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04-29-2011, 07:55 AM   #1
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Will a cheap split prism not allow me higher apertures?

Read reviews of the jinfinance split prism and while they were good, positive reviews, one of them said they could on go to 5.6 aperture because things would get too dark otherwise.

Is this just the jinfinance? I'm thinking that one day I might want to get above/below 5.6.

04-29-2011, 08:05 AM   #2
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The aperture setting does not come into play through the viewfinder on your K-x unless you are shooting with an older lens that requires you to stop down manually, or if you are using extension tubes or a reverse mounted lens where the auto aperture cannot be leveraged.

The viewfinder displays wide open with modern 'auto' lenses in typical shooting modes, so not sure what the comment was referring to.
04-29-2011, 08:07 AM   #3
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Split screens tend to become dark around f5.6 or 8- that's just their nature (if the lens is stopped down). It shouldn't affect your photos, however.
04-29-2011, 08:13 AM   #4
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So far as I know high f-stop function for split prism focus detection is poor. The fundamental reason is that the split prism method relies on off-axis light rays. As f-stops increase off-axis light rays decrease hence the method can't work well.

It isn't so much a matter of low light levels at high f-stop; it is the f-stop itself that becomes the limiting factor. One can accommodate a little by centering your eye "just right" in the viewfinder but that only extends the range of f-stops a little in my experience.

Some prism designs may be a bit better than others but they are all fundamentally limited to low f-stop use.

This only applies to using manual lenses of course; if you use an automatic lens which focuses wide open then stops down at photo time there's little problem unless the lens is limited to above f:5.6-f:8. A DA 55-300mm lens works just fine at 300mm with my chinese split prism even though the minimum aperture is f:5.8.


Last edited by newarts; 04-29-2011 at 08:20 AM.
04-29-2011, 08:13 AM   #5
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Which is better for bad eyes (not blind but need glasses)? Microprism or split-prism?
04-29-2011, 08:16 AM   #6
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QuoteOriginally posted by crewl1 Quote
The aperture setting does not come into play through the viewfinder on your K-x unless you are shooting with an older lens that requires you to stop down manually, or if you are using extension tubes or a reverse mounted lens where the auto aperture cannot be leveraged.

The viewfinder displays wide open with modern 'auto' lenses in typical shooting modes, so not sure what the comment was referring to.
This is important, just wanted to make sure it didn't get lost. With all but M42/screwmount lenses, the lens stays wide open until you actually take the shot. Meaning even if you shoot at f/32, the viewfinder is still f/4 or whatever the maximum aperture of your lens is.

So it's only lenses with a *maximum* aperture of worse than f/5.6 (or whatever) that might give trouble.
04-29-2011, 08:18 AM   #7
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Don't worry. What happens is that when not enough light reaches the split prism, it turns black and therefore is useless. It happens with all of them; maybe some are better then others, I don't know. As said, it's the amount of light. So during the day the problem can happen at f/5.6 and in the evening even at f/1.4 (with no artificial light).

With your M42 lenses, you focus wide open, next close the aperture to 'any' setting you want, set shutter speed (if in M) and take the photo.
04-29-2011, 10:50 AM   #8
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QuoteOriginally posted by sterretje Quote
Don't worry. What happens is that when not enough light reaches the split prism, it turns black and therefore is useless. It happens with all of them; maybe some are better then others, I don't know. As said, it's the amount of light. So during the day the problem can happen at f/5.6 and in the evening even at f/1.4 (with no artificial light).

With your M42 lenses, you focus wide open, next close the aperture to 'any' setting you want, set shutter speed (if in M) and take the photo.
For my K20D, i had the Katzeye "brite treatment" applied when i ordered it. My impression is that it really does look brighter, because after my friend's wife looked into my Pentax camera, then her husband's Canon 40D, she asked why my camera was brighter. I've loaned my K20 out to a friend while his K20 is in the repair shop. When he gets his repaired camera back, i'll ask him to compare his K20 to mine to see if the brite finish works.

Its an expensive option, $50 or more, i'm not sure, but it seems to work. I'm not installing a split lens in my K5 because i'm starting to use and appreciate the spot metering for the kind of work i do now.

04-29-2011, 11:07 AM   #9
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QuoteOriginally posted by justtakingpics Quote
Which is better for bad eyes (not blind but need glasses)? Microprism or split-prism?
Don't think it has anything to do with eyes.

If the object being photographed has straight lines, split prism is better. If you focus on the texture of the object, micro prism is better.

But most screen has a split prism surrounded by a ring of micro prism
04-29-2011, 11:11 AM   #10
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all split image finders will go dark eventually, and as others have said, some sooner than others,

Additionally, the black out is very very sensitive to the allignment your eye has, so if you see that it has blacked out, moving slightly can reduce the problem.

