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Old Jun 5, 2007, 10:44 AM   #1
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I was testing my kit lens 18-55 against my A 50 f.2 lens at 5.6, 8.0, 11.0 and 22.

The kit lens at 50mm would only go down to 5.6 the prime goes down to 2 which is nice. BUT...when I ran my DL on aperture priority at these settings with the two lenses the kit lens shutter speed was FASTER.

I can't figure it out. Maybe the kit lens meters different or it said 50mm but was a tad wider? It was but by only a fraction.

Am I off base thinking under the same lighting conditions the shutter speed on both lenses should be the same at the same fstop and ISO?

Oh, shooting some stuff on my dresser drawer from maybe 5 feet away I did not find any difference in image quality between the kit lens and the prime. That I could see anyway.

What conditions might I see an image quality difference?
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Old Jun 5, 2007, 12:27 PM   #2
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Hi corvairfan,

I really can't say for sure, but realize that the camera was metering the scene wide open for both lenses, so at f5.6, it was essentially metering at actual aperture with the kit lens and calculating the exposure with the 50 f2. Whether or not that makes a difference, I don't know. . . If you're worried about it, and the difference was significant (more than 1/4 stop), take the same shot with both lenses (camera mounted on a tripod, artificial light to keep things constant) and compare the histograms. If the exposures are significantly different, you'll know that you have to compensate the Ev accordingly when using that lens. Also try the different metering modes (spot, center weighted, and matrix) to see if using any of these make a difference.

The IQ differences will likely be most evident when using the wider apertures of the 50 f2. You'll be able to control DOF more with the faster lens with wider apertures, and I would guess that the bokeh would be more pleasing (smoother)with the prime at wider apertures. Another area where the prime should be better is in distortion. I'm only guessing, but it would probably be pretty safe to say that the kit would have more pincushion distortion than the prime. I would also guess that the prime should capture significantly more resolution at equivalent apertures. Casual shots might not show this, but taking careful comparison shots of detailed subjects probably will.

The DOF control and bokeh qualities should be pretty easily evident, the distortion probably won't. I would think that the differences in resolution should be relatively easy to see, but camparisons between two good lenses are sometimes subtle.

The ability to gather 3 times the light wide open should make for significantly more available light shooting opportunities with the prime.

Scott


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Old Jun 5, 2007, 3:25 PM   #3
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corvairfan wrote:
Quote:
I was testing my kit lens 18-55 against my A 50 f.2 lens at 5.6, 8.0, 11.0 and 22.

The kit lens at 50mm would only go down to 5.6 the prime goes down to 2 which is nice. BUT...when I ran my DL on aperture priority at these settings with the two lenses the kit lens shutter speed was FASTER.
I'm not sure I understand what you mean. Are you saying the kit lens used faster shutter speeds at f/5.6 than the 50mm lens at f/2? If that was the case, I'd check to see if auto ISO was on.

If you're saying that you set the apertures to be the same for both lenses but the kit lens was getting faster shutter speeds, that's something I've observed myself. Hayward once pointed out that the image circle on the DA lenses is smaller and therefore concentrates the light into a smaller area, creating a brighter image. I tested this by setting the camera up on a tripod and shooting the same object with the kit lens and my 50mm 1.7, both at f/8, and I did find that the kit lens seemed to capture an image that was about half a stop brighter.

I'm not sure if there may have been any flaws in my testing, and I might want to try again. I did an outdoor test as well recently using similar methodology, capturing the same scene with both lenses at f/8 1/125, but this time the kit lens was only marginally brighter, not enough to make a difference.
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Old Jun 16, 2007, 3:22 AM   #4
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Yes a DA lens concentrates ALL the light on the exposure area... where as a FF lens has a 1/3 spillage outside the the imager area. (and why DA lenses will NOT work on a FF camera or severely vignetted anywhere near wide open.

So yes f/stop should be f stop.... but not when imaging area differs. But Both lenses actually are but for different image areas its focused on. The DA concentrated on the smaller area vs the FF lens on a digital 1/3 or so going to waste outsiide it

So in a digital sense a FULL FRAME 2.8 lense is actuall only functionally a 3.5 on a digital, and would likely be matched by the kit DA lens at its min 3.5, exposure wise.

JUST THINK OF HOW A FLASH LIGHT BEAM BEHAVES....

Farther you spread it dimmer it effectively is... yet bulb gets no dimmer... you concentrate it all in a specific registered area quite bright..... works the same way with lenses.

And also why a cheap FF lens might be fine on a digital. but maybe vignettes on a FF cam.

And my diappointment with the cheap NOT ES 2.8 Sigma but the 18-50 3.5-5.6 DC lens (Their term for DA digital only).... wide open vignetted significantly, MUCH WORSE than anyone complains of the Pentax kit 18-55mm (and since aquired one and much more minimal).... on a FF it would lkely look like an uncurved fisheye with TOTALLY black corners.

And again but for the 1/2 stop light loss where inexpensive FF lenses look quite good on on the digitals while maybe not on FF.
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Old Jun 16, 2007, 7:10 AM   #5
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hayward,
it would be nice if people could make sense out of 50% of what you post.
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Old Jun 18, 2007, 12:15 AM   #6
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That makes no sense at all, just bacause you are looking at a subject through a smaller window does not make it any brighter.

