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01-07-2011, 01:50 AM   #1
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Tested focus on lens

Thought I would start a new page to save all the scrolling. I have included a pic of the distance I was from target. ( Which was manual and set at f8 1/60 iso 200.) For the first test I set the f stop for f3.5 on and automatic exposure which set itself for 1/1000 no idea why and had to lighten it. the focal distance was 28. The second shot I zoomed in 'cos I wasn't sure if I should. I did the same setting but the camera put itself on f5.6 as I zoomed in and automatically exposed at 1/500 and the focal distance was 63 iso 200. You may now give your expert opinions on the results! I think I might have forgotten one step?

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01-07-2011, 07:47 AM   #2
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On your first shot, it looks like you are hitting it at around front 40mm (as read on the chart). Your second shot looks pretty spot on in the middle.

That's just what it looks like. Before I would draw any conclusions however, what does the first shot look like at the focus line if you zoom in on it (I cannot, photo too small)? Also, what happens if you Manual focus on that point?

01-07-2011, 08:42 AM   #3
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It still looks pretty blurry on larger view. Seems funny to think I need to zoom in to get sharper focus. will try manual tomorrow.
01-07-2011, 08:55 AM   #4
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it looks to me like the camera angle is too steep...more than 45degrees.

when at the wide end. raise the target and take it closer to the lens in that 45 degree ark..just so you can see the chart more clearl

with you test shots the long end seem ok..short end is FF..but that may be user error due to the fact the target is so far away at that wide angle and hard focus on the true target

01-07-2011, 10:08 AM   #5
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You should also use as large an aperture as you can ..... F5.6 is too much really if you want to accurately note where focus is (don't zoom in .. move closer or just enlarge the shot later).

Note - I did think that there was a degree of FF in your previous pics (look at the flowers in the shot of the couple ?).
01-07-2011, 11:44 AM   #6
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QuoteOriginally posted by Frogfish Quote
You should also use as large an aperture as you can ..... F5.6 is too much really if you want to accurately note where focus is (don't zoom in .. move closer or just enlarge the shot later).

Note - I did think that there was a degree of FF in your previous pics (look at the flowers in the shot of the couple ?).

At that end, f5.6 IS wide open. This is apparently a cheapo Sigma, not an f2.8 lens.

01-07-2011, 04:44 PM   #7
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If I could interject my thoughts on this, I think that this sort of test, while amusing to do, is not especially telling of how the equipment will perform in general use. For myself, I believe focus tests should be done at more normal subject distances, a couple of meters or so at least.
This is just a gut feeling, I don't have any particular quantitative reasoning in this regard.

Carry on.
01-07-2011, 07:19 PM   #8
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at F5.6 the DOF extends pretty equally F/B on the chart..so Id say that the focal point is pretty spot on. and at the F stop the DOF will mask any reall world problem with FF/BF alignment.

Wheaty

Trish has done just that,,in another thread which shows some real world examples of a soft focus...she's now trying to see if its a FF/BF problem or just a crap lens....some other examples taken with the istd the lens looks better than the KX she is now using..although Id expect that the Better sensor of the KX shows the lens faults more than the previous body..{more Resolution}

trishs other thread


https://www.pentaxforums.com/forums/general-technical-troubleshooting/128139-...-new-lens.html

01-09-2011, 12:13 AM   #9
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I think you have all been just brillliant at helping me here. with the comments on f5.6, I did set it to f3.5 and automatic exposure, and when I zoomed in the f stop went to 5.6. Anyway I think I am going to take the plunge and put a new lens on laybye (for non Australians, that is not a quiet parking bay, it is put down a deposit and then pay what you can until paid off) What I will do is keep the cheap sigma, and have it on the 1stD as a second camera. Now though, the big decision; Sigma 17-70, tamron 28-75 or Pentax 17-70, though thommat (?) does say the tamron can have some focus probs too. OK. I shall await your thoughts.
01-09-2011, 09:33 AM   #10
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The 28-75 is probably the best lense (and fastest) but it really depends on whether you need the wide end or not.
01-09-2011, 04:50 PM   #11
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QuoteOriginally posted by trishytee Quote
Anyway I think I am going to take the plunge and put a new lens on laybye (for non Australians, that is not a quiet parking bay, it is put down a deposit and then pay what you can until paid off).
In the USA, we call laybye, layaway. Good luck in choosing your new lens. My next lens will be the Tamron 28-75.
01-12-2011, 09:32 PM   #12
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Hmm I must say chosing a new lens is a worry. I mainly want the new one for weddings, as the other is OK for the studio ones. can I ask if you prefer the Tamron over the Pentax DA17-70? I like the idea of the larger aperature on the tamron for window shots etc.
01-12-2011, 11:28 PM   #13
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Trish

I think if I were you Id have F2.8 as a must for any new medium zoom that you may buy..that will then take a few out of the equation and you can stop thinking about them...

the reason I love a F2.8 lens is for a much brighter view finder..and the increased flexibility it gives you for either a low light situation or creating a portrait with good Bokeh .

once that is a done deal..then work on a focal length zoom that will be most use full to you..perhaps go through some of you past work and see what FL you can not do without....

once you have both the F stop sorted and the FL of the lens..then start to look at lenses within those parameters ....with Lenses you also only get what you pay for..

Id advise you to look at some of the sigma range....the reason behind that is the sigma distributor in Australia will match any overseas or grey import price you can find..the Guys at Camera electronics in perth will be able to help you with that, as they are a full bottle on it
01-13-2011, 12:30 AM   #14
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You do make a lot of sense, but now I am wary of going sigma. sadly I am in Mandurah and while I get good prices here because I am in the camera club, they are nowhere as full bottle as the chaps at cannington. I still have your list. I really don't need that much zoom as funnily enough my Sigma 70-300 is not bad at all. So I am now at the Tamron or Sigma stage I guess as the Pentax DA is not f2.8. Well I am now off to the casino to win some money for the lens I will finally get!! Sob sob. As you might have guessed I am a bit vague about some things so: what is the dif between 28-75 and 17-70 in terms of the photos I posted forever ago.
01-13-2011, 02:23 AM   #15
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if you want a Pentax lens that is in that FL range , thats is also 2.8..then it will be a 16-50 DA*..

anther great pentax lens is a 50-135 DA*.. I personally feel that would be a great wedding lens .

Ive just bought a Sigma 17-50 OS..fantastic lens ..
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