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01-07-2012, 07:39 AM   #1
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Riddle me this...50mm fixed vs stock 18-55

So.. today was the first day I took my 50mm prime out to the hockey game. Previously I've used my 70-300 tamron, and pre-game I sometimes use the stock 18-55.
One of my issues has always been light. The lights aren't great in the arena, and I usually have to crank the ISO up very high to get a non-blurry picture. 800 or more on my k-m.

Today, just for fun, I decided to shoot a bit with my 50mm prime. It's not really big enough to take the shots I like, but upon getting it going I noticed something.
My K-m was set for aperture priority
I set the lens to F8 and set up a shot and the automatically chosen shutter speed was 1/125 or 1/250
that's odd I thought. I took the shot and voila, the picture came out fine. I didn't recall being able to take that fast of a picture with my other lens.
So I put the 18-55 back on, and zoomed to 50mm. Give or take. F8, and took the same shot in the same spot (pre-game so nothing was changing or moving)
automatically chosen shutter speed? 1/30


I'm going to be honest here, I don't know a lot about lens theory. I'm a very interested hobbyist at best. Not a scholar or student of photography. Obviously the the 50mm prime is much better at accepting light. My questions are this:
1) Do prime lenses accept light better and are thus generally faster?
2) The prime is a manual focus, are manual focus lenses better at accepting light?
3) is there some larger problem with my stock lens? I'll be honest when I put it on, I felt it looked a little dark in comparison to the 50mm prime I'd just had on there. It doesn't look dirty

How can I use this to my advantage? Could I get a big zoom like my tamron 70-300 that might be manual, and cheaper, but faster because it takes more light?

01-07-2012, 07:45 AM   #2
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A prime lens when set to the same aperture are the stock 18-55 should not give you that much of a difference in shutter speed. At most 1 stop if that due to lens construction.

Is this said prime lens an K/M series? Perhaps you have a M series or K which does not allow for aperture priority. Which would shoot wide open all the time in that setting. An A series would be a different story.
01-07-2012, 07:52 AM   #3
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QuoteOriginally posted by epqwerty Quote
Perhaps you have a M series or K which does not allow for aperture priority. Which would shoot wide open all the time in that setting
I'm willing to bet that's what's going on.
01-07-2012, 07:56 AM   #4
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QuoteOriginally posted by epqwerty Quote
A prime lens when set to the same aperture are the stock 18-55 should not give you that much of a difference in shutter speed. At most 1 stop if that due to lens construction.

Is this said prime lens an K/M series? Perhaps you have a M series or K which does not allow for aperture priority. Which would shoot wide open all the time in that setting. An A series would be a different story.
I believe it's an M, specifically it's this:
HorusBennu 50mm F1.7 MC (PK) Lens Reviews - Miscellaneous Lenses - Pentax Lens Review Database

Both shots with the 50mm prime and the 18-55 set to 50mm seem to have come out more or less equal.
The 50mm prime, slightly soft, but that was a focus issue. It seems all the way to infinity seems to leave everything a touch blurry, if I come back 1/2 a mm on the ring things slightly distant focus much better.

Here are the two images. The one showing clock time 11:37 is the 18-55 stock, shot at 1/20/f8 and the other one is the 50mm prime listed above shot at 1/250 (manually set to f8) both at ISO200




01-07-2012, 08:07 AM   #5
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QuoteQuote:
I'm willing to bet that's what's going on.
Okay, be that as it may, the shot still turns out more or less the same with a much faster shutter speed, which is great for sports.
is this unique to prime lenses or all lenses? if I had an M series zoom would I notice the same thing?

I've just tested this again here in the office, a shot of the corner gives me the following results:
ISO-200, f8
50mm - 1/30 shutter speed
18-55 stock - 0.3" shutter speed
This is a significant difference.
Not only that I'm noticing some slight color changes between the lenses.



