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Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 10
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 09:14 am: | 
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High all and happy holidays!
I have a focusing screen question that is slightly off topic but I am guessing that many of you will have experience with this, so here goes...
It is very common for dSLR users that use old MF lenses to replace the stock screen with split prism, micro prism or others for easier focusing. The two common techniques are to buy professionally cut-down Canon EC-L (or a or b) or Nikon K3 screens.
My question is to those of you who have used K3 screens, either on a dSLR or on older film cameras. I am perticularially curious as to blackout of the prism as I have several long lenses that are slower and more prone to blackout. I read in one place that the K3 should not blackout with lenses faster than about f/8. Can anyone confirm or check this for me?
Sorry to be slightly off topic but this forum just seems like such a good place to ask. Best wishes for a New Year!
Dennis |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 753
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 12:18 pm: |
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All split image prisms will black out when the aperture drops below the design limit of the screen - this is usually around f/5.6 or f/8. I can't speak specifically for the Nikon K3 screen. Some screens are designed with a shallower prism wedge to delay blackout to a smaller aperture ... the price of this is a less precise split image, so it is done only when considered necessary for use with slower lenses. |
Glenn
Tinkerer
Username: Glenn
Post Number: 474
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 02:12 pm: |
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Dennis,
I think you have been searching in most of the sites I personally know about - seen your post on one. Some of these sites seem to be inhabited by brain dead idiots, so I would ignore what you read. The telling word in your quoted source is 'about', from memory some long lenses in the f5.6 range caused some 'shading'. However by viewing properly down the middle of the optical axis reduced this to the minimum.
You should also note that there are some very badly formed aftermarket 'K3' screens advertised these days. In general terms, the split prism circle is not centered properly on these screens. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 11
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 02:12 pm: |
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Thanks Rick. To be honest, any aftermarket or replacement screen is better than the stock screen on the D40. Many people replace with $25 Chinese knockoffs. These have the lowest quality of all screens used for replacement but are about twice as good as the stock screen for manual focusing.
My research indicates that the K3 was used on F3Ma film cameras. Anybody shoot with one of these?
The difference between the K3 and the Canon is the K3 uses a horizontal split prism in the center surrounded by a microprism ring. The canon uses a four way split only in the middle. Having never really used either type, I am sure I would quickly adapt to either version.
The biggest question for me is which screen delays blackout longer? A slight loss in precision is OK with me as a lot of focusing will be done with the matte outer portion. This part of either screen is reportedly way better than the stock screen so any change is a good change there. Still, I do use f/5.6 lenses a lot, so blackout is a major concern.
Thanks! |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 12
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 02:36 pm: |
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Thank you Glenn for your input. I would be buying this from focusingscreen.com which, by all accounts, is second best only to Katzeye, but the price is better. I would not be buying from any of the countless ebay knockoff places with really cheap prices.
Are you saying that your memory of using a K3 screen is some shading that is fixed by proper eye placement?
Do you have any experience with the split vs microprism and which experiences blackout when?
Thanks! |
Glenn
Tinkerer
Username: Glenn
Post Number: 475
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 02:38 pm: |
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Dennis,
Your last post appeared as I was writing mine. I prefer the Canon screen, I have never liked nor used 'in anger' a microprism ring or microprism centre screen. You may find that a bit of experimentation will help you decide, the cost of purchasing a couple of screens for modification is not out of this world. No different really to the T90 owner purchasing two or three screens, to get the best out of his range of lenses. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 13
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 04:35 pm: |
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Thanks again Glenn.
May I ask what you mean by "in anger"? As to experimentation, unfortunately at this time I am limited. One screen will be about $75 (unless I buy the original screen locally and cut it down myself). Two screens are out of the question now, hence my plethora of questions and reading.
Dennis |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 754
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 27, 2008 - 06:59 pm: |
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Typically, on any given screen, the split image and microprism will both black out at the same point, as the entire screen will be made to the same specification. Eye placement is critical when using a split image screen, especially as you approach its design limit. A screen that blacks out at f/8 might be made to black out much sooner by off-center eye placement, so if you see it begin to black out try moving your eye to see if it improves.
