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07-28-2012, 04:46 PM   #1
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50 mm f1.8 with 49 mm filters

Ok, so after I got the new 50 mm f1.8, I quickly became curious about trying my current 49 mm filters with it. Although it has a 52 mm filter thread, there is quite a distance from the thread diameter to the optical elements, so I thought I would test it out. I took some photos at the widest aperture, 1 - with the naked lens; then 2- with just the step down ring 52-49 mm and no filter; then 3- step down ring with a 49 mm UV filter; then 4 - the lens with a 52 mm UV filter; then 5 - with the step down ring again but with a CP filter. I focussed on a distant tree branch and then just pointed to the clear blue sky with the same exposure settings all across the photos, all at f1.8 since I believe the widest aperture challenges the lens vignetting characteristics the most. I thought since I was interested in this, others may be too. I don't mean to start a debate about the use of filters, just to get a quick look at the vignetting characteristics. This was of interest to me since most of my lenses are 49 mm threads, and it sure is handy packing one set of filters be they UV, CP or NDs. I think the results are quite interesting and I find that there is very little vignetting with the step down ring and the UV filters (ND would be the same size). In fact it looks like the vignetting with the ring and 49 filter is no worse than with the 52 mm UV filter and no ring. Although the "naked" lens is the best, there is not that much difference with the uv filters and I was quite surprised to find that the 52 mm filter is no better (maybe a little worse?) than the 49 mm filter with the step down ring. Of course the CP filter is the worst due to the size of the filter ring, standard Hoya CP - not a slim version - but it did clean up nicely with a little devignette in Aperture (not shown). I don't think I would be using a CP filter much with this lens anyway, but you never know. All these were taken with the K5. I think that your current 49 mm filters with a 52-49 mm step down ring are useable on this lens if you wish. After I was all done, I definitely felt like I had too much time on my hands today!!!

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Last edited by fotogaffer; 07-28-2012 at 05:18 PM.
07-28-2012, 05:10 PM   #2
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Looks like you are getting a very, very slight vignetting, but not enough to be concerned about. I'd use the filter if I thought that I needed it. Hardly enough even be noticed under all but the most demanding of circumstances. It's not something I'd be concerned with.
07-28-2012, 07:24 PM   #3
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The slight vignetting (if there is any) is likely due to the lens being at f/1.8. To check vignetting due to the step down ring, filter, and/or hood, focus at infinity and use the smallest aperture (f/16 or f/22).
07-29-2012, 01:18 AM   #4
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The vignetting is definitely not because of the filter, it is a full frame lens used on crop sensor, so you can put even smaller filter without affecting the picture.

07-29-2012, 06:57 AM   #5
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QuoteOriginally posted by dms Quote
The slight vignetting (if there is any) is likely due to the lens being at f/1.8. To check vignetting due to the step down ring, filter, and/or hood, focus at infinity and use the smallest aperture (f/16 or f/22).
dms, thanks for your reply. I understand about testing at a focus of infinity, and for practical purposes, I felt the AF at a distance was close enough to infinity for the purposes of the test. I was initially confused by your suggestion of stopping down the lens though. So, that got me digging into this a little deeper. I now appreciate the difference between optical and mechanical vignetting. I think what I said initially, about opening the lens up to the widest aperture to push the vignetting characteristics applies to optical vignetting. Also, I think you are correct in pointing out that to test mechanical vignetting the lens should be stopped down. So thanks for the nudge in the right direction. Since the purpose of all this was to be sure there would be no mechanical vignetting from putting narrower threaded filters and accessories on the factory threads, I shot a couple of more photos with the lens and the 49 mm filters. I only took 2 photos - the worst case scenarios - with the step down ring and UV filter in the first photo, and the step down ring and the CP filter in the second photo. These were taken on the K5 manual focus at infinity at f22. Allowing for the uneven sky (especially with the CP filter, very sunny today), there is still no mechanical vignetting so I think the narrower filters can be used in the new DA 50 mm f1.8 if you wish. Thanks again for pointing this out.
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Last edited by fotogaffer; 07-29-2012 at 08:08 AM.
07-29-2012, 07:13 AM   #6
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QuoteOriginally posted by simbon4o Quote
The vignetting is definitely not because of the filter, it is a full frame lens used on crop sensor, so you can put even smaller filter without affecting the picture.
Thanks for your reply simbon4o. Myself, I'm confused about the specs on this lens and whether it is full frame compatible or not. All it says on the Pentax site is that it was developed specifically for Pentax digital SLRs. The "other forum" release announcement on this lens was that it was designed exclusively for Pentax K-mount interchangeable lens digital cameras. The only reference I can find to full frame is on the B&H site where it is noted under format compatibility for 35 mm full frame and DSLR APS-C. Even this site lists it in the lens database description as designed for the APS-C sensor format. Do you have any other information about it being full frame compatible?
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