It as marc clarified, only at issue with lenses that have a slow maximum aperture, or with manually activated apertures (M42 lenses and some K but not many mounts)

It also happens when it gets dark.

It is usually not an issue because the reason you are shooting manual focus lenses normally is because you have old fast primes. ( OK, I can already hear people screaming about their old F5.6 200 mm lenses etc)

The other poiint to note is that it will impact the camera's spot metering, AND may cause changes to the camera metering overall, causing potentially a minor EV shift when using "A" lenses or different metering than the stock screen.
04-29-2011, 01:04 PM   #11
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QuoteOriginally posted by SOldBear Quote
Don't think it has anything to do with eyes.

If the object being photographed has straight lines, split prism is better. If you focus on the texture of the object, micro prism is better.

But most screen has a split prism surrounded by a ring of micro prism
I don't understand how the microprism works. I thought it just went clear whenever something was in focus. I've never seen one but is it just the focus point, the whole screen, what does it look like? Maybe someone has a link. I couldn't find a picture.
04-29-2011, 01:56 PM   #12
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QuoteOriginally posted by justtakingpics Quote
I don't understand how the microprism works. I thought it just went clear whenever something was in focus. I've never seen one but is it just the focus point, the whole screen, what does it look like? Maybe someone has a link. I couldn't find a picture.
microprism focusing screens work much the same as split prism screens and rely on off-axis light rays. Since off-axis rays go away as the f-stop is increased they stop functioning properly. In practice one side of the prism(s) goes black.

QuoteQuote:
Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras.... the classical split image arrangement gives a direct, sensitive indication of imperfect focus of the camera by visible misalignment of the two adjacent image portions—in particular, by causing an apparent dislocation in the image of any line crossing from one side of the circle to the other. We have also seen that, if the aperture of the lens (in the viewing and focusing mode) is too small, the arrangement will not be usable.

{RE the} the microprism field system. Simplistically, imagine that we had a portion of the focusing screen not in ground glass form but rather covered with tiny prism pairs, conceptually like the split image system we discussed here at length. If the focus is correct, the two prisms of each tiny pair produce to the viewer two aligned virtual images of adjacent parts of the scene. The result is that a clear virtual image is seen over the entire prism field. ...
The prism's going black as f-stop is increased has nothing to do with the lighting level. Of course the viewfinder image gets dimmer as the available light gets dimmer but the way split prism focus detectors function has nothing to do with how bright the incident light is - if there is any light at all and the aperture is open they will work.

If a split prism works for you at f5.6 for a brightly lit scene it will work at f:5.6 in a dimly lit scene.

Last edited by newarts; 04-30-2011 at 04:40 AM.
04-29-2011, 01:57 PM - 1 Like   #13
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QuoteOriginally posted by justtakingpics Quote
I don't understand how the microprism works. I thought it just went clear whenever something was in focus. I've never seen one but is it just the focus point, the whole screen, what does it look like? Maybe someone has a link. I couldn't find a picture.
I don't have a photo. This is from the manual of the K1000. Hope it helps:

04-29-2011, 03:46 PM   #14
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Note that the standard Katzeye is a split prism in the center, surrounded by a "collar" of microprisms. You get three ways to focus because the rest of the screen is similar to the stock screen. I have never liked just microprisms as pictured above in the K1000, but I found they were useful in really low light. They also black out.

I rarely had blackout with my Katzeye, even with an f8 mirror lens. I saw it sometimes with a 400/5.6 when I used a 2X teleconverter, and definitely with the mirror lens and 2X teleconverter (f16). The whole viewfinder is pretty dark then. Still, it was workable.
04-30-2011, 12:43 AM   #15
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QuoteOriginally posted by justtakingpics Quote
Which is better for bad eyes (not blind but need glasses)? Microprism or split-prism?
I absolutely hate microprism focusing "aids". I've never had any success in trying to use them, and find they just get in the way. I don't believe they can ever be precise enough for accurate focus. I would go so far as to say that I can probably focus more easily and accurately with my K-m's stock screen than with my MX's microprisms!

On the other hand, the split-prisms seem to get the job done - but I think you have to be very careful to get the split images to line up exactly. The implication here is that, whichever replacement screen you fit, there'll be a distinct possibility that you'll need to re-shim. This will entail a lot of checking with a focus test chart.

As has been said, you will get darkening of the split-image for lenses with smaller maximum apertures, but, as long as you have your eye well centred, you'll be OK up to f4-5.6. In any case, the replacement screen's matte area will be significantly better for focusing than your DSLR's stock screen.
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