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08-10-2010, 08:11 PM   #1
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Pentax A 50mm F2 questions

I just bought a pentax P30T with two lens and carry case for $60. all of it looked to be in great shape, the focus is extremely smooth and im pleased with the feel of it. However i cant get it to perform up to my expectations. Im not talking about sharpness either, im talking about brightness.

I bought this setup for my Pentax K-X because i wanted a faster lens on the cheap to play with. Mostly ive been shooting my 18-55mm at F4.5 around 24mm. I assumed this 50mm at F2 would be a LOT brighter right? In my house with all the lights on it seems to have less light and require a much slower shutter speed to match my 18-55mm is it because my 18-55mm is shooting at 24mm instead of 50mm??

Here is some samples.





08-10-2010, 08:39 PM   #2
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The EXIF info is gone from your pictures, so it's hard to say what you are doing here. But f/2 is two and a half stops brighter than f/4.5. So if you're shooting at f/4.5 on the 18-55, then shooting at f/2 on the 50 should give you a shutter speed 6 times faster. If you're not shooting the 50 at f/2 but are instead matching the f/4.5 you had been shooting the 18-55 with, then you should get the same shutter speeds, more or less - assuming you are shooting the same kind of scene. The examples you posted are actually quite different. One is a large white space with a small black object in it. The other is a large black object surrounded by a small amount of white space.
08-11-2010, 05:08 AM   #3
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The picture on the left was not at F2 but F11 and a 10 second shutter speed. My 18-55mm would of had that much or more brightness with 3 second shutter speed. The picture on the right with the blokeh (sp?) was at F2 and shutter speed of 1 second i think. iso = 200.

Both pictures were taken at the same spot but the angle is different. the left is on a tripod so its about 22-25 inches above the old pentax. the one on the right my K-X is resting on the counter facing the old pentax.
08-11-2010, 05:43 AM   #4
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Make sure you test the 18-55 at 50mm to be consistent. Also check metering and ensure they are both the same (matrix, center weight or spot)
The same scene has more light available at wider angles.

Thanks,

08-11-2010, 09:29 AM   #5
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To compare, you should shoot the same objects on the same spot with the same conditions.
The first photo has a bright background which would make the meter work different from the second photo which might just meter the black of the cap and it would meter differently too!

Put your test camera on a tripod pointing at one direction and subject only then just change the lenses only.
..also as stated with the above posts, zoom the 18-55mm to 50mm too since you are comparing it to a 50mm so that the meter would also just read the same areas ans not be as wide as the 18mm which might get more ambient light 'coz it is not zoomed-in (50mm) to the subject.
08-11-2010, 09:32 AM   #6
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Ill try that when i get home from work.
08-11-2010, 02:49 PM   #7
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QuoteOriginally posted by buttons Quote
The picture on the left was not at F2 but F11 and a 10 second shutter speed. My 18-55mm would of had that much or more brightness with 3 second shutter speed.
Without seeing both pictures - taken back to back at the same ISO - I'll just say I find that unlikely.

QuoteQuote:
Both pictures were taken at the same spot but the angle is different.
Which is my point. That change in composition changed a lot as far as metering goes.
08-12-2010, 12:51 AM   #8
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QuoteOriginally posted by Marc Sabatella Quote
One is a large white space with a small black object in it. The other is a large black object surrounded by a small amount of white space.
And there's me thinking they were called cameras.

08-12-2010, 05:11 AM   #9
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Ok, i took two pictures last night both at 50mm, from a tripod at F5.6 because thats the lowest i could go on the 18-55mm da lens. Both were 1/5 shutter speed.

The 18-55mm appears to be brighter and a tad sharper then the pentax-a smc 50mm/f2. Pictures taken with the 50mm f/2 have a "yellow" mask -- is this UV damage that happens with age? Its easily fixed with post processing and i dont really notice it until i compared images back to back.

Ill try to upload them on my lunch break.
08-12-2010, 08:04 AM   #10
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At that shutter speed, most likely you're seeing a little camera shake, even with tripod, unless it's a very sturdy tripod, and you don't say if you used the mirror lockup feature. When performing these tests, it's better to do so in good light where you can decent shutter speeds. Also, with the 50, obviously, you had to focus it manually, so that introduced another variable.

The 50/2 should definitely be sharper than the 18-55 when both are at f/5.6. Color difference could be due to the lenses themselves but could just as easily be a matter of the camera choosing different WB for the shots. So it's best to set WB manually when comparing.

There might indeed be slight differences in brightness between two lenses both at f/5.6, but there shouldn't be anything even remotely close to the two stop difference you described earlier. At most, maybe a quarter of a stop.
08-13-2010, 12:16 AM   #11
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I am no expert on this, but oddly I was doing some experimenting today with the same lenses as you. From what you are describing, two things come to mind. And if I am wrong the experts here can correct me and I learn along with you:

1) The yellowness in the second photo appears to be from tungsten lighting? If you set the WB to correct for that, it goes away.
2) Are you talking about contrast and not absolute exposure? The A 50mm will have much more contrast than the zoom.

I took the following photos today. ist DS, same camera settings for each, ASA 200, f8, and 1/125 sec. The two zooms were set to 50mm. They were offhand and not mounted. Jpg mode and not altered.

1) Pentax A 50mm 1:2


2) Pentax DA 18-55mm


3) Hanimex 35-70mm

Last edited by Oro; 08-13-2010 at 12:50 AM.
08-13-2010, 08:33 AM   #12
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QuoteOriginally posted by buttons Quote
Ok, i took two pictures last night both at 50mm, from a tripod at F5.6 because thats the lowest i could go on the 18-55mm da lens. Both were 1/5 shutter speed.

The 18-55mm appears to be brighter and a tad sharper then the pentax-a smc 50mm/f2. Pictures taken with the 50mm f/2 have a "yellow" mask -- is this UV damage that happens with age? Its easily fixed with post processing and i dont really notice it until i compared images back to back.

Ill try to upload them on my lunch break.
Let's remove another couple variables from your test (for future reference). Set the camera to Manual focus with the kit zoom mounted. That may answer the Sharpness question. Second, set your camera to Spot meter and use the same spot for both exposures. That should largely negate the affect of the surroundings, shooting angle, etc.

Lenses that yellow, or at least with my understanding of it, do so because they were made with radioactive materials either in the coatings or the glass itself. A Yellow cast, in this case, is more likely caused by white balance error and not so much the lens. If your lens were yellowing, you could probably see it on the lens.

08-13-2010, 10:48 AM   #13
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As far as I know, yellow lenses due to radiation in the coating are very very old, from the M42 screwmount era. I think that was used only on a handful of lens models. I don't think there were any K mount lenses with that coating.
Can anybody confirm or deny this?

One more thing: In the pic with the 18-55: Was the hood used? Since the front element is not as recessed as in the A50 f2, a hood can help reduce glare and improve contrast.

Thanks,
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