Pentaprism and pentamirror
We
have discussed about the anatomy of a SLR camera. So we have to know about the
optical element called mentaprism.
A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry
beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image
through a right angle without inverting it (that is, without changing the
image's handedness) as an
ordinary right-angle prism or mirror would.
The reflections inside the
prism are not caused by total
internal reflection, since the beams are incident at an angle less than the critical angle (the minimum angle for total internal
reflection). Instead, the two faces are coated to provide mirror surfaces. The two opposite
transmitting faces are often coated with an anti reflection
coating to reduce spurious
reflections. The fifth face of the prism is not used optically but truncates
what would otherwise be an awkward angle joining the two mirrored faces.
A variant of this prism is the
roof pentaprism which is commonly
used in the viewfinder of single-lens
reflex cameras. The camera lens
renders an image that is both vertically and laterally reversed, and the reflex
mirror re-inverts it leaving an image laterally reversed. In this case, the
image needs to be reflected left-to-right as the prism transmits the image
formed on the camera’s focusing screen. This lateral
inversion is done by replacing one of the reflective faces of a normal pentaprism
with a "roof” section, with two additional surfaces angled towards each
other and meeting at 90°, which laterally reverses the image back to normal.
(A perspective drawing showing how a
roof pentaprism corrects a laterally reversed SLR image.)
Pentamirror
A pentamirror is an optical device used in
the viewfinder systems of various SLR cameras instead of the
pentaprism. It is used to reverse again the upside-down and laterally reversed
image coming from the reflex mirror.
Instead of the solid block of glass of the prism in
pentaprism system, here 3 mirrors are used to perform the same task. This is
cheaper and lighter, but generally produces a viewfinder image of lower quality
and brightness.
This optical device is often (more precisely) referred
to as roof pentamirror because of
the roof like ridge.
(A roof pentamirror contains 3 mirrors.)
Pentaprism vs pentamirror
As we mentioned before SLRs permit the photographer to see through the lens. Pentaprisms use a prism to redirect the light from the lens to the view finder vs the camera sensor. Pentaprisms are the highest quality viewfinders - they provide the highest resolution and most natural representation of the photo about to be taken. Pentaprisms are higher quality than penta-mirrors and thus are found largely in most professional DSLRs, and also in some entry level and mid range DSLRs (Nikon D90, Nikon D7000, Nikon D7100) they allow more light to pass through permitting a brighter view of the scene being photographed.
Most entry-level DSLRs use penta mirrors which, like pentaprisms permit the image being captured by the lens to be redirected (in this case by a series of mirrors) through the viewfinder. This permits full resolution and a natural immersive view of what you are taking a photograph of. Penta-mirrors provide the same function as a pentaprism by are typically constructed of plastic and cheaper to produce; on average they produce a darker image in the viewfinder.
When we compare these two….,
The pentamirror is around 1/4 of the weight of the pentamirror. (pentaprism about 43 grams.., pentamirror is about 13 grams ) Pentaprisms are made from high quality chunk of glass and pentamirrors are hollow constructions with some of the walls having reflective surfaces. so it becomes the camera heavier when it has a pentaprism instead of a pentamirror.
if someone ask the question.., “which one is better..?? "
Well ‘better’ is subjective. The image produced by a pentamirror tends to be a little darker — making manual focusing harder under some conditions.
but Autofocus has absolutely nothing to do with the pentamirror in the viewfinder. The autofocus runs through the reflex mirror (the one in the lens mount), and it will behave identically regardless if you have a pentamirror or a pentaprism in the viewfinder.
you can again have a look at the diagram of the optical components of a typical SLR camera shows how the light passes through the lens assembly
However, a pentamirror is lighter and cheaper. This makes the cameras that use them more affordable and lighter to carry. For me.., it’s not useful that much.
If we summarized all above..,
pentamirror =
- cheap to produce.
- slightly darker.
- better for auto focus.
pentaprism =
- bulky heavy and more expensive.
- brighter.
- easier to manual focus.
So which is better? That depends. If you can’t afford a DSLR that has a pentaprism but can afford one that uses a pentamirror, you could argue a pentamirror is better. If you plan to use manual focusing a lot then you may find it easier to focus using a pentaprism especially in dim
conditions. I personally like to focus manually. Because I need to capture the picture.., always when I half press the shutter release button.., myself is telling me.., that don’t let the camera to take that photograph.., make it by yourself. (letting the camera to take the photograph and by just pressing the button tagging my name on the photograph doesn't make me feel good. :/)
And there is a very important thing to know.., If the design of the pentamirror and its housing is such that moisture and/or dust can enter the chamber, then the mirrored surfaces may degrade over time and dust could become a permanent annoyance.
So it’s so important to look after our camera always as our eyes. (original Sinhala term
“as deka wage raekagannawa..”) if you really love photography, your most valuable non- living thing should be your camera or a lens. ( something like 50mm f1.8g)
When I was planning to buy my first DSLR…, I loved both Nikon D5200 and Nikon D7000. When I was comparing these two bodies, I knew that the d7000 has a pentaprism viewfinder when the D5200 has a pentamirror ( D90 has a pentaprism too.., D90 was a good deal…, but I thought that it's not a very good deal for its price. Cameralk price was about SL Rs98000/- with 18-105mm kit. ) The question that came to my mind is why they didn’t put a pentaprism in to d5200. The answer that I found was this.
“Pentamirrors are cheaper to make as they can be made from thin walls of plastic. Due to the extra cost in manufacture, pentaprisms tend to be found in higher end cameras. Putting a pentaprism in to a very cheap camera would probably add too much to the selling price and reduce its commercial viability.”
But again and again I worried about the features that D7000 had and D5200 doesn't.., then I have got the best answer for all. That was the exact answer i needed.
“ If you don’t have much money a camera with a pentamirror will take much better photos than not having a camera at all……!!”
SOURCES,
http://www.youtube.com/watch?v=jRnnPozwenk
http://en.wikipedia.org/wiki/Pentamirror
http://snapsort.com/learn/viewfinder/type
http://photography.incyder.info/pentaprism-vs-pentamirror-which-is-better/
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