The stock lens is the slightly blurry one
01-07-2012, 08:22 AM   #6
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If you have the camera on aperture priority the 50 is probably shooting wide open and not f8. Set it on manual, ap on f8 and hit the green button, see what it gives.
01-07-2012, 08:36 AM   #7
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QuoteOriginally posted by bluestringer Quote
If you have the camera on aperture priority the 50 is probably shooting wide open and not f8. Set it on manual, ap on f8 and hit the green button, see what it gives.
The 50 has a manual aperture ring, it's set to f8
The KM doesn't have a green button
01-07-2012, 08:42 AM   #8
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Oh I see now..okay..the riddle is solved.. apparently I'm an idiot..
seems I wasn't really figure out how to use my wonderful new lens after all..even though I was pretty happy with the pictures I was getting
I seem to have misunderstood the instructions for using a manual lens on a KM. The metering button comes from pressing down the +/- button..

anyone know what f-stop it would have been defaulting to?

01-07-2012, 08:46 AM   #9
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f1.7 (wide open)
01-07-2012, 08:51 AM   #10
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QuoteOriginally posted by manacho2005 Quote
f1.7 (wide open)
How can it do that if it's a manual lens? Does it somehow limit it on the body side of things? That actually makes this lens a little bit less useful if I have to manually meter before each shot. Okay I guess if light conditions don't change, but I find the light in either end of the rink to differ slightly.
01-07-2012, 09:10 AM   #11
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sorry no choice you have to +/- each time if the lighting changes. However after you use it enough it becomes second nature and you can guess the appropriate shutter speed. As for the difference when you shoot wide open the depth of field is reduced. However in the case of your pictures you were at infinity and therefore not as noticeable. If you were to do somewhat close up shots the two lenses will become night and day in terms of isolating the subject.
01-07-2012, 09:14 AM - 1 Like   #12
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I believe this is the logic behind it.

K-mount lenses have that little paddle in the back that closes the aperture when a picture is taken. So, the lens aperture is wide open until the picture is taken. This allows the photographer to focus with the lens wide open instead of stopped down (this results in a brighter viewfinder and lower depth of field, resulting in the ability to focus more accurately). The result is that the camera meters with the lens wide open, and performs a simple calculation to find the correct exposure for the lens when stopped down.

On a manual lens, the camera does not know the aperture, and thus cannot do the calculation. Thus all of the metering is only valid when the lens is wide open. Because of this, the camera's logic says when you have a manual lens attached, and are in aperture priority, it will only shoot wide open no matter what f-stop the lens is set to (to get correct metering).

What I do not understand is why Pentax did not give the user the opportunity to dial the aperture into the camera when using manual lenses. As long as the number dialed into the camera matched the aperture selected on the lens aperture ring, the camera should be able to perform the necessary calculations.
01-07-2012, 09:25 AM   #13
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QuoteQuote:
What I do not understand is why Pentax did not give the user the opportunity to dial the aperture into the camera when using manual lenses. As long as the number dialed into the camera matched the aperture selected on the lens aperture ring, the camera should be able to perform the necessary calculations.
That would be ideal. I could set it to f8 or whatever and leave it and let it autocheck the light without pushing the button..do any of the other model cameras do that? or is that the same across the line?

QuoteQuote:
sorry no choice you have to +/- each time if the lighting changes.
Right, in sports sometimes you have to get a shot fast, so not ideal unless I confine my shots to one end of the rink at once so I can meter and then shoot a few shots, then meter for the other end as light changes.
01-07-2012, 09:59 AM   #14
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QuoteOriginally posted by crossmr Quote
Right, in sports sometimes you have to get a shot fast, so not ideal unless I confine my shots to one end of the rink at once so I can meter and then shoot a few shots, then meter for the other end as light changes.
Well the light fluctuations at one end of the rink will not change drastically from one moment to another. What you could do is meter both ends of the rink and remember the speed and just dial it in. Even if the lighting changes slightly you can also post process it by 1/2 or maybe even 1 stop without blowing out the picture. In sports I usually find the issue being focus then exposure. However, that is only an issue when your not using infinity.
01-07-2012, 10:14 AM   #15
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The camera doesnt stop down the aperture blades until the pic is taken because it will be hard to see through the viewfinder if you are shooting with a small aperture.
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