] exposures of 1/15 - 1/2 s maybe?? are most vulnerable. I haven't noticed a difference in using mirror-lockup or not at 1/30 on a Bronica 6x6 and the mirror really has some momentum on one of those. I use lockup for slower speeds but have never compared. Use a cable release or self-timer as fingers are a worse culprit.

[El_Sid]

If this does'nt work do you think it may be mirror slap iv'e read about on the forum.

Possibly though it depends a lot on the lens and shutter speed in use. The only definite case of mirror slap that I can truly identify as such was a quite extreme case where I had a 400mm lens mounted, with extension tubes, on a Nikon D50. In this case I knew I was getting some mirror slap as I could actually see the front end of the lens wobble as the picture was taken and even then increasing the ISO to shorten shutter time helped significantly. That said I do tend to use mirror lock where available.

There can be other causes too. If the exposures are longish I've found that even fairly light breezes can vibrate the system enough to soften the image somewhat and the longer the lens the more likely this is.

Another softening effect is the aperture setting. Beyond a certain point reducing the aperture size can have an increasingly softening effect due to diffraction physically induced by the small size of the hole the light has to pass through. The effect is particularly problematical with very short focal lengths where the aperture can be an extremely small hole indeed. The effect is the same one as you get with a pinhole camera: great depth of field but an inherently soft image - although the end result is not likely to be as extreme as a true pinhole it can still be quite significant.

[ianbullivant]

Hi chaps
Been out today found a sheltered spot on Grimsby docks. Used the auto focus, clicked to manual and marked the lens.(sacrelige)I know. I attached the grey grad making sure my marks were in line and took the shot. Great sharpness compared to earlier efforts mentioned, but iffy exposure. I soon had that sorted out though.
So thanks for all the advise and feed back.
P.S I found F11 - F16 at ISO 200 worked best but did sacrifice some depth of field.
Once again cheers guys. Big.E