In my Canon XTi (400D) I use a Virtual Village screen - one of the "$25 Chinese knockoffs", though I think it was $35. The split image prism in this screen is usable at f/8 (checked in both a 50/3.5 OM Macro lens and a 400/6.3 Spiratone), but not at f/11. I did have to do some shimming to make the focus perfect with this screen, a process which involved installing and removing the screen several times. I can't speak for the D40, but in the XTi the screen changing procedure is not such as to encourage swapping among multiple screens once you find an arrangement that you can live with. I have settled on the VV screen as much better than the original factory screen for my purposes (but I'm sure it will not be to everyone's tastes) |
Glenn
Tinkerer
Username: Glenn
Post Number: 477
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 28, 2008 - 10:54 am: |
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Dennis,
By 'in anger' I mean in my job as a professional photographer, but obviously I have 'looked through' plenty of different screen types. Rick's last post says it all; eye placement can be very critical to get the most out of some screens as you approach the blackout point, and ease of screen replacement is a major factor in getting choice correct first time - if one is not very happy with diy camera bashing. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 14
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 28, 2008 - 12:01 pm: |
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Thanks again Rick and Glenn. Searching around the net I found two things. First, http://yipal.blogspot.com/2008/10/fitting-k3-focus-screen-to-d70s.html is a blog post from someone who cut down their own K3 screen for a D70. Coincidently, I found a cheap K3 screen at Adorama for $13 that is supposed to be in excellent condition. having dealt with Adorama several times before, I know if there is a problem with the screen it can go back. I also figure for $13 cutting down the screen myself will be no problem. I have lots of tools and experience with them so no worries there. Second, if I decide I don't like the K3, I can still buy a EC-L and cut it down. If that happened, the price would be less than buying one screen from focusingscreen.com so I will not be "out" anything.
Oh- I love DIY as I'm poor but also it's a lot more fun and this is a hobby for me.
I'll update w/my experience when I'm all done.
Thanks again all! |
Glenn
Tinkerer
Username: Glenn
Post Number: 479
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 28, 2008 - 04:57 pm: |
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Dennis,
Found that blog a while ago - sound instructions and method - so you should have no problems. Coincidently; I nearly suggested earlier that you pick up a cheap second hand screen to play with, even if not perfect you will see what actually happens. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 15
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 28, 2008 - 07:30 pm: |
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Thanks again.
Near as I can tell from lots of internet searching, all the D40/50/60/70 screens are exactly the same size. I have asked some forum acquaintances to double check this for me.
BTW, does anyone know what the EC-L is made of? I assume all newer screens are plastic but just trying to double check. |
Glenn
Tinkerer
Username: Glenn
Post Number: 480
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 29, 2008 - 08:34 am: |
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All modern fresnel screens are acrylic formulations. You will find that good quality wet & dry abrasive sheets - use dry - are the best 'tool' for trimming down to final size. I am fortunate to have a miniature 50mm disc table sander - very cheap micro tools item, runs off 12v - but it is very good for roughing the screens down to size, prior to the final 'wet & dry' trim. You should use a very fine saw - if you use one, however many people score the screen and then break off the excess over a straight edge. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 757
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 31, 2008 - 06:22 am: |
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It depends on how much you need to remove... for small adjustments I lay a flat file on the bench top and slide the screen on it edgewise, rotating 180 degrees periodically and checking often with a caliper to make sure it's staying square as I go. A power sander is a great tool but you have to use care not to go too fast as the heat will distort the optics of the screen near the edge. Scoring and snapping is fast for removing large amounts of material, but unless you have machined fixtures it can be very hard to get the score line exactly where it needs to be - plus the risk of the break failing to follow the score line, resulting in a destroyed screen. This depends very much on the quality of the molding operation when the screen was made, something that can be difficult or impossible to judge until it's too late. A saw is safer, but with a similar risk of destroying the screen if you don't have solid, accurate fixtures and the saw slips from the intended position. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 17
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 31, 2008 - 09:29 am: |
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Thanks Glenn and Rick for the input and sizing tips. I'm not to worried about trimming the screen as I've been a professional furniture maker for a long time and much of my work involved hand cut joinery, inlays, etc. My biggest thought is about protecting the screen during the process. I see people mention 3M magic tape so maybe I'll give that a try, testing it first on the parts that will get cut off anyway.
Best,
Dennis |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 759
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 31, 2008 - 03:51 pm: |
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For a plastic screen, I score it heavily and grip the edge up to the score line in a parallel jaw vise (like the sort you'd use on a drill press or milling machine table). I have a metal plate that I have covered on one side with velvet-flocked paper, which I use to apply uniform pressure against the screen without bending or scratching it. This works well if the screen is well molded, there is still some risk of a bad break if it has a lot of molding stress in it. I have found, for example, that Mamiya RB67 screens break well every time, but Mamiya RZ67 screens do not. |
