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Old Nov 18, 2009, 2:43 PM   #1
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Default Manual focusing with large aperture lenses

I recently picked up a Pentax-A 50mm f1.4 lens and while haveing no problems at f2.8 and up - when i start opening it any wider the autofocus indicators frankly aren't working very well and are saying its in focus when its not

oddly in the viewfinder the dof field is barely noticeable when looking through it but press the shutter release and bang - blade thin dof and usually not where i want it

now i've ordered a O-ME53 1.2 magnifying eye-piece (not arrived yet sadly) hoping this will help somewhat but does anyone have any tips for getting accurate focus with this lens at wide apertures? will more light on the subject while focusing help the autofocus sensors tell me when its in focus? (manual focus lens) or should i stop it down to focus before opening the aperture again?
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Old Nov 18, 2009, 2:52 PM   #2
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You may want to consider a split prism type focusing screen if you do a lot of manual focus work, versus trying to rely on the AF sensors and making adjustments for a specific lens. Here's one example of a focusing screen available for the K7.

http://www.katzeyeoptics.com/item--P...--prod_K7.html
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Old Nov 18, 2009, 3:11 PM   #3
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expensive but interesting - would cost as much as my 50mm lens =)

how would something like this effect normal operation of the camera though?
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Old Nov 18, 2009, 3:13 PM   #4
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Every once in a while, you'll see someone mention metering problems with a different focus screen (i.e., you may need to use a small amount of exposure compensation to account for differences in transmission quality between focusing screens). But, that's going to be more screen/camera specific (how much the focusing screen differs between the one that the camera manufacturer installed). Other camera operation should not be impacted (no change in Autofocus operation, etc.).
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Old Nov 18, 2009, 3:48 PM   #5
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hmmmmm well sounds like might be worth considering then if the effects are negligible for normal autofocus shooting but make focusing manually easier

argh more money to find - this is an expensive hobby =)
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Old Nov 18, 2009, 5:07 PM   #6
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Quote:
Originally Posted by John.Pattullo View Post
expensive but interesting - would cost as much as my 50mm lens =)

how would something like this effect normal operation of the camera though?
Hi John,

There are a number of other sources, depending on the model, though Katzeye makes a great focusing screen, IMO. Here are links to a couple that I've found:

http://haodascreen.com/sifs.aspx

http://www.focusingscreen.com/index....7d2fcb9f9e77d2

. . . and a number of ebay sources, mostly in China or Korea. I also have a diagonal split screen that I got from JNFinance on ebay.

Katzeye used to state that their split screen with microprism collar could throw spot metering off as much as a couple of stops, and that was my experience with one on a DS. I believe that all of them will effect metering to some extent, but if you don't use spot metering much, it's really not much of an issue as there was little or no effect on center weighted or matrix metering, and you can always use Ev compensation to make up the difference, since the metering would be consistent.

I would tend to prefer the Katzeye screens (though they are pricey) because they have been designed to be less likely to "blackout" one side of the split screen at smaller apertures, which can be a problem when using slower lenses or TCs which reduce light transmission. The blackouts are very dependent on the precise angle of your eye to the VF, so this can be worked around, but I rely on "slow" lens/TC combos like the FA*300/4.5+1.7x AFA (f7.7) and the FA*300/2.8+1.4x+1.7x AFA (f6.7) much of the time, which makes split screens less practical for me.

My main problem with the split screens that I've seen is that the areas of the ss/mpc is just too large and obscures too much of the focusing screen as they are mostly made from 35mm SLR screens, and aren't proportional to APS-C viewfinders, but YMMV. Focusingscreen.com has a wide range of possible focusing aids, and I'm tempted to have them make one with just a small microprism central spot like the F6J or ECA, but I'm still on the fence. . .

If you decide that you want to try one, I'd suggest researching some of the less expensive alternatives, then if you find this works for you, consider one of the better quality screens as a permanent replacement.

Scott
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Old Nov 18, 2009, 5:34 PM   #7
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Blackouts? that doesn't sound good!! From what you are saying only an issue with smaller apertures but i have a number of lenses that aren't overly fast so could be an problem.

on the plus side i have discovered a work around for the problem - at least for now...
Liveview - gives accurate representation of the dof and can even zoom in and check focus - obviously no autofocus sensors to help but certainly for still life shots on a tripod think i can get the focus accuracy i want from there - for non tripod shots hmmmm well dont like shooting from liveview handheld but in the short term will suffice until i can descide if these focus screens are the way to go or not

just curious how easy are they to change? if i was planning on doing some low light manual focus shots would it be feasible to change the focus screen for specific session?
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Old Nov 18, 2009, 7:25 PM   #8
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Hi John,

I mainly shoot birds, and they're uncooperative more often than not, so speed is always a factor for me. Having to reposition my eye, even though it doesn't take a lot of time, is a frustration that I'd rather do without, so I'm not really an average shooter when it comes to preferences, and I'd consider my opinions with that in mind.

As far as ease of changing -- I wouldn't consider it practical to change focusing screens very often. They are very easy to scratch, and even the cheapest ones aren't what I'd consider throwaways. I managed to scratch my original DS screen, and I was very careful with it -- Even though I'm pretty mechanically inclined, and have pretty steady hands, I consider this a fairly scary operation considering the very fragile surfaces (mirror and focusing screen) that are right there.

That being said, depending on the particular camera, they're not really hard to change, but some bodies require shims to position them properly. Katzeye has downloadable PDF installation guides on their site for each of the cameras that they support, complete with good pics. It would be a good idea to go over the one for your model before deciding to purchase -- just to see if you'd be willing to tackle the job.

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Old Nov 18, 2009, 7:43 PM   #9
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hmmm i think you've just put me off them - i'm getting more and more interested in shooting wildlife photography so i wouldn't want to have this in all the time since i cant afford fast long glass so blackouts would be a problem

with changing them being such a delicate operation seems that switching them isn't a viable option either - guess will have to stick with liveview for now and hope the o-me53 is enough when it arrives

thanks for the advice though - if i start shooting manual more with fast glass then will reconsider
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Old Nov 29, 2009, 9:08 AM   #10
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New to this forum but I have a K2000 with a split image screen, I wouldn't shoot manual focus lenses without it, Not to bad to install, Only a problem with blackout is on my 300mm f5, only 1/2 of the split image blacks out the rest of the screen still works
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