24-70 F4 blows away my kit lens

Started 7 months ago | Discussions thread
ianbrown
Senior MemberPosts: 1,920
Like?
Re: 24-70 F4 blows away my kit lens
In reply to Fred Briggs, 7 months ago

Fred Briggs wrote:

dave92029 wrote:

My 28-70mm "kit lens" has been producing many sharp images, and therefore, I'm not motivated to spend 4X the money on a lens of essentially the same focal range.

I'll use the money I save to purchase the 70-200mm f4 in two months.

Most of these "test shots" don't IMHO, select subjects, actually there is no clear subject in most of the test images, so it is really difficult to know what is suppose to be "in focus". The selected images do not make evaluating any lens easy. Nothing looks very sharp, in these low contrast images.

I totally agree - and DPR's test shots also often guilty of this. There is no point in making test shots of a scene with great front to back depth and then using this to pronounce on lens sharpness in corners v centre when these are often nowhere near being in the same plane.

The problem is that people don't really understand that DOF is a very artificial construct, and depends very much on viewing conditions such as magnification. Many seem the think that everything in the estimated DOF is equally sharp, and that the DOF remains a constant regardless of viewing size, both of which are incorrect.

In fact there is only one plane which is actually in focus - everything in front and behind that is out of focus to varying degrees, depending on how far from the actual focal plane it is. DOF just reflects how much "appears" to be in focus if viewed small enough, or from far enough away.

You can confirm this with a simple experiment. Take a picture of a brick wall at about 45 degrees, using a large aperture setting. View the picture at standard size on screen and estimate how much of the brick is in focus. Now increase the size of the image and check again - the apparent DOF will have reduced. Keep doing this until you reach 100% magnification and observe how small the DOF has become.

The lesson from this is that if you are going to view 36MP images at 100% (i.e. 1 image pixel to one screen pixel) then bear in mind that you had better be pretty sure that the parts of the image you are comparing are in the same plane with respect to the plane of the sensor, otherwise any judgements on lens sharpness across the frame will be fundamentally flawed.

Fred

-- hide signature --

"We are so small between the stars, so large against the sky." Leonard Cohen c1967

Hi Fred

Thanks for this and I am sure you are correct, however surely if I am comparing using the same apertures etc then its a fair comparison,ie like for like, although I accept that its not 100% scientific. It still remains that one lens is better than the other using the same setting???

Thanks

Ian

Reply   Reply with quote   Complain
Post (hide subjects)Posted by
(unknown member)
GeesNew
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow