[mee]

I was testing sharpness across frame between my DA* 16-50mm and a DA 50 f/1.8 this evening (just to see how sharp the zoom is comparatively to a known sharp prime). Something that caught me off guard during this little test was the field of view is different at 50mm between the two! I suspect my zoom is not a true 50mm and the prime is. The difference is noticeable... I think the zoom is really a 16-47 or so

And, yes, I was for sure full to 50mm on the zoom ring... isn't going any further and is aligned with the 50 text.


Now I want to try the 16mm end with an ultrawide and see if they match.. but I suspect already they will not. Also might test the 50-135 .. but then I can test the 18-135 too...

this is pretty interesting to me as it has an effect on what one is able to reproduce if needing a more exacting level of care on maintaining FL between lenses.

[Alex645]

It would make sense that the zoom is optically 3x or a 16-48mm. The manufacturing world is full of approximations as a 32GB USB flash drive can't hold 32GB and 3 liter engine is often something like 2962 ml and not 3000 ml. Of course the implications would extend to the aperture as well.

[Adam]

Because the 16-50mm focuses internally, the focal length will vary depending on your subject's distance But it would make sense for there to be wiggle room beyond that.

[Na Horuk]

but I suspect already they will not. Also might test the 50-135 .. but then I can test the 18-135 too.

Ah yes, same thing will happen. Many such lenses are notorious for not really being as long as they claim to be. You can try focusing to infinity, and the difference from the prime will be smaller; but if you focus close up (near the minimum focus distance), it will be bigger (the focal length is rated at infinity focus). IF lenses will all act like that, though some lenses use a slight combination of both internal and regular focusing (moving the lens elements). Focus breathing. Here is another thread:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/309669-60-2...-60-200-a.html

But there is another thing - the 50mm prime might also be a little "off". I remember some time ago I read that one of the Pentax 50mm models is actually closer to 52mm.. problem is, its all about labeling conventions.

Edit: I've edited the post a little, so its a bit different from the quote below. I added the thread and a longer explanation

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Last edited by Na Horuk; 04-05-2016 at 06:33 PM.

[mee]

Because the 16-50mm focuses internally, the focal length will vary depending on your subject's distance

Hmmm.. I'm afraid I don't understand what you are saying there. I mean why would the FL vary depending on distance of subject? I'm all eyes for learning something new..

---------- Post added 04-05-16 at 08:29 PM ----------

Ah yes, same thing will happen. Many such lenses are notorious for not really being as long as they claim to be. You can try focusing to infinity, and the difference from the prime will be smaller; but if you focus close up (near the minimum focus distance), it will be bigger (or is it the other way around? now im not sure)

But there is another thing - the 50mm prime might also be a little "off". I remember some time ago I read that one of the Pentax 50mm models is actually closer to 52mm.. problem is, its all about labeling conventions.

ahhhhhhhhhhhhhhhhh!! This is really throwing a wrench into the works

I was shooting at infinity tonight so that is my reference point for my OP just fyi. Now I'm tempted to go through ALL my lenses and check for differences..

[ScooterMaxi Jim]

Following up on Adam, the typical internal-focus lens has at least some focus breathing, so close focusing can lose 10% (45mm) pretty easily. Fact is, most zooms cheat the range by roughly 5% on both ends - even at infinity. Some brands hedge more so than others.

Remember too, the more complex and more elements, the more transmission loss. A lot of supposedly "pro" lenses are far closer to f/3.2 than the stated f/2.8. Most primes are much closer to actual stated f-stop due to far fewer elements.

[Digitalis]

the 50mm prime might also be a little "off"

You would be astonished by the number of 50mm lenses in my collection that aren't 50mm exactly. Nikkor 50mm lenses tend to be longer 52~59mm, canons 50mm tend to be wider 49~53mm. Zeiss and Leica 50mm lenses tend to be around 51mm. Pentax 50's are rather consistent, Especailly the 50mm f/1.4 lenses that have a distinct lineage. But the 55's and some of the slower 50mm lenses from pentax vary quite a bit. For instance the 50mm f/1.7 is 53.4mm the SMCP-K 50mm f/1.2 is 51.8mm

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Last edited by Digitalis; 04-06-2016 at 12:19 AM.

[mee]

For academic purposes, is there a means to determine what is precisely 50mm on our crop body cameras at say.. infinity? Or is this like tilting at windmills?

[dms]

Modern Camera used to test two lenses, and publish:
-- actual FL range
-- fully open aperture, although not sure if they meant true (geometric) aperture or t-stop (most of us would be interested in t-stop)
-- resolution and contrast (at 30 line/mm), and as I recall did so at macro distance for macro lenses as well
-- bench test observations/basically more qualitative results (e.g., comma)

I don't see all this fundamental information in current lens reviews--well most do resolution and some (one?) do comma--but not the others. You cannot judge a lens only by the numbers, but neverthless a lens test should measure those things that can be reasonably easily measured (by the testing organization and not by most individuals), and not subject to interpretation, and are likely to be of importance.

[Digitalis]

For academic purposes, is there a means to determine what is precisely 50mm on our crop body cameras at say.. infinity? Or is this like tilting at windmills?

Have fun with the windmills.

-well most do resolution and some (one?) do comma

Don't you mean Coma?

[zapp]

For academic purposes, is there a means to determine what is precisely 50mm on our crop body cameras at say.. infinity? Or is this like tilting at windmills?

I don´t see a problem in the small offsets. Standards allow for 1% offset of the reportet focal length value. Some manufacteres used to provide focal length number down to 0.1 mm, most don´t care. For practical purposes you get what you get. If a manufcateres gets a nice solution for a 48.89 mm lens he will call it 50 mm and is not blamed for it.You can determine precise focal length for each focus position if needed - for scientific purposes.
Some people calculate comparable focal length down to 0.1 mm based on some conversion factor. This is all pretty much pointless.

[dms]

Yes, thanks for the correction--it is "coma." BTW for some readers that may not be aware of it--LensTip.com does coma testing as part of their lens reviews. I don't know that any of my (all) older lenses were tested by them--and thus I have no feel how realistic is their testing procedure/are their results.

[Digitalis]

s I have no feel how realistic is their testing procedure/are their results.

I think their testing methodology is fine, at least they are open on exactly how they arrive at their testing figures unlike DxO.

[bxf]

It would make sense that the zoom is optically 3x or a 16-48mm. The manufacturing world is full of approximations as a 32GB USB flash drive can't hold 32GB and 3 liter engine is often something like 2962 ml and not 3000 ml. Of course the implications would extend to the aperture as well.

Sorry for going OT here, and for being pedantic, but to clarify:

The 32GB which is not quite 32GB is typically explained by the inconsistent use of the terms KB/MB/GB. Technically, the units are supposed to be ordered by a factor of 1024, but this is often replaced by a factor of 1000. Also, some uses of the drives have implicit overheads which result in reduced capacities as seen by the user.

The reason car engines are just under the stated or implied size is due to taxation factors in some countries. For example, a 2000CC engine would be taxed at a higher rate than one of 1999CC.

[Simen1]

Focal lengths are defined at infinity focus, and they are usually shorter at shorter focusing distances. For instance, some Nikon 70-200 lenses have been criticised for being only around 160-170 mm in the long end at portrait distances.

In lens patents the focal length is often precise down to 3 decimals, again at infinity focus, and usually rounded to a near round number. Zooms are usually a little narrower then marketed. Aperture numbers are usually rounded down. I wouldn’t be surprised if the Pentax DA 16-50 uses a patent of for instance 16,543 - 48,765 mm and f/2,890 - 2,901. And if it measures "worse" at closer distances.