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03-27-2013, 01:33 AM   #16
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Location: Croydon
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QuoteOriginally posted by Not a Number Quote
According to the EXIF data all the photos except the second one (shot with the Pentax lens) have a shutter speed of 1/160th of a second. The photo in question shows a shutter speed of 1/125.

All the photos except the third photo shot with the Pentax show an ISO of 100. The third photo shows an ISO of 200.

The net effect is that there is only a 1/3 stop difference between the 2nd and 3rd photos.

If you keep the ISO and shutter speed constant the first set of photos shot with the Pentax lens should look like those shot with the SIgma.

What settings are you using on your camera? And how are you metering the shots?
I'm so sorry, I realized what mistake I've made. I've changed the ISO and shutter speeds to keep the photo in proper exposure. I'll re-shoot with constant ISO and shutter speeds this time.

03-27-2013, 01:36 AM   #17
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QuoteOriginally posted by stevebrot Quote
I did a quick eye ball of the six shots and don't know that the Sigma did that poorly.

Lens comparisons between different focal lengths are a bit tricky. It is usually best to provide equivalent magnification which means that the shorter lens needs to be closer to the subject. (I generally use 20x the focal length.) Doing so introduces the issue of different perspective, so it is usually best to choose a flat, highly detailed, high contrast subject (a sheet of classified newspaper ads often works just fine.) Then you can compare apples with apples.

It is possible to shoot from the same distance and crop, but then you are asking a lot more from the shorter lens.

FWIW, it might have been good to start a new thread regarding the performance of your Sigma. There are several users on the forum who are fans of your lens and who might be willing to offer their insight as to how high your expectations should be.

Another mistake. Shooting with different focal lengths to compare. I'll re-shoot with a same focal lengths and from same distance, crop etc.
03-27-2013, 02:52 AM   #18
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Many thanks for all the responses. This time I've taken same focal length lenses, Miranda 28mm f2.8, Petri 28mm f2 and Sigma 28mm f2.8. I maintained the same distance, same ISO and same speed for all the 3 lenses on a tripod. Hope, this helps for better illustration. First set with Miranda, second with Petri and third with Sigma with ISO 400 and f4, f5.6, f8 respectively.

Last edited by vass; 09-09-2014 at 09:35 PM.
03-28-2013, 02:16 PM   #19
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Join Date: Jan 2013
Posts: 19
Original Poster
Well the lens came in today, first pentax k mount lens for me

QuoteOriginally posted by maverick75 Quote
Got a 28mm 2.8 macro today, it's minty fresh! and only $20 shipped with both caps, brand new case and adapter from eBay.

The only mods I had to do is bend the aperture tab and adjust the adapter, both were easy.

Seems a tiny bit soft at 2.8, but that could just be me not focusing it right

Shots some jpegs with it:

28mm, @2.8, 0.3 meters

28mm, @2.8, 0.3 meters

28mm, @2.8, 1:4 macro


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