It is hard to figure out just what the kit lens is doing because it doesn't have an aperture ring and the f/stop listed in the viewfinder is a close estimate of what it really is, but it could be off a bit each way.

If your exposures are correct with both lenses then it doesn't matter what it says in the viewfinder anyway.

Tom
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Old Jun 19, 2007, 2:01 AM   #7
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robar wrote:
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hayward,
it would be nice if people could make sense out of 50% of what you post.
OK robar and ennacac

Sorry if optical physics are a bit beyond you....

A DA or other digital ONLY lense concentrates ALL the light on the APC(S?....or whatever the format was in film I never gave a crap about as digital was already there... and if I still did film it would be FF 35mm) image area vs FF 35mm area.

By NATURE that is going to be brigther as there is no spilled light, and even if they fit would be SEVERLY vignetted on a FF camera.

A full frame lens on the other hand spills about a third of its light outside the imagining area on a dighital (hense the FL x factor cropping the pic and loosing a lot of light), so while AE compensates for, it is about a 1/2 stop slower in reality.... IE a 3.5 DA will equal a FF 2.8 in AE shutter selected speed.

And at the same time a cheap FF lens that might have minor vignetting issues on a FF would not on a digital.

And my beef.... the cheap digital lens DO have that problem like the Sigma basic 18-50mm DC (NOT the ES 2.8 ) , even the Pentax kit does NOT... they cut corners so close to that edge.... well wide open and flat background is there but not near level to the basic Sigma 18-50mm wide open would be vignetted.







An
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Old Jun 19, 2007, 2:23 AM   #8
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Hayward wrote:
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Sorry if optical physics are a bit beyond you....
After telling us all that there are no FA*-lenses, that the *-series is about weather sealing, that the FA 50 1.4 is worthless, that there is no real difference in quality between 10 bucks and 1000 bucks lenses, that Canon and Nikon don't know anything about camera manufacturing, now you teach us about physics. Is there anything else you don't know anything about you can teach us?

Kjell
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Old Jun 19, 2007, 6:12 AM   #9
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bilybianca wrote:
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Hayward wrote:
Quote:
Sorry if optical physics are a bit beyond you....
After telling us all that there are no FA*-lenses, that the *-series is about weather sealing, that the FA 50 1.4 is worthless, that there is no real difference in quality between 10 bucks and 1000 bucks lenses, that Canon and Nikon don't know anything about camera manufacturing, now you teach us about physics. Is there anything else you don't know anything about you can teach us?

Kjell
Funny how selective your memory is.....

OK I'll give you at one point I didn't know what a * lens was.... have I said any thing since? Did I get nasty and DEFENSIVE about it???? LIKE MANY OF you DO? (Like what I am replying here to now?)

No I said USED FA 50 1.4 were going for as much as new or MORE and that is STUPID. (Nor is it a GREAT LENS) Though also did say were in the 50mm range a used 50mm A is a much better bargain and likely better optics... if you can handle MF.... a zoom AF is very useful... a short prime... MF is an easy thing to work with... and likely are at least semi static subject as well as DOF. And F not even FA.

But hey you want to come down to Key West with a FA 1.4..... I'll put my A 1.4 against it, I'd bet I'd win in a non fast action situation.... which again is rather odd NECESSSITY for a 50mm.

And again I only question that a 135 A * lens is really 8X better than a non star you could easily pick up for 1/8th $1400 And the person who responded got it right.... sort of like having a Paul McCartney Autograph on something.... very nice lense but given the lesser (which were much more expensive in their time but bargains now) The * factor rules for those who are willing to spend rediculous amounts of money on an old A lens.... yeah its the best of the genre.... but again 8x10x better... except in bragging rights?

Of course Canon and Nikon know something..... but they do seem do be dozing.... and again maybe I'd guess considering the formidable thing especially the Pentax K10D presented them while they were.... to my knowledge NEITHER has released XTi was just before...NOTHING since (even upgrades... again Pentax has done twice already Canon has never done pretty much certainly not for added functionallity.... that had to be HACKED on the 300D... they could have offered) or announced anything new. Maybe just hoping Hoya will bury Pentax?

And its not even about quality better but overall functionallity/useability.... even just by the dual function wheels on the 10D I blow away what I can do at instant finger tip 30D and 80/200 users can't and have to dig into menues for let alone the dozen added functions Pentax added in the first firmware upgrade.... neithe rever does just maybe begrudging fixes. (which Pentax also did for odd situations)

Other than the XTi how old is the Canon line now and not even updated?

But no OK argue optic physics with me DA (digital) vs FF lenses.... go ahead.
Since I knew the physics... I didn't feel like doing the tests (plus at the time I was without having a DA lens as my original K10D and lens had been stolen) But Corpsy did and found just what I had said to be true... a DA lens will be about a 1/2 stop faster (shutter AE) than a FF lens at same f/stop.




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Old Jun 19, 2007, 6:57 AM   #10
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Hayward wrote:
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OK I'll give you at one point I didn't know what a * lens was.... have I said any thing since? Did I get nasty and DEFENSIVE about it????
Not since but before... and if you relax a bit you mightunderstand that that is my point here – "mr know it all". Is SCREAMING the only volume you master?

With this I rest my case and promise everyone to ignore Haywards provocations and spend my time here on the forum on the positive side. Sharing fun and knowledge, not to talk of the beautiful pictures that are posted.

Kjell

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