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02-28-2013, 05:27 PM   #1
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difference in lenses

hello everyone,
first off, i feel so lost and confused. i can't seem to fathom the differences in many lenses such as the 15mm, 21mm, 35mm, 50mm, etc. i don't understand the different outcomes of the lenses. i'm sure this question is redundant, but i need clarification, thanks. next problem. i have a k30, with an 18-135 wr. it seems very practical for my usage, anything and everything. my first foray into photography was 30 plus years ago. i had a k1000 with only one lens, smc pentax-m 1:1.7 50mm. being a novice, i loved what the results were with that lens. i have since tried the lens on my k30. it recognizes when its in focus, a beep and the light, but it doesn't seem like it recognizes the different fstops, info is blank on setting. do i need to enable (disable) a certain setting?
thanks for listening,

02-28-2013, 05:37 PM   #2
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This should help:

Yes, you need to turn on a setting in the menu. It is "use aperture ring" or similar. Not sure what menu # it is on the k-30. Also you will need to use the camera in M mode and use stop down metering. All is explained in the article.
02-28-2013, 05:42 PM   #3
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thanks, i'll give that a try
02-28-2013, 05:47 PM   #4
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All SMC-M will show F - - as the camera cannot determine the aperture without an electronic connection. That's one of the reasons that SMC-A are more popular here and will cost more than an SMC-M despite giving equally nice images (they also allow more automation since the camera can adjust the aperture).

As to why so many lenses and especially primes, look at it this way. You can buy a 21mm f/3.2, 35mm f/2.4, and 50mm f/1.7 but it will cost more than the 18-55 f/3.5-5.6 kit lens. What you gain is much more light capability especially comparing the possibilities at 50mm, where you can use f/1.7 (or f/2.8, or f/4) against f/5.6. While the 21mm f/ratio looks similar to the 18-55 at 21mm, the prime lens will still give a better result because it was engineered to be great at 21mm. The zoom lens is optimized for the best results over the full range, so it will do better and worse with distortions and image quality - especially wide open.

It's a choice to carry a zoom or several primes, but those who carry primes made the decision that the extra burden will yield superior images. If convenience is most important, or your images are plenty good enough with the kit or another zoom lens, that will be your choice. I really liked the 18-135 for a while, now I have 15-24-40-70 and telephoto zooms and I like the results enough to feel quite happy about it!

02-28-2013, 06:35 PM   #5
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You sound like to need to pick a place to start learning again, if you take on everything at once you get buried. I am also not entirely certain how much you don't understand so I will not make assumptions.

Way impossible to explain everything in one post, but here are some basics:

Higher mm lens = zoom in, narrower visible angle
Lower mm lens = zoom out, wider visible angle
The mm size of a lens is its "focal length" the focal length of a lens also effects "depth of field" but don't think about that yet or your brain will break (getting to understand DOF sucks) more on DOF in a second.

The 1.7 on your 50mm is its maximum aperture (the widest the hole inside the lens between the shutter blades will go), lower number is wider opening letting more light in at once. If a lens has a range of apertures listed on it (like your 18-135 is 3.5 to 5.6) then it is variable aperture meaning it is F3.5 max when it is set to 18mm but it is only F5.6 max when zoomed to 135mm, don't try to understand why at this point but it allows them to make the lens at a much lower cost with much smaller lenses than if it were the faster F3.5 through the whole range.
All the effects of aperture are the topic of a book, not just one post so I will be half assed and skip that.

Depth of field (DOF) is the distance near to far that objects are actually in focus. when you focus on something you are moving the center of your DOF to that point, though if you wanted more in focus behind it you would put your object at the nearest part of your DOF or if you wanted more in focus in front then you would focus so that your target is at the farthest away part of your DOF.
A narrow DOF is nice if you want to take a picture of a person because the sharply focused person will stand out from the fuzzy background. If you wanted a picture of a person in front of scenery or something you would use a wide DOF.

Some of the things that change DOF:
-lens focal length (mm) for various reasons
-distance to target (farther away you are the more stuff in front of and behind your target will also be in focus)
-aperture (wider aperture (lower number) gives a narrower DOF while smaller aperture (higher number) gives much more front to back depth in focus)

I'm assuming you remember ISO (film speed) from your earlier days. Still the same only now you are turning the power up and down on the sensor in the camera instead of getting different film. Same as with the film higher ISO absorbs light faster though the higher the number ISO the worse the picture because you get a digitally noisier image the more you crank up the power on the sensor, newer cameras just like newer types of film are better about that.
So low ISO = better image quality
higher ISO = faster light absorbtion so you have more freedom to work in lower light.

Shutter speed used to be how long the camera kept the film exposed (not to be confused with the mirror flipping up or down which is unrelated). On a DSLR your sensor is always exposed (though the mirror will block your view of it normally) but the computer in the camera tells it how long to turn on and off based on what your shutter speed is set to.
Faster shutter speed freezes moving things, slower shutter speed blurs them.

Exposure is too complicated to explain also because it is effected by ISO, Aperture and Shutter Speed. Kinda like 3 legs of a stool its all a balancing act.

If you want to get a lot of DOF (much depth in focus) you could among other things close the aperture. But that lets less light in. You could then compensate by using a slower shutter speed to give the sensor more time to absorb the reduced amount of light. But if you have moving objects then they will blur (sometimes for a neat effect). So if you wanted to prevent blur and still get the extra front to back focus depth of field you would have to turn up the sensor (higher ISO) which is ok to a point but eventually costs you image quality (often written as IQ).
Its all a balance between depth of field (aperture) motion blur (shutter speed) and ISO (image quality) to get the right amount of light to the sensor (or film) to make the final picture correctly exposed.

Hmm, I just felt like rambling today I guess. I'm sure I missed a ton of crap and probably could have just provided a link to the same info somewhere.

EDIT: oh and if you want to make your life simpler stick with A lenses or newer instead of older M or K lenses because your camera will display the aperture info and light meter correctly when they are set to the A setting on the aperture dial on the lens. Plus you can also manually control the aperture with the dial on the camera instead of having to turn the physical dial on the lens (the camera will close it to what you set when you take the picture then open it again) or you can use the camera on full auto like your 18-135 (though without the auto focus).

Last edited by PPPPPP42; 02-28-2013 at 06:45 PM.
02-28-2013, 09:27 PM   #6
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Some years ago, a relative asked me to explain photography terminology to them, so I wrote a long email. Then I saw other people asking and I polished that message and passed it to them. And later, when I decided to start a blog, I used it for the first post:

Laur's photo blog: An introduction to photography

But this aside, the only way to understand all this is to go out and fiddle with your camera and pay attention to what results you get with what settings - there is no easy way around it.
02-28-2013, 09:50 PM - 3 Likes   #7
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02-28-2013, 10:14 PM   #8
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QuoteOriginally posted by jatrax Quote
This should help:

Yes, you need to turn on a setting in the menu. It is "use aperture ring" or similar. Not sure what menu # it is on the k-30. Also you will need to use the camera in M mode and use stop down metering. All is explained in the article.
That is not easy to find. Page 259 in the manual, [ Custom Setting 4], [23. Using Aperture Ring], [2 Enable].

The lens thing may not be obvious at first. You're just turning the zoom ring until the stuff you want is in the picture. All kinds of details are changing at the same time. You can ignore a lot of that stuff for now, concentrating on focus, exposure and framing. When those start to work for you, then work on other details, one or two at a time. It can help to look at many other people's shots for what you like and how that was accomplished.

02-28-2013, 11:20 PM   #9
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Zafar Iqbal ... What a useful demonstration. Very